DOT/FAA/AM-00/23

Van B. Nakagawara

Ronald W. Montgomery

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

July 2000

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

Gender Differences in a

Refractive Surgery Population

Of Civilian Aviators

Office of Aviation Medicine

Washington, D.C. 20591

U.S. Department

of Transportation

Federal Aviation

Administration
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-00/23

4. Title and Subtitle 5. Report Date

Gender Differences in a Refractive Surgery Population of July 2000

Civilian Aviators 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Nakagawara, V.B. and Montgomery, R.W.

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aeromedical Institute

P.O. Box 25082

Oklahoma City, OK 73125 11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aviation Medicine

Federal Aviation Administration

800 Independence Ave, S.W.

Washington, DC 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This work was performed under task AM-A-99-TOX-203.

16. Abstract

INTRODUCTION. Refractive surgical procedures performed in the United States have increased in recent years and

continued growth is projected. Postoperative side effects can affect the quality of vision and may be unacceptable in a

cockpit environment. The scientific literature suggests certain females (pregnant, menopausal, elderly) are more likely to

experience complications and have less than optimal visual performance after refractive surgery. This study reviews the

civil aeromedical experience with refractive surgery by gender. METHODS. A list of airmen with Federal Aviation

Administration (FAA)-specific pathology codes 130 (radial keratotomy) and 5179 (general eye pathology with surgical

prefix), during the period 1 January 1994 through 31 December 1996, was generated from FAA medical databases. The

records of airmen with pathology code 5179 were reviewed and those identified as having refractive surgery were collated

into a database with those who had pathology code 130. The records were then stratified by class of medical certification

and gender, and analyzed using demographic data extracted from FAA publications. RESULTS. There were 3,761

airmen identified as having had refractive surgical procedures during the study period. The prevalence rate for refractive

surgery was found to be significantly higher (p < 0.05) for female (8.74/1,000) than for male (6.06/1,000) aviators.

Prevalence rates for all classes of FAA medical certification were also found to be significantly higher for female aviators.

CONCLUSIONS. Higher prevalence rates for female aviators with refractive surgical procedures suggest that they view

refractive surgery to be a more viable alternative for correcting refractive error than do their male counterparts. With the

more frequent post-surgical complications for selected females with these procedures, further research is recommended to

investigate the potential for operational problems in the aviation environment. Continued monitoring may determine

whether there is an increased risk of performance loss associated with normal age-related ocular changes.

17. Key Words 18. Distribution Statement

Aviation; Vision; Refractive Surgery; Gender; Document is available to the public through the

Aeromedical Certification; Occupational Health National Technical Information Service,

Springfield, Virginia 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 11

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

1

GENDER DIFFERENCES IN A REFRACTIVE SURGERY

POPULATION OF CIVILIAN AVIATORS

INTRODUCTION.

Optimum vision is essential for pilots who must

detect and identify airborne traffic as well as hazards

that may be on runways and taxi lanes. Additionally,

cockpit instruments and printed materials, such as

flight manifests, charts, and maps, need to be clearly

visible to ensure that proper flight procedures and

safety are maintained.

Refractive error is an optical defect that prevents

light rays from being focused as a clear, single image

on the retina. Common refractive conditions that

include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (irregular corneal curvature) (see Figure 1) are normally corrected with

ophthalmic lenses (eyeglasses, contact lenses). There

are approximately 145 million Americans (54.6% of

the U.S. population) who are dependent upon corrective lenses to achieve an acceptable quality of

vision for their daily needs.

Civil airmen with radial keratotomy (RK) refractive surgical procedures have been allowed to obtain

Federal Aviation Administration (FAA) medical certificates since the early 1980s. RK requires a surgeon

to make a series of spoke-like incisions around the

periphery of the cornea that, once healed, flattens the

cornea and reduces myopia. In 1995, the Food and

Drug Administration (FDA) approved the use of the

excimer laser to perform refractive surgical procedures (1). Laser procedures, such as photorefractive

keratectomy (PRK) and laser-assisted in situ

keratomileusis (LASIK), are being performed on a

rapidly growing number of people, including civilian

pilots. PRK reshapes the cornea through a process

called photoablation, which utilizes the excimer laser

to vaporize the cornea’s outer tissue. LASIK uses a

specially designed surgical scalpel (microkeratome)

to slice a thin horizontal flap from the top of the

cornea, leaving it connected by a small hinge of

tissue. The flap is folded aside, and the excimer laser

is used to remove tissue from the corneal stroma. The

corneal flap is then returned to its original position

(see Figure 2).

