Screening for Open-Angle Glaucoma in the

Elderly

October 1988

NTIS order #PB91-192872
Screening for Open-Angle Glaucoma in the Elderly

by

Elaine J. Power

Judith L. Wagner

Brigitte M. Duffy

Health Program

Office of Technology Assessment

Congress of the United States

Washington, DC 20510-8025

October 1988

A Staff Paper

in OTA’s Series on

Preventive Health Services Under Medicare

The views expressed in this Staff Paper do not

necessarily represent those of the Technology

Assessment Board, the Technology Assessment

Advisory Council, or their individual members.
CONTENTS

Chapter Page

1.

2.

3.

4.

5.

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Summary of Findings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

The Epidemiology of Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Incidence and Prevalence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Natural Course of the Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Treatment for Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Treatment Outcomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Treatment Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........17

Screening Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

Screening Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Description of Screening Technologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Screening Test Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Medicare Coverage of Screening for Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Costs to Medicare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Implications of Scheduled Frequency of Screening and Screening

Utilization for Medicare Costs . . . . . . . . . . . . . . . . . . . . . . . . ......................32

Costs and Effectiveness of Screening in Preventing Blindness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Problems in Implementing Medicare Coverage of OAG Screening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Appendix

A. Advisory Panel--Project on Preventive Health Services Under Medicare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

B. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

C. The Costs of Screening for Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . 53

Tables

Table Page

1. Estimated Prevalence and Five-Year Incidence of Open-Angle Glaucoma

in Framingham, Massachusetts, 1973-1975 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2. Common Forms of Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Distribution of Intraocular Pressure in the Elderly, Framingham,

Massachusetts, 1973-1975 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Development of Manifest Open-Angle Glaucoma in Individuals With

Untreated Ocular Hypertension In At Least One Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Three Estimates of Rate of Visual Impairment for Eyes of Patients

With Treated Open-Angle Glaucoma. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

ii
6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

Summary of Evidence Regarding the Effectiveness of Treatment for

Ocular Hypertension in Preventing Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Studies Relating Long-Term Outcomes of Treatment to Lower Intraocular

Pressure in Patients Without Visual Field Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

A Comparison of Estimates of Accuracy for Three Glaucoma Screening

Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Calculation of Estimated Cost per Confirmed Case of Open-Angle Glaucoma

and Number of Cases Diagnosed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Calculation of Incidence of Open-Angle Glaucoma

(OAG) in the Elderly Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Glaucoma Screening Test Accuracy Assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Cost Assumptions . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........45

Number of Open-Angle Glaucoma Cases Diagnosed Under Extreme

Assumptions of Test Accuracy and Followup Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

High and Low Bounds of Total Costs of Screening and Confirmatory Followup

for Open-Angle Glaucoma in Initial Years of a Screening Program ...................................47

High and Low Bounds of Cost Per Confirmed Case of Open-Angle

Glaucoma in Initial Years of a Screening Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

High and Low Bounds of Total Costs of Screening and Confirmatory

Followup for Open-Angle Glaucoma In Later Years of a Screening Program ...................49

High and Low Bound of Cost Per Confirmed Case of Open-Angle

Glaucoma in Later Years of a Screening Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Assumptions for Low- and High-Cost Bounds of a Model for Estimating

Costs of Screening for High Intraocular Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Boxes

Box Page

A. Components of Screening Test Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

B. Estimation of Theoretical Upper Limits of Accuracy of Tonometry (21 mm Hg or

Greater) for Identifying People With Open-Angle Glaucoma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

iii
1. SUMMARY

Introduction

Open-angle glaucoma (OAG) is the second largest single cause of blindness in the

elderly, afflicting an estimated 2 to 3 percent

of this age group at any time. Approximately

4,600 elderly people will go blind from some

form of glaucoma this year, many from OAG

(84).

1 Many other elderly people have substantial visual disability short of blindness as

a consequence of the disease.

There are many different forms of glaucoma, all characterized by progressive loss of

vision associated with increased intraocular

pressure (IOP) and subsequent deterioration

of the optic nerve. OAG, the most common

type of glaucoma and the only one currently

targeted by mass screening programs, is the

subject of this paper.

Despite considerable study, the exact

cause of OAG remains elusive. It is a disease

strongly associated with old age; OAG is

very rare in people under age 40, but it is

common among the elderly. Although the

exact incidence of OAG is not known, a “best

guess” based on prevalence data is that at

least 2 in every 1,000 elderly people develop

the disease each year.

The single most important predictor of

future OAG is the existence of elevated IOP,

commonly defined as pressure within the eye

of 21 mm Hg or greater. Between one-tenth

and one-fourth of the elderly have this characteristic. Of those in whom the elevation is

modest (2 1 to 30 mm Hg), about one-third

will go on to develop OAG over the next 20

years. For those with very high IOPs (greater

than 30 mm Hg) the risk is much greater; it

I This number is based on rates reported by the

Nat i ona 1 Society to Prevent B 1 i ndness but is updated based on an est i mated 19B8 elderly population

of 30,263,000.

will take only about 5 years for OAG to become manifest in about one-third of this

population.

In classic OAG, the next sign of disease,

after elevated IOP, is abnormality of the optic disc (the area where the optic nerve enters

the eye). This abnormality is a result of

damage to the optic nerve. As nerve damage

becomes more extensive, defects--dim or

blind areas--develop in the field of vision.

These visual field defects initially go unnoticed by an individual but eventually become extensive enough to cause visual impairment, including blindness.

Not all people with OAG follow this

classic model, and diagnosis of early OAG is

often not a simple matter. First, elevated

IOP and OAG are not always related. Many

people with elevated IOPs will not develop

any other signs of OAG. In addition, some

people develop OAG despite having apparently normal IOPs. It appears that some

people have eyes that can cope well with

modest elevations in IOP, while other people

have eyes that are particularly susceptible to

damage at seemingly normal IOP levels.

Second, an optic disc that appears somewhat abnormal is not necessarily indicative of

OAG. The disc may in fact be normal for

that individual, or the abnormality may be

related to some other eye disorder. There are

certain characteristics that are especially

strongly linked to OAG--for example, a disc

whose central area occupies an abnormally

large proportion of the disc as a whole. But

since the assessment of these features is subjective, the accuracy of an examiner in diagnosing OAG based on them depends heavily

on the examiner’s skill and experience.

Third, not all people in whom visual

field defects are detected have OAG. Their

fields of vision may be deteriorating due to

some other cause.

I
2 ● Screening for Open-Angle Glaucoma in the Elderly

Thus, when a person has all the classic

signs of manifest OAG, and when other possible causes of these signs (e.g., another form

of glaucoma) have been ruled out, the examiner can be reasonably confident of a diagnosis of OAG. A diagnosis inconsiderably

more difficult when only one or two signs are

present--for example, when a person has

elevated IOP and apparent abnormalities of

the optic disc, but no visual field defects.

Such a person may be termed an O A G

suspect, with a definitive diagnosis held in

abeyance until visual field defects develop,

making it clear to the examiner that the eye

is truly suffering from manifest disease.

The larger group of people who have

elevated IOPs but no other signs of disease

are often said to have ocular hypertension

(OH), to distinguish them from people with

more specific signs of OAG. This group includes some people for whom a moderately

elevated IOP is normal (or at least welltolerated by the eye); some in whom OAG is

beginning to develop; and some for whom the

elevated IOP is a sign of abnormality or disease, but not of OAG.

The complexities of establishing that

OAG is present are reflected in the definitions of the disease used in various studies in

the literature. Some studies define a patient

as having manifest OAG only if both an abnormal optic disc and visual field defects are

present ( 13). Others, however, consider subjects to have OAG not only if they have

these features but also if they have less conclusive signs-- for example, subjects who have

an IOP of greater than 30 mm Hg, but no

other signs of disease (24). These differing

definitions are a partial explanation for the

varying results among studies discussed in

this paper. Greater weight has been given in

this analysis to studies in which visual field

defects were measured and used as a criterion

for a diagnosis of OAG. This was done on

two grounds: first, that the development of

visual field defects where none previously

existed is unequivocal evidence that disease

exists; and, second, that this criterion is less

subjective than the assessment that an optic

disc is abnormal and thus is more comparable

among studies.

Summary of Findings

Screening is the detection of disease in

asymptomatic people. To be an appropriate

disease for screening, OAG must meet three

minimum criteria:2

1.

2.

3.

OAG must have a recognizable latent or

early stage, during which persons with

the disease can be identified before

symptoms develop. If there is no such

early stage, screening is useless, since

patients will appear when they develop

the symptoms in any case.

There must be an accepted and effective treatment for patients with OAG.

Furthermore, the treatment must be

more effective at preventing visual impairment and disability when initiated

in the early (symptomatic) stage than

when begun in the later, symptomatic

stages of the disease.

There must be an appropriate, acceptable, and reasonably accurate screening

test.

These three criteria are discussed below.

Early-Stage OAG

OAG does have an early stage at which

the disease can be diagnosed but the patient

has no outward signs. Once both optic disc

changes and visual field defects have developed, a diagnosis of OAG can be made with

reasonable confidence, even though the

patient does not usually notice a change in

visual ability until the defects have become

fairly extensive.

Although they precede actual visual impairment, visual field defects do not develop

until the disease is well established and has

already caused injury to the eye. Consequently, there has been great interest in

identifying people with OAG at earlier

stages- - when abnormalities of the optic disc

first appear, or even when the disc still apZ These three criteria are derived from the basic

principles of screening first set out by Wilson and

Junger twenty years ago (125).
Screening for Open-Angle Glaucoma in the Elderly ● 3

pears normal but elevated IOP is present.

The difficulties with these criteria for

identifying people with OAG were mentioned

earlier. People with OH are at higher risk

for OAG than are others in the population,

but they are not all destined to develop

manifest OAG within their lifetimes. People

with abnormalities of the optic disc are often

designated OAG “suspects, ” but not all of

them have that particular disease. Still, if

treatment is safe and sufficiently effective in

preventing impairment in those who have

OAG, people may wish to be treated upon

the finding of signs associated with the earliest stages of the disease, even if some of

those people do not, in fact, have OAG and

are therefore treated unnecessarily. Thus we

are led to the next question: effective is

treatment?

Treatment

Treating people with OAG has been established medical practice since the nineteenth century. Standard treatment consists

of medication to lower IOP. If medication

alone is insufficient in achieving this, a

physician may perform surgery to create an

artificial opening through which fluid can

flow out of the eye, lowering IOP. New

drugs (with fewer side effects) and laser

surgery

3 (with fewer complications) have

been added to the array of therapeutic

alternatives over the past decade, but the

basic treatment framework remains.

Many eye care practitioners treat people

they consider to be at high risk of OAG as

well as those with manifest disease (i. e.,

visual field defects). A patient might be

considered “high risk” in the opinion of that

particular practitioner if the patient has a

high IOP, suspicious abnormalities! of the optic disc, a family history of OAG, or any

combination of these and other factors.

3 The most promising form of laser surgery, argon

laser trabecu(oplasty (ALT ), is not quite analogous

to traditional surgery to louer IOP. Rather than

creating new openings to enhance fluid flow from

the eye, it scars the tissue around the existing

natural openings, stretching the openings and enabling better out flow.

Just as the assessment of who is sufficiently “high risk” to be treated is considered

a matter of individual judgment, treatment

policies and preferences vary considerably

among physicians and optometrists. 4 For example, one physician might prescribe medical

treatment for a patient with an IOP of 24 mm

Hg and a family history of OAG, even if no

more definitive signs of OAG are present.

Another might treat only patients in whom

highly abnormal optic discs and/or visual

field defects are detectable. Both would be

within the range of currently accepted medical practice.

The difficulty of deciding who should be

treated is compounded by the uncertainties of

treatment effectiveness. On the one hand,

treatment is inconvenient, costly, and uncomfortable; for a few patients, treatment is

itself a risk to health. It has distinct drawbacks even for those with manifest OAG. On

the other hand, if treatment is very effective

at preventing visual impairment from OAG,

and if it is more effective if initiated earlier

in the course of the disease, it may be

desirable to treat some or all people with OH,

even at the expense of treating many of them

unnecessarily y.

That treatment of both OH and manifest

OAG is effective is an accepted tenet of the

medical profession, 5 but it has not been documented in the literature to date. The relationship between IOP and OAG is wellestablished; it has been shown that higher IOP

is associated with higher risk of OAG, and

that a rapid rise in IOP to very high levels

induces glaucoma (5,38,95). What is absent is

sufficiently convincing evidence that the

reverse is true: that lowering IOP by conventional means also lowers the risk of OAG in

those with OH, and that lowering IOP slows

or prevents visual impairment in those who

have manifest OAG.

4 Thirteen States permit optometrists to treat

giaucoma patients with medications. In 11 of those

States , the opt omet r i s t need not consu ( t a

physician before initiating treatment (122).

5 One medical researcher has recently questioned

the effectiveness of OAG treatment (32).
4 ● Screening for Open-Angle Glaucoma in the Elderly

No adequate studies comparing treated

with untreated patients with manifest OAG

exist, because physicians have considered it

unnecessary and unethical to mount such a

study. Consequently, one must turn to the

results of studies of treatment for patients

with OH (i.e., elevated IOP only) and infer

the results to treatment for manifest OAG.

A few studies comparing treated and untreated patients with OH have been conducted; their findings are mixed, ranging

from effective to no effect to harmful. All

have deficiencies in design that either make it

difficult for the study to find an effect (for

example, small sample sizes or short study

duration), render the conclusions suspect, or

make the findings inapplicable to the larger

population.

Two very recent clinical trials, whose

final results are still unpublished, may establish the effectiveness of treatment in preventing OAG in persons with OH. Preliminary

results from these studies suggest that if an

effect exists, it may be substantial. It

remains to be seen whether the final study

results will reach statistical significance. (It

is possible for even a substantial difference in

outcomes between groups to be due to

chance; if the results of these studies reach

statistical significance, this explanation is unlikely and a true effect of treatment probably

exists). Both studies are nearing completion

(68,71).

If treatment of OH is shown effective in

preventing or delaying the onset of manifest

OAG, and if treatment of manifest OAG is

therefore presumed effective in preventing or

delaying visual impairment resulting from the

disease, the degree of effectiveness may still

depend heavily on the criteria used to decide

when the current level of treatment is sufficient. Under the treatment practices of the

past, most patients with manifest OAG still

suffered gradual visual deterioration. These

treatment practices included acceptance of the

idea that if an elevated IOP could be lowered

to the high end of normal--e. g., from 30 mm

Hg to 21 mm Hg-- treatment was adequate.

In the past decade, as evidence has accumulated that people suffered visual deterioration

despite this level of treatment, many ophthalmologists have emphasized more aggressive

treatment, striving for even lower IOPs if any

signs of deterioration occur. They believe

that current treatment practice is thus more

effective than in the past. This difference

has not been documented; it is an important

hypothesis to investigate, both in order to assess the potential gains and in order to

determine whether more aggressive treatment

leads to an increase in the severity or frequency of side-effects.

Screening Technologies

Three technologies have been used in

OAG screening:

● tonometry, which measures IOP by calculating the resistance of the eye to a

force;

● ophthalmoscopy, a tool enabling an examiner to scrutinize the optic disc of a

patient and detect abnormalities characteristic of OAG, and

● Perimetry, which detects defects in the

visual field through a patient’s

responses to dots of light introduced at

various points in the field.

Each of these three technologies is

designed to identify a somewhat different

group of people. Tonometry is designed to

detect people with elevated IOPs who are thus

at risk of OAG, and it is effective at doing

so. It is much less effective at identifying

people who already have manifest OAG

(since some people have the disease without

having elevated IOP). Thus, when tonometry

is used to screen for elevated IOP, it has a

relatively small false positive rate (it is not

zero, because some people whose IOPs were

slightly high at the screening are normal upon

reexamination). When tonometry is used to

screen for manifest OAG, however, it has a

high false positive rate, because most people

with elevated TOPS do not have manifest

OAG.

Both ophthalmoscopy and perimetry also

may produce false positive tests for manifest

OAG, because people may have apparently

abnormal optic discs or visual field defects

for reasons other than OAG. However, the

rate of false positives under ideal conditions
Screening for Open-Angle Glaucoma in the Elderly ● 5

is much lower for both of these tests than for

tonometry.

Still, despite the potentially greater accuracy of these two technologies in detecting

manifest OAG, neither is without drawbacks.

