prescribed since the eye may become painful 1 to 2 hours
post-operatively and may be extremely painful during
the first 8 to 12 hours. However, current use of topical
non-steroidal anti-inflammatory agents greatly reduces
the pain thus requiring fewer analgesics. By the follow-
ing day, the pain is reduced considerably and the cornea
is re-epithelialized in most patients within 48 hours. A
topical corticosteroid and antibiotic regimen is nor-
mally initiated at the same time as soft contact lens or
patch and is continued after their removal (95). Vision
is usually considerably improved within 3 to 4 days, and.
most patients become slightly over corrected for a few
weeks before beginning to settle. Correction normally
becomes stabilized within 3 to 6 months for low myopia
and 6 to 18 months for high myopia (96,97). Regular
follow-up visits are necessary for the first year (95).

1. Epithelial removal. Removal of the epithelium
before photorefractive keratectomy increases the pre-
dictability of results, since its thickness or homogeneity
is unknown. Methods used for de-epithelization in-
clude mechanical debridement with a blunt spatula or a
sharp blade (98); chemical de-epithelization by using
topical synthetic anesthetics, cocaine, or alcohol (99);
and photoablative de-epithelization with the excimer
laser (100). The most commonly used method employs
a mechanical debridement with adjunctive chemicals,
such as topical anesthetics.

2. Ablation zone diameter. During the clinical
evolution of PRK, ablation zone diameters have pro-
gressively increased from 3.5-millimeters (mm) to 6.5-
mm or more (101). One major reason for this increase
was the high incidence of symptomatic halos under
night-driving conditions (102,103,104,105,106). A
larger optical zone reduces the effect of optical irregu-
larities at the junction of the ablation zone and un-
treated cornea, which is thought to cause symptomatic
halos (107) and may affect objective and subjective
post-operative visual outcomes (108). Modern excimer
lasers are able to create ablation zone diameters of at least
6-mm for myopia. (Note: The amount of myopic
change is directly related to the depth of tissue removed
(109) and inversely related to the diameter of the zone.
Therefore, deeper ablation and/or smaller zones are
required for higher myopia (100).) Patients with larger
ablation zones also achieve a more rapid visual recovery
with less variation in refractive outcome (110,111).
Since patients are not as likely to develop over correc-
tions (more common to smaller ablation zones), they

can return to full activities within a few days (112).
Currently studies are underway using larger ablation zones
on simultaneous bilateral procedures.

3. Centration of ablation zone. Precise centration
of the laser over the entrance pupil is important, as the
regularity and centration of the ablated corneal optical
zone is imperative to clear, crisp vision (102,113,114,
115). Previously, a hand-held suction ring was used
during ablation by some surgeons to stabilize the eye
(116). Currently, self-fixation by the patient is used
during surgery (101). The Summit and VISX lasers only
use self-fixation. Centration errors can occur if the eye
and laser are not perfectly aligned, there are small
involuntary movements by the patient and/or surgeon,
the patient loses fixation, or the pupil is irregular
(113,117). (Note: Decentration occurs in about 20% of
PRK treatments (118).) Self-fixation may be adequate
for wide ablation lasers, such as the Summit or VISX
lasers. However, techniques that use scanning laser
beams and eye-based masks (e.g., the Meditec System)
previously required a suction ring-assisted fixation of
the eye but are currently using an eye tracking system
(82). Decentered or eccentric ablations can generate
monocular diplopia, glare, and irregular astigmatism
(75,96,105,119,120,121). (Note: A rigid contact lens is
normally the only option for correcting this type of
astigmatism, so a patient who has previously been
unable to wear contact lenses is at greater risk for a
decrease in uncorrected and BCVA (96,122,123,124).)
(Note: There have been reported losses of two or more
lines of BCVA with ablation zones that are significantly
eccentric (102).) When both the pupil and ablation
diameter are 4.5-mm and the ablation is displaced .5-
mm, 14% of the optical zone is outside the ablation area,
while a displacement of 1-mm will result in 31% of the
optical zone being outside the ablation area, which
could induce halos and affect visual function
(125,126,127). The halo effect is generally proportional
to the amount of correction attempted and is the result
of light passing through both the treated and untreated
portions of the cornea. In studies of $ 5.0-mm ablation
zones, as many as 78% of PRK patients reported halos
at night (114,128,129). Larger ablation zones (26-mm)
have less critical centering requirements when being
used for mild to moderate myopia (114,128,130,
131,132). However, when larger ablation zones are used
for higher refractive corrections, centering is critical due
to the steep and deep transition zone (133). (Note:
Hyperopic eyes may be more sensitive than myopic eyes


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by FDA panel. AOA News. Nov 1, 1994;
33(9):1,12-13.

27

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PRKs total 13,000 in first 6 months. AOA News.
Apr 8, 1996; 34(19):1,4,8.

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on the rate of epithelial wound healing after
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310(6985):979-85.

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keratectomy in high myopia. Ophthalmol. Sep
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Ruiz LA, Slade SG, Updegraff SA. Laser in situ
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Editor. LASIK wins FDA panel nod. AOA News.
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COMPOUND HYPEROPIC
ASTIGMATISM
Light Rays
COMPOUND MYOPIC MIXED
ASTIGMATISM ASTIGMATISM
Light Rays Light Rays
SY XN

FIGURE 3

II. REFRACTIVE TECHNIQUES

A. NON-SURGICAL TECHNIQUES AND
SURGICAL PROCEDURES NOT
INVOLVING THE CORNEA
Over the centuries, many techniques have been used

with limited success in an attempt to alter or reduce
refractive error including nutritional, pharmacological,
and visual training (7). The better known of these
procedures includes orthokeratology, cycloplegia, clear
lens extraction, and scleral reinforcement.

1. Orthokeratology. This procedure uses tight-
fitting rigid contact lenses that are changed to progres-
sively flatter ones to reduce the curvature of the cornea.
Reported complications include induced keratoconus,
corneal ulceration, and irregular astigmatism. An ad-
vantage of this procedure is its reversibility, as the cornea
will return to its original curvature if retainer contact
lenses are not worn (8) (see Appendix A).

2. Cycloplegia. This procedure involves the daily
instillation of a cycloplegic to prevent ciliary spasm,
caused by near work, by paralyzing the ciliary muscle in
pre-adolescents. Reported complications include pho-
tophobia, due to dilated pupils and the inability to
predict accurately at what age myopia will develop (8)
(see Appendix A).

3. Clear lens extraction. This procedure was inves-
tigated in the 1980s, but discontinued due to a large
number of retinal detachments and other complications
that could permanently decrease vision. There has been
renewed interest by surgeons using an extracapsular lens
extraction procedure with a low power posterior cham-
ber intraocular lens (8) (see Appendix A).

4. Scleral reinforcement. With this procedure, the
scleral shell is strengthened with a graft to curb the
progression of axial elongation. Reported complica-
tions include anterior uveitis and retinal detachments

(8) (see Appendix A).




Advantages: Treatment of keratoconus and high
myopia (14), safety, reversible in-office procedure, no
sophisticated instruments (7), and corrected powers up
to 37.0 diopters (8).

Disadvantages: Problems with predictability, diffi-
culties with freezing and unfreezing lenticules (14), and
centering problems with the lenticules (15).

STROMAL THERMOKERATOPLASTY
Original procedure was introduced by Lans nearly
100 years ago. Gasset and Fyodorov refined the proce-
dure in the 1970s (7). Initially, this procedure consisted
ofa nichrome tip probe placed at a depth of 80% of the
corneal stroma and heated to 600° C for 0.3 seconds to
shrink stromal collagen and flatten the cornea (7,16). A
modified procedure is undergoing FDA investigational

study using a Holmium:YAG laser as an alternative
instrument (17) (see Figure 12). There are other
studies using this procedure after RK in patients with
disabling spherical hyperopia, but so far results have
been less than promising (333).

Complications: Loss of BCVA (17), regression
(17), epithelial thinning, Bowman’s membrane al-
terations, recurrent corneal erosions, iritis, and
stromal melting (7).

Advantages: It can correct hyperopia and astigma-
tism (334).

Disadvantages: Failure rate of 50-80% (7) and
substantial regression, due to problems controlling
the amount and depth of delivered heat (17).

Myopia Correction Pattern

270

STROMAL THERMOKERATOPLASTY

Hyperopia Correction Pattern

270

Treatment at the 3.0- to 3.5-mm zones produces central corneal flattening (left), whereas
treatment at beyond the 5.0-mm zone results in central corneal steepening (right).

FIGURE 12

A-4



operative conditions (161). Factors related to the etiology
of this problem include:

a) Post-operative haze: Present in most pa-
tients at 2 to 3 months (196,197), it disappears in most
eyes at 1 year (178). One study reported that at 12
months 30% of eyes had a trace and 4% had mild to
moderate haze (186). Another study reported 85% of
eyes had more than a trace of haze at 24 months
(198,199). Variables that influence the severity of cor-
neal haze include: size of ablation zone (75,200) (greater
haze with smaller zones (135)), deeper ablation depths
(148,200), single ablation zone procedures (200), male
gender (135), higher myopic correction (128,135,171,
196, 201,202,203, 204,205), shorter re-epithelializa-
tion time (148), fibroblast proliferation (206), differen-
tial healing response (103,207,208), myopic regression
(128,171,204), and dry eye (tear fluid components are
known to stimulate corneal healing) (209). More sig-
nificant haze with the absence of steroid use or discon-
tinuation of corticosteroids to modulate the haze response
has been reported (135,196). However, even with the
use of steroids, corneal haze was reported present in
some cases at 6 months (179).

b) Irregular astigmatism: Astigmatism in-
duced by PRK is relatively uncommon compared to
other refractive procedures (210). It can occur with
decentered ablation zones and/or asymmetric healing
(100). In one study, > 0.75 D of induced cylinder was
observed in 2.5% of cases at 1 year post-operatively but
declined to 0.0% at 2 years after surgery (142).

c) Corneal scarring: A leading cause for loss
of visual acuity (29,102,105,115,118,125, 174,176,
211,212), corneal scarring reduces optical clarity (213)
and produces glare (104,126,202). It is more prevalent
in PRK patients with corrections > 4.00 D (118,
125,171).

d) The accuracy of refractive correction is
diminished on traumatized corneas (214) and as larger
corrections are attempted (173). Over corrections in-
crease in multi-zone ablation procedures (215).

e) Approximately 15% of PRK patients may
experience over correction, and resulting near vision
disturbances or increased corneal haze and rapid myopic
regression (216).