There was an approximate 100% increase in the

number of U.S. laser refractive procedures from 1996

to 1997 (from 94,500 to 200,000 procedures). It is

estimated that one million laser refractive surgical procedures will be performed annually in this country by

Hyperopia

(farsightedness)

Point

of Focus

Astigmatism

(irregular corneal curvature)

Points

of Focus

Lens

Cornea

Light

Rays

Fovea

Point

of Focus

Myopia

(nearsightedness)

Figure 1: The diagrams above illustrate the three most common forms of

refractive error.
2

the year 2000. Presently, applicants with refractive

surgery may obtain a medical certificate without a

waiver for a particular class provided they meet the

vision standards for that class, and an eye-care specialist verifies that their vision is stable, healing is

complete, and no glare intolerance is present.

Success rates of 90% or better have been reported

for patients with low-to-moderate myopia who have

had laser refractive procedures. However, the criteria

for success used in many of these studies permit

uncorrected visual acuity (VA) as low as 20/40 (2).

While VA in this range may be adequate for the

general population, professional pilots, who must

safely perform a wide variety of vision-related flight

operations under visually demanding environmental

conditions, may find these results unacceptable. Aviation safety concerns remain regarding the quality of

the resulting vision correction, side effects, and the

potential for surgical complications.

Before choosing refractive surgery, a pilot should

be aware of the possible problems associated with

these procedures (3). Complaints following refractive surgery include reduced contrast sensitivity, increased glare sensitivity, and loss of best corrected

visual acuity (2). Some patients, who report seeing

well in normal lighting conditions, have found night

driving to be difficult or impossible due to halos,

aberrations, and glare from oncoming head lights (4).

These complaints may be associated with residual

corneal haze and/or de-centered ablation zones. Furthermore, under-correction of the refractive condition can result in poor distant vision, while

over-correction can produce “premature” presbyopia

(i.e., the need for reading glasses).

Clinical studies have suggested that a number of

factors can influence the results of refractive surgical

procedures, including the magnitude of refractive

correction, age, and gender. Several studies have

concluded that the predictability of the resulting

refractive correction decreases with increased refractive error (5,6,7). Other research has suggested that

the outcome of refractive procedures performed on

older patients can differ significantly from those of

younger patients who have had similar procedures to

correct equivalent refractive error (8). Furthermore,

some studies suggest that females may not respond as

favorably to surgical correction when compared with

males who have undergone similar surgical procedures to correct comparable amounts of refractive

error. The reasons for these gender-specific results

may be associated with oral contraceptive use and

hormonal changes during pregnancy and menopause

(6,9,10). Finally, clinical trials suggest poorer outcomes for older women who undergo laser vision

correction (11).

Photorefractive

Keratectomy (PRK)

Ablation Zone

Laser

Pulses

Radial Keratotomy (RK)

Radial Incisions

Laser-assisted in situ

Keratomileusis (LASIK)

Corneal Cap

(flap) Laser

Pulses

Hinge

Figure 2: The diagrams above illustrate the most common forms

of refractive surgery.
3

This epidemiological study reviewed the prevalence of civil aviators who fly with refractive surgical

procedures by gender and class of medical certification for the study period, 1994-96.

METHODS

A list was generated of active airmen with FAAspecific pathology codes 130 (radial keratotomy) and

5179 (general eye pathology with surgical prefix) for

the period from 1 January 1994 through 31 December 1996. This information was extracted from the

Consolidated Airman Information System medical

database, maintained by the FAA’s Application Systems Division at the Mike Monroney Aeronautical

Center in Oklahoma City, OK. Medical records of

airmen with pathology code 5179 were reviewed to

identify those airmen who had refractive surgery.

(Note: The 5179 code is assigned to applicants with

various eye surgeries, including refractive surgery.)

Data from the medical records of airmen identified as

having had refractive surgery and those with pathology code 130 were collated into a common database

and stratified by gender and class of medical certification. The refractive surgery data were then compared with airman population data extracted from

the Aeromedical Certification Statistical Handbook,

which is published annually by the Civil Aeromedical

Institute’s Aeromedical Certification Division (12).