Ophthalmoscopy can detect the first visible

signs of deterioration due to OAG--i.e. ,

degeneration of the optic nerve. But since

evaluations of the optic disc are subjective, it

has a high inherent variation that depends on

the experience of the examiner. Perimetry is

designed to detect defects in the visual field,

the most definitive outward sign of manifest

OAG. Its most significant inherent limitation

is that it cannot identify patients with OAG

before visual field defects occur (although it

can identify them before visual impairment

occurs).

In practice, the relative accuracies of

these three technologies, alone or in combination, in correctly identifying people with

manifest OAG are very poorly documented.

Only one study was found that compared the

effectiveness of all three technologies in

detecting manifest OAG in the same population. The study has not been published (although an abstract of results is available), and

because the setting of the study was mass

screening performed in community facilities

(e.g., churches), its findings may not be applicable to screening performed in the offices

of physicians and other eye care practitioners.

A few other studies exist of the accuracy of

specific technologies alone, but the study designs and the results vary widely. Consequently, it is possible to make some reasonable guesses about the relative accuracy of

the three technologies and the factors that affect their accuracy, but major questions

remain about average effectiveness in practice.

Implications for Medicare

The prevalence of OAG is sufficiently

high to warrant considering the disease a substantial health problem. In an ongoing

screening program, however, it is not the total number of existing cases but the number

of new cases that the program will detect.

Because new cases of manifest OAG are relatively rare, even among the elderly --a few

cases per 1,000 elderly per year- -any screening program will incur considerable costs to

detect those cases.

To gain a sense of the magnitude of

potential costs that might be incurred by

Medicare if an OAG screening benefit were

offered, the average costs of identifying and

confirming a case of manifest OAG in an

every-other-year screening program were

estimated. Under the assumptions of the

model, and assuming that Medicare paid 80

percent of all allowed charges associated with

the screening and follow up/confirmatory

visits, it is estimated that Medicare costs for

an OAG screening program would range from

approximately $160 million to $800 million

per year. These costs do not include the costs

of treating the glaucoma cases detected by the

program. The costs of a screening program

to identify those with elevated IOP (most of

whom would have only OH, but some of

whom would also be found to have manifest

OAG) would be similar. Again, however, the

costs cited here do not include either the

costs of treating OH patients or the costs of

long-term followup of untreated OH patients.

The potential number of OAG cases

diagnosed as a result of a screening program

for manifest OAG is highly uncertain. Such

a program might detect anywhere from

10,000 to 90,000 cases of manifest OAG per

year in an ongoing program, depending on

factors such as the accuracy of the screening

tests and the true incidence of OAG in the

elderly. (In the initial years of such a program, when a backlog of undiagnosed OAG

cases exist, anywhere from 50,000 to 340,000

cases might be detected. ) If the goal of the

program was to identify all individuals with

high IOP, the program would ultimately

detect between 30,000 and 350,000 cases per

year (with between 300,000 and 3 million per

year detected in initial years).

The accuracy of the screening tests has a

particularly important impact on costs. The

costs of a screening program depend heavily

on the number of people with positive tests

who are referred for a followup visit, and

whether a substantial proportion of those are

false positives. If many people test positive,

and thus many undergo a comprehensive fol-
6 ● Screening for Open-Angle Glaucoma in the Elderly

lowup visit, total costs will be high. If most

of these are true positives, however, the cost

per case found will be relatively low; if most

of them are false positives, the cost per case

found will be high.

These screening costs should be compared to the costs of treatment and the economic and net health benefits resulting from

treatment. OTA has not attempted such a

comparison because of the major uncertainties associated with treatment effectiveness. The higher the screening costs, the

greater the benefits of treatment must be to

justify screening. Potential benefits include

improved visual function, improved quality

of life, lessened dependency on others, and

lower expenditures for public programs that

provide financial and personal assistance to

the disabled.

Apart from the potential benefits of

screening itself, the potential benefits of

Medicare coverage for OAG screening depend

heavily on how many people would be encouraged to undergo screening if the service

were covered. Over 50 percent of the elderly

already undergo frequent OAG screening; for

these people, Medicare coverage will bring

some relief from out-of-pocket costs but no

additional benefits in preventing visual impairment. (A small benefit is possible if, as a

result of Medicare coverage, these people

were screened in settings where test accuracy

was higher than wherever they receive the

service at present. )

Elderly people who would not have undergone screening without Medicare coverage,

on the other hand, could potentially improve

their health. OTA assumed that total utilization of a Medicare biannual screening program would be 75 percent-- i.e., one-third of

those screened would not have undergone

screening without Medicare coverage. If this

assumption is correct, then between 3,000 and

30,000 people per year would have manifest

OAG diagnosed earlier (i. e., before they

would otherwise be screened or developed

symptoms), and between 10,000 and 100,000

people per year might have high IOP

identified (if tonometry were the screening

test used), if Medicare covered screening for

OAG.

This analysis examined only the costs and

yield of glaucoma screening as a single service. It did not examine the benefits of

detecting disorders other than OAG as a

result of the glaucoma screening visit. Nor

did it examine the effectiveness and cost of a

comprehensive eye visit in its entirely.

Conclusions

An OAG screening program for the

elderly, whether aimed at detecting only those

with manifest disease or also those at risk due

to high IOPs, is likely to be fairly expensive.6

The potential benefits of such a program are

substantial, but whether those benefits can

actually be realized is still highly uncertain.

It depends heavily on two unknown factors:

first, on the true accuracy of the various

screening tests in the settings in which they

would be used; and, second, on the effectiveness of treatment in preventing, halting, or

delaying the progression of visual impairment

due to OAG.

Most critical to the question of whether

screening for OAG is useful is whether treatment of people with OH (if screening for

high IOP) or OAG (if screening only for

manifest disease) alters the course of the disease. The evidence in the literature to date

leads neither to the conclusion that treatment

is effective nor that it is ineffective; by and

large, the few relevant studies are too small

to detect an effect even if one exists. A consensus panel of eye care professionals would

undoubtedly conclude that treatment is effective, although such a panel would likely also

acknowledge that support for this belief has

been inadequately documented. The opinions

and personal experiences of eye care experts

are compelling, but they are also subjective.

In light of the preliminary results

reported from unpublished research, it is

likely that treatment does reduce the incidence of OAG among those at high risk

6 As a rough measure of comparison, the annual total cost of screening for OAG would be approxi -

mate 1 y one- tenth to one- third of the $3 bi 11 ion

annual cost of the Medicare End-Stage Rena 1 Disease

program ( 116).
Screening for Open-Angle Glaucoma in the Elderly ● 7

and, probably, delays visual impairment

among those in whom OAG is already

man if est. If this expectation is borne out,

screening would be effective in reducing

visual impairment due to OAG. That said,

certain reservations regarding glaucoma

screening deserve reiteration:

●

●

●

The amount of impairment that might be

prevented through a screening program

is unknown. The degree of effectiveness of such a program depends on the

unknown degree of effectiveness of

treatment in preventing or delaying the

development of OAG (in those with

OH) or the development of visual impairment due to OAG (in those with

manifest disease). It also depends on

the accuracy of the various potential

screening tests in the settings in which

they might be performed, which is unknown except within a very broad

range.

Consequently, both the benefits of

screening -- i.e., visual impairment

prevented -- and the costs of screening

and resultant treatment are highly uncertain. OTA’s estimates, which include only costs to Medicare for the

screening episode and confirmatory

visit, cover a five-fold range (from

about $160 million to $800 million).

Treatment costs for those who are

detected with the disease prior to onset

of symptoms would add to the total cost

of screening.

The field of OAG screening and treatment is one that deserves more selfexamination and critique than it has

received. The potential research agenda

for this field is large, and some important areas are outlined below.

Research Needs

Sustained investment in clinical research

on OAG treatment is crucial to evaluating the

effectiveness of current treatment technologies and protocols and to developing better ones. For example, a clinical trial currently sponsored by the National Eye Institute

comparing standard medical treatment with a

new treatment-- early argon laser trabeculoplasty (ALT)--may give some insight into

treatment effectiveness. Although “no treatment” is not an option in this trial, the trial

does include rigorous documentation of the

outcomes of two different types of treatments. Since ALT is a relatively new technology but appears to be widely used, documenting its effects is critically important.

The theory that outcomes will be improved if treatment is more aggressive also

deserves in-depth examination. Current expert opinion supports the idea of increasing

the intensity of treatment whenever visual

deterioration is suspected, regardless of the

absolute level of IOP. This more aggressive

treatment practice has merit considering the

history of deterioration of many treated

patients in the past. Its incremental effectiveness over more conservative drug therapy

should be studied to determine whether very

low IOPs can be sustained for long periods of

time, and whether such a consequence

reduces vision loss. An examination of this

practice could also illuminate any increase in

side-effects that might result from it. If the

effectiveness of this practice is demonstrated,

the information should be disseminated so

that treatment practices of eye care practitioners can be changed.

The natural history of OAG is still perplexing. It is still unknown which people

with OH will develop OAG and how to

identify people without OH who nevertheless

will develop the disease. Researchers have

tackled these questions with some energy over

the past decades, but they are still far from

the answers.

Useful information could be obtained on

one important aspect of the natural history of

OAG without a major investment o f

resources. The natural course of untreated

OAG is unclear and subject to a great deal of

individual interpretation. Documented case

studies of patients who were untreated for

personal reasons would help establish a

baseline against which treatment outcomes

could be assessed.

The tradeoff between immediate inconvenience and discomfort and long-term

prevention of impairment is one faced by

every eye care practitioner and every patient
8 ● Screening for Open-Angle Glaucoma in the Elderly

with elevated IOPs contemplating the initia- by research. Such an information base would

tion of treatment to decrease the risk of de- help physicians predict which patients are

veloping OAG. A more precise estimate of most likely to be aided by treatment. But the

the magnitude of the costs and benefits of essential dilemma of how to balance the

that tradeoff- -for example, a quantified benefits and drawbacks of treatment for a

compilation of the frequency and intensity particular individual is likely to remain for a

with which different side-effects of the vari- very long time.

ous medications occur--can be greatly aided
2. THE EPIDEMIOLOGY OF OPEN-ANGLE GLAUCOMA

Incidence and Prevalence

The exact incidence and prevalence of

open-angle glaucoma (OAG) in the elderly

are unknown. Studies in the United States

and Europe have reported that between 0.4

and 4.1 percent of the general population has

some form of glaucoma (72,118), The broad

range of study findings reflects the different

study designs, populations, disease definitions, and measurement methods.

A study of the population of Framingham, Massachusetts helps to narrow this range

down as it applies to OAG in the U.S. elderly. In this study, approximately 1.2 percent

of the population over age 52 had OAG

(58,60). This number is based on a conservative definition of OAG and is considered by

the researchers to be an underestimate (60).

The risk of developing glaucoma increases dramatically with age (see table 1).

OAG in people under age 40 is very rare

(72), but in the Framingham, Massachusetts

study, approximately 0.9 percent of people

aged 65-69 and 4.4 percent of people aged

80-84 had the disease (94). Given this trend

and the figures discussed above, a reasonable

estimate of the total number of people over

age 65 with OAG today is thus about 2 to 3

percent, or between approximately 600,000

and 900,000 elderly peop1e2

.

The National Society to Prevent Blindness estimates that about 8 percent of visual

impairment and about 14 percent of blindness

in the elderly is due to some kind of glaucoma (84). 3 If these figures are correct, approximately 4,600 elderly people are blind as

a result of glaucoma; presumably, the majori1 Incidence is the number of new cases during a

spec i f i ed per i od of t i me; preva I ence is the tota 1

nurher of cases during a period of t i me.

z These figures are based on an estimated 1988

population of 30,263,000 people over age 65.

3 These estimates are based on reports by certain

States in 1969 and 1970.

ty of them have OAG, the most common

form of glaucoma.

Precise data on the number of new OAG

cases diagnosed each year in the Unites States

do not exist. Incidence estimates derived

from the Framingham study suggest that approximately 0.5 percent of 65-year-olds and

1.1 percent of 75-year-olds will develop the

disease within 5 years (94). These estimates

are consistent with the findings of a Swedish

study, in which 2 percent of elderly people

(over age 62) developed some form of glaucoma during a 9-year period ( 12).

OAG does not affect all subpopulations

equally. Blacks, diabetics, and people with a

family history of glaucoma are much more

likely than others to have the disease (72).

The high prevalence of advanced OAG in

blacks suggests that they might be a particularly important population to screen for this

disease. Preliminary results from a study of a

Table 1. --Estimated Prevalence and

Five-Year Incidence of Open-Angle

Glaucoma in Framingham, Massachusetts,

1973-1975

Age Prevalence Incidencea

55-59 0. 5% o .2%

60-64 0.7 0.3

65-69 0.9 0.5

70-74 1.7 0.7

75-79 2.0 1.1

80-84 4 .4b .-

Total 1.2

a Incidence estimates are approximate and based on

calculations described by the authors below. They

are reported in the source as five-year incidence

estimates for the lowest age in each interval in

this table (e. g., Podgor et al. report a incidence

b of 0.2% at age 55).

The number of persons in this age group in the

sample five-year Population was very small.

SOURCE : M.J. Podgor, M. C., Leske, and F. Edererr

llIncjdence Estimates for Lens Changes,

Macular Changes, Open-Angle Glaucmna, and

Diabetic Retinopathy, N Am. J.

Epidemiology 118(2):206-212, August 1983.

9
10 ● Screening for Open-Angle Glaucoma in the Elderly

predominately black population suggest an

OAG prevalence of 5.5 percent in people

overage 40 in this group (112). Furthermore, OAG may be more severe in blacks;

data from a 1970 blindness registry suggest

that blacks are more than eight times as likely

as whites to go blind from glaucoma and go

blind at an earlier age (48).4

Other characteristics associated with

OAG are less well established. Low levels of

thiamine in the blood have been correlated

with OAG; dark eye color and nearsightedness have been correlated with high intraocular pressure, which is itself a major risk

factor for OAG (see below) (7,28,49,124).

Behavioral factors may have a small influence

on risk. Some studies have found correlations

between OAG and sedentary lifestyles,

cigarette smoking, poor diet, and alcohol use,

although the findings for smoking and alcohol

use are inconsistent (7,40,59,124).

There is similarly conflicting evidence

regarding the relative likelihood of OAG in

men and women. Most studies have found

that men and women of a given age get OAG

with equal frequency, but the Framing ham

study found a higher prevalence of OAG in

men than in women (58,76).

The potential links between hypertension

(high blood pressure), cardiovascular disease,

and OAG have been of particular interest to

researchers because of the theory that OAG is

caused by inadequate blood supply to the optic nerve. In a review of the literature, Leske

lists numerous studies that found associations

between sudden reduction of blood pressure

(from antihypertensive drugs or from blood

loss) and onset of OAG (72). However, she

notes that these studies do not rule out the

possibility that hypertension itself, not just

treatment of the condition, may b e

responsible for OAG risk. Some studies have

4 One hypothesis to expiain the greater prevalence

and sever i ty of OAG in blacks is that this population is less likely to receive early, ad~uate vision care. Houever, various studies (reviewed by

Leske and Rosenthal) suggest that less adequate

care does not completely explain the greater

prevalence and severity of disease in blacks than

in whites (76).

found associations between OAG and systolic

blood pressure but not between OAG and

diastolic blood pressure (40, 124).5,6

Other studies have found associations between hypertension and high intraocular pressure but not between hypertension and OAG

itself (59,74). Thus, it is well established that

a relationship between OAG and blood pressure exists, but the relationship does not seem

to be a simple one.

Natural Course of the Disease

In a normal eye, the fluid in the anterior

(front) portion of the eye is naturally

maintained at a pressure that averages about

16 mm Hg.

7

This fluid, which helps

maintain the shape of the eye, is formed in

an area under the lens and then flows forward through the pupil into the anterior portion of the eye (between the pupil and the

cornea). Excess fluid flows out through a

sieve-like opening into a duct that leads back

to the bloodstream, maintaining a constant

balance of pressure in the eye.

In an eye with glaucoma, the normal

fluid balance is disrupted, usually because the

outflow of fluid is inhibited. When this happens, the pressure in the eye (the intraocular

pressure, or IOP) increases. It is presumed

that the increase in pressure eventually

damages the optic nerve, causing blind spots,

tunnel vision, and, potentially, total blindness

as vision becomes progressively more impaired (34,67).

s Systolic blood pressure is the pressure at the

height of the heartbeat pulse; diastolic blood

pressure is the pressure at the lowest point in the

pulse.