¢ Decrease in contrast sensitivity
a) Loss of contrast sensitivity may not be
detectable under conditions typically used for measur-
ing Snellen visual acuity (100). Sher et al. (120), using

14

the Vistech MCT-8000 and Pelli-Robson chart, found
no significant difference in contrast sensitivity function
(CSF) at 3 months after PRK. However, others have
found reduction in CSF during exams at the 12-month
post-operative period (175,189,217). Ambrosio et al.
(218), using stationary or temporally modulated sinu-
soidal gratings, found post-PRK patients with low myo-
pia to have a reduction in both static and dynamic
contrast sensitivity at 1 month. At 6 months, static
sensitivity had recovered, but there was a persistent loss
of dynamic sensitivity.

b) Reduction in contrast sensitivity seems
proportional to the amount of corneal haze (109,203).
However, post-operative reduction in contrast sensi-
tivity and glare may occur in eyes with “crystal clear”
corneas, suggesting that visual loss and “halo effect”
are not attributable to reduced corneal clarity alone.
These effects may stem from an apical corneal surface
that differs substantially from sphericity (124,219),
since the main retinal image is accompanied by a
“ghost” image.

c) Ithas been speculated that larger ablation
zones (minimum of 6-mm) may lessen the amount of
induced CSF loss (114,128, 130,131,132). However, a
recent study reported that even with ablation zones of 6-
mm, there was a loss of daytime contrast sensitivity with
peripheral glare (220).

* Ultraviolet-C (UV) exposure
The long-term effects of UV-C exposure from the

excimer laser are unknown at this time. Such exposure
has been reportedly linked to:

a) Formation of free radial byproducts
(221,222)

b) Mutagenicand cataractogenic effects (221)

c) Premature expression of certain age-related
conditions (217)

d) Central corneal thinning that may resultin
keratoconus type anomalies (223)

e) Corneal graft rejection (224)

f) Cataracts (102)

4. Side-effects
¢ Thermal damage

a) Although the excimer laser photoablation
process is non-thermal, temperature changes on the
corneal surface occur as molecular bonds break (130).
Berns et al. (84), using a thermal camera, showed an
average increase in corneal temperature of 20°C. With
pulsed laser energy, higher temperature spikes occur on


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Mar 1996; 133(3)127-33.


FOREWORD

In the past 10 years, numerous refractive surgical procedures have risen to levels of transient
popularity, only to quietly disappear with few publications stating why they were abandoned. The urge
to use the latest surgical technique is particularly strong in refractive surgery because development is
rapid, surgeons want to offer patients the most current advantages, there is a prevalent attitude of hype,
competition is intense to gain professional leadership, and the economic stakes are high for both
practitioners and industry. These factors push unproven techniques into clinical use before their
advantages and disadvantages are well defined, creating a pattern of free market madness that is

unhealthy for refractive surgeons and patients (1).
—George O. Waring III, M.D.

iii


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to ablation zone decentration (134).) In general, the
visual and refractive outcomes of PRK depend on
good centration.

4. Epithelial and stromal changes. The epithelium
is normally healed within 4 to 5 daysafter PRK (95,100),
although there have been some reported instances of
epithelial wound healing occurring as late as several
months after the procedure (135). Studies have shown
that normal epithelial barrier function may not return
during the first month after ablation, suggesting the
importance of minimizing corneal trauma (e.g., eye
rubbing, applanation tonometry) (136). When it regen-
erates, the epithelium is initially thinner than normal.
Over time, the epithelium thickens, and may become
hyperplastic with greater than normal numbers of cell
layers and thickness. This epithelial hyperplasia is com-
mon in rabbits (137), non-human primates (138), and
humans after PRK (87,139,140), which, if sufficient,
may cause a return of myopia (100). This myopic
regression is most common in higher myopic patients
(128,141,142,), and is rarely seen in ablation depths less
than 50-microns ({im) (130,143). After PRK, there is
little risk of corneal perforation (88,144), corneal rup-
ture (145,146), continued refractive change (88), and
abnormal corneal epithelial healing after traumaticabra-
sion (147). However, corneal ruptures may occur with
very deep ablations (31), although the major risk is an
increase of corneal opacities (29,148). A smooth corneal
surface at 1 to 3 months is no guarantee of a smooth
surface at 6 months, as the extent of the deposition of
new tissue remains unpredictable (131,149). Recurrent
epithelial breakdown and epithelial adhesion problems
appear to be extremely rare after PRK (114,144,150).

Bowman’s layer, which is partially or completely
excised during the PRK procedure, does not regenerate.
The clinical significance of the absence of Bowman’s
layer is uncertain. From a biomechanical perspective,
there is evidence that the cornea is not substantially
altered (151). However, itis unknown ifits absence may
predispose certain individuals to a more rapid stromal
involvement in developing corneal infections.

Stromal changes may evolve for months or even
years after PRK. Recent observations of late regression
beyond 18 months and up to 26 months post-PRK have
raised clinical concerns that stromal healing may be
much slower than previously assumed (152). Initially,
the anterior stroma beneath the treatment zone is
hypocellular, with fewer than normal anterior stro-
mal keratocytes after 24 hours (99,138). Vision

11

immediately after surgery is limited, due to the exposed
corneal stroma absorbing fluid from the precorneal tear
film (153). In about 2 hours, most undamaged
keratocytes within 500-[1m of the ablation area are
activated and begin intense metabolic activity. This
activity transforms the keratocytes into fibroblasts (con-
nective tissue cells) that migrate into the treated region,
causing the subepithelial thickness of 10- to 15-[Lm to
become hypercellular (87,138). These activated
keratocytes synthesize new collagen (protein substance
of connective tissue) and extracellular matrix (contains
type III collagen [anchoring fibrils], type IV collagen
[basement membrane], type VII collagen and newly
produced keratan sulfate), all of which may contribute
to the corneal haze observed post-operatively (154).

In patients with delayed epithelial healing, the cor-
nea is at risk for developing infectious keratitis (95),
ulceration, and greater post-operative corneal haze
(100,121). Conditions reported to delay healing in
PRK patients include: 1) keratoconjunctivitis sicca (tear
deficiency resulting in dry eyes), prompting most spe-
cialists to recommend the tear film be assessed and all
dry-eye conditions be treated pre- and post-operatively
(100); 2) epithelial toxicity, which may be related to
topical anesthetic agents applied before epithelial re-
moval, topical anti-inflammatory agents, and prophy-
lactic antibiotic therapy (100); 3) reduced oxygen
availability to the corneal epithelium (i.c., bandage
therapeutic soft contact lenses fit excessively tight)
(100); and 4) an enlarged corneal epithelium debrided
area (100).

5. Corneal endothelial changes. Endothelial dam-
age, documented with very deep (about 90% depth)
linear excimer laser excisions, is probably due to acous-
tic or shock waves from the laser (155,156). In rabbits
and non-human primates, the corneal endothelial cells
appear to be unaffected after wide area superficial cor-
neal ablations from PRK (86,138,157,158,159,160).
No significant change in human endothelium cell den-
sity and morphology has been found up to 12 months
after PRK (correction between -2.50 to -17.0 D) (44).

B. SELECTION CRITERIA
1. Post-operative vision.

* Potential patients should be aware that PRK is
an elective procedure and there are alternatives for
refractive correction (eyeglasses, contact lenses,
orthokeratology, and other refractive surgery) (161).


severe haze and regression at 1 year post-operatively,
noncompliance with steroid use was found to be the
cause in approximately 50% of these cases (114,162).

c) Complications from corticosteroids in-
clude: foreign body/dry eye sensation (178,191,209),
tear film alteration (178), herpetic keratitis recurrence
(253), anisocoria, loss of accommodation (254), ocular
hypertension (191), and cataract formation from long-
term usage (227).

d) Elevated intraocular pressure is the major
complication from using topical corticosteroids, affect-
ing 10-20% of PRK patients who use them
(105,139,170,186,255). This percentage may be as
high as 50% in high myopes (102). Usually, intraocular
pressure is reduced with discontinuation of corticoster-
oids, but some (about 1.7% (75)) require therapy for
ocular hypertension over a limited period of time (100).
There have been reports (up to 22%) of induced open
angle glaucoma due to corticosteroid use after PRK
(103,171,174).

e) At this time, there is no consensus on the
benefits of pre-and post-operative corticosteroid therapy
on the outcome of PRK (95).

* Non-steroidal anti-inflammatory agents

a) These agents have been used to decrease
post-operative pain after PRK. Studies have shown that
those treated with non-steroidal anti-inflammatory
agents had less initial and overall pain, photophobia,
burning/stinging, and require fewer oral narcotics (243).

b) Since soft contact lenses are commonly
used with these agents (243), care must be taken to
reduce the risk of possible lens complications (e.g., tight
lens syndrome, infectious keratitis (243)).

c) Currently, there is ongoing research on the
use of non-steroidal anti-inflammatory agents that re-
tard fibroblast activity to control the post-operative
corneal cellular response (100,256). However, some of
these drugs have been associated with inhibition of

epithelial wound healing (256).

* Diurnal fluctuation in vision

a) A common complication of RK, diurnal
fluctuation in vision, has not been reported in most
PRK studies (144,176).

b) One study did report fluctuations of 14%
of PRK patients at 6 months and 4% at 12 months
(178). In contrast to post-RK eyes, which tend to show
increasing myopia, the post-PRK refraction shifted
toward hyperopia during the day (158).

17

* Central islands

a) Central islands are topographic non-
uniformities of the cornea following PRK. Analysis
reveals a region of higher refractive power, compared to
the flattened central cornea. With a corneal topography
system, the flat cornea is projected in blue while the
steeper cornea is projected in green, giving an “island in
the sea” picture.

b) Central islands have been reported with all
brands of excimer lasers (75) and incidence varies,
depending on the defining criteria and post-operative
time. Rates as high as 60% of patients (117,186,257)
have been reported in the first few months after surgery,
decreasing to about 3% at 6 months (117).

c) Central islands are reportedly more preva-
lent with ablations > 5-mm in diameter (with each 0.5-
mm increase in diameter, there is a > 2X increase in the
odds ratio for central islands) and eyes treated without
nitrogen gas-blowing (16-88%) (258). (Note: Nitrogen
gas-blowing is primarily used with the VISX laser.)

d) Symptoms of central islands include: un-
der correction, reduced BCVA (257,259,260), diplopia
(259), glare (259), and distortion (261).

e) The etiology of central islands includes: 1)
shock wave formation (262,263), 2) ejection ofa plume
gaseous and particulate debris interfering with the pas-
sage of succeeding laser pulses (264), 3) optical prob-
lems within the laser delivery system or a variation in
laser beam homogeneity leading to less ablation of tissue
centrally (100,265), 4) differential hydration of the
corneal tissue during the PRK procedure (100), 5)
differential corneal healing process (100), and 6) in-
complete corneal de-epithelialization resulting in raised
areas on the stroma (98).

f) Mostcentral islands resolve spontaneously;
however, corneal contours may continue to change
from 1 to 6 months (257). When islands persist,
retreatment with a second laser ablation may be required
(261). (Note: The optimal time for reablation should be
performed only after the patient demonstrates a stable
refraction and corneal topography (215,266).)

g) New software has been developed in an
attempt to decrease the occurrence of central islands
with increasing ablation zones (100). Multi-zone and
multi-pass ablations and aspheric algorithms have been
proposed to make a smoother corneal surface
(267,268,269). Their value has yet to be established in
clinical trials.