Frequency, prevalence, and average age were calculated

for both the refractive surgery and general civil airman

populations. Hypothesis testing was performed to determine whether the prevalence rates for refractive surgery differed significantly for male and female airmen.

RESULTS

A total of 3,708 airmen with pathology code 130

and 202 airmen with pathology code 5179 were

identified. A review of the medical records for those

with pathology code 5179 found 53 airmen had

refractive surgical procedures. When these were combined with those who carried pathology code 130, a

total of 3,761 airmen (3,454 males, 307 females) had

refractive surgery from 1 January 1994 through 31

December 1996. During the study period, the average total civil airman population was 605,296

(570,185 males; 35,111 females). Figures 3 and 4

show both the refractive surgery and average total

airman populations by class of FAA airman medical

certificate.

The prevalence rate of refractive surgery per 1,000

airmen by class and gender are presented in Figure 5.

Females had higher prevalence rates for refractive

surgery in all three classes of medical certification for

the study period. Utilizing the Chi-square test, the

total prevalence of female aviators (8.74/1,000) with

refractive surgery was found to be significantly higher

(p < 0.05) than that of males (6.06/1,000). Further

statistical analysis using the Cochran-MantelRefractive Surgery Airman Population

0

350

700

1,050

1,400

1,750

2,100

Class I 487 34

Class II 919 62

Class III 2,048 211

Males Females

Figure 3: This chart presents the frequency of refractive

surgery in the civil airman population by class of medical

certification and gender for the period 1994-96.
4

Average Airman Population

0

50,000

100,000

150,000

200,000

250,000

300,000

Class I 138,664 5,930

Class II 149,574 7,077

Class III 281,947 22,103

Males Females

Figure 4: This chart presents the average airman

population frequency by class of medical certification

and gender for the period 1994-96.

Prevalence of Refractive Surgery

2

4

6

8

10

Class I 3.51 5.73 3.60

Class II 6.14 8.76 6.26

Class III 7.26 9.55 7.43

Total 6.06 8.74 6.21

Males Females Total

Figure 5: This chart presents the prevalence of

airmen with refractive surgery in the civil airman

population (per 1,000 airmen) by class and gender

for the period 1994-96.

Average Age of Refractive Surgery Airmen

30.0

33.0

36.0

39.0

42.0

45.0

Class I 38.9 34.4 38.6

Class II 43.2 39.3 43.0

Class III 44.3 43.3 44.2

Total 43.2 41.4 43.1

Males Females Total

Figure 6: This chart presents the average age of airmen

with refractive surgery in the civil airman population by class

and gender for the period 1994-96.
5

Haenszel method found the association between female airmen and refractive surgery to be significantly

higher (p < 0.05) for all classes of medical certification when compared with that of males.

The average age of the refractive surgical population by class of medical certificate and gender is

reported in Figure 6. The average age of females

(41.4) in the refractive surgery population was slightly

less than two years younger than their male counterparts (43.2) (mean difference = 1.8; Std. Dev. = 1.9).

DISCUSSION

The use of eyeglasses to correct refractive problems

may have disadvantages for pilots. Spectacle frames

can reduce the field of vision, be uncomfortable,

become displaced due to G-forces encountered during flight maneuvers, and may be incompatible with

communication and protective breathing equipment.

Spectacle lenses can also be dislodged in flight, and

fogging may occur with sudden changes in humidity

and air temperature.

Contact lenses can have inherent advantages for

pilots over spectacle correction, including more natural vision, full field of vision, no lens fogging or water

droplet accumulation, no discomfort due to weight,

and no annoying obstruction from the frame or

distracting reflections from the lenses. However, difficulties have also been reported with contact lens use

in the aviation environment. Contact lenses have

been displaced off the cornea and even ejected from

the eye in flight. Airmen have also reported having to

remove a contact lens while in flight due to dryness or

a foreign body beneath the lens (13). Furthermore,

the aviation environment includes low barometric

pressure and relative humidity, which may affect

contact lens wear. Corneal edema has been reported

in seemingly well-fit contact lens wearers who experience altitude hypoxia (14). With the use of soft

contact lenses in the 10-15% relative humidity of an

aircraft, there is evidence of less lens movement and

increased conjunctival injection (redness), both indications of an excessively tight fit (15). Soft contact

lenses can dehydrate in low humidity, resulting in a

vision performance (reduced low-contrast visual acuity) loss (16).