6 Exemplifying the fact that the relationship between blood pressure and OAG is complex, these two

studies found opposite effects of systolic blood

pressure. Goldberg et al. found a high prevalence

of low systolic pressure in patients with lowtension OAG (40). In contrast, Wilson et al.

found a correlation between untreated high systolic

blood pressure and OAG risk (124).

‘7 Millimeters of mercury (mm Hg) is a standard

measurement of pressure.
Screening for Open-Angle Glaucoma in the ElderIy ● 11

There are several general types of glaucoma (see table 2). In some cases, glaucoma

is the secondary result of trauma or some

condition (such as a malignancy) that causes a

sudden, drastic rise in IOP. In other cases,

the condition is present at birth (congenital

glaucoma). Or, the glaucoma is the result of

abnormal eye anatomy which can result in

sudden blockage of the area through which

fluid flows out of the eye (closed-angle glaucoma). The most common form of glaucoma,

however, and the target of current glaucoma

screening programs, is OAG.

Table 2. --Common Forms of Glaucoma

Primary Glaucoma

Chronic open- angle

high tension

Low tension

Closed - angle

acute

chronic (recurring attacks )

Congenital

Secondary Glaucoma

May be secondary to:

trauma

infection

tumors

intraocular hemorrhage

other causes

SOURCES : Adapted from D. Campos-Outcalt and J.M.

Carmi chae 1, llNew perspectives on Glaucoma

Screen ing, ” J. Fami IY Practice 12(3):451-

457, 1981; and R. Berkow, cd., The Merck

Manual, 15th edition (Rahway, NJ: Merck

& CO., 1987).

OAG accounts for 50 to 80 percent of all

glaucoma (9,52). In contrast to some other

forms of glaucoma, there are no sudden

events precipitating OAG. Rather, the primary predisposing factor for this disease is a

gradual increase in IOP. An IOP of 21 mm

Hg is statistically two standard deviations

above the mean of 16 mm Hg and is the

cutoff often used by eye care professionals to

categorize a person at high risk of visual loss

from OAG. People with IOPs greater than

this cutoff, but without any other signs of

OAG, are frequently described as having

ocular hypertension

An estimated 7

eral population has

of OH is higher

Framingham study,

(OH).*

to 13 percent of the genOH(72). The prevalence

in the elderly; in the

nearly one-fourth of the .

elderly had IOPs of 20 mm Hg or more (58)

(see table 3).

OH is the single greatest risk factor predicting future OAG. The extent of risk

posed by OH has been the subject of a

8 The use of the term ‘ocular hypertension” is inconsistent in the literature and controversial

among ophthalmologists. In this paper, the term is

used to mean elevated intraocular pressure without

evidence of visual field defects. Some studies use

the term to include patients with optic disc

changes (but no visual field defects); others do

not. Where the studies clearly group patients uith

optic disc changes separately from those without

them, OTA has done so as well and includes on[y the

latter group in the analysis of the risk posed by

OH.

Table 3--- Distribution of Intraocular Pressurel in the Elderly,

Framingham, Massachusetts, 1973-1975

Number Percent distribution

Age screened <20 mm Hg 20-24 mm Hg 25-29 mm Hg 30+ mm Hg

65-74 780 75.5 19.4 4.0 1.2

75-85 383 75.9 19.1 2.9 2.1

1

Higher value in mm Hg of right or left eye.

SOURCE: H.A. Kahn, H.M. Leibowitz, J.P. Ganley, et al., 1lThe Framingham Eye Study,” Am. J. EPidemiolo~

106(1):17-32, July 1977.
12 ● Screening for Open-Angle Glaucoma in the Elderly

number of studies, summarized in table 4. 9

In each of these studies, patients with OH

were monitored without treatment. Outcomes

vary considerably, as do the length of the

9 None of these studies focused exclusively on the

over -65 popu( at ion, but because of the increased

prevalence of disease in the elderly, many study

subjects were in this group.

study and the precise definitions of OH (e.g.,

above 20, 21, or 22 mm Hg). The geographic

populations studied are also very different,

including people of Japan, Sweden, England,

the United States, Australia, South Africa,

and Norway.

Nonetheless, there is enough consistency

among studies to suggest that people with OH

have the following probabilities of developing

Table 4.- -Development of Manifest Open-Angle Glaucoma in Individuals with Untreated

Ocular Hypertension In At Least One Eye

Percent of ocular hypertensives in specified IOP range developing OAGa,b

—

Source < 20 mm Hg 20-24 mm Hg 25-29 mm Hg 30 mm Hg or greater

l-Year Studies

Kitazawa, 1981

aThese ranges do not correspond exactly to the ranges reported in each study. For exanp(e, the results of a

study reporting the nunber of OAG cases developing in individuals ~ith IOP of 21-25 mn Hg in at least one

eye would be placed in the second colunn (Iabelled 20-24 m Hg).

bA line is used to indicate the results of a study that apply to a broader range of IOP than the individual

categories in this table. For example, Kitazawa (1981) reported only on the nurdxr of OAG cases developing in patients with IOPS of greater than 25 mm Hg. Thus, his results apply to a group that inc(udes

both the last two co[uins in this table (the colunns covered by the line).

cThe patients in Cockburn’s study uere apparently all untreated, but the author is not exp(icit on this

~poigt”

Patients in this study were reported to be under observation for 1 to 11 years, with an average of 41

months.

‘These authors included criteria other than visual defects when determining whether a patient had developed

~ OAG (e.g., changes in the optic disc or IOP > 30 m Hg were also considered sufficient criteria).

This figure includes all simple glaucoma, not just OAG.

SCWRCES: See references.
Screening for Open-Angle Glaucoma in the Elderly ● 13

manifest OAG--i.e., glaucomatous visual field

defects--in one or both eyes within 5 years:

● less than 1 percent of screened people

with “normal” IOPs (20 mm Hg or less),

● approximately 3 to 10 percent of all untreated people with IOPs greater than 20

mm Hg,

● approximately 6 to more than 16 percent

of all people with IOPs greater than 25

mm Hg, and

● approximately 33 percent of people

whose IOPs exceed a cutoff of 30 mm

Hg.

These estimates may be too high. One

study listed in table 4 found an unusually low

prevalence of visual defects developing in

people with OH, even lower than the lower

bounds suggested here (4). It found no visual

defects developing in any persons with IOPs

of 30 mm Hg or greater, in contrast to the

results in other studies.

Two recent studies have found that over

the very long run, people with OH are about

7 times more likely than people with normal

IOPs to develop OAG. Swedish researchers

found that 34 percent of untreated OH

patients developed OAG within 20 years of

screening, compared with only 5 percent of

people whose IOPs were normal at the time

of screening (8 1). (People with IOPs greater

than 30 mm Hg were not included in this

study, so the overall likelihood of people with

OH developing OAG was probably even

higher than reported. ) Researchers in Norway found similar results. In their study, 30

percent of untreated OH patients (including

people with IOPs greater than 30 mm Hg) developed OAG over a 17 to 20 year period

(55).

Some people develop OAG without ever

having high IOPs. 10 These people, often

designated “low-tension glaucoma” cases, ap10 I n some cases these individuals do have abovenorma 1 I OPS at some t i me i n the day, but the i r

pressures are norma [ when measured i n an office

set t i ng. However, at 1 east a few peop[ e seem to

have t rue low- tension OAG, in which there is no

discernible cause of visual damage and the IOP

never exceeds 21 nwn Hg (30).

..,, -1 { { - ,-< -

pear to be unusually prone to optic nerve

damage even when the measured IOP is

within the range generally considered normal

(30). Hollows and Graham found that about

one-third of all OAG cases in their survey

had IOPs below 21 mm Hg at the time of

first examination, although some of these

patients had higher IOPs at later followup

visits (52). In Armaly’s ten-year study, four

people developed OAG; at the time visual

field defects developed, two had average IOPs

of 21 mm Hg and two had higher average

IOPs (4).

At the other extreme, some people can

have IOPs of greater than 30 mm Hg for

many years without developing visual field

defects. Armaly found no evidence of visual

field defects in any of the eyes in his study

that had IOPs of 30 mm Hg or greater at the

initial examination; six of these eyes were

followed for 9 years (4).

It appears that at least two factors are at

work: one that affects the level of IOP, and

one that determines how sensitive the optic

nerve is to the effects of that pressure. 1 1

Thus, there is a clear increase in the risk of

developing OAG as pressure increases (5),

with an especially high risk at very high

pressures (greater than 35 mm Hg) (95).

Nonetheless, a few people may develop the

disease at quite moderate pressures, while the

majority of people with mildly elevated pressures do not develop visual field defects even

after many years.

The classical clinical course of OAG

progresses from high IOP to a characteristic

cupped or other abnormal appearance of the

optic disc (the area of the retina where the

optic nerve enters), to the development of

visual field defects--dim or blind spots that

appear as portions of the optic nerve die out

(initially detectable only by tests and, as they

11 Other theories to exp[ a in the inconsistencies

betueen pressure and vi sua 1 i mpa i rment have been

proposed; for exanp(e, that ( OW - tens ion glaucoma is

actually a different disease from high-tension

open-angle g[aucoma. Given the lack of evidence

for this hypothesis, the idea that the two types

are simply manifestations of different tolerances

for a given IOP has considerable appeal (30).
14 ● Screening for Open-Angle Glaucoma in the Elderly

become more extensive, causing actual visual

impairment). In the absence of other potential causes, the existence of these visual field

defects confirms a clinical diagnosis of OAG.

The dilemma of how to diagnose and

treat people with OAG as early as possible

without treating many people unnecessarily

has dogged eye care professionals for many

years. Although it is clear that the majority

of individuals with elevated IOPs at the time

of first screening do not go on to develop

OAG, even after 20 years (55,81), it is equally clear that the disease is present before it is

manifested in a decrease in visual capacity.

Quigley and colleagues have demonstrated

that up to 50 percent of optic nerve fibers

are already lost by the time changes in the

visual field are detectable (98).

Researchers have used such varying

definitions of OAG onset as: IOP greater

than 30 mm Hg; suspicious changes in the

optic disc; abnormal results on tests designed

to measure the fluid outflow in the eye;

changes in the visual field that follow a particular pattern; and a multitude of combinations of these and other criteria. In assessing

the literature on outcomes associated with

OAG, OTA has chosen to give greatest

weight to those studies that include visual

field defects as a necessary criterion for a

definite diagnosis of OAG. There are two

reasons for doing so. First, visual field

defects can be measured more objectively

than can changes in the optic disc, and their

existence is hard evidence of established disease. When visual field defects are measured

in a study and are included as a necessary

criterion for a diagnosis of OAG, it is unlikely that study subjects who are in fact

healthy will be classified as diseased. Second,

individuals with OAG do not suffer visual

disability until after they develop visual field

defects. Nonetheless, even before visual

defects arise, the disease process is well underway.
3. TREATMENT FOR OPEN-ANGLE GLAUCOMA

Description

Patients may be treated for potential or

confirmed open-angle glaucoma (OAG) at

any of four stages:

1)

2)

3)

4)

after the intraocular pressure (IOP) has

reached a level suspected to be intolerable to that individual’s optic

nerve, but before any other characteristics of glaucoma appear;

after changes in the optic disc have appeared, but before any visual field

defects have occurred;

after defects are apparent to the

physician but before the patient is

visually impaired in any way; or

after some visual impairment occurs, to

prevent further impairment.

Treatment cannot reverse impairment; it

is prescribed on the assumption that it can

prevent visual deterioration by lowering the

pressure in the eye and preventing further

damage to the optic nerve. Eye care professionals believe that the earlier treatment is

begun, the greater the likelihood that visual

impairment can be prevented. This belief has

been bolstered by evidence that a substantial

proportion of the optic nerve dies before a

patient becomes visually impaired (98).

Treatment for OAG follows a wellestablished pattern (34,67). Initial treatment

nearly always consists of topical application

of one of three drugs: epinephrine, pilocarpine, or timolol. Although these drugs act in

different ways, the goal of each is to lower

the IOP (either by decreasing formation of

fluid or by enhancing outflow), thus

presumably preventing further damage to the

optic nerve. If one of these drugs is inadequate in lowering pressure, they may be

combined, given at higher dosage, and/or

substituted with similar, alternative drugs. If

pressure still remains high, a stronger, systemic drug with more side effects (e. g.,

acetazolamide) may be added. Finally, if

even maximum tolerable medication is inadequate, an ophthalmologist will perform

laser or filtering surgery to enhance outflow

of the ocular fluid.

The medications used to lower IOP must

be taken for life, and all have numerous

common side effects (e. g., blurred vision,

headache, nausea, and increased blood pressure and heartbeat (33)). Some medications

also increase the risk of cataract formation

(8,101). These side effects and sequelae, plus

the cost of the medications and the inconvenience of applying them up to 4 times

per day, have resulted in noncompliance rates

of up to 58 percent in various studies (6,61).

Timolol, one of the only two new medications

to be approved for glaucoma treatment in

recent years, 1 has become a popular first

medication because it is better tolerated by

patients than epinephrine or pilocarpine.

(Unfortunately, an initial lack of understanding of timolol’s full effects led to several

deaths in glaucoma patients with respiratory

and cardiovascular diseases exacerbated by

the drug (86)). Other topically-applied drugs

that are chemically similar to timolol are under investigation in the hope that they may

be more effective or have more limited systemic effects (2, 15,17,22,121 ).

Despite the disadvantages of glaucoma

medications, they are still usually considered

preferable to surgery. Traditional filtering

surgery--the creation of an artificial opening

through which fluid can flow out of the

eye--is reported to be successful in lowering

IOP in 60 to 90 percent of patients, depending on patient characteristics (57). However,

filtering surgery also carries the risks of

permanent damage to the eye from infection,

excessive drainage (causing soft, shrunken

eyes), and hemorrhage (57). OAG patients

who have undergone filtering surgery are

much more likely than other OAG patients to

1 The second relatively new medication for glaucoma

is d i pi va [ y ( epi neph r i ne, a form of epi nephr i ne

that becomes act i ve on[y after interact ion Hi th the

eye and thus causes fewer side effects (119).

15
16 ● Screening for Open-Angle Glaucoma in the Elderly

develop cataracts (110).

2 Furthermore ,

eventual return to medication and/or additional future surgery may be necessary in

some patients for continued control of IOP

(37).

In recent years, argon laser tra -

beculoplasty (ALT) has become a frequent

intermediate step for patients whose IOP is

uncontrolled by medication (57). ALT consists of making tiny laser burns within the

trabecular network. It is unclear why this

laser scarring facilitates fluid outflow, but

ALT has been shown to decrease IOP

(1 14,1 17). Among the benefits of ALT are

an avoidance of some of the risks of filtering

surgery (e. g., infection), but unlike surgery,

most patients must continue taking medication even after undergoing the procedure

(37). The long-term effects of ALT scarring

are unknown. The National Eye Institute is

currently conducting two clinical trials of the

procedure: one of ALT (instead of medication) as primary treatment in patients with

early evidence of OAG, and one comparing

ALT with filtering surgery in patients with

advanced OAG (120).

Treatment Outcomes

A number of studies have reported the

proportion of OAG patients whose visual

2 Cataract development is espec i a 1 [y common i n

those glaucoma surgery patients in uhom surgeryre[ated c~lications arise (110).

field deteriorated while under treatment

(2,25,43,45,46,69,82,83,87,97,109). The

reported outcomes vary considerably, with

anywhere from 11 to 82 percent of patients

in these studies suffering further deterioration while under long-term treatment.

Patients with advanced OAG suffe r

deterioration more rapidly than patients with

only minor visual field defects (69), perhaps

because the loss of additional optic nerve

fibers in people with advanced disease leads

to proportionately greater impairment (98).

In general in these studies, longer followup

results in more patients deteriorating. 3 It appears that about one-fourth of all patients

with existing defects suffer deterioration

within 4 years (87, 109). However, another

one-fourth of patients suffer no deterioration

even after many years (83).

Surprisingly little information exists on

the rate at which people with OAG, treated

or untreated, actually become visually impaired. Table 5 summarizes the results of

three studies that reported on rates of visual

deterioration in treated OAG patients. Of

these reports, the one that can be interpreted

most directly found that 75 percent of

patients with manifest OAG went blind in the

affected eye within 20 years, even when

treatment was begun soon after the detection

of visual field defects (43). (Of patients who

3 Fol (owup in these studies ranged from 1.4 to 42

years.