10. Pupil size

+ Patients with large pupils are likely to experi-
ence a “halo effect” around lights, especially at night
(171,172,176). Patients undergoing corrections greater
than -4.00 D with pupil diameters larger than 6-mm in
subdued lighting should be counseled concerning this
complication (95).

* Younger patients, who normally have larger
pupils than older persons, may experience significant
halo problems while driving at night.

* Current FDA-approved lasers provide abla-
tion zones diameters of 6-mm, which should minimize

halo effects (95).
C. CLINICAL STUDIES

1. Stability

* The post-operative time required for a stable
refraction may vary due to the age of the patient, amount
of attempted myopic correction, and type of laser used.
Stable refractive correction has been reported at 3 (173),
6 (171,178,179), and 12 months (99). A study of 146
PRK patients (193 eyes) followed for 2 years reported
that the stability between the 1- and 2-year visits was
excellent, except in the subgroup that had pre-opera-
tive refraction of > -9.00 D (142).

* PRK retreatment rates range from 0.64-9.1%
(139,168,141). Retreatment is used to correct corneal
scarring, under correction of higher refractive error
(180), myopic regression, corneal haze, and topographic
irregularities (181). Retreatment enhances the results of
PRK, although it is less accurate than the initial PRK
procedure (182). A higher retreatment rate was observed
after astigmatic correction than after spherical correc-
tions (182).

2. Efficacy
* The efficacy after PRK is generally reported as:
a) The percentage of eyes that achieve a post-
operative refraction within + 1.00 D of emmetropia.
b) The percentage of eyes that achieve 20/40
or better uncorrected distant visual acuity.

Such data may be helpful in assessing efficacy and
predictability of a procedure, but they are not absolute
measures of “refractive success.” Patients with residual
refractive errors achieving 20/30 or 20/40 acuity may
continue to report they need a vision correction for a
substantial part of the day. (Note: The international

standard for normal vision is 20/20 + 1 line of Snellen
visual acuity. Therefore, many surgeons are now sug-
gesting that uncorrected acuity after refractive surgery
should meet this criterion to be successful.) Patients
with high myopia may considera refractive procedure to
be successful even with substantial residual myopia.

* Due to the sometimes extended corneal heal-
ing with PRK, itis believed that clinical reports of visual
performance at < 12 months have little clinical rel-
evance. Therefore, we have focused on reporting data
from those studies with post-operative data> 12 months.

* At 12 months, post-operative uncorrected vi-
sion was 20/20 or better in 58-75% of eyes (164,183),
20/25 or better in 74% of eyes (117), and 20/40 or
better in 85-95% of eyes (117,128,164,173,
174,178,184,185,186,187).

* At 12 months, post-operative refractive error
was within + .50 D of emmetropia in 48% (183) and
within + 1.00 D of emmetropia in 70-98% of myopes
of less than -3.00 D (102,117,128,164, 173,174). For
myopes of -3.00 to -6.00 D, 60-92% were within + 1.00
D of emmetropia at 12 months (96,121,168,169,
178,186,188,189).

* At 2 15 months, post-operative uncorrected
vision was 20/20 or better in 66% of eyes (190), 20/30
or better in 81.5% of eyes (97,191), and 20/40 or better
in 71-100% of eyes (97,173,174,189).

* At215 months, 71.7% were within + .50 D of
emmetropia (97) and 71-92% were corrected to within
+ 1.00 D of emmetropia (97,173,189,190,191).

* At 2 years, post-operative vision of 20/20 or
better was reported in 58% of eyes and 20/40 or better
in 94% of eyes (161).

* At 3 years, a study of 117 eyes with PRK
reported post-operative uncorrected vision of 20/40 in
83% of eyes, 20/20 in 37% of eyes, and 88% of eyes were
corrected to + 1.00 D of emmetropia (192).

* At this time, the clinical stability and long-
term side-effects are unknown.

3. Safety

* Loss of BCVA is a major safety issue of the
PRK procedure. Clinically significant visual loss is
generally reported as the loss of 2 or more lines of BCVA
(193,194). Data from previous FDA-supervised refrac-
tive surgery trials report that 1-3% of patients undergo-
ing PRK lost 2 or more lines of Snellen acuity (195),
while other more recent FDA data indicate that 42 of
480 eyes (8.8%) declined by more than 1 line from pre-


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107(4):506-11.

Kraff MC, Sanders DR, Karcher D, Ragnan M,
DeLuca M, Neumann G. Changing practice pat-
terns in refractive surgery: Results of a survey of
the American Society of Cataract and Refractive
Surgery. J Cataract Refract Surg. Mar 1994;
20(2):172-8.

Editor. Radial keratotomy for simple myopia.
JAMA. Jul 8, 1988; 260(2):264-7.

Editor. Can surgery free you from glasses? Consum

Rep. Feb 1994; 59(2):87-90.

Parmley V, Ng J, Gee B, Rotkis W, Mader T.
Penetrating keratoplasty after radial keratotomy:
A report of six patients. Ophthalmol. Jun 1995;
102(6):947-50.

Lynn MJ. Waring GO III, Kutner MH. “Predict-
ability of refractive keratotomy.” In: Waring GO
Ill, ed. Refractive Keratotomy for Myopia and
Astigmatism. St Louis: Mosby Year Book;
1992:341-80.

Department of Transportation/Federal Aviation
Administration. Guide for aviation medical ex-
aminers. Washington, DC:1996; FAA Office of
Aviation Medicine.

Aron-Rosa D, et al. Excimer laser surgery of the
cornea: Qualitative and quantitative aspects of
photoablation according to energy density. J Cata-
ract Refract Surg. Jan 1985; 12(1):27-3.

Trokel SL. Evolution of excimer laser corneal
surgery. J Cataract Refract Surg. Jul 1989;
15(4):373-83.

Aron-Rosa D. Worldwide update on the excimer
laser. Eyecare Technol. May-Jun 1994; 4(3):52-7.

Editor. VISX submits 2-year excimer clinical data
and updates FDA filing status. AOA News. Aug 1,
1994; 33(3):8.

Hahn TW, Sah WJ, Kim JH. Phototherapeutic
keratectomy in nine eyes with superficial corneal
diseases. Refract Corneal Surg. Mar-Apr 1993;
9(suppl):S115-8.

Hersh PS, Spinak A, Garrana R, Mayers M.
Phototherapeutic keratectomy: Strategies and re-
sults in 12 eyes. Refract Corneal Surg. Mar-Apr
1993; 9(suppl):S90-5.


Radial Keratotomy Per 1,000 Civil Airmen

(1) , : T T r T r T T T
1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994
Class! + | 0.0608 | 0.5267 | 1.4829 | 1.917 | 2.2145 | 2.9382 | 24919 | 2.5893 | 2.5636 | 2.7069 | 28508 | 3.0405
Class il + | 0.0206 | 0.2901 | 0.7279 | 1.1869 | 1.4295 | 1.7262 | 2037 | 21758 | 24104 | 2.7655 | 3.079 | 3.7583
Class iN @| 0.0213 | 0.2111 | 0.6886 | 0.9278 | 1.2166 | 1.4058 | 1.4661 | 1.6012 | 1.7189 | 2.0585 | 2.3528 | 2.9497
FIGURE 5

2. Considerations to aviation. In 1983, RK was
assigned its own pathology code (i.e., 130) by the
FAA. To expedite the tracking of this condition by
the FAA’s Aeromedical Certification Division, this
code is placed on the medical records of all airmen
applicants with RK. Figure 5 presents the prevalence
rates (per 1,000 airmen) of RK by class of airman
medical certificate held for the period 1983-94. RK
has increased in prevalence in each year of the period
in the total airman population, and essentially in each
year of the period for all classes of certificate holders.
RK had the highest prevalence rates in first-class
airmen until the end of the period. In 1994, there
were over 1,900 civil airmen with RK. While it is
expected that airmen who feel threatened by a loss of
physical or sensory skills may self-select themselves
out of hazardous activities, such as flying, the data
suggest that airmen with RK continue to fly and may
not perceive a substantial visual problem from any
side-effects typically associated with RK.

IV. PHOTOREFRACTIVE KERATECTOMY

A. HISTORICAL AND PROCEDURAL PROCESS

Excimer lasers have been used in ophthalmic and
refractive applications since the early 1980s. Laser tech-
nology has come a long way since these early applications.
Modern lasers are not only considered safe, but they also
have the precision capability of literally splitting hairs.
The excimer laser’s beam can remove 0.25 millimicron-
sized portions of corneal tissue with each pulse (3), and
the ophthalmic surgeon can actually alter the corneal
curvature. The excimer laser, which employs a 193-
nanometer (nm) ultraviolet-C light, is emitted as an
excited dimer of argon fluoride gas mixture. The high-
energy laser light causes an almost instantaneous vapor-
ization of the cornea by direct photochemical disruption
of molecular bonds, with theoretically minimal impact
on neighboring ocular tissue (73,74,75).

The excimer laser was initially approved by the
FDA for phototherapeutic keratectomy (PTK) (76).
During PTK the laser is used to perform a superficial





incomplete healing, and corticosteroids often have seri-
ous side effects. One of the most serious is induced
ocular hypertension (105,139, 186,252), suggesting
that intraocular pressure tests be performed on all PRK
applicants. (Note: A recent trend is the reducing of
steroid use and substituting non-steroidal anti-inflam-
matory agents for shorter periods of time. However,
steroids are more effective in minimizing post-PRK
inflammation and corneal haze, and preventing post-
operative refractive regression. Their clinical use seems
unavoidable, as they continue to be prescribed by most
refractive surgeons.)

* Since the primary surgeon may be less candid in
identifying complications as a result of their PRK pro-
cedure, an evaluation using the FAA Form 8500-7
(Report of Eye Evaluation) by a neutral eye specialist is
recommended.

* Due to some unique symptoms associated with
PRK, an expanded case history is recommended. Ques-
tions about symptoms associated with PRK, such as
fluctuating visual acuity, glare, halos, dry eyes, etc.,
should be included.