The frequent need to meet strict occupational or

recreational vision standards, not normally required

of the general population, can be a powerful motivating factor in a pilot’s choice of refractive correction.

Civilian pilots may also be more likely to consider

such surgical procedures due to an actual or perceived

operational concern accompanying the use of spectacles and contact lenses in the aviation environment.

In this study, female aviators were approximately

30% more likely to have had refractive surgery than

male pilots and had significantly higher (p < 0.05)

prevalence rates for refractive surgery for all classes of

medical certification. This suggests that female aviators consider refractive surgery a more viable alternative for refractive correction than do their male

counterparts. It also contradicts one study, which

found that males generally express a greater interest in

refractive surgery than did females (17). Social-economic factors may influence the decision to have

refractive surgery, since female pilots may be more

financially capable of affording these relatively expensive procedures than females who do not fly.

Females, especially the elderly, may have poorer

results following refractive surgery. According to the

Ophthalmic Device Panel of the FDA’s Medical

Device Advisory Committee, women of middle age

and older tend to have less favorable results with laser

vision correction for reasons that are not totally

understood (11). The predictability of refractive procedures may be more complicated for older females

due to hormonal shifts associated with menopause

(18). However, according to other studies, hormone

replacement therapy may have little or no influence

on the outcome of laser refractive procedures. The

poorer outcomes seen in older female laser patients

appear to be age-related (19). Additional clinical

research involving older female patients has been

recommended, which may result in revised guidance

for laser refractive surgery for this population. Further research may also help determine whether menopause and the use of hormone supplements in the

years following refractive surgery have any adverse

affects on vision performance or if age-related changes

in vision (cataracts, corneal dystrophies, keratoconjunctivitis sicca) are exacerbated by the presence of

refractive surgery (20,21).

Younger women should be aware that pregnancy is

a contraindication for refractive surgery (22,23).

Women undergoing hormonal changes during pregnancy can experience relatively large shifts in refractive error, and hormonal changes may also affect the

corneal healing process. If this occurs, surgery could

result in an increased incidence of under- or overcorrection of refractive error, corneal haze, glare
6

disability, and myopic regression (9,10). Furthermore, oral contraceptives have been associated with

increased myopic regression after PRK (6). This

study found female aviators with refractive surgery to

be relatively young (34.4 to 43.3 years of age); therefore, many of these women would still be in their

child-bearing years and possibly using oral contraceptives for birth control.

Long-term effects of newer laser refractive surgical

procedures are still unknown. However, the longterm effects observed for RK may provide some clues

regarding possible problems that female aviators with

laser refractive surgical procedures may face in the

future. At ten years after RK, women were found to

have benefited less from their refractive surgery than

men. The study also showed that women had poorer

uncorrected and best-corrected visual acuity with less

preoperative myopia, suggesting a possible biologic

gender effect (24). In addition, women have been

shown to have a higher prevalence of dry eye syndrome (25). This may adversely influence corneal

healing and interfere with optimal results following

refractive surgery, due to the reduction in the tear

film layer (21). Since the average age of female airmen

with refractive surgery in this study was 41.4 years of

age, it is likely that these aviators will continue to fly

for decades. Over time, it is conceivable that vision

problems could develop as a direct result of refractive

surgery, which may affect operational performance.

In summary, if the increase in the number of

civilian pilots who elect to have refractive surgical

procedure follows that projected for the general population, such procedures will become substantially

more prevalent. Female pilots have significantly higher

prevalence rates for refractive surgery, suggesting that

they consider it a more viable alternative for correcting refractive error than do their male counterparts.

Given their higher prevalence and the reports of

gender-specific post-surgical complications women

may experience, further research is recommended to

investigate the potential for operational problems in

the aviation environment. Instability of refractive

correction associated with normal age-related ocular

changes could also become an issue, as aviators with

surgical correction mature. Special monitoring is

warranted to evaluate whether unknown long-term

side effects associated with laser refractive surgery

could adversely affect aviation safety.

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8