Table 5. --Three Estimates of Rate of Visual Impairment for Eyes of Patients with Treated

Open-Angle Glaucoma

Percent of Condition at

Initial condition eyes that end of measured

Source Time period of eyes deteriorated time period

Kronfeld and McGarry, 5 years

1948 5 years

5 years

Hart and Becker, 10 years

1982

Grant and Burke, 5 years

1982 10 years

20 years

“early” OAG 16% “advanced” OAG

"moderate" OAG 50% "advanced” OAG

"moderate" OAG 20% blindness

82% of all eyes with OAG suffered Insignificant visual loss”

(not necessarily synonymous with further impairment)

“early” OAG 25% blindness

“early” OAG 38% blindness

“early” OAG 75% blindness

SOURCES : See references.
Screening for Open-Angle Glaucoma in the Elderly ● 17

began treatment after changes in the optic

disc but before the onset of visual field

defects, 50 percent went blind in the affected

eye within 20 years.)

4 The incidence of

blindness was fairly constant across time in

this study.

By their very nature, even recent reports

of very-long-term outcomes of treated

patients with OAG reflect the treatment patterns of many years ago. Some ophthalmologists believe that outcomes are better

now than under treatment practices of the

past. In the past, they argue, ophthalmologists were content to maintain treatment

without change when a patient’s IOP had

been lowered to a certain level, even if the

patient’s visual field continued to deteriorate

at that level ( 105). Now, they maintain,

patients are treated more aggressively if their

visual condition is not stable under the current treatment regimen, and patients

deteriorate less rapidly. Continuing documentation of long-term outcomes could both

support this contention, assuming it is true,

and improve the dissemination of knowledge

regarding the most appropriate treatment

practices.

Treatment Effectiveness

A necessary condition for OAG screening

to be effective is that treatment is effective.

One might choose to screen for an OAG risk

factor (i.e., high IOP), for probable early

OAG (i.e., suspected optic nerve damage), for

fully developed OAG as manifested through

visual field defects, or not to screen for OAG

or its risk factors at all. Which screening

policies should be considered depends heavily

on whether and at what stage treatment is effective in preventing visual deterioration.

The assumption that early treatment can

prevent visual field defects pervades the literature. Studies of patients with ocular hypertension (OH) have tended to reinforce the

assumption by emphasizing how few patients

who were treated early suffered visual

deterioration. (In fact, only a small proportion of such patients would be expected to

suffer measurable deterioration even without

treatment. ) The belief in the importance of

early detection and treatment has continued

almost unabated despite the fact that a few

eminent researchers pointed out the inconsistencies between documented evidence

and clinical practice as early as the 1960s

(23). Their conclusion, that the efficacy of

treatment for OH and OAG was undocumented, has been reiterated by others in

recent years (3 1 ). The Canadian Periodic

Health Examination Task Force likewise concluded that the evidence for effectiveness of

treatment for OAG consisted of the opinions

of respected authorities (2 1,39).

What exactly is the evidence regarding 1)

the effectiveness of treating OH to prevent

OAG, and 2) the effectiveness of treating

manifest OAG to prevent or delay functional

visual impairment? Not surprisingly, there is

no direct evidence regarding the effectiveness

of treating manifest OAG. There have been

no studies of comparable groups of treated

and untreated patients with visual field

defects, because the standard of care is to

treat all such patients. However, it is possible to review the evidence for the effectiveness of treating OH and assume that if

treatment of OH is effective in preventing or

delaying the development of visual field

defects, then treatment of manifest OAG is

likewise effective. It is also possible to examine other indirect evidence of the effectiveness of OAG treatment.

Q The lower incidence of b( indness in those treated

before onset of vi sua( field defects could ref [ect

one or both of two poss i bi 1 i t i es: 1 ) that i t nat -

ura I [y takes a longer time for those with on[y optic disc changes to reach blindness, since they are

identified at an earlier stage in the disease than

are those uith visual field defects; or 2) that

treatment of those with abnormal optic discs was

more effective because it was initiated earlier in

the stage of the disease.

Evidence of the effectiveness of treating OH

to prevent OAG

To be considered direct evidence of the

effectiveness of treatment of OH for preventing OAG, a study must, at a minimum, meet

three criteria:
18 ● Screening for Open-Angle Glaucoma in the Elderly

1.

2.

3.

Long-term followup of the study population (at least 1 year, and preferably

much longer if differences are to be

detectable),

Monitoring of visual field changes in

the study population, and

Existence of well-defined treated and

untreated groups of patients (or eyes of

patients). Ideally, these patients (or

eyes) should be randomized prospectively into the two groups, although

studies in which patients are matched

for salient characteristics also provide

useful evidence. The study must control

in some way for differences in the level

of IOP among treated and untreated

patients or must be reported in such a

way that the evaluator can control

retrospectively for this factor, because

people with high IOPs are more likely

to get OAG than those with low IOPs

irrespective of treatment.5

Although study size per se is not one of

the criteria, the number of subjects studied is

crucial to the ability to detect differences and

attribute them to treatment. For example, if

the incidence of OAG among all people with

OH were 2 percent per year, and treatment

reduced this by 50 percent --i.e., to an incidence of 1 percent per year--a 1 -year study

would require hundreds of subjects to show

this result with a probability of less than 5

percent that the result is due to chance (even

assuming full compliance of all subjects).

Despite the large published literature

relating to OAG, OTA could identify only

seven studies of OH treatment (two published

only in abstract form) that meet these three

5 As a case in point, two c omnon(y c i ted reports of

a Danish epidemi 01 og i c glaucoma study (87, 104)

reported the number of OH patients who went on to

develop visual field defects. I n both reports,

treated pat i ents were more [ i kely than untreated

patients to develop visual field defects. The

average IOP levels in the two groups (treated and

untreated) uere not stated in either report. The

authors si~ly reported on the outcome of patients

under standard medical care, and it is extremely

like[y that certain patients were treated because

their higher IOPS or other factors placed them at

an especially high risk of disease. Thus, these

studies cannot be used to eva[uate the effectiveness of treatrwnt.

basic criteria. The results of these studies are

summarized in table 6. The studies are of

two types: those that compared treated with

untreated patients, and those in which one

eye of each patient was treated, while the

other eye was left untreated.

Of the three studies that compared

treated with untreated patients, the one finding the most positive effect of treatment is

also the most recent. Preliminary results of

this study (still ongoing), which employs a

prospective, randomized design, suggest that a

statistically significant positive effect of

treatment may be found (70). A less recent

study, in which matched patients were prospectively assigned to treatment or placebo,

found a positive but not statistically significant effect (only 12 placebo and 15

treated patients completed the study) (64).

Finally, the oldest study, which neither randomized nor matched patients, found that

treated patients were actually more likely to

develop OAG (27). When patients in this last

study are grouped by IOP, it appears that

treated patients with the lowest IOPs (21-25

mm Hg) were significantly more likely to develop OAG than untreated patients; the differences in development of OAG between

treated and untreated patients with initial

IOPs of 26-30 mm Hg and over 31 mm Hg

are not significant.

Studies that use eyes rather than patients

as the unit to be treated pose some problems

in interpretation, because treatment of one

eye could affect the outcome of the untreated

eye. Of the four studies that compared

treated with untreated eyes, three found a

significant positive effect of treatment (see

table 6). The fourth study found a negative

effect, but the difference between the treated

and untreated groups was not statistically significant (77). In the study showing the

greatest positive effect, the patients selected

for the study were thought to be the subgroup most likely to benefit from the particular treatment (because the patients had previously demonstrated an IOP response to the

medication chosen for the study) (102). In

this same study, however, the untreated eyes

did particularly poorly (i.e., a higher propor-
Screening for Open-Angle Glaucoma in the Elderly ● 19

+ + I + 1 + +

-0

a

. .

N*

N.-4

- -

.-l U

FYo

- -

Cno

U-J(O

in

r- A

-1

0

4

4

;

-4

a
20 ● Screening for Open-Angle Glaucoma in the Elderly

tion developed OAG than did the eyes of un- 0

treated OH patients in the studies discussed

earlier).

In addition to the above studies, several

investigators have reported the outcomes of o

treating patients who had either OH or

changes in the optic disc (but no visual field

defects). These studies had no clear control o

groups. Relevant results of the studies are

summarized in table 7; they are useful

primarily as contextual information for assessing the outcomes of treatments in the

comparative studies.

Indirect evidence of the efficacy of treatment

for manifest OAG

A number of other studies and observations provide indirect evidence of the efficacy of treatment for manifest OAG in

delaying or preventing further visual field

defects. For example:

Studies in animals in which IOP was

artificially raised have been able to induce glaucomatous changes in the eye

(38), implying that the level of IOP is

causally related to damage to the eye.

Several studies have found improvement

in the appearance of the optic disc after

treatment (44,62,91 ).

Some researchers have observed, in

retrospect, that patients whose IOPs were

maintained at relatively low levels while

under treatment (e.g., under 20 mm Hg)

suffered less loss of vision over time

than patients whose IOPs remained relatively high despite treatment (66). It

may be that a drastic lowering of IOP is

necessary in some patients before treatment is effective (62); it is possible that

some studies have not detected an effect

of lower IOP because the treatment was

inadequate.

Table 7--- Studies Relating Long-term Outcomes of Treatment to Lower Intraocular Pressure in

Patients Without Visual Field Defects

Percent of Selected

Treatment treated patients characteristics

Source duration developing OAG and limitations
Screening for Open-Angle Glaucoma in the Elderly ● 21

Most convincing to ophthalmologists,

however, is their own experience with glaucoma patients, in whom visual deterioration is

proceeding rapidly until the patients are successfully treated (36, 106). Although dramatic

treatment effects are most common with

closed-angle glaucoma patients, ophthalmologists’ experience with these patients leads

them to believe that lowering IOP is very

beneficial in OAG patients as well.

Compliance as a factor in the effectiveness

of treatment

One possible explanation for the lack of

documentation of treatment effectiveness is

inadequate patient compliance with the longterm treatment regimens prescribed in the

studies. In one early study, for example, only

20 percent of patients on treatment at the beginning the study remained on treatment for

the full 4 to 5 years; the remainder dropped

out of treatment, primarily due to side effects

(10). Researchers have noted that patients

with established OAG are more compliant

(i.e., keep appointments and take medications

as scheduled) than patients with OH but no

visual field defects ( 18, 107). Patient noncompliance with treatment regimens means

that treatment effectiveness (in actual practice) may differ substantially from treatment

efficacy (in research or ideal situations).

Conclusions

Considering both the inadequacies and

contradictions in the literature and the experience and opinions of practicing eye care

professionals, the following conclusions

regarding treatment effectiveness seem warranted:

1.

2.

Most people with modestly elevated IOP

but no visual field defects upon initial

screening will not develop OAG in the

near future, even if left untreated (see

chapter 2).

Justification for the current mode of

treatment for OH and OAG is based on

theory, personal experience, and the

postulates shared among physicians

rather than on direct evidence documented in the literature.

3.

4.

5.

6.

7.

8.

The evidence regarding the efficacy of

medical treatment to prevent OAG by

lowering IOP is sparse, conflicting, and

largely of poor quality. Two very

recent studies, yet to be published, are

likely to provide more convincing evidence than currently exists in the literature. It is likely, based on preliminary

results, that both will show treatment of

OH patients to be efficacious in preventing or delaying the onset of

manifest OAG.

If treatment of OH is shown to be efficacious, further research will still be

needed to clarify which groups of OH

patients are most likely to benefit from

treatment, which are likely to suffer as

much harm as good if treated, and what

the most effective treatment regimen is.

Patient compliance with medical treatment is highly variable and can be very

poor, leading to potentially poor realworld effectiveness of treatment even if

treatment is shown to be efficacious.

If treatment of OH is shown to be effective in preventing visual field

defects associated with OAG, then

treating manifest OAG is probably effective in preventing or delaying visual

impairment. The extent of effectiveness is unknown and cannot be inferred

directly from the effectiveness of OH

treatment, since the degree of effectiveness may depend on when treatment

is begun.

Even with more aggressive medical

treatment than in the past, and even

with early treatment of patients, it is

unlikely that treatment will prevent

eventual visual impairment in all

patients. However, to the extent that

treatment delays blindness, it is valuable in enabling many elderly people to

live out their lives with sight.

The knowledge base for treatment of

manifest OAG would be improved with

research on comparative long-term effectiveness of different treatment

modalities, establishing the most effec-
22 ● Screening for Open-Angle Glaucoma in the Elderly

tive overall strategy (including criteria course of the disease when patients

for when current treatment of a patient with OAG are not treated (e. g., when

is insufficient), and delineating more the patient’s religious beliefs prohibit

clearly the best treatment at different treatment) would also be extremely usestages of the disease and in different ful in describing the natural course of

types of patients. Documenting the untreated OAG.
4. SCREENING TECHNOLOGIES

Screening Considerations

Screening for open-angle glaucoma

(OAG) has two potential objectives:

o to identify people with manifest OAG so

that affected individuals can be treated

before becoming visually impaired, and

o to identify people with ocular hypertension (OH), the major risk factor for

OAG, so that these individuals can be

treated and thus reduce their risk of developing OAG.

Although current screening programs

frequently combine these two objectives, they

are important to distinguish for three reasons.

First, the accuracy of a screening test at correctly identifying people depends on which of

these characteristics is the purpose of the

screen. (See box A for a description of the

basic components of screening test accuracy).

A test that is very good at identifying people

with OH may be very poor at identifying

people with manifest OAG, and vice versa.

Second, OH is much more prevalent in the

population than OAG, and the yield of an

OH test (i.e., the proportion of all positives

who are true positives) is thus likely to be

much higher. And third, the potential costs

and medical benefits of screening depend on

which groups of people are identified. This

last consideration is discussed further in

chapter 5.

Description of Screening

Technologies

OTA reviewed three different screening

technologies that have been used, alone or in

combination, in large-scale screening for OH

and OAG. These are:

o

0

0

tonometry, which measures intraocular

pressure (IOP);

ophthalmoscopy, which enables the examiner to see abnormalities in the optic

disc; and

perimetry, which measures the extent of

visual field loss.

Tonometry is the most familiar of the

three screening methods. It may be performed by physicians, optometrists, or (less

commonly) by opticians. 1 Tonometers work

by measuring the resistance of the eye to a

force, which may be applied by direct contact

with the eye or by shooting a puff of air

(non-contact tonometry). Tonometry is the

method most often used in large screening

programs (78,93) and is often part of routine

visits to eye care professionals.

Ophthalmoscopy requires the analysis of

the appearance of the surface of the optic

nerve by a physician or optometrist using an

ophthalmoscope (a tool that enables the examiner to look through the pupil at the

retina). This procedure can identify characteristics of the optic disc indicative of OAG.

It can be performed by trained non -

ophthalmologic physicians (e.g., family practitioners).

2 It has been used in community

screening efforts in the United States, but it

is usually used in conjunction with tonometry

(78). Its use in this role has been promoted

in at least one textbook on glaucoma (67).

In contrast to tonometry and ophthalmoscopy, perimetry identifies actual visual loss.

In perimetry, dots of light of varying brightness are introduced at a pattern of points in

the visual field. The patients’ responses to

these stimuli are recorded by the perimetrist

(in manual perimetry) or by a computer (in

automated perimetry). A lack of response

(i.e., the patients’ inability to see the light)

indicates a blind spot in the visual field.

(When the visual field defect is less severe,

the patient may see the light but only if it is

very bright. )

Perimetry can be time-consuming and

expensive and has only rarely been used as a

screening tool (63). It is most commonly

1 Opticians can perform ort(y non-contact tonometry.

z However, not a 11 f ami ( y practice residencies requi re some ophthalmologic 1 training, and only twothirds i nc 1 ude routine g [aucoma screening as a part

of the care residents must provide ( 113).

23
24 ● Screening for Open-Angle Glaucoma in the Elderly

Box A--- Components of Screening Test Accuracy

Two basic attributes are used to compare the accuracy of a screening test in identifying

patients for followup: the sensitivity of the test (expressed as the proportion of people with the

condition--i.e., OH or OAG--who actually test positive) and the specificity of the test (expressed as the proportion without the condition who actually test negative). Sensitivity and

specificity do not depend on the prevalence of OH or OAG in the population.