* FAA Form 8500-7 may be inadequate for moni-
toring PRK. Specific clinical tests for PRK may include:
retinoscopy to evaluate corneal integrity and pupillary
dilation impinging on the ablation border; keratotmetry
to evaluate induced microscopic astigmatism; biomi-
croscopy to evaluate corneal haze and fluorescein pool-
ing in the central cornea; and corneal topography to
evaluate long-term changes.

* The addition of more sensitive vision perfor-
mance tests may be needed to supplement the current
aeromedical certification examination. For example,
the Danish Civil Aviation Administration uses glare and
contrast sensitivity to evaluate all RK applicants for
aeromedical certification (325).

* The potentially large number of civil airmen that
may opt for PRK requires that aeromedical certification
issues be carefully studied, and that airmen with PRK be
appropriately screened and prospectively followed for
any future problems associated with this ophthalmic
condition in aviation activities.

* Proper educational material for airmen consid-
ering PRK should be developed and distributed. (Note:
A brochure developed by the Vision Research Team at
CAMI has been approved for publication.)

* There is a need to continue monitoring and
providing periodic feedback to the Office of Aviation
Medicine on these and other new refractive surgeries

23

that may impact aviation safety. The literature on PRK
is difficult to analyze, because treatments and proce-
dures are continuing to be developed and refined.
Historically, it is similar to the improvements in RK
since the original Prospective Evaluation of Radial
Keratotomy Study (or “PERK Study”) of 10 years ago.
Since refinements to PRK and new procedures are being
studied and used successfully in clinical settings in foreign
countries, their use in the United States is inevitable.

VI. SUMMARY

Refractive errors do not indicate that eyes are un-
healthy. Therefore, it is important that refractive sur-
gery procedures be safe as they are treating healthy eyes
that can obtain clear and useful vision by other correc-
tive modalities (glasses, contact lenses). Absence of
complications and long-term safety should accompany
a good refractive outcome. The goal of refractive surgery
is to provide the patient with a stable corneal surface and
the best optical performance possible. In the reporting
of PRK clinical studies, it is important that other test
results (glare, CSF) be included, since visual acuity and
refraction alone do not fully describe the optical perfor-
mance of the eye.

RK has increased in prevalence in civil airmen each
year from 1983 through 1994. This increase, even in
years in which RK was not recommended by many eye
care practitioners due to a multitude of post-operative
symptoms being reported, suggests that airmen do not
feel threatened by any loss of sensory skills often associ-
ated with RK. Based on the history of RK in the civil
airman population, it is believed that pilots will opt for
excimer laser PRK. It is estimated that about 75 airmen
will have had PRK by the end of 1996, and over 1,200
civil airmen may choose to have PRK by the year 2000.

Due to the inherent risks of refractive surgical pro-
cedures, aeromedical certification issues concerning PRK
will be carefully studied. Airmen must be appropriately
screened and monitored to identify problems associated
with PRK and the performance of aviation activities.
Factual educational materials highlighting the
advantages and disadvantages for airmen considering
PRK will be provided to allow airmen to make informed
choices based on how this procedure may affect their
aviation activities and careers. This approach is respon-
sive to today’s patients who feel a need to be proactive
in their medical diagnosis and treatment.


¢ PRK applicants are required to submit a com-
pleted “Report of Eye Evaluation” (FAA Form 8500-7)
after full recovery from their refractive procedure.

* PRK applicants must meet the visual stan-
dards for the class of airman medical certificate applied
for as stated in Federal Aviation Regulations Part 67.

There are over 1,200 laser systems installed world-
wide, and more than 900,000 PRK/LASIK procedures
have been performed (318). About 500,000 laser proce-
dures are performed annually outside the United States.
PRK has overtaken RK as the preferred refractive surgi-
cal procedure (319). It is estimated that 200,000 laser
refractive procedures will be performed in the United
States by the end of 1997 (320).

Projected total airmen who may opt for PRK was
calculated using a mathematical model (see Figure 8)
similar to that used for total PRK predictions by laser
manufacturers. Our calculations project a conservative
estimate that 75 airmen opted to have PRK by the end
of 1996. Five years later, more than 1,200 civil airmen
may elect to have PRK.

The promotional campaign for PRK has been far
greater than anything associated with RK. The high-
tech glitz of the “L-A-S-E-R” has enticed many indi-
viduals to have PRK procedures, without their full
knowledge of the apparent risks and that safer and more
suitable treatment procedures are close to being devel-
oped. It is reasonable to assume that the PRK procedure
currently approved by the FDA will be replaced by more
sophisticated technology and procedural applications
already in development. The long-term detrimental
effects and reduced vision performance resulting from
PRK may adversely affect aviation safety and be an
unacceptable risk for pilots, particularly air transport pilots.

PRK has been used in foreign countries for years.
However, its popularity has waned as it has largely been
replaced by the LASIK procedure as the refractive
surgery modality of choice. Civil airmen should be
advised that the FDA-approved PRK is being replaced
by newer procedures and techniques that may be less
detrimental to their vision and ability to obtain an FAA
airman medical certificate. FDA approval for active
marketing of LASIK is expected shortly.

ESTIMATED AIRMEN WITH PRK

1,400,
w 1,200)
<
= 41,0004
5
i 800;
2 600;
s
> 4004
2
2004
0.
1996 1997 1998 1999 2000
PRK 75 215 427 791 1,245
YEAR
FIGURE 8

20


TABLE OF CONTENTS

Page
|. INTRODUCTION ed
A. Myopia 4
B. Hyperopia 3
C. Astigmatism nd
Il. REFRACTIVE TECHNIQUES 4
A. Non-Surgical Techniques and Surgical Procedures Not Involving the Cornea . 4
1. Orthokeratolog wed
2. Cycloplegia .. 4
3. Clear lens extractio 4
4. — Scleral reinforcement 4
B. Refractive Procedures Involving the Cornea . od
1. Keratomileusis eee)
2. Keratophakia .. 5
3. Epikeratophaki: 5
4. Stromal thermokeratoplasty 5
5. Intrastromal corneal ring (ICR) ete}
Ill. RADIAL KERATOTOMY te)
A. Historical and Procedural Process 5
B. Complications .... 6
1. Anesthesia 6
Operative . 6
3 Intraocular and eyelid complications . 6
4 Corneal... 6
5. Refractive wT
6. Visual performance 7
C. Advantages ............. 7
1. Rapid recovery 7
2. Established safety . 7
3. Procedures for astigmatism 7
4. Cost...... 7
D. Disadvantages 7
1. Refractive 7
2. Limitations ... 7
3. Predictability 7
E. Certification of Pilot Applicants with RK 7
1. Certification proces: a7
* Historical 7
+ Present .. 7
2. Considerations to aviation 8

IV. PHOTOREFRACTIVE KERATECTOMY ..
A.

Historical and Procedural Process
1. Epithelial removal ......
2. Ablation zone diameter

boli ot
Ronee)





PHOTOREFRACTIVE KERATECTOMY (PRK)

er
Ses

Ablation
Zone

Corneal Tissue
Removed

FIGURE 6

keratectomy (smoothing of the corneal surface) remove
scars, pterygiums, band keratopathies, and superficial
corneal diseases (77,78,79).

Originally for refractive surgery, excimer lasers were
used to make semi-radial and linear transverse excisions
in the cornea in an attempt to improve traditional
diamond-knife RK (80,81,82). However, the laser exci-
sions created wider wounds because tissue was removed
(29,83), and provided less control over wound depth
(28,84). The cost and sophistication of the laser system
precluded its use in this application (85). Consequently,
attention was directed toward a wide-area, lamellar
stromal excision procedure, termed “photo-refractive
keratectomy” (86,87). In PRK, the excimer laser re-
shapes the anterior curvature of the cornea, essentially
“sculpting” the cornea to correct for any refractive error
(see Figure 6). After programming the amount of in-
tended refractive change required and baseline eye ex-
amination data, a computer-assisted algorithm
determines the excimer treatment parameters. Unlike
RK, PRK is not surgeon-dependent (“robotic” surgery)
(88,89). The excimer laser removes the corneal epithe-
lium (if mechanical debridement of epithelium has not
been performed), basementmembrane, Bowman’s mem-
brane, and portions of the corneal stroma (90).

Since the approval of the Summit laser by the FDA
for PRK in October 1995 and the VISX laser in March
1996, lasers previously approved for PT Kare upgradable
for PRK with the addition of a new software chip (91).
Summit and VISX, in an unprecedented move, formed
the Pillar Point Partners (PPP) to resolve a lawsuit over
ownership of the rights to the excimer laser refraction
process (92). Asa result, as lasers are upgraded for PRK,
a special card is required for the laser device to function.
These cards are purchased in batches of 10 for $250.00
each, so that Summit and VISX can collect a royalty
each time the laser is used to perform PRK (93).

Photorefractive keratectomy is an outpatient proce-
dure, which usually requires only topical anesthesia, and
is initially painless. The entire procedure, including the
manual debridement of the corneal epithelium before
initiating the laser, takes about 10 minutes. The actual
laser beam exposure is dependent upon the amount of
myopia to be treated (average 30 seconds). Immediately
after treatment, most surgeons instill an antibiotic com-
bined with a corticosteroid and non-steroidal anti-
inflammatory agent. Most surgeons insert a disposable
bandage contactlens that is removed when the epithelial
defect is healed (2 to 3 days); however, a few still use a
firm eye patch for 24 hours (94). Oral analgesics are


Vi.

Vil.

Vill.

SUMMARY ono. ccscccscsececesesesessesensessesnsnssnsescesensacacsuessnessseseevavssesssssescsceusscssssaeacsesesesesavacavense 23

APPENDIX A .oo.ecececseeeseeeecesesesteseseseseeneneeeeees A-1

A. Non-Surgical Procedures . At
Orthokeratology A-1
Cycloplegia ...

Surgical Procedures Not Involving the Cornea.
Clear Lens Extraction .
Scleral Reinforcement
Surgical Procedures Involving the Cornea .
Keratomileusis
Keratophakia
Epikeratophakia
Stromal Thermokeratoplasty
Intrastromal Corneal Ring ....

LIST OF ACRONYMS

.Automated Lamellar Keratoplasty
est-Corrected Visual Acuity
..Contrast Sensitivity Function
..Diopter

Federal Aviation Administration
..Food & Drug Administration

.. Instrument Flight Rules
..Intrastromal Corneal Ring

. Laser-Assisted in Situ Keratomileusis
Laser Thermokeratoplasty

illar Point Partners
hotorefractive Keratectomy
..Phototherapeutic Keratectomy
..Radial Keratotomy

- Ultraviolet

vii


the surface. While these spikes may not be sufficient to
denature the surface corneal collagen, they may affect
keratocyte activation and corneal wound healing (130),
which also may influence the severity of corneal haze
(225). Current studies investigating ways to reduce the
amount of post-operative haze use irrigation and reduc-
tion of the excimer laser pulse width to cool the cornea

(130,225).

5. Specific complications.
* Healing responses

a) Normal responders, or Type I (85-95%),
demonstrate a slight hyperopic over correction at 1
month with gradual regression toward emmetropia
(normally within 6 months). Approximately 26.5%
have a trace of haze and 7.1% have moderate to marked
haze at 6 months (216,226)).

b) Inadequate responders, or Typell (3-11%),
show a clear cornea and marked hyperopic over correc-
tion (> 1.00 D) at 1 month with minimal regression at
6 months. About 30.8% havea trace of haze at 6 months
(216,226).

c) Aggressive responders, or Type III (1-4%),
display an early over correction with rapid myopic
regression. Approximately 20% havea trace of haze and
80% have moderate to marked haze at 6 months
(216,226). Type III healing response has been linked to
such variables as age, race, and eye color (227). Aggres-
sive healing response can occur at any point within the
first 2 years after PRK, indicating corneal remodeling is
ongoing (226,228). Late haze marks the onset of Type
TI response, which can produce irregular astigmatism
and visual disturbances (226,228). Type III response
can occur after some sort of corneal insult or irritation
(e.g., corneal abrasion, UV keratitis, dryness, contact
lens wear, eye rubbing, acute or allergic conjunctivitis,
and topical eye medications) in patients who have
undergone metabolic stress (pregnancy, lifestyle change
(227)) or in patients who have PRK after having had
previous refractive surgery (227). Eye doctors now
recommend that contact lenses be discontinued for
post-PRK correction until corneal healing is complete
(about 1 year) and anesthetics should be avoided (such
as for applanation tonometry) (226).

* Post-surgical corneal opacities
a) Partial loss of corneal clarity is common
after PRK (142,229,230,231,232,233). The rate and
severity of opacities are proportional to the attempt-
ed correction (29,135,139,148,178, 202). Corneal

15

scarring, sufficient to reduce visual acuity by 2 or more
lines, occurs in about 1% of PRK patients (139).

b) Corneal haze is time-dependent. Initially
clear, corneal haze is usually maximal between 3 and 6
months (115,120,135,162,172), with progressive clear-
ing normally at 1 year post-operatively (120,162).

c) The incidence of patients with a corneal
opacity severe enough to warrant reoperation is 0.81-
3.7% (102,139,171,173). There is currently no method
for predicting which patients might develop an exagger-
ated wound-healing response resulting in corneal opaci-
fication.

d) In more than 80% of PRK eyes, a central
round intraepithelial iron deposit is visible 1 year after
surgery (102,234). This deposit has minimal effect on
visual function. In otherwise clear corneas, the central
iron spot is the only clinical sign of a previous PRK.
However, the definitive tool for following PRK patients
through the healing process is a corneal topography
system, which is expensive and not available in many
ophthalmic practices.

* Glare/Halos

a) Mild to moderate night glare sensitivity
and halos have been reported in as many as 27% of PRK
patients (103,106), the severity of which often prevents
driving at night during the first few months after
treatment (201). At 1 year, 10% of patients reported
persisting effect, and 50% of those reported it severe
enough to interfere with night driving (141,176). Inone
study, 10% of PRK patients declined to have the fellow
eye treated because of disturbances in night vision and
associated symptoms of glare (217). This phenomenon
is difficult to measure, since there is no known mecha-
nism to define a patient's tolerance to glare (106,235).

b) Of PRK patients, approximately 60% re-
ported reduced quality of vision in dim lights
(121,141,178), 38% reduced quality of vision in artifi-
cial light compared to daylight (141,178), and 14%

fluctuation of vision (178).

* Pain
a) Pain can be moderate to severe (130,236)
or up to significantly intense, 10 to 24 hours after
surgery (75,237). In most patients, pain is normally
gone at 3 days post-operatively (175,237).
b) Pain may be more significant in females,
which could be due to gender differences in epithelial
healing (238).


DOT/FAA/AM-98/25

Office of Aviation Medicine
Washington, D.C. 20591

The Aeromedical Certification
of Photorefractive Keratectomy
in Civil Aviation:

A Reference Guide

Van B. Nakagawara
Kathryn J. Wood

Civil Aeromedical Institute
Federal Aviation Administration
Oklahoma City, OK 73125

September 1998

Final Report

This document is available to the public
through the National Technical Information
Service, Springfield, Virginia 22161.

Q

U.S. Department
of Transportation

Federal Aviation
Administration




MYOPIA

Lens

Cornea

L~

Light Rays

Parallel rays of light come to

focus in front of the retina.

HYPEROPIA

Cornea

Light Rays

Lens

Parallel rays of light come to
focus behind the retina.

FIGURE 1

Sex

Male 28.4 Female
Race % Myopic % Myopic
Black 25.1 Non-black 32.7
Region % Myopic ‘% Myopic % Myopic % Myopic
Northeast 32.4 Midwest 35.7 South 24.1 West 33.4
Family Income % Myopic Family Income % Myopic
<$500 168 $4,000 - $4,999 315
$500 - $999 20.9 $5,000 - $6,999 312
$1,000 - $1,999 26.5 $7,000 - $9,999 32.3
$2,000 - $2,999 26.7 $10,000 - 14,999 36.8
$3,000 - $3,999 28.1 $2$15,000. 35.1
Grade in School ‘% Myopic Grade in School ‘% Myopic
<6th 26.4 10th 32.2
7th 28.9 1th 35.4
8th 313 2 12th 35.0
oth 32.8
‘Age (years) % Myopic ‘Age (years) % Myopic

12 29.9 1s 319

13 315 16 33.0

14 312 17 33.2
Reading Test (deciles) % Myopic Reading Test (deciles) % Myopic

1 (low) 19.7 6 32.4

2 24.9 7 310

3 216 8 38.1

4 30.9 9 37.5

5 313 10 (high) 453.
Time spent reading magazines, books, newspapers in a typical day

‘% Myopic ‘% Myopic % Myopic

<1 hour 27 1-3 hours 32.8 >3 hours 34.6

‘Source of data: National Health Examination Survey of 12 to 17 year olds, 1966-1970,

(Reprinted from Stein HA, Cheskes A, Stein RM. The excimer: Fundumental and clinical use. ‘Thorofare, NJ: Slack Inc. 1995, 22.)



THE AEROMEDICAL CERTIFICATION OF
PHOTOREFRACTIVE KERATECTOMY IN CIVIL AVIATION:
A REFERENCE GUIDE

I. INTRODUCTION

Refractive error is a defect in the eye that prevents
light rays from being brought to a single focus on the
retina. Refractive errors are common conditions in
which corrective lenses (e.g., glasses, contact lenses) are
needed to see clearly. There are three refractive condi-
tions: myopia (nearsightedness), hyperopia (farsighted-
ness), and astigmatism (visual blurring at all distances).
Although not a refractive condition, presbyopia (de-
creased focusing ability), which normally occurs around
age 40 years, results in blurred vision at near distances,
requiring bifocals or reading glasses.

There are approximately 145 million Americans
who are dependent upon spectacles or contact lenses to
achieve a quality of vision satisfactory for their daily
needs. This conservatively represents 54.6% of the
United States population. About 10%, some 26.5 mil-
lion Americans, wear contact lenses (2).

Itis anticipated that, in the next decade, there will be
increasing market pressure to provide those with refrac-
tive error a lifestyle free of spectacles and contact lenses.
Lifestyle improvement is a major factor influencing
patients to seek alternative methods of refractive correc-
tion (i.e., refractive surgery). An industry analyst pre-
dicts that, within the next 5 years, between oneand three
million photorefractive keratectomy (PRK) procedures
will be performed annually, with about three million
Americans having been treated (3). One leading refrac-
tive surgeon expects PRK to become a rite of passage like
braces, a driver’s license, college, and then PRK (3).

A. MYOPIA

Myopia, or nearsightedness, is a condition in which
parallel rays of light from an object being viewed come
to focus at a point just in front of the retina (see Figure
1). There are two types of myopia: 1) axial, in which the
eye is too long for the normal refractive power of the lens
and cornea; and 2) component, which results from a
change in thecurvature of the cornea/lens or in the index
of refraction of the cornea/lens. For example, diabetes
(in which the crystalline lens loses water due to the high

level of blood sugar in the anterior chamber) and
cataract (an opacity or cloudiness of the crystalline lens)
may change the index of refraction of the lens. Almost
everything has been blamed as a cause of myopia,
including diet, obesity, allergy, light conditions, vita-
min deficiencies, and even wearing glasses too much or
too little. Controversy and heated debate continues on
whether excessive close work or reading is a primary
cause of myopia. Myopia is rare at birth, normally
manifesting itself after the 4th year of life, with progres-
sion relatively constant until the time of puberty, when
it may progress rapidly. Normally, myopia becomes
stable when full maturity is reached. Therefore, between
the ages of 20 and 40 years, the myope’s correction may
remain essentially unchanged (4).

Even a relatively small amount of myopia results in
distant objects being considerably blurred. For example,
a-1.00 Diopter (D) myopic refractive error would result
in unaided vision of 20/40 to 20/60 Snellen acuity.
While eyeglasses or contact lenses are the primary treat-
ment for myopia, they can be a hindrance in some
occupations or recreational activities. In addition, high
myopic lenses have thick edges and the optical image is
reduced by minification and optical aberrations. It is
estimated that about 25% (75 million) of the popula-
tion in North America are myopic and, of these, more
than 60 million have < 6.00 D of myopia (5).

Myopia is a frequent phenomenon in all social
categories, but incidence varies depending on many
factors (see Table 1). Myopia has been found to be
patterned in its occurrences in different races and ethnic
groups. Blacks, whether in Africa or in the United
States, have a low prevalence of myopia. Asians have
been found to have a prevalence of myopia as high as
40%, compared to only 20% in whites. The severity of
myopia is associated with educational attainment. Myo-
pia has also been found to be positively associated with
social class, degree of urbanization of place of residence,
and level of economic development of the region or
country of residence. There is an increase in the


252.

253.

254,

255.

256.

257.

258.

259.

260.

261.

262.

O’Brart DPS, et al. The effects of topical corticos-
teroids and plasmin inhibitors on refractive out-
come, haze, and visual performance after
photorefractive keratectomy. Ophthalmol. Sep
1994; 101(9):1565-74.

McDonnell PJ, Moreira H, Clapham TN, D’Arcy
J, Munnerlyn CR. Photorefractive keratectomy
for astigmatism: Initial clinical results. Arch

Ophthalmol. Oct 1991: 109(10):1370-3.

Meyer D, Gimbel HV, Van Westenbrugge JA.
Adrenergic/anticholinergic responses after
photorefractive keratectomy. J Cataract Refract
Surg. Nov 19915 17(6):862.