Sensitivity and specificity often vary inversely. A test that is very sensitive (i.e., identifies

most of the people with the disease) often also falsely identifies many people who actually do

not have the disease, giving it a low specificity. However, this is not always the case. Some

tests are both more sensitive and more specific than others. And, an inexperienced examiner

may cause a test performed by that examiner to be both less sensitive and less specific than the

same test performed by a more experienced examiner. Sensitivity is generally considered more

important in OAG screening than specificity, since false positives (people falsely identified as

having glaucoma) can be eliminated in the diagnostic workup, but false negatives (people falsely

identified as not having glaucoma) are not referred and thus are not diagnosed. However, if

specificity is very low, a program can incur substantial followup costs, lead to unnecessary

treatment, and cause much distress for people who are disease-free.

Two other attributes used to compare screening tests are the positive and negative predictive values of a test, the ability of the test to correctly predict disease or health. The predictive

value of a positive test is the proportion of all test-positives who actually have the disease,

while the negative predictive value is the proportion of all test-negatives who do not. Thus, a

positive predictive value of 5 percent means that of 100 persons who test positive, 5 have the

disease.

Unlike sensitivity and specificity, predictive values do depend on the prevalence of the

condition. “Given a fixed level of sensitivity and specificity, predictive values increase as the

prevalence increases...[S]creening will lead to a large number of overreferrals if carried out in a

population where the disease is rare; conversely, false-positives are greatly reduced when

screening is done in population where the disease is common” (71). When a condition is very

rare, even a very sensitive and specific test may have a modest positive predictive value.

Since true positives are a higher proportion of total positives when a condition is common

in the population, the cost per case detected is lower than would be the case if a large number

of false positives incurred followup costs. Thus, high prevalence is one factor leading to low

cost per case detected through a screening program. (Other factors can also lead to low costs

per case detected by compensating somewhat for large numbers of false positives referred for

followup--for example, low followup visit costs.) b
Screening for Open-Angle Glaucoma in the Elderly ● 25

used, in conjunction with ophthalmoscopy, as

a diagnostic procedure (e. g., to confirm that a

person testing positive on tonometry, due to

high IOP, actually has OAG). Visual field

defects detected by perimetry are used as the

“gold standard” definition of OAG when

evaluating the accuracy of tonometry and

ophthalmoscopy in identifying people with

OAG.

Screening Test Accuracy3,4

Although glaucoma screening has been

common for many years, there have been few

rigorous studies of the accuracy of screening

tests in correctly identifying either OH or

manifest OAG. Table 8 summarizes relevant

estimates of the accuracy of tonometry,

ophthalmoscopy, and perimetry as screening

tools for detecting OH (by tonometry) and

manifest OAG (for all three technologies).

Most notable in this table is the scarcity of

estimates available and the enormous variation among the estimates that do exist. Particularly lacking are methodical studies of

variation in accuracy among different types

of examiners. The estimates that do exist,

and the characteristics of the respective technologies that may affect accuracy, are described in greater detail below.

3 “Accuracyi’ i s used here i n i ts more genera (

sense, the abi ( i ty to identify something correctly,

rather than i n i ts strict stat i st i ca [ meaning.

o These accuracies generally apply to the over-40

popu[ at i on. The diagnostic accuracy of the various

screening technologies has not been reported in the

[ iterature for the e[der[y alone (over age 65).

Table 8--- A Comparison of Estimates of Accuracy for Three Glaucoma Screening Technologies

Technology Sensitivity Specificity Set t i rig/context

Tonometry >21 mm Hg

accuracy compared to

elevated pressure on 71% 97% study of Schiotz

Goldmann tonometry tonometry

accuracy compared to

“glaucoma”, defined 75% 81% study in hospital

in various ways clinic

accuracy compared to

confirmed visual 50% . . population survey

field defects 72% 30% mass screening study

Ophthalmoscopy

accuracy in live 72% 64% mass screening study

eyes compared to 76% . . population survey

confirmed visual 44- 53%a 69- 77%b study of examiner accuracy

field defects 84% 97% study of ophthalmoscopy

accuracy using expert

examiners

100% . . population survey with

expert examiner

Perimetry

accuracy compared to

confirmed visual

field defects

96% 89% manual perimetry in

population survey

93% 88% study of automated perimetry

92% 46% automated perimetry in

mass screening study

Source

Bengtsson, 1972

Packer et al., 1965

Leske et al., 1982

Ford et al., 1982

Ford et at., 1982

Leske et al., 1982

Wood and Bosquanet, 1987

Hoskins and Gelber, 1975

Graham, 1969

Rock et al., 1972

Sommer et al., 1987

Ford et al., 1982

:44% for consultants, 53% for junior doctors.

69% for junior doctors, 77% for consultants.

SOURCES: See references
26 ● Screening for Open-Angle Glaucoma in the Elderly

Tonometry

Tonometry may be used for either of the

two different screening purposes: to identify

people with OH who, due to their high IOPs,

are at high risk of developing OAG; and to

identify people with manifest OAG. It is

fairly successful at identifying people with

OH, since IOP is the characteristic it is

designed to measure. It is much less accurate

in identifying people with manifest OAG,

since only a small proportion of people with

elevated IOP also have manifest OAG (and,

conversely, a substantial minority of people

with OAG do not have elevated TOPS at the

screening visit).

Screening for OH. Since high IOP--the

definitive characteristic of OH--is the quality

measured by a tonometer, tonometry might be

expected to be quite accurate in identifying

people with OH. Although the potential accuracy is high, in practice a number of factors can have a substantial impact on the

number of individuals in whom OH is correctly identified.

These factors fall into two categories:

those associated with the technology, and

those associated with the nature of IOP itself.

Technology-associated factors that affect the

accuracy of tonometry include the type of

tonometer and the person performing the test.

Of the three main types of tonometers

(Schiotz, applanation, and non-contact), applanation tonometry is generally considered

the most accurate (128), although all have

been used for screening (56,78,93). As might

be expected, tonometry is more accurate

when performed by someone who does it frequently, yielding fewer false positive readings

(108).

Even when applanation tonometry is performed by skilled examiners, however, factors associated with the intrinsic nature of

IOP affect tonometric accuracy in identifying

OH. For example, there is substantial daily

variation in IOP, and two tonometr y

measurements of the same person during different times of the day can lead to quite different conclusions (4).

The net result of these factors, combined

with random errors, is an accuracy far below

the ideal. A 1968 review of studies of the

prevalence of OH found five studies that

reported both the number of people with OH

found at the screening visit and the number

in whom OH was confirmed at a followup

v is it. In these studies, the proportion of

people with unconfirmed OH--i. e., false

positives --ranged from 27 to 86 percent of

the people referred . Sensitivity an d

specificity could not be calculated, since

people testing negative were not retested (96).

A Swedish study attempted to estimate

false negatives as well as false positives. In

this study, readings on a Schiotz tonometer

were compared with readings with an applanation tonometer (considered the “gold

standard” for the purposes of this study).

The investigators found that the Schiotz readings had a 71 percent sensitivity and 98 percent specificity for identifying high IOP (11).

The accuracy of modern applanation

tonometry in screening for OH remains unclear. Although it is considered more accurate than Schiotz tonometry, it still produces false positives and negatives due to

factors such as interexaminer variation and

the variation of IOP with the time of day.

Thorburn found that when two examiners

performed separate applanation tonometry

tests, the readings differed by 2 mm Hg or

more in 40 percent of the paired measurements(lll). The variation did not depend

on the level of IOP. Armaly found that approximately 30 percent fewer eyes had IOPs

of 20 mm Hg or greater in the afternoon than

in the morning (4).

Screening for manifest glaucoma. Although most people with OAG have elevated

IOP, tonometry alone cannot distinguish between people suffering optic nerve damage

and those with equivalent IOPs who are not.

Thus, it has a naturally high false positive

rate for manifest OAG (and a low

specificity). This rate can only be reduced

by raising the IOP designated as the cutoff

criterion for referral (which, in turn, raises

the false negative rate and decrease s

sensitivity).
Screening for Open-Angle Glaucoma in the Elderly ● 27

In an early study of the accuracy of

tonometry, Packer et al. found that tonometry

was 75 percent sensitive and 81 percent

specific at identifying persons with glaucoma

at a cutoff level of 22 mm Hg (90). These

results are not directly applicable to identifying manifest OAG, since the definition of

“glaucoma” used by the investigators in this

study was quite broad and included many

characteristics not necessarily indicative of

OAG (e.g., people with narrow angles, abnormal disks, or certain results on other tests

were considered to have glaucoma for the

purposes of the study (89)). However, the

study does provide a good example of the effect on test accuracy of changing the cutoff

level for referral. The investigators found

that raising the screening cutoff level from 22

mm Hg to 26 mm Hg decreased sensitivity

from 75 to 59 percent, while specificity increased from 81 to 95 percent (90).5

Two more recent studies of the accuracy

of tonometry used glaucomatous visual field

defects to define OAG. In a community

screening study in New Orleans, tonometry

demonstrated a 72 percent sensitivity for

OAG by this definition, but only a 30 percent specificity (cutoff level 22 mm Hg)

(35).

6 Leske et al., in a study of the

Framingham population found that tonometry

(cutoff level 22 mm Hg) had a 50 percent

sensitivity for detecting OAG in glaucoma

suspects (i.e., the group of people testing positive on one or more OAG screening tests),

implying a somewhat lower sensitivity for the

s The community screening programs organized by

the Nat iona[ Society Prevent to B1 i ndness usual (y

use a cutoff of 24 mm Hg to keep the number of

referra(s manageable (95).

6 The results from the study by Ford and colleagues have been publ i shed only in abstract form.

According to one of the investigators, all subjects

screened subsequently underuent a comprehensive

ophthalmic examination in order to determine the

true disease status of the sujects and calculate

the sensitivity and specificity of the screening

procedures (130). A similar, larger study isnou

ong o i ng. It will be of interest to see if the

results of the larger study confirm those of the

original one, since the study cited above found

surprisingly high sensitivity but surprisingly (OW

specificity.

entire screened population (75). Specificity

could not be estimated, since people who

tested negative were not rescreened.7

Ophthalmoscopy

Ophthalmoscopy can identify people in

whom OAG has already caused visible nerve

damage. It can identify people who have developed OAG earlier than can perimetry,

since considerable nerve damage occurs before defects in the visual field become apparent (98). Its primary disadvantage is that

analysis of the optic disc is highly subjective,

and abnormalities can be very difficult to interpret. Consequently, it is difficult to be

confident that a person reported to have an

abnormal optic disc actually has OAG until

visual field defects have also become apparent.

The sensitivity and specificity of

ophthalmoscopy in identifying people with

manifest OAG (i.e., visual field defects)

depend on such factors as:

c

●

8

whether the pupils are dilated for the

procedure (less common in community

screening settings, more common in office settings);

the criteria used to define a positive test

(e.g., a cup:disc ratio of .6, or an examiner’s overall impression that the disc is

“abnormal”); and

the skill and experience of the examiner.

Ford et al. found the sensitivity of

ophthalmoscopy for identifying manifest

OAG to be 72 percent and specificity to be

64 percent in community mass screening (35).

Leske et al. examined several specific test

criteria and found that vertical optic cup:disc

ratios gave the best results for the Framingham population ; this criterion had a

sensitivity of 76 percent (75).

‘7 The positive predictive value of IOP in this

study was 5 percent (i.e., of the people with IOP

of 22 nwn Hg or greater, 5 percent had visual field

defects) (75). A large proportion of the people in

this study were elderly.
28 ● Screening for Open-Angle Glaucoma in the Elderly

Examiner skill is crucial to the accuracy

of ophthalmoscopy in identifying manifest

OAG. In studies in which glaucoma

specialists performed ophthalmoscopy, or

studied photographs of the optic nerve, accuracy is generally much higher than that

reported in the two screening studies above.

American investigators have reported a

sensitivity of 84 percent and a specificity of

97 percent for ophthalmoscopy performed by

glaucoma specialists (54). Studies in which

photographs of optic discs were examined by

specialists have found sensitivities of 71 to 88

percent and specificities of 75 to 97 percent

(29,50,53,54). An English population survey

to determine glaucoma prevalence found that

all cases of manifest OAG identified in the

survey were detectable by ophthalmoscopy

(performed by a glaucoma specialist) (42).

One exception to the reports of high accuracy when ophthalmoscopy is performed by

specialists is a recent English study of examiner skill. In this study, ophthalmoscopy performed by consultants was more specific for

OAG--but less sensitive--than the same procedure performed by junior physicians (126).

Perimetry

Perimetry can identify people in whom

actual visual field defects have occurred, but

who are not yet visually impaired. It is for

people with visual field defects that a diagnosis of OAG can be made with the most

confidence. In part for this reason, this

paper has considered OAG to be manifestly

present only if visual field defects are among

the signs of disease. However, it should be

noted that the disease is established, and optic

nerve damage is underway, before visual field

defects occur. Thus perimetry will naturally

be more accurate than ophthalmoscopy at

detecting manifest OAG, but it detects the

disease at a later stage.

The accuracy of perimetry for screening

is determined by comparing screening results

with multiple comprehensive perimetric examinations. Accuracy depends heavily on

how much of the visual field is screened and

the manner in which it is done. The accuracy of manual perimetry is more variable

than that of automated perimetry, since it

depends more on the skill and consistency of

the person performing the test.

A method of large-scale screening by

manual perimetry was developed by Armaly

(4) and subsequently modified by Drance and

colleagues (99). The modified test is reported

to have a sensitivity for manifest OAG of 96

percent, a specificity of 89 percent, and a

positive predictive value of 83 percent (99).

These numbers are probably maximums; in

general practice, the accuracy of manual

perimetry could be much lower depending on

how the examiner carried out the procedure.

Automated perimetry has some advantages over manual perimetry for mass

screening (63), although it is not necessarily

more accurate. It has been reported to have a

sensitivity varying from 80 to 96 percent,

depending on how extensive a test is performed; the main problem is a potentially

high false positive rate (as high as 33 percent)

if test conditions are not properly established

(63).

8 The type of perimeter also affects accuracy.

Three studies demonstrate the variation

in accuracy that may be found when automated perimetry is used to detect OAG. Under careful research conditions, Sommer et al.

found automated perimetry to have a

sensitivity and specificity of 93 and 88 percent, respectively (103). Ford et al., on the

other hand, found that automated perimetry

in a community screening program had a 92

percent sensitivity but only a 46 percent

specificity (35). According to one of the authors in this latter study, the difficulty of

many subjects in understanding how to

respond correctly when undergoing perimetry

for the first time led to a high false-positive

rate in the community setting ( 130).

Bengtsson and Krakau found that 3 percent

of eyes in a careful mass screening program

in Sweden tested positive on automated

perimetry, but only half the positive eyes ac8 For example, false posit ives can be reduced by

automat i 11 y retest i ng any point i n the vi sua 1

field not reported as seen by the patient on the

first try.
Screening for Open-Angle Glaucoma in the Elderly ● 29

tually had visual field defects that were glau- ly tested normal and therefore could not

comatous or meriting medical attention (14). report completely on the sensitivity and

The authors did not retest people who initial specificity of the test.
5. MEDICARE COVERAGE OF SCREENING

FOR OPEN-ANGLE GLAUCOMA

Costs to Medicare

At present, neither screening for all

people with high intraocular pressure (IOP)

nor for those with only manifest open-angle

glaucoma (OAG) is covered by Medicare, although tonometry, ophthalmoscopy, and

perimetry are all covered procedures when

provided as diagnostic services or in the

course of management of established disease.1

If Medicare were to initiate a policy of

covering screening for manifest OAG or for

high IOP, what would be the implications for

program costs and patient benefits?

As the previous chapters demonstrate,

there is great variability and uncertainty surrounding the accuracy of screening tests and

the effectiveness of treatment for either

ocular hypertension (OH--high IOP without

other signs of disease) or manifest OAG.

This uncertainty makes a precise estimate of

the costs and effectiveness of glaucoma

screening infeasible. Nevertheless, OTA constructed a simple model of a hypothetical

biannual glaucoma screening program in order to estimate the likely magnitude of the

annual costs of such a program. The model

incorporates a wide range of reasonable assumptions based on the available evidence. It

is applied here, first, to screening for

manifest OAG using the various technologies

available; and second, to screening for high

IOP with tonometry.