Lohmann CP, Gartry DS, Kerr-Muir MG,
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Talamo JH, Gollaumundi S, McDonnell PJ. The
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Lin DTC, Sutton HF, Berman M. Corneal to-
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Gilbert ML. Corneal topography: In search of the
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Parker PJ, Klyce SD, Rayan BL. Central topo-
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Puliafito CA, Stern D, Krueger RR, Mandel ER.
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Campos M, et al. Ablation rates and surface
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than the risk with extended wear contact lens use and is
25-75X greater than the risk with daily wear soft contact
lenses (324).

*  Epithelial/subepithelial integrity is questionable
after PRK. It is known that the corneal epithelium
regenerates, and that it normally reattaches to the cornea’s
basement membrane and Bowman’s layer. PRK re-
moves Bowman’s layer and the basement membrane
over the central cornea forcing the epithelium to re-
epithelialize over anterior corneal stroma. The long-
term effect of this alteration of the cornea is unknown.
There was some original concern that such an altered
cornea might result in recurrent erosions, but the short-
term experience has not supported that concern (152).

* The corneal endothelial cell layer is vital to
corneal clarity. There has been no conclusive evidence
of substantial problems with the corneal endothelium
after PRK. However, longer-term follow-up is required
to determine the clinical significance, if any, of the
effects of PRK on this cell layer (152).

¢ Uncorrected visual acuity after PRK may not be
adequate for safe operation of an aircraft. In 5-15% of
PRK patients, uncorrected visual acuity of 20/40 or
worse has been reported. A considerable amount of
residual refractive error may persist after PRK and
require correction either by glasses or contact lenses.
Though approximately 75% of the post-PRK patients
have refractions within + 1.00 D of emmetropia, it
appears that 40% of these will still require some type of
visual correction (152). The use of contact lenses by
airmen with PRK poses other clinical problems. At this
time, the health of the corneal epithelium and its
capacity to support the use ofa contact lens is unknown
(152). It is possible that post-PRK eyes wearing contact
lenses may be at increased risk for corneal ulceration or
complications because of the alteration in the histologi-
cal relationship of the cornea. Civil airmen with PRK
may not be able to tolerate contact lenses as long and
under the same conditions that they did prior to PRK.
The aviation environment is already a hostile one to
contact lens wearers with low relative humidity, hy-
poxia, acceleration, etc., affecting the fit and comfort of
the lenses. Additional research is needed before the full
benefits from contact lens use following PRK can be
realized (152).

¢ A “double pupil effect,” resulting from the edge
of the ablated optical zone being overshadowed by the
dilated pupil under reduced lighting conditions, can be
detrimental to aviators. This phenomenon causes reti-
nal image degradation, glare, blurred vision, and visual

22

confusion. This is an important issue that deserves
careful consideration, especially with night and low-
light flight operations.

* Induced anisometropia, a form of monovision,
may bea planned procedure for early presbyopes having
PRK. Monovision, normally performed using contact
lenses, is contraindicated in the Guide for Aviation
Medical Examiners (72). Monovision may affect depth
perception and reduce BCVA. Aviators should be in-
formed of these side effects, and that ophthalmic lenses
to correct PRK-induced monovision may be required.

* When bilateral PRK procedures are required, the
FDA recommends that PRK be performed on one eye at
a time. This allows adequate vision for the patient
during the extended healing process. The healing re-
sponse on the operated eye may be used by the surgeon
to refine the PRK procedure on the second eye. The
induced anisometropia between eye surgeries may sub-
stantially affect depth perception and require the use of
a contact lens. This may become a serious problem for
those patients who are contact lens intolerant. Follow-
ing first eye PRK with resultant anisometropia, aero-
medical certification of applicants should be delayed
until the procedure is completed bilaterally.

* Aviation-related environmental and flight ma-
neuver effects on PRK need further studies. The effects
of gravito-intertial, accelerative or decelerative, and vibra-
tional factors on PRK have yet to be determined (121).

D. OPTIONS UNDER CONSIDERATION BY
FAA FOR EVALUATION OF APPLICANTS
WITH PRK
*  Asaresult of possible delayed corneal healing, an

application for an airman medical certificate should be

initiated no earlier than 6 months after the final post-
operative eye procedure. The Guide for Aviation Medical

Examiners states that healing is usually complete within

6 to 12 weeks after refractive surgery (72). However, it

takes about 1 year for the post-PRK refraction to stabi-

lize, and this period may be longer in the correction of
higher myopes. (Note: In some countries, a waiting
period of 12 months after RK is required before
obtaining an aeromedical certificate. In France, any
person who has refractive surgery is automatically clas-
sified as unfit to fly and must request a special dispen-
sation; only 1 of 3 applicants is successful in obtaining

this dispensation (223).)

* Medical certificates should not be issued while
an applicant is using corticosteroids to modulate the
corneal healing process. The use of such drugs suggests


APPENDIX A

A. NON-SURGICAL PROCEDURES

ORTHOKERATOLOGY

First practiced in the 1970s, it uses flat, tight-fitting,
rigid contact lenses to decrease the curvature and refrac-
tive power of the cornea. Progressively flatter lenses are
used to mold the cornea. When the cornea sufficiently
flattens, “retainer” contact lenses are normally worn
intermittently to preserve the modified shape (8). Cur-
rently there are studies with PRK patients, with less-
than-satisfactory results, who are using orthokeratology
to correct their residual myopia (326).

Complications: Keratoconus, ulceration of the cor-
nea, and irregular astigmatism (8).

Advantages: Cornea elasticity and memory allow it
to return to its original shape (8).

Disadvantages: Changes to the cornea are not per-
manent (8).

CYCLOPLEGIA

Used primarily in young children to prevent ciliary
spasm caused by near vision, by paralyzing the ciliary
muscle, it was thought to prevent the progression of
myopia (8). Treatment involved the daily instillation of
a cycloplegic in early developing myopes until matura-
tion is complete. Bifocals or reading glasses are used for
near vision to prevent further ciliary spasm (8).

Complications: Photophobia due to dilated pupils
and exposure to excessive solar radiation (8).

Advantages: None.
Disadvantages: It is difficult to predict which pa-

tients are going to develop high myopia. Dilated
pupils are cosmetically unsuitable (8).

B. SURGICAL PROCEDURES NOT
INVOLVING THE CORNEA

CLEAR LENS EXTRACTION

Developed by Fukula in the 1980s, it was discontin-
ued due to the large number of subsequent retinal
detachments. There has been renewed interest in Eu-
rope, using modern techniques of crystalline lens extrac-
tion and improved intraocular lenses (8).

Complications: Retinal detachments (8,327), cys-
toid macular edema, posterior capsule opacification,
motility disturbances, and secondary glaucoma (8).

Advantages: Initial visual improvements and cor-
rection of high degrees of myopia.

Disadvantages: Complications could cause perma-
nent vision impairment (8).

SCLERAL REINFORCEMENT

Introduced by Borley in 1958, it is the only surgical
procedure that corrects the cause (axial elongation)
rather than the effect of refractive error (8). The proce-
dure consists of strengthening the scleral shell with
fascia lata or homologous scleral graft that theoretically
curbs the progression of axial elongation (8).

Complications: Anterior uveitis, motility disorders,
and retinal detachment (8).

Advantages: Studies have reported that staphylomas
(bulging or protrusion of cornea) was reduced and
myopic progression was stopped or partially reversed in
some patients (8).

Disadvantages: Due to the limited scleral area rein-
forced by the narrow graft, long-term reports state that
some patients have had an increase in myopia (8).


B. FACTORS FAVORING PRK IN CIVIL

AIRMEN

+ Age: PRK candidates should be between 18 to 60
years of age, and civil airmen are preponderantly in this
age group category. (Note: Older patients tend to have
better PRK results than younger patients (196).)

¢ Refractive error: Corrections between -1.00 to -
6.00 D are most predictable. About 17% of the U.S.
population are within this group (< -6.00 D myopia)
(8); while only 3.2% have myopia of > -6.00 D. A large
number of civil airmen have received flight training
through the military, effectively prescreening them to
this lower to mid myopic range.

* Economic considerations: Aviation is an expen-
sive avocation and lucrative occupation, so civil airmen
have the economic means to afford this procedure.

* Gender Preference: Studies suggest that males
express greater interest in PRK than females (321). Civil
airmen are preponderantly male (16 males to 1 female).

* Historical interest in refractive surgery: Civil air-
men have opted for RK in substantial numbers since 1983.

C. AEROMEDICAL CERTIFICATION

RECOMMENDATIONS

The successful PRK procedure is dependent upon a
variety of objective and subjective criteria. Surgeons
often define success as a function of the final visual
acuity achieved and the lack of complications. Patients,
on the other hand, may define success as the ability to
exist without the encumbrances of thick glasses or
contact lenses, Given these alternatives, they may toler-
ate associated subjective problems from PRK. However,
when approached from an aeromedical safety view-
point, these perceived minor annoyances resulting from
refractive surgery may be magnified and become more
important in flight activities. Early research protocols
excluded pilots due to their demanding occupational
vision requirements and the possibility of complica-
tions (171,172,176). For that reason, post-operative
signs and symptoms need to be carefully scrutinized
from the perspective of their effect on aeromedical
certification (152).

¢ Night vision problems, which include glare,
halos around lights, haze, starbursts, and dim lighting
difficulties, may impair the pilot during Instrument
Flight Rules (IFR) conditions (103,121,141, 322,323).
Gimbel et al. (141), reported that as high as 50% of
bilateral PRK patients have reduced quality of vision in
dim light, 38% reduced vision in artificial lighting versus
daylight, and 50% reported night driving difficulties.

21

* Reduced contrast sensitivity may occur from
both glare and corneal haze. The visual acuity standards
for civil airmen are based on high-contrast Snellen
targets viewed in controlled environments. In the natu-
ral environment, visual contrast sensitivity becomes
critical to optimal performance where weather elements
(fog, haze, and rain) reduce contrast between targets and
the background environment. Although most countries
have no current aviation medical standards with respect
to contrast sensitivity, an individual's ability to perceive
contrast has been recognized as a critical element in
overall visual performance. A procedure, such as PRK,
with the potential to degrade contrast sensitivity must
bescientifically evaluated to determine its impact (152).
Contrast sensitivity performance may improve, as cor-
neal haze is reduced. Prematurely accepting ill-defined
reduced contrast sensitivity performance in aviators, until
it is fully understood, may be aeromedically risky (152).

* The structural integrity of the eye after PRK is
unknown. PRK removes a portion of corneal tissue up
to 10% of the total corneal depth. Although it is
assumed that any structural weakening of the eye in-
duced by PRK would be much less than in RK, one
cannot predict with absolute certainty what the corneal
rigidity will be after PRK (152).

* The stability of refraction may be a problem.
There is a hyperopic shift (164,197) and a period of
refractive instability that slows over a 3- to 6-month
period, but may continue beyond 1 year. Studies have
shown that refractions may continue to be unstable for
up to 26 months (102,105,213, 249,309). The post-RK
cornea (3 to 5 years) has been shown to be susceptible to
refractive changes in altitude (309). While the surgical
mechanism is different between RK and PRK, it cannot
beaccurately predicted what the aviation environment’s
impact will be on post-PRK corneas without further
investigation (309).

* Reduced BCVA may affect medical certification
of professional pilots who have more stringent vision
standards (102,105,114,125,174,176,211, 212). FDA
trial data reports 1-3% of patients undergoing PRK lost
2 or more Snellen lines of best-corrected acuity. If only
5% of the 60 million myopes in the U.S. undergo PRK,
there will be 30-90,000 PRK patients with significant
acuity losses (195). When these numbers are compared
to use of extended wear contact lenses, the differences
are significant. Studies have shown that 0.02% develop
a contact lens-related infection that results in loss of
BCVA. By a conservative estimate, this would mean
that the risk of loss of BCVA after PRK is 5-15X greater


Cc.

D.

V. AEROMEDICAL ISSUES

oOm>

Centration of ablation zone ...
Epithelial and stromal changes
Corneal endothelial changes.
election Criteria...
Post-operative vision
Sex
Age
Ocular/systemic disease .
Amount of myopia .
Astigmatism .
Binocularity ..
Contact lens weai
Occupation
Pupil size
tinical Studies
Stability .
Efficacy .
Safety ...

« Loss of BCVA

+ Decrease in contrast sensitivity .

*  Ultraviolet-C (UV) exposure

4. — Side-effects
* Thermal damage

5. Specific complications .

OARw

SOEPNAARWNS

oO

eN=Q

« Healing responses . 15

¢ Post-surgical corneal opa

+ Glare/Halos

Pain...

* Corneal ulceration .

* Corneal sensitivity ........

¢ Sterile corneal infiltrates 16

« Regression ............ 16

* Over/Under correction 16

¢ Corticosteroids ...... 16

¢ Non-steroidal anti-inflammatory agents 17

* Diurnal fluctuation in vision............... 17

* Central islands.............. 17

* Subretinal hemorrhages 18

¢ Other complications 18
Future Developments... ccesceesesssesssetseseseseseseecceecsesesavesacacseesseescsnsescescecsvavecanavacavees 18

Certification of Pilot Applicants .
Factors Favoring PRK in Civil Airmen.......

Aeromedical Certification Recommendations
Options Under Consideration by FAA for Evaluation of Applicants With PRI

vi


* It is recommended that perfectionist patients
with high expectations for perfect post-operative un-

aided vision be excluded (95).

2. Sex. The only gender-based exclusion is
pregnant women (95).

3. Age

* Patients should be = 18 years of age with the
VISX Laser and 2 21 years ofage with the Summit Laser.
(Note: The difference in the approved age for PRK by
the FDA was based on investigational data supplied by
Jaser manufacturers.)

* Near presbyopic patients should be warned
that glasses may be required for near vision after
surgery (95).

* There is a trend to greater over correction
(induced hyperopia) in older patients for the first 6
months after surgery (162,163,164,165).

4, Ocular/systemic disease
* A complete eye examination before surgery is
required to identify contraindications such as:

a) Collagen vascular disease (e.g., corneal ul-
ceration or melting) (146,166)

b) Ahistory of ocular disease or an abnormal-
ity (e.g., dry eye, keratoconus, glaucoma or glaucoma
suspects, incipient cataracts, herpes simplex keratitis,
amblyopia) (95,167)

c) Systemic disorders that may potentially
affect wound healing (e.g., diabetes, rheumatoid arthri-
tis, systemic lupus erythematosus, lagophthalmos, pol-
yarteritis nodosa) (146, 162,166)

d) A history of steroid intraocular pressure

response (146,166)

5. Amount of myopia

* Predictability of refractive outcome is in-
versely related to the degree of myopia (relatively
good S-4.00D, less predictable > -6.00 D) (95,168,169,
170). However, it has been reported that even low
myopic patients may achieve only partial refractive
correction and may even experience complete myopic
regression (171,172).

* Variability in corneal wound healing and the
incidence of complications (e.g., glare, over correction,
scarring, regression) increases with greater amounts of
myopia (95,97,120,128,142,144,164,165,170,171,
172,173,174,175,176).

12

6. Astigmatism. The FDA recommendation that
only patients be treated with refractive astigmatism
of $ 1.00 D with the VISX Laser and $ 1.50 D with the
Summit Laser was based on the investigational data
originally submitted by each laser manufacturer (95,163).
In July 1997, the FDA approved the VISX Excimer
Laser System to perform moderate levels (< 4 D) of
astigmatic PRK, Summit currently has investigational
data before the FDA Ophthalmic Devices Advisory
Panel to allow their laser to treat moderate levels of
astigmatism.

7. Binocularity
* Current FDA guidelines recommend a mini-
mum of 3 months between treatments while others
recommend longer periods of up to 6 months for higher
myopia. However, some surgeons are currently doing
simultaneous bilateral surgery.

* Time between surgeries allows fora stabilization
of refraction and manifestation of complications in the
first eye (95), but may greatly affect binocularity. The
healing response of the first eye may be used to modify
treatment in the second eye.

8. Contact lens wear

* Contact lens wearers are good PRK candi-
dates, since they can wear a contact lens in the untreated
eye to achieve comfortable binocular vision during the
interval between treatments.

* Patients with fluctuating refraction due to
corneal distortion from contact lens wear should be
excluded until stabilized (95).

* Unsuccessful contact lens patients may have
underlying lid disease and dry eyes that could interfere
with post-surgical healing if untreated (163).

9. Occupation

* Caution should be exercised when considering
treating individuals with demanding occupational vi-
sion requirements (e.g., pilots, truck drivers) (95), since
early research protocols excluded such patients
(171,172,176).

* Refractive surgery is disqualifying for entry
into the U.S. military, but may be performed on active
duty personnel. It is disqualifying for all members on
flight status and some special schools in the armed forces
(177).

* Local police or fire departments may reject an
applicant who has had refractive surgery (95).


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The corneal endothelium after myopic excimer
laser photorefractive keratectomy. Arch Ophth-
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Del Pero RA, et al. A refractive and histopatho-
logic study of excimer laser keratectomy in pri-

mates. Am J Ophthalmol. Apr 1990; 109(4):419-29.

Marshall J, Trokel SL, Rothery S, Krueger RR.
Photoablative reprofiling of the cornea using an
excimer laser: Photorefractive keratectomy. Laser

Ophthalmol. Jan 1986; 1(1):21-48.

Editor. FDA details required reading for PRK
candidates. AOA News. Apr 8, 1996; 34(19):9.

Seiler T, Wollensak J. Myopic photorefractive
keratectomy with the excimer laser: One-year fol-
low-up. Ophthalmol. Aug 1991; 98(8):1156-63.

Epstein AB. Test yourself on PRK. Optom Man-
age. Jan 1996; 31(1):29,32-33,36-38.

Dutt S, Steinert RF, Raizman MB, Puliafito CA.
One-year results of excimer laser photorefractive
keratectomy for low to moderate myopia. Arch

Ophthalmol. Nov 1994; 112(11):1424-36.

30

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Talley AR, et al. Use of the 193-nm excimer laser
for photorefractive keratectomy in low to moder-
ate myopia. J Cataract Refract Surg. Mar 1994;
20(2):239-42.

Snyder R, Klein P. Laser vision correction: Part 2
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Lavery FL. Photorefractive keratectomy in 472
eyes. Refract Corneal Surg. Mar-Apr 1993;
9(suppl):S98-100.

Brancato R, et al. Excimer laser photorefractive
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Taylor HR, McCarty CA, Aldred GF. Predict-
ability of excimer laser treatment of myopia. Arch

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Gartry DS, Kerr-Muir MG, Lohmann CP,
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on refractive outcome and corneal haze after
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Gartry DS, Kerr-Muir MG, Marshall J.
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tide excimer laser: A clinical study. Refract Cor-
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laser photorefractive keratectomy for myopia.

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pia. Ophthalmol. Sep 1994; 101(9):1548-57.

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keratectomy for myopia: 12 month follow-up.

Eye. 19935 7: 617-24.
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laser photorefractive keratectomy: 18 month follow-

up. Arch Ophthalmol. Aug 1992; 99(8):1209-19.

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tive changes during prolonged exposure to alti-
tude following refractive surgery. Ft. Belvoir, FL:
U.S. Army Medical Research and Materials Com-
mand; Oct 1994; Defense Technical Information
Center Publication No. 94MM4576.


* Subretinal hemorrhages

a) There have been reports of spontaneous
retinal hemorrhages in pathologic myopes (Fuchs’ spots),
resulting in severe post-operatively vision loss (100).
Whether the incidence of these subretinal hemorrhages
is increased with PRK is unknown. During PRK, me-
chanical stress waves travel through the eye and may
irritate or disrupt the fragile subretinal vessels (100).

¢ Other complications

a) There have been reports of mild bleph-
aroptosis (drooping upper eyelid) in some PRK pa-
tients, which may be related to use of lid speculum
during the procedure. These cases spontaneously re-
solved in 3 months (100).

b) About 5% of PRK patients have anisocoria
(pupils of unequal diameter) for up to 12 months after
surgery (100).

c) Patients who subsequently develop a cata-
ract following PRK may have difficulty in getting the
correct power for intraocular lens implants (270). Addi-
tionally, patients who develop myopic regression are at
increased risk for measurement errors in their intraocu-
lar lenses (270).

D. FUTURE DEVELOPMENTS

In theory, PRK corrections of up to 20.0 D are
possible (201,271,272) (compared to 8.00 D for RK)
(29). The impact of epithelial and stromal wound
healing on corneal clarity define the upper achievable
refractive limits (272). Meanwhile, there are several new
developments that may further improve laser refractive
surgery techniques.

Studies are being conducted using multi-zone/multi-
pass procedures for high myopia (117,215,269,
273,274,275,276). The multi-zone techniques are used
to limit ablation depth, while multi-pass techniques
reportedly smooth the ablated surface (276).