The model, presented in appendix C

along with detailed assumptions and results,

calculates both total program costs and the

costs of detecting a case of OAG (or high

1 The current procedure codes for perimetry, on

which payment is based, are intended to represent

diagnostic, rather than screening, procedures. It

is possible that costs and charges would be lower

for screening perimetry, resulting in lower total

cost estimates than those here. Simi(arly, there

is no code for ophthalmoscopy that appears to be

appropriately app~ied to screening for glaucoma;

OTA’S assu~tion that the costs of ophthalmoscopy

wou[d be the same as for tonometry may well be incorrect. The code for tonometry is not intended

for screening, but in this case the procedure itself is the same regardless of the purpose.

IOP). Total costs include both the costs of

the screening episode and the costs of a fol10WUP visit for all individuals testing positive,

to confirm or deny the test result. To calculate the average cost per case of OAG (or

high IOP) detected, all screening and followup costs are loaded on true cases--i.e., on

those with confirmed positive tests.

The costs of an ongoing screening

program--and the number of cases detected

as its result- -depend fundamentally on

whether the program is newly implemented or

has been ongoing for some time. In initial

years, the number of cases that can be

identified through the program will approach

the prevalence of the condition. The

prevalence of high IOP in the elderly is quite

high, and OAG itself is not uncommon, occuring in about 2 to 3 percent of the elderly.

Consequently, costs per confirmed case of

high IOP or OAG will be relatively low in

initial years of a screening program (since

costs per case are total costs divided by total

number of cases found).

On the other hand, new cases of OAG

are comparatively rare --on the order of 2 per

1,000 elderly per year. Thus, an ongoing

program to detect OAG can identify only a

very small number of cases. It has a correspondingly high cost per true positive case

identified through the program. Note that,

with true new cases of manifest OAG being

comparatively rare, a high proportion of

people referred for followup will in fact be

false positive cases, even if the screening tests

are quite accurate.

As discussed in the appendix, the uncertainties surrounding several crucial assumptions of the model preclude a precise

estimate of costs. These assumptions include:

the accuracies of the different screening

procedures as performed by different

examiners in different settings,

the incidence and prevalence of OAG in

the elderly,

the costs attributable to screening,

31
32 ● Screening for Open-Angle Glaucoma in the Elderly

● the extent to which people utilize the

program, and

● the proportion of people testing positive

who will show up at the confirmatory

physician visit.

Because the uncertainties associated with

these factors are so great, OTA has estimated

and presented here only the likely upper and

lower bounds of the costs and number of

cases likely to be found through a screening

program. These bounds encompass a very

wide range. Nonetheless, the range is a useful indicator of the order of magnitude of

costs likely to be incurred and number of

cases cases likely to be identified through a

screening program for the elderly.

In the initial years of an every-otheryear program to screen for manifest OAG in

the elderly, it would cost between $1,000 and

$16,000, on average, to detect and confirm a

case of glaucoma. Between 50,000 and

340,000 cases of OAG would be detected annually in the first two years, depending on

the exact prevalence of OAG, the accuracy of

screening tests, and the skill of the examiners

using them. 2 In the later years of such a

screening program, it would cost between

$3,000 and $81,000 per confirmed case of

OAG to detect between 10,000 and 90,000

cases per year. Annual total screening program costs would be between roughly $200

million and $1 billion in both initial and subsequent years (see app. C).

In a similar program screening for high

IOP, costs would be between $100 and $1,700

per confirmed case of high IOP in initial

years, and between $300 and $14,600 in later

years. Total annual costs of such a program

would likely be between $100 million and

z These cases would not all be previously unknown.

Since OTA’s calculation is based on prevalence, it

includes the implicit assumption that a[( peop(e in

the population would be screened, regardless of

whether they were already known to have OAG. (In

f act , a substantial number of people who

voluntarily appear at contnunity screening clinics

actually have been told previously that they have

OAG (56).) After the first 2 years the model assumes that previously diagnosed cases will not

appear for screening, and the calculations are

based on incidence.

$300 million initially and between $250 million and $500 million in subsequent years.

This screening program would detect between

300,000 and 3 million people with high IOP

per year in initial years and between 30,000

and 350,000 per year in later years. The

cases of confirmed high IOP would consist

primarily of people with OH--high IOP but

no other signs of OAG--but would include a

minority of individuals who had manifest

OAG.

Medicare pays 80 percent of allowed

charges after the beneficiary has met the deductible. Assuming that Medicare pays 80

percent of the total program costs delineated

above leads to the conclusion that total

Medicare costs of an ongoing program to

detect OAG in the elderly would likely be

between approximately $160 million and

$800 million per year. Total Medicare costs

of a similar program to screen for high IOP

would be between $80 million and $400 million per year. These costs do not include the

costs of treating detected cases of OAG or

high IOP. Nor do they include the costs or

the benefits of detecting conditions other

than OAG (or high IOP) as a result of the

screening visit.

The full benefits of a screening program

depend fundamentally on the effectiveness of

treatment. Potentially, these benefits include

additional years of vision, lessened dependence on assistance in everyday tasks, and

reduced expenditures for programs providing

social services and support for people with

disabilities. Because of the uncertainties

about treatment effectiveness, the extent to

which these potential benefits can be realized

is unknown at present.

Implications of Scheduled Frequency of Screening and Screening Utilization for Medicare Costs

The scheduled frequency with which

screening occurs and the utilization rate of

glaucoma screening among the elderly have

little impact on the average cost of identifying a case of OAG through the screening

program. They do, however, have enormous

implications for the total number of cases

detected and for total program costs if the
Screening for Open-Angle Glaucoma in the Elderly ● 33

Medicare program were to cover the service.

In the model presented in appendix C, OTA

assumed that 75 percent of the population

would participate regularly in a program in

which people were screened every 2 years. A

less frequent screening schedule would result

in lower total program screening costs per

year, since fewer people would be screened

and diagnosed each year. These lower total

costs would come at the expense of fewer

diagnosed cases and cases that would be more

severe when diagnosed.

Screening utilization would affect case

detection and Medicare costs in three important ways:

Current utilization. Approximately 50

percent of elderly people report that they

have been screened for OAG within 2 years

(118). For this group, Medicare coverage

simply means a shift in the cost of screening

from the individual patient (or provider, or

non-profit organization) to the Medicare program. No new health benefits accrue, since

OAG (and OH) cases in this group would

have been diagnosed regardless of Medicare

coverage.

In some cases, Medicare coverage would

replace screening currently provided free of

charge to the patient. The National Society

to Prevent Blindness, a nonprofit organization

that often coordinates with local hospitals or

service organizations to provide glaucoma

screening, screened 46,889 people age 65 and

over in 1985 (0.16 percent of the population

in that age group) (85). The American Academy of Ophthalmology operates the National

Eye Care Project, which refers needy elderly

people to ophthalmologists who volunteer

their services. Since 1986, this project has

referred over 137,500 people, of which at

least 77,500 have seen an ophthalmologist and

had an eye examination as a result (129).

About 5 percent of patients seen were diagnosed with glaucoma (19).

New utilization. Presumably, Medicare

coverage would encourage people to be examined who otherwise would never have been

screened for OAG. The cost model OTA has

used assumes 75 percent utilization, or a 50

percent increase over the current utilization

rate of 50 percent. 3 This new group would

obtain new health benefits that would not

have accrued in the absence of Medicare

coverage. For example, under a scenario in

which screening by perimetry in an office

setting would detect 50,000 cases of OAG per

year (about the middle of the range estimated

in the model), the utilization rate of 75 percent would mean that one-third of these

cases- - nearly 17,000 of them--would not

have been detected as rapidly without

Medicare coverage because these people

would not have been screened. The remaining 33,000 cases would have been diagnosed

without Medicare coverage, but Medicare

now pays their screening costs.

If screening were less frequent, the new

additional utilization due to Medicare

coverage would quite likely be less as well.

For example, while only about 50 percent of

the elderly population currently receive glaucoma screening every 2 years, a total of 75

percent receive screening at least every 3

years (1 18). Thus, if Medicare covered

screening every 3 years, the additional

utilization might be perhaps 10 percent over

current levels (for a total of 80 to 85 percent

utilization), and could be no more than 33

percent higher than at present.

Current diagnostic visits. Under the

present Medicare system, a screening visit (in

which the patient is asymptomatic) would not

normally be reimbursed. Despite this policy,

however, some current utilization is probably

already supported by Medicare. For example,

patients may be screened for OAG during a

visit that was reimbursable for other reasons

(e.g., evaluation of a cataract). Since a substantial number of elderly people have eye

conditions other than OAG, and visits to the

physician due to these conditions are often

reimbursable, it is possible that a substantial

amount of glaucoma screening is already

being done during Medicare-reimbursed

visits.

3 This represents a sinpl istic assumption that the

50 percent of the e[derly reporting that they have

been screened ui thi n 2 years are i n fact routinely

screened every other year.
34 ● Screening for Open-Angle Glaucoma in the Elderly

In summary, the benefits of Medicare

coverage for OAG screening depend heavily

on how many people would be induced to

undergo screening if the service were covered. For the over 50 percent of the elderly

assumed to be already undergoing frequent

screening, coverage will bring some relief

from out-of-pocket costs but no additional

benefits in preventing impairment. Elderly

people induced to undergo screening due to

Medicare coverage represent the greatest

potential social benefit to the service. Under

the assumptions of our model, this means that

somewhere between 3,000 and 30,000 people

would have manifest OAG diagnosed earlier

if Medicare covered OAG screening than under the current financing scheme.

Costs and Effectiveness of

Screening in Preventing Blindness

The above discussions of screening for

OAG and high IOP include only the costs of

detecting and confirming a case, not the expense of treating the cases found. OAG

treatment can be expensive. Rough estimates

by two researchers in 1980 suggested that

each person with a diagnosis of OAG incurred annual charges of between $180 and

$460 for medications and followup, depending on the number and type of drugs prescribed (41,4127). (Individuals with OH who

are treated to lower their IOP would incur

similar chargers. ) OAG patients requiring

filtering surgery were estimated to incur

charges of $2,400 to $3,000 in the year they

received surgery.5

Because of the uncertainties regarding

the effectiveness of treatment for OH and

OAG, OTA did not extend the costeffectiveness analysis to the full effectiveness

of screening in preventing visual disability.

The authors of one study in the literature,

however, did attempt such an analysis (41).

The baseline assumptions in that analysis

were generally optimistic, including high

sensitivity and specificity for screening tests,

low per-person costs of screening, and generous assumptions regarding the effectiveness

of treatment. 6 However, when less generous

assumptions were made--for example, when

less favorable treatment outcomes were

assumed- - costs per year of vision saved were

up to 40 times greater than the lowest cost

under baseline assumptions. Thus, just as

this OTA analysis reports a wide range of

potential program costs per OH and OAG

case detected, that analysis demonstrated the

extreme sensitivity of cost-per-year-ofvision-saved to assumptions regarding the effectiveness of treatment.

Problems in Implementing

Medicare Coverage of OAG

Screening

Covering OAG (or OH) screening as a

Medicare benefit would present two problems

concerning payment policies:

1. Paying for screening in community

settings. If Medicare were to cover glaucoma

screening, an immediate policy decision

would have to be made regarding who would

be paid to provide it. At present, a considerable amount of screening is provided inexpensively by non-profit organizations in

community settings (e.g., at churches, schools,

or hospitals). However, mass screening in

community settings is a controversial issue

among eye care professionals. Such efforts

make glaucoma screening available to a broad

spectrum of people who might otherwise not

receive the service. On the other hand, a

negative glaucoma test in a community

screening clinic may sometimes encourage an

individual not to seek any further eye care of

any kind. Policy makers would have to decide

if glaucoma screening were to be covered

d Although this paper was published in 1983, the

treatment cost estimates i t contained uere 1980

estimates from an ear [ i er, unpubl i shed paper.

5 Unti 1 1991, Medicare ui11 not cover any part of

the costs of outpatient drugs to treat OH or OAG.

Med i care does pay a proport ion of hospi ta 1,

physician, and anc i 1 lary charges charges associated

Hi th surgery for OAG.

6 The authors assumed, first, that persons with

field loss at the screening uould become blind in

7.5 years without treatment (it was assumed that

only a small number of ‘treatment failures” would

go blind if treated); and, second, that people with

elevated IOP but no visual defects at screening

would go blind an average of 12.5 years after

screening if untreated (41).
Screening for Open-Angle Glaucoma in the Elderly ● 35

only in community settings, only in traditional health care settings (e. g., physicians’

and optometrists’ offices), or in both.

There is at present no mechanism by

which Medicare pays for medical services offered in community facilities such as churches or senior citizens’ centers. If Medicare

covered OAG screening done in these settings, the Health Care Financing Administration (HCFA) would have to develop reimbursement policies for them. For example,

HCFA might pay community screening program sponsors -- hospitals, nonprofit vision

societies, etc. --- a set rate per patient for all

Medicare beneficiaries screened. Although

feasible, this policy would take some time to

establish. Processes for developing payment

rates, designating eligible clinic sponsors, and

regulating dangers and problems in community facilities would have to be developed.

2. Paying for components of office visits.

The two most widely used OAG screening

technologies, tonometry and ophthalmoscopy,

when used as part of a routine physician office visit, are not billed separately (3), If

Medicare covered OAG screening but not

screening for other vision conditions, either:

1)

2)

3)

examiners must be able to bill separately

for these procedures,

Medicare would have to establish a policy

of paying for part of a visit charge, or

Medicare would pay for a visit designated

to include only glaucoma screening (for

example, a “limited visit,” as used in the

cost model).

Ultimately, a Medicare decision to cover

glaucoma screening would probably require

the development of new codes to designate

tonometry, ophthalmoscopy, or perimetry

used for that purpose.
Appendix A: ADVISORY PANEL--PROJECT ON PREVENTIVE

HEALTH SERVICES UNDER MEDICARE

Gordon De Friese, Panel Chair

Health Services Research Center

University of North Carolina, Chapel Hill, NC

Marianne C. Fahs

Department of Health Economics

Mt. Sinai Medical Center

New York, NY

John Frank

Department of Preventive Medicine &

Biostatistics

University of Toronto

Ontario, Canada

Gary D. Friedman

Epidemiology and Biostatistics Division

Permanence Medical Group, Inc.

Oakland, CA

Lawrence Gottlieb

Clinical Guidelines Program

Harvard Community Health Plan

Brookline Village, MA

Mary Knapp

John Whitman and Associates

Philadelphia, PA

Risa Lavizzo-Mourey

Geriatrics Program

University of Pennsylvania

Philadelphia, PA

M. Cristina Leske

Department of Preventive Medicine

SUNY at Stony Brook

Stony Brook, NY

Donald Logsdon

INSURE Project

New York, NY

Mildred B. McCauley

American Association of

Retired Persons

Washington, DC

Peter McMenamin

Chevy Chase, MD

Meredith Minkler

Center on Aging

University of California, Berkeley

Berkeley, CA

Marilyn Moon

Public Policy Institute

American Association

of Retired Persons

Washington, DC

George Morley

Department of Obstetrics/Gynecology

University of Michigan Medical Center

Ann Arbor, MI

Gilbert Omenn

School of Public Health &

Community Medicine

University of Washington

Seattle, WA

George Pickett

Department of Public Health Policy

School of Public Health

University of Michigan

Ann Arbor, MI

Donald Shepard

Department of Health Policy

and Management

Harvard School of Public Health

Boston, MA

Barry Stults

Division of General Internal Medicine

University of Utah Medical Center

Salt Lake City, UT

Advisory Panel members provide valuable guidance during the preparation of OTA reports.

However, the presence of an individual on the Advisory Panel does not mean that individual

agrees with or endorses the conclusions of this particular paper.

37
Appendix B: ACKNOWLEDGMENTS

The OTA staff would like to express their appreciation of the following people for their

advice and assistance. This acknowledgement should not be

the individuals agree with or endorse the conclusions of this

Frank S. Ashburn, Jr.

Eye Associates of Washington, D. C., P.C.

Washington, DC

Morris F. Cohen

Kaiser-Permanante Medical Care Program

Oakland, CA

Peter Dudley

National Institutes of Health

Bethesda, MD

David M. Eddy

Duke University

Durham, NC

Alan Garber

National Bureau of Economic Research, Inc.

Stanford, CA

Douglas Gaasterland

Georgetown University Medical Center

Washington, DC

Susan Gleeson

Blue Cross and Blue Shield

Chicago, IL

H. Dunbar Hoskins, Jr.

The Foundation for Glaucoma Research

San Francisco, CA

Allan D. Jensen

Baltimore, MD

Terry Kay

Health Care Financing Administration

Baltimore, MD

Allan E. Kolker

Washington University School of Medicine

St. Louis, MI

Glory Tenney Kosmatka

National Society to Prevent Blindness

Schaumburg, IL

Joseph H. Krug, Jr.

Massachusetts Eye & Ear Infirmary

Boston MA

38

construed in any way to imply that

paper.

Benjamin Littenberg

Veterans Administration Medical Center

Palo Alto, CA

Stephanie Mensh

American Academy of Ophthalmology

Washington, DC

Samuel Merrill, Jr.

Congressional Research Service

Washington, DC

Karl Michelitsch

Georgetown University Medical Center

Washington, DC

Angela Mickalide

U.S. Preventive Services Task Force

Washington, DC

Steven Moore

Office of the Surgeon General

Rockville, MD

Marvin Podgor

National Institutes of Health

Bethesda, MD

M. Bruce Shields

Duke University Medical Center

Durham, NC

George L. Spaeth

Wills Eye Hospital

Philadelphia, PA

Steven H. Woolf

U.S. Preventive Services Task Force

Washington, DC

John Whitener

American Optometric Association

Alexandria, VA

Thorn J. Zimmerman

University of Louisville

Louisville, KY
Appendix C: THE COSTS OF SCREENING FOR OPEN-ANGLE

GLAUCOMA

To fully analyze the costs and effectiveness of alternative screening methods in preventing visual disability, one must know:

●

●

●

the costs of screening, diagnostic

workup, and treatment,

the effectiveness of the screening technologies in identifying established or

potential cases of open-angle glaucoma

(OAG), and

the effectiveness of treatment in preventing disability in the identified cases.

None of these factors is adequately

known. Some tenuous but reasonable assumptions regarding cost can be made about

the cost of screening and diagnostic workup.

There is a basis for estimating the effectiveness of the three screening technologies in

identifying OAG (or its precursor, high intraocular pressure (IOP)), although there is

great uncertainty about the estimates, especially as they apply to different settings and

different examiners. Most uncertain of all is

the effectiveness of treatment. Treatment is

probably effective, but how effective it might

be is not yet established.

Because of the tremendous uncertainties

in the basic assumptions necessary to a full

cost-effectiveness analysis, particularly the

uncertainties regarding treatment effectiveness, the analysis forming the bulk of this

appendix is limited to the comparative costs

of identifying and verifying a case of

manifest OAG. Following the analysis of the

cost of screening for manifest OAG is a

parallel analysis of the cost of screening for

high IOP with tonometry.

The purpose of these analyses is to

estimate the rough magnitude of the total

national health care costs of screening for

OAG in the over-65 population, and the

number of cases of OAG or high IOP that

might be detected through such a program.

The analyses are structured so that these cost

estimates, in turn, can be used as a basis for

estimating the likely magnitude of Medicare

program costs in the event of a decision to

cover OAG screening for the Medicare population. OTA has reasonable confidence that

the true costs of a screening program to

identify OAG cases (or cases of high IOP) lie

between the upper and lower bound s

specified here, but there is at present no factual basis for assessing where in that range

the true costs lie.

The 

Table 9 describes the steps of the simple

model used to estimate the number of OAG

cases detected in a screening program and the

average costs of detecting an OAG case. In

this model, the screening program includes

both the screening episode itself and the

workup of all people testing positive. Three

basic assumptions of the model do not vary.

These are:

●

●

●

the size of the overall elderly population

(31 ,697,000),

the frequency of screening (every 2

years), and

the proportion of the population participating in the program (75 percent).

Changes in these assumptions have little effect on the average cost per OAG case

detected, although they do affect the total

number of cases found and the total cost of a

screening program. 1 The program implications of these assumptions are described in

chapter 5.

1 Screening less frequently would have three major

effects. First, total program costs wou(d be less

because fewer people uould be screened each year.

Second, the average severity of OAG cases detected

through the screening program uould be greater,

because the length of time betueen screening visits

is greater. Th i rd, it uould become more likely

that cases of manifest OAG would be diagnosed due

to the onset of synptoms (e. g., decreased vision),

rather than being detected when a symptomatic

through the screening program. This last factor

does affect the average cost per case detected in

the screening program, since these cases are not

diagnosed as a resu(t of the program. However, the

magnitude of the effect is likely to be small unless screening is very infrequent.

39
40 ● Screening for Open-Angle Glaucoma in the Elderly

Table 9--- Calculation of Estimated Cost per Confirmed Case of Open-Angle Glaucoma

and Number of Cases Diagnosed

Step Calculation Description

1. (population) x (utilization rate) x (screening frequency rate) Number of people screened

2. (prevalence or incidence) x (sensitivity of screening test) Rate of true positive cases

3. [1 - (prevalence or incidence)] x [1 - (specificity)] Rate of false positive cases

4. (STEP 2) + (STEP 3)

5. (STEP 4) x (cost per followup visit)

6. (sTEP 5) + (cost per screening episode)

7. (STEP 1) X (STEP 6)

8. (STEP 1) X (STEP 2)

9. (STEP 7) / (STEP 8)

SOURCE: Office of Technology Assessment, 1988.

Other variables in the model are described

below.

Prevalence and Incidence

In the beginning of a screening program,

relatively few cases of OAG are known; most

await detection by the program. Thus, the

estimated costs in the first 2 years of an

every-other year program are based on the

prevalence of OAG. OTA estimated costs

based on both 2 percent and 3 percent

prevalence of disease in the elderly.

In the subsequent years of a screening

program, only new cases and a small, constant

number of false negatives from previous

screenings exist to be detected by the program. The incidence is thus the basis for cost

estimates of an ongoing program.

A summary of OTA’s calculation of the

incidence of OAG in the elderly is presented

in table 10. In this calculation, OTA first

derived annual age-specific incidenc e

estimates from the five-year incidence rates

for ages 65-79 estimated by Podgor et al. (see

Proportion of all screenees

testing positive and referred

for followup

Followup cost rate

(followup cost averaged across

all screened persons)

Screening cost plus followup

cost rate per screened person

Total cost

Total number of true positive

cases

Cost per true confirmed

positive case

ch. 2, table 1) (94). (For example, where

these researchers estimated the five-year incidence of OAG at 5 cases per thousand population for the age 65-69 cohort, OTA assumed an annual incidence of 1 cases per

thousand). No OAG incidence estimates for

people overage 79 exist. OTA assumed that

the incidence continues to rise at an increasing rate with age and arbitrarily chose annual

incidence rates of 3.4 and 5 per thousand for

ages 80-84 and 85 and over, respectively.

(These rates are consistent with the trend in

the younger age groups.) From these agespecific incidence rates, OTA then calculated

the overall incidence rate for the elderly.

This rate was 2 cases per thousand per year

(4 cases per thousand per screening period

for the model case of every-other-year

screening).

This incidence rate is likely to underestimate the true number of cases available

to be detected by an ongoing screening program, for two reasons. First, the overall incidence of OAG in the elderly is likely to increase over time as the proportion of the

elderly in the oldest cohorts increases. Sec-
Screening for Open-Angle Glaucoma in the Elderly ● 41

end, the OTA model does not adequately account for the OAG cases that eluded detection in previous screening years (i. e., were

false negatives) but might be detected in the

current year. Thus, OTA also estimated

screening costs under the assumption that, for

every thousand population, 4 OAG cases per

year (8 cases per screening period) exist to be

detected by the screening program.

Screening Accuracy

An examiner screening for OAG would

most likely use one of five possibl e

combinations of screening technologies:

tonometry alone;

ophthalmoscopy alone;

both tonometry and ophthalmoscopy,

referring for workup all persons testing

positive on either test (henceforth designated “tonometry/ophthalmoscopy in

parallel”);

both tonometry and ophthalmoscopy,

referring for workup only persons positive on both tests (henceforth designated

“tonometry/ophthalmoscopy in series”); or

perimetry alone.

The sensitivity and specificity of OAG

screening depend heavily on which technology is used, how it is used, and who uses it.

There are, unfortunately, very few studies of

the accuracy and predictive value of OAG

screening tests, and the results of those

studies cannot be inferred to different examiners and different settings. OTA has thus

chosen only to examine the extremes of the

values presented in the literature for these

technologies, recognizing that the true value

is unknown and greatly depends on who performs the screening and the conditions under

which they do so. Table 11 lists the high and

low bounds of sensitivity and specificity for

each technology that OTA used in this analysis.

Tonometry, as a test for manifest OAG,

has a natural limit to accuracy because most

people with high IOPs do not have manifest

OAG, and many people with OAG do not

have high IOP when they are screened. OTA

estimated a theoretical upper limit for the accuracy of tonometry (see box B) and used this

as the upper bound in the analysis.

Table 10 .--Calculation of Incidence of Open-Angle Glaucoma (OAG) in the Elderly

Population

Number of OAG

2-year OAG Total number cases per year

Age Population incidence of cases in screened

group i n 1990= per thousand per 2 years population c

65-69 9,996,000 2.0 19,992 7,497

70-74 8,039,000 2.8 22,509 8,441

75-79 6,260,000 4.4 27,544 10,329

80-84 4,089,000 6.8 27,805 10,427

85+ 3,313,000 10.0 33,130 12,424

TOTAL 31,697,000 4.1 130,980 49,118

aEst i mates from U.S. Bureau of the Census, Current Populat ion Reports, Series P- 25, No. 952, Projections of

the Population of the Uni ted States, by Age, Sex, and Race: 1983 to 2080 (Washington, DC: U.S. Govern-

~ment Printing Office, 1984).

Cases per year of open-angle g[aucoma in the re(evant age group. Estimates for ages 65-79 from M.J.

Podgor, M.C. Leske, and F. Ederer, Ii Incidence Estimates for Lens Changes, Macu(ar Changes, @en- Ang~e

Glaucoma, and Diabetic Retinopathy,ll Am. J. Epidemiology 118(2):206-212, August 1983. Estimates forages

80 and over are undocumented assumptions of the Office of Technology Assessment.

cAssunes that 75 percent of the e[der[y in each age group would avail themselves of the screening benefit,

and that ha(f of this group wou(d be screened each year.

SOURCE : Office of Technology Assessment.
42 ● Screening for Open-Angle Glaucoma in the Elderly

!-(

e

0

0

0

0

0

!-4
Screening for Open-Angle Glaucoma in the Elderly ● 43

Box B--- Estimation of Theoretical Upper Limits of Accuracy of Tonometry (21 mm Hg or

Greater) for Identifying People with Open-Angle Glaucoma

A natural upper bound to use for the accuracy of tonometry in identifying cases of

manifest OAG is the theoretical maximum accuracy of high IOP in predicting manifest OAG.

Since most people with high IOP do not have manifest OAG, and many people with OAG do

not have high IOP, tonometry is clearly less than 100 percent sensitive and specific when used

to identify cases of manifest OAG, even if the technology itself accurately identifies all people

with high IOP.

To estimate the theoretical limits to accuracy of tonometry used for this purpose, first recall

that approximately 24 percent of the elderly in one study were found to have IOPs of 20 mm

Hg or greater. This figure provides a maximum-- probably a considerable overestimate--for the

proportion of elderly people in the United States as a whole who would test positive by

tonometry at the slightly higher cutoff level of 21 mm Hg.

Next, one needs to know the proportion of this group that actually has OAG (true positives). Recall from table 4 (chapter 2) that approximately 3 to 10 percent of all individuals with

IOP 20 mm Hg or greater will get manifest OAG within 5 years. This group therefore

represents the absolute maximum number of people with high IOPs that actually have manifest

OAG already. Take the higher end of this range and assume that, at the absolute maximum, 10

percent of the elderly with high IOPs also have manifest OAG. Multiplying .10 by .24 (the

proportion of elderly people with high IOPs) gives a total of .024, the maximum proportion of

all elderly who have high IOPs and manifest OAG.

Now, recall (also from table 4, chapter 2) that less than 1 percent of all individuals with

normal IOPs will get OAG within 5 years. Again, assume that 1 percent therefore represents

the absolute maximum number of elderly people with normal IOP that could have manifest

OAG at the time of screening. Since 76 percent of elderly people have normal IOPs, the maximum rate of false negatives is (.76)*(.01), or .0076.

Adding false negatives and true positives yields the total proportion of elderly in the population with OAG. The maximum proportion of the elderly with OAG is thus

(.024)+(.0076)=.0316; the proportion without OAG is (l)-(.0316)=.9684.

Lastly, one needs to know the proportion of true negatives (i.e., people with normal IOPs

and no OAG). This proportion is all negatives minus false negatives, or (.76)-(.0076)=.7524.

Then, the upper bounds of tonometry (cutoff 21 mm Hg) for detecting OAG are:

True positives .024

Sensitivity = = = 76%

All persons with OAG .0316

True negatives .7524

Specificity = = = 78%

All persons without OAG .9684
44 ● Screening for Open-Angle Glaucoma in the Elderly

Cost

OTA analyzed five different potential

screening settings: mass screening in community facilities (e.g., screening clinics at

senior citizens’ centers), and the offices of

family practitioners, internists, ophthalmologists, and optometrists.

In the model, costs for services provided

in offices settings are based on Medicare

average allowed chargesl for office visits and

procedures. For the purpose of estimating

the social costs of screening, these average allowed charges are assumed to represent real

resource costs, although the extent to which

this assumption is true is unknown.

The true cost of OAG screening done in

physicians’ or optometrists’ offices depends

on how much of the visit is due to the

screening procedure. If screening is only one

of many services performed during the visit,

the cost of screening is only the cost of the

screening procedure itself. If, on the other

hand, the visit is made only for screening,

then OAG screening must bear the entire cost

of that visit. OTA thus tested two basic cost

alternatives: one in which the screening cost

is simply the Medicare average allowed

charge for the procedure itself, and one in

which the cost is the charge for a brief visit

plus the charge for the screening procedure.

Table 12 outlines these cost assumptions in

greater detail.

Within either of these two basic assumptions, costs vary depending on who does the

screening. A visit to a family practitioner,

for example, costs less (on average) than a

visit to an ophthalmologist. OTA used the

examiner-specific charges when running the

model to give the most accurate estimation of

1 Medicare average allowed charges are used as the

basis of Medicare payments to physicians; Medicare

pays a proportion of the allowed charge for all

covered services. Since the most recent charge

data available are from 1985, OTA updated all

charges by the Medicare Economic Index for participating primary-care physicians. Medicare average

allowed charges underestimate actual average

physician charges, since some physicians charge

more than the level allowed for Medicare payment.

the extremes. However, the true interexaminer costs per case diagnosed cannot be

compared with existing information. An examiner who charges more for screening, for

example, may also be more skilled, resulting

in fewer false positives referred and consequent lower cost per diagnosed case overall.

Since the actual relative accuracy of testing

among examiners is unknown, specific inferences about which setting results in the

overall lowest true costs cannot be drawn.

The use of Medicare average allowed

charges applies only to office settings. For

community facilities, OTA relied on a cost

analysis of charitable glaucoma screening

programs in northern California (56). These

costs underestimate true resource costs, since

the programs rely in part on volunteer labor.

The baseline cost from this source was assumed to apply to either tonometry or

ophthalmoscopy (see table 12). The programs

do not currently use perimetry for screening;

OTA assumed that, as with office-based examiners, perimetry would be about twice as

expensive as the baseline cost. Since it was

assumed that, in offices, the cost of performing both tonometry and ophthalmoscopy

would be double the cost of providing either

one alone, the same assumption was made for

community facilities.

Followup rates

In the model, it was assumed that all

persons with a positive screening test result

would be referred to an ophthalmologist for a

comprehensive visit, at which the definitive

diagnosis (OAG/no OAG) would be made.

(For patients screened by an ophthalmologist,

it was assumed that the patient would return

for a confirmatory comprehensive visit).

OTA tested two extreme alternative assumptions of the rate at which people referred

would actually show up for the visit: a 100

percent compliance rate, in which all people

show up, and a 40 percent rate. The lower

bound is slightly less than that reported for a

current mass screening program (85). OTA

assumed that compliance was independent of

whether an individual had a true or false

positive result on the screening test.
Screening for Open-Angle Glaucoma in the Elderly ● 45

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46 ● Screening for Open-Angle Glaucoma in the Elderly

Results

Number of OAG Cases Diagnosed

As shown in table 13, the number of

cases that might be diagnosed annually by an

every-other- year screening program range

from approximately 50,000 to 340,000 cases

in initial years and from 10,000 to 90,000

cases in later years. The actual numbers

depend heavily on the true prevalence and

incidence of OAG in the elderly.

In addition, the effectiveness of a

screening program in identifying cases

depends on the sensitivity of the screening

procedure in whatever setting it is used.

Perimetry and tonometry/opthalmoscopy in

parallel (anyone testing positive on either test

referred for follow up) are equal in their

potential to detect a maximum number of

cases. Tonometry/ophthalmoscopy in series is

likely to detect the least number of cases under anyone set of assumptions. This result

leads to the conclusion that, while tonometry

and ophthalmoscopy used in combination

have the potential to be highly effective in

identifying previously unknown cases of

OAG, maximum effectiveness is likely to be

achieved only if all persons testing positive

on either test are referred for followup.

cost

Tables 14 and 15 set out the high and

low estimates of the model for total screening

program costs and for average cost per case

of manifest OAG detected, respectively, in

the initial years of a screening program.

Tables 16 and 17 present the same information for subsequent years. In the tables,

setting-specific charges were used, resulting

in five double columns for the five settings.

In each double column, the low-cost estimate

assumes:

100 percent followup rate for people

with positive tests,

maximum test accuracy for the respective screening procedure,

high incidence (4 per thousand per year)

or prevalence (3 percent), and

screening episode costs that include only

a procedure-specific charge.

The high-cost estimate assumes:

Table 13--- Number of Open-Angle Glaucoma

40 percent followup rate for people with

positive tests,

minimum test accuracy for the respective screening procedure,

low incidence (2 per thousand per year)

or prevalence (2 percent), and

screening episode costs that include a

visit charge as well as a procedurespecific charge.

Cases Diagnosed Under Extreme

Assumptions of Test Accuracy and Followup Rates

Low-cost model High-cost model

100% followup, 40% followup,

high prevalence/incidence, low prevalence/incidence,

high test accuracy low test accuracy

Initial Later Initial Later

Technology years years years years

Tonometry 271,000 72,300 68,500 13,700

Ophthalmoscopy 300,000 80,000 68,500 13,700

Tonometry/ophthalmoscopy

in series 228,000 60,700 49,300 9,900

Tonometry/ophthalmoscopy

in parallel 343,000 91,400 87,600 17,500

Perimetry 342,000 91,300 87,500 17,500

SOURCE : Office of Technology Assessment, 1988.
Screening for Open-Angle Glaucoma in the Elderly ● 47

Table 14--- High and Low Boundsa of Total Costs of Screening and Confirmatory Followup for

Open-Angle Glaucoma in Initial Years of a Screening Program

(In millions of dollars)

Technology

C ommunity

Family Practice Internist Optometrist Ophthalmologist Facility b

Low High

—

Low High Low High Low High

Tonometry $363

Ophthalmoscopy (276)C

Tonometry/ophthalmoscopy

in series (476)C

Tonometry/ophthalmoscopy

in parallel (692)C

Perimetry 462

$613 $409

493 (276)C

604 (476)C

803 (692)C

748 481

$720 $338 % 0 9 $440

600 (276)C 489 276

757 (476)C 574 476

956 (692)C 774 692

828 405 711 531

$732 $700 $321

612 403 201

799 371 229

998 429

859 634 334

aHigh bound assumes lowest re(evant reported sensitivities and specificities of the respective screening

procedure, 40% fo[[owup rate for positive cases referred, prevalence rate of 2 percent per year, and

screening episode costs that include both visit and procedure-specific charges (for office settings). Low

bound assumes highest relevant reported sensitivities and specificities of the respective screening procedure, 100% fol(owup rate for positive cases referral, prevalence rate of 3 per thousand per year, and

b screening episode costs that include only procedure-specific charges (for office settings).

High and low bounds for conwnunity facilities vary only by assumptions regarding followup rates and incidence rates assuned; costs per screening episode and test specificities and sensitivities did not vary.

Because of this, the average per-case cost varies litt(e between the bounds, but the total nurber of cases

varies drastically. Thus, unlike the other settings, the model variant producing the most nunber of cases

diagnosed (“lowU variant) actually produces much higher total costs than does the model variant producing

the least nunber of cases (llhighH).

cFor procedures invo(ving ophthalmoscopy, OTA assumed that only very experienced ophthalmologists could attain the ieve(s of sensitivity and specificity used for the IOW bound. Thus, despite the fact that ophthalmologists’ charges for ophthalmoscopy are higher than charges of other professionals, OTA assumed that

other specialties could perform the procedure no less cheap{y overall than could optha[mologists. The

lowest cost per case found for ophthalmologists was thus assuned to be also the lowest attainable cost per

case for any other profession.

SOURCE: Office of Technology Assessment, 1988.
48 ● Screening for Open-Angle Glaucoma in the Elderly

Table 15. --High and Low Bounds a of Cost Per Confirmed Case of Open-Angle Glaucoma in Initial Years of a Screening Program

Communityb

Family Practice Internist Optometrist Ophthalmologist Facility

Technology Lou High Lou High Lou High Low High "Low" "High"

Tonometry $1,300 $9,000 $1,500 $10,500 $1,200 $8,900 $1,600 $10,700 $2,700 $4,700

Ophthalmoscopy c (900) 7,200 (900) 8,800 (900) 7,100 900 8,900 1,600 2,900

Tonometry/ophthalmoscopyc

in series (2,100) 12,200 (2,100) 15,400 (2,100) 11,700 2,100 16,200 2,000 4,700

Tonometry/ophthalc

moscopy in parallel (2,000) 9,200 (2,000) 10,900 (2,000) 8,800 2,000 11,400 2,600 4,900

Perimetry 1,300 8,600 1,400 9,500 1,200 8,100 1,600 9,800 1,900 3,800

aHigh bound assumes lowest relevant reported sensitivities and specificities of the respective screening procedure, 40% followup rate for positive cases referred, incidence rate of 2 per thousand per year, and

screening episode costs that include both visit and procedure-specific charges (for office settings). Low

bound asstxnes highest relevant reported sensitivities and specificities of the respective screening procedure, 100% followup rate for positive cases referred, incidence rate of 4 per thousand per year, and

~ screening episode costs that include only procedure-specific charges (for office settings).

High and low bounds for comnunity facilities vary only by assumptions regarding followup rates and incidence

rates assuned; costs per screening episode and test specificities and sensitivities did not vary.

cFor procedures involving ophthalmoscopy, OTA assumed that only very experienced ophthalmologists could attain

the levels of sensitivity and specificity used for the low bound. Thus, despite the fact that opthalmologistsl charges for ophthalmoscopy are higher than charges of other professionals, OTA assuned that other

specialties could perform the procedure no less cheaply overall than could opthalmologists. The lowest cost

per case found for ophthalmologists was thus assumed to be also the lowest attainable cost per case for any

other profession.

SOURCE: Office of Technology Assessment, 1988.
Screening for Open-Angle Glaucoma in the Elderly ● 49

Table 16--- High and Low Boundsa of Total Costs of Screening and Confirmatory Followup

for Open-Angle Glaucoma in Later Years of a Screening Program

(In millions of dollars)

Communityb

Family Practice Internist Optometrist Ophthalmologist Facility

Technology Low High Low High Lou High Lou High "Low" "High"

Tonometry $231 $613 $277 $720 $206 $609 $308 $732 $700 $321

Ophthalmoscopy (240)C 491 (240)C 598 (240 )

C 4 8 6 240 610 396 199

Tonometry/ophthalmoscopy

in series (458)C

602 (458 )

C 7 5 5 (458 )C 5 7 3 458 797 366 228

Tonometry/ophthalmoscopy

in parallel (543)C 802 (543)C 956 (543 )

C 7 7 3 543 998 428

Perimetry 382 746 401 826 325 709 451 857 626 333

aHigh bound assumes lowest relevant reported sensitivities and specificities of the respective screening

procedure, 40% follouup rate for positive cases referred, incidence rate of 2 per thousand per year, and

screening episode costs that include both visit and procedure-specific charges (for office settings). Low

bound assumes highest relevant reported sensitivities and specificities of the respective screening procedure, 100% fo((owup rate for positive cases referred, incidence rate of 4 per thousand per year, and

~ screening episode costs that include only procedure-specific charges (for office settings).

High and low bounds for community facilities vary only by assumptions regarding followup rates and incidence rates assuned; costs per screening episode and test specificities and sensitivities did not vary.

Because of this, the average per-case cost varies little between the bounds, but the total n-r of cases

varies drastically. Thus, unlike the other settings, the model variant producing the most nun-bet- of cases

diagnosed (II(OWII variant) actua[[y produces much higher total costs than does the model variant producing

the least ntir of cases (nhighH).

cFor procedures involving ophthalmoscopy, OTA assuned that only very experienced ophthalmologists could attain the levels of sensitivity and specificity used for the low bound. Thus, despite the fact that opthalmologistsi charges for ophthalmoscopy are higher than charges of other professionals, OTA assumed that

other specialties could perform the procedure no less cheap(y overall than could opthalmo[ogists. The

lowest cost per case found for ophthalmologists was thus assuned to be also the lowest attainable cost per

case for any other profession.

SOURCE: Office of Technology Assessment, 1988.
50 ● Screening for Open-Angle Glaucoma in the Elderly

Table 17--- High and Low Bounds a of Cost Per Confirmed Case of Open-Angle Glaucoma in

Later Years of a Screening Program

Communityb

Family Practice Internist Optometrist Ophthalmologist Facility

Technology Low High

—

Low High Low High Low High "Low"

Tonometry $4,900 $44,800 $5,500 $52,600 $4,500 $44,400 $5,900 $53,500 $1O,2OO $23,400

Ophthalmoscopy (3,200)C 35,900 (3,200)C 43,700 (3,200)C 35,500 3,200 44,500 5,800 14,500

Tonometry/ophthalmoscopy

in series (7,600)C 61,100 (7,600)C 76,600 (7,600)C 58,100 7,600 80,900 7,400 23,100

Tonometry/ophthalmoscopy

in parallel (7,400)C 45,800 (7,400)C 54,500 (7,400)C 44,100 7,400 56,900 9,900 24,400

Perimetry 4,900 42,600 5,100 47,200 4,300 40,500 5,600 49,000 7,200 19,000

‘High bound assumes lowest relevant reported sensitivities and specificities of the respective screening procedure,

40% followup rate for positive cases referred, incidence rate of 2 per thousand per year, and screening episode

costs that include both visit and procedure-specific charges (for office settings). Low bound assumes highest

relevant reported sensitivities and specificities of the respective screening procedure, 100% followup rate for

positive cases referred, incidence rate of 4 per thousand per year, and screening episode costs that include only

~Procedure-specific charges (for office settings).

High and low bounds for conmnunity facilities vary only by assumptions regarding followup rates and incidence

rates assumed; costs per screening episode and test specificities and sensitivities did not vary.

cFor procedures involving ophthalmoscopy, OTA assumed that only very experienced ophthalmologists could attain the

levels of sensitivity and specificity used for the tow bound. Thus, despite the fact that ophthalmologists’

charges for ophthalmoscopy are higher than charges of other professionals, OTA assumed that other specialties

could perform the procedure no (ess cheaply overall than could optha(mologists. The lowest cost per case found

for ophthalmologists was thus assuned to be also the lowest attainable cost per case for any other profession.

SOURCE: Office of Technology Assessment, 1988.
Screening for Open-Angle Glaucoma in the Elderly ● 51

For procedures involving ophthalmoscopy, it was assumed that only a few

ophthalmologists who were glaucoma

specialists might actually attain the test accuracies implicit in the lowest-cost estimate.

Thus, despite the higher charges of ophthalmologists, OTA assumed that the lowest-cost

estimate for procedures involving ophthalmoscopy could be no lowe r for non -

ophthalmologists than for ophthalmologists.

As the tables 14 and 16 demonstrate, total annual health care costs of a screening

program would be somewhere between $200

million and $1 billion. Clearly, the benefit

derived from such a program--the number of

OAG cases identified--vary greatly within

these bounds. A more accurate reflection of

how efficiently the program might detect

cases can be expressed in the average amount

of money that must be spent to identify and

confirm a case of OAG. As tables 15 and 17

show, this amount lies somewhere between

approximately $1,000 and $16,000 per case

of OAG detected through the program in initial years and between approximately $3,000

and $81,000 per case in later years. The actual cost, within these ranges, depends upon

the variables listed above and upon the examiner and the screening procedure used. These

costs do not include the costs associated with

treatment.

The wide ranges of the above estimates--

nearly 30-fold, in the last case--is indicative

of the uncertainty surrounding the estimates.

For example, in order to know more about

the relative costs of screening as performed

by different examiners in different sites, one

must know not only their relative charges but

their relative accuracies in identifying cases

when using the different screening procedures. Since almost nothing about the relative accuracies of different examiners is

known, OTA can conclude only that the true

costs associated with each type of examiner

probably lie within the extremes above. It is

not known whether, on average, screening

would be cheaper if done by optometrists,

ophthalmologists, or other physicians. Nor is

it known which procedure would, on average,

be cheapest. At best, some hypotheses can be

suggested - -for example, that if bot h

tonometry and ophthalmoscopy are performed, there may be a substantial difference

in cost in some settings depending on the

referral criterion. The results of OTA’s

model suggest that referring for followup any

person testing positive on either test results in

more cases found and lower per-case costs

than referring only those testing positive on

both. The model results also suggest that

mass screening in community facilities is not

necessarily the least expensive method of

screening; if office-based examiners can perform screening more accurately and/or have

higher followup rates, the cost per OAG case

detected in these settings would be lower.

Applying the Model to Screening

for High Intraocular Pressure (IOP)

The model described above to estimate

the costs and yield of a screening program

for manifest OAG can also be applied to a

program whose goal is detecting high IOP.

The population with high IOP consists

primarily of people with ocular hypertension

(OH) --high IOP but no other signs of OAG--

but includes a minority of people who, at the

confirmatory visit, would be found to have

manifest OAG.

Estimating the screening program cost

per confirmed case of high IOP involves uncertainties similar to those for manifest OAG

screening. Both the accuracies of the tests

and the settings in which they would most

often be performed (and, hence, the cost of

the screening episode and the type of

tonometer used) are largely unknown. The

precise incidence and prevalence of high IOP

in the U.S. elderly population is also still uncertain.

OTA estimated the likely upper and

lower bounds of the costs of a screening program to detect elderly people with high IOP

using the same basic model as for manifest

OAG screening. Assumptions regarding population size, utilization of the program, and

frequency with which screening would occur

are the same as for the OAG screening

model. Again, OTA calculated the average

cost per confirmed case of high IOP (i.e., all

persons with high IOP at the screening

episode would be referred for a confirmatory,

comprehensive visit; total costs are divided by
52 ● Screening for Open-Angle Glaucoma in the Elderly

confirmed high IOP cases to yield the average

cost of detecting a case of high IOP). Costs

and medical benefits occurring after the confirmatory visit were not calculated due to the

uncertainty regarding the likely benefits of

treatment. Tonometry was the only screening

technology considered. Table 18 presents

other assumptions for the high and low

bounds of the estimated cost per confirmed

case of high IOP and the rationale for each

assumption.

OTA estimates that a program to screen

elderly people for high IOP with tonometry

would cost between $100 and $1,700 per

confirmed case of high IOP in initial years,

and between $300 and $14,600 in later years.

Total costs of such a program would likely

be between $100 million and $300 million initially and between $250 million and $500

million in later years of an ongoing program.

2 Because the cost of the confirmatory

visit is so great compared to the screening

visit, and because high IOP is fairly common

(with a large number of positive tests), the

proportion of people testing positive who actually show up for the confirmatory visit is

especially crucial to costs and to the number

of cases identified.

This screening program would detect between 300,000 and 3 million people with

high IOP per year in initial years and between 30,000 and 350,000 per year in later

years. The cases of confirmed high IOP

would consist primarily of people with OH

but would include a minority of individuals

who had manifest OAG.

z Here, as with a program to detect only manifest

OAG, high per-case costs do not lead to proport i onately high tota 1 costs, because the h i ghest

per-case costs occur when there are relatively few

cases detected.

Table 18--- Assumptions for Low- and High-Cost Bounds of a Model for Estimating Costs of

Screening for High Intraocular Pressure
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