An erodible mask (184,277,278,279,280) is being
developed that allows the excimer laser to correct astig-
matism (134,171,180,278,281,282,283,284,285) and
hyperopia (124,180). The mask is composed of poly-
methyl! methacrylate, which is the material used in rigid
(non-gas permeable) contact lenses, and a quartz sub-
strate. The mask absorbs the laser light and is eroded
(vaporized) in the process, while also transmitting
undisturbed portions of the UV-C light. The surgeon
uses these specifically contoured masks to control where
and how the laser energy is applied to the cornea,
allowing selectively contoured corneal surfaces. How-

18

ever, use of the erodible mask and lower ablation rate
increases the total time needed for the PRK procedure,
thus increasing the risk ofan imperfect treatment due to
decentrations and micromovements (286). Besides these
erodible masks, iris (247,287) slit (288,289), and oval
apertures (188) of different sizes may be used with the
excimer laser to created toric ablations to correct astig-
matism.

Currently, there are ongoing studies on the use of
PRK to correct residual myopia after refractive surgery
(290,291,292,293,294,295,296,297), to correct irregu-
lar astigmatism (resulting from previous refractive surgery,
pterygium removal, penetrating keratoplasty, cataract sur-
gery) (298,299,300), and to treat keratoconus (301).

The excimer laser used with laser-assisted in situ
keratomileusis (LASIK) has great promise for correcting
higher myopia (11,302,303,304,305,306,307). The
LASIK procedure can be performed with the laser used
to perform PRK. LASIK removes tissue from the inner
corneal layers, rather than from the exposed epithelial
surface (see Figure 7). A specially designed knife blade
(microkeratome) slices a thin, horizontal flap (100- to
200-j1m in depth) (308) off 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 flap is then replaced (termed
“flap and zap”). LASIK has significantly less scarring
and regression, since Bowman’s layer of the corneais not
ablated (308,309,310). Post-operative patients typi-
cally are more comfortable, stabilize faster, do not
require extended use of topical steroids, and have fewer
long-term complications and side effects, compared
with other types of refractive surgeries (308,310,
311,312). LASIK appears also to be effective in the
treatment of astigmatism.

The major complaint of LASIK is foreign bodies in
the corneal interface and epithelium of the corneal flap.
Operative risks include: 1) ocular perforation, 2) dis-
lodged or detached corneal flap (313) (A study of
patients undergoing repeat procedures at 6 months
reported that, by removing the epithelium at the inci-
sion, the corneal flap could be folded back and another
ablation performed. This suggests that even at 6 months
post-operative the flap has not yet adhered to the
stromal bed. (313)), 3) poorly aligned or positioned flap
resulting in irregular astigmatism (about 1%), 4) epithe-
lial ingrowth (2%), 5) foreign bodies in the stromal bed,
6) slower healing rate (314)), and 7) microkeratome-
induced subconjunctival hemorrhages (310).




323.

324.

325.

326.

327.

328.

Lohmann CP, Fitzke FW, O’Brart DPS, Kerr-
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myopes? A comparison between spectacles, con-
tact lenses, and photorefractive keratectomy. Re-
fract Corneal Surg. Mar-Apr 1993; 9(suppl):S72-5.

Zadnick K, Mutti DO. PRK vs. contact lenses:
The risk is relative. Contact Lens Spectr. Jan 1996;
11(1):12,

Schallhorn SC, et al. Preliminary results of
photorefractive keratectomy in active-duty United
States navy personnel. Ophthalmol. Jan 1996;
103(1):5-22.

Garber JM. Orthokeratology after PRK: Portend
for the future? Contact Lens Spectr. Feb 1996;
11(3):37-40.

Barraquer C, Cavelier C, Mejia LF. Incidence of
retinal detachment following clear-lens extrac-
tion in myopia patients. Arch Ophthalmol. Mar
1994; 113(3):336-9.

Nordan LT. Keratomileusis. Int Ophthalmol Clin.
Winter 1991; 31(1):7-12.

37

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334,

Polit F, Ibrahim ©, Maghraby AE, Salah T.
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Slade SG, Updengraff SA. Complications of au-
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Sep 1995; 113(9):1092-3. Letter.

Lyle WA, Jin GJC. Initial results of automated
lamellar keratoplasty for correction of myopia:
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Guimaraes RQ, Rowsey JJ, Guimaraes MFR,
Reis PP, Castro RD, Tirado BR. Suturing in
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Charpentier DY, Nguyen-Khoa JL, Diplessix M,
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inadequate for over correction following radial
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10(1):34-5.

Thompson V. Holmium: YAG laser thermo-
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fract Corneal Surg. Mar-Apr 1994; 10(suppl):S293.


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Dausch D, Klein R, Schroder E, Dausch B.
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liminary report of six cases. J Cataract Refract
Surg. Sep 1993; 19(5):590-4.

Kim JH, Hahn TW, Lee YC, Sah WJ. Clinical
experience of two-step photorefractive keratec-
tomy in 19 eyes with high myopia. Refract Corneal
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Pop M, Aras M. Multi-zone/multi-pass photor-
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ract Refract Surg. Nov 1995; 21(6):633-43.

Brancato R, Carones F, Trabucchi G, Scialdone
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Cherry PM, Tutton MK, Bell AM Neave C,
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Friedman MD, et al. OmniMed II: A new system
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Am J Ophthalmol. Feb 1988; 105(2):117-24

Editor. Lasers proving useful in treating astigma-
tism. AOA News. Jun 1, 1992; 30(23):5,8.

Taylor HR, Guest GS, Kelly P, Alpins NA. Com-
parison of excimer laser treatment of astigmatism
and myopia. Arch Ophthalmol. Dec 1993;
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Lipshitz I, Lowenstein A, Lazar M. Comparison
of excimer laser treatment of astigmatism and
myopia. Arch Ophthalmol. Dec 1994; 112(12):
1509-10.

Colliac JP, Shamas HJ, Bart DJ. Photorefractive
keratectomy for the correction of myopia and
astigmatism. Am J Ophthalmol. Mar 1994;
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35

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G, Asfar L, Waring GO III. A rotary slit delivery
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Jan-Feb 1996; 12(1):163-70.


B. REFRACTIVE PROCEDURES INVOLVING

THE CORNEA

Refractive procedures have concentrated on modi-
fying the anterior surface of the cornea. Of the four
physical surfaces responsible for refractive power of the
eye, the anterior corneal surface is by far the most
important, supplying 44 of the 66 diopters (2/3) that
comprise the total refractive power of the eye. These
procedures include keratomileusis, keratophakia,
epikeratophakia, stromal thermokeratoplasty, and in-
trastromal corneal ring.

1. Keratomileusis. Unique in that it was the first
medical procedure where a part of the body was re-
moved, modified, and returned to its original location
(9), keratomileusis can correct very high myopia (e.g.,
16.0 to 18.0 D) (8). Reported complications of over and
under correction (7), corneal perforation during lathing
(7,10), irregular astigmatism (4,10,11,12,13), corneal
opacities (9), the complexity of the procedure (7),
sophisticated equipment (7), cost (12), and unpredict-
ability (13) have resulted in this procedure being discon-
tinued (see Appendix A).

2. Keratophakia. A donor corneal tissue is placed in
the stromal bed, with the patient’s anterior corneal
tissue sewn back in place, to steepen the cornea and
correct aphakic refractive errors (7). Capable of correct-
ing powers greater than +10.0 D, keratophakia has
reported complications with induced astigmatism, loss
of best-corrected visual acuity (BCVA), and limited
supply of donor corneas (7,14). The development of
new and improved intraocular lenses to correct aphakic
refractive error contributed to the discontinuance of
this procedure (see Appendix A).

3. Epikeratophakia. Currently, this is the most
widely practiced lamellar refractive keratoplasty tech-
nique. Safer and simpler than keratophakia, freeze-
dried lenticules are pre-lathed to correct a particular
refractive error and sutured into a small epithelial at-
tachment site made in the peripheral cornea, which
allows the host epithelium to cover the anterior surface
of the donor lens (7). This procedure can be used in the
treatment of keratoconus and high myopia (up to -37.0
D) (8). Epikeratophakia has reported complications
with neovascularization (7), infections (7), reduction in
contrast sensitivity, and difficulty in precise centering of
lenticules (15) (see Appendix A).

4. Stromal thermokeratoplasty. Initially, a probe
placed at 80% depth of the stroma was used to shrink
stromal collagen and flatten the cornea (7,16). How-
ever, currently a modification of this procedure is
undergoing U.S. Food and Drug Administration
(FDA) investigational studies using a Holmium YAG
laser (Laser Thermokeratoplasty [LTK]) (17). The
procedure is used to treat hyperopia and astigmatism.
There have been reported complications of corneal
changes that include epithelial thinning, recurrent
erosions, stromal melting (7), loss of best-corrected
acuity (17), regression (17), and unpredictability
(17) (see Appendix A).

5. Intrastromal corneal ring (ICR). ICR is under
FDA investigation. The procedure uses an implanted
corneal ring in the mid-peripheral stroma to change
corneal curvature (i.e., rings of different thickness are
used to induce different corneal changes) (18). There is
no direct contact with the central cornea (19), and the
ring is easily removed for complete reversibility (18).
There have been reported increases in intraocular pres-
sure (IOP) due to corticosteroid use in the post-opera-
tive healing phase (18) (see Appendix A).

Of all refractive surgical techniques, the two most
widely used are radial keratotomy (RK) and
photorefractive keratectomy (PK). These will be dis-
cussed in the next two sections.

III. RADIAL KERATOTOMY

A. HISTORICAL AND PROCEDURAL

PROCESS

RK was first used on a wide scale by Sato of Japan in
1953. Itwas considerably refined by Fyodorov of Russia
in the early 1970s. Since the late 1970s when it was first
introduced in this country, RK has been performed on
well over one million Americans. Most recent statistics
indicate that RK is increasing in popularity. In 1990,
only 50,000 RK operations were performed; in 1992,
about 250,000 RK operations were performed; and in
1994, an estimated 300,000 RK procedures were per-
formed on Americans (20). The procedure consists of
making incisions radially on the peripheral cornea using
a diamond surgical blade, usually to a depth of 90-95%
of the central corneal thickness, leaving the central
optical zone untouched (see Figure 4). These incisions
weaken the peripheral cornea and allow intraocular
pressure to push the cornea out, flattening the apex and


* Excessive limbal scarring (8) (Note: Currently,
RK is no longer carried out to the limbus.)

* Neovascularization (36,46) (Note: Normally
occurs when incisions are carried out to limbus.)

* Late-onset bacterial corneal ulcers (36,47)

* Central corneal scarring (48)

* Stellate iron deposits (49)

5. Refractive.
* Over correction (i.e., refractive hyperopia)
(26,50,51,52,53)
* Under correction (26,54)
* Anisometropia (unequal refractive state for
the two eyes) (21,26,54)
* Induced astigmatism (8,26,36)

6. Visual performance.

* Glare (36,55,56,57,58)

* Changes in contrast sensitivity (55,59,60)

* Long-term instability (21,51,61)

¢ Fluctuating acuity/diurnal variations (56,58,
59,62,63,64)

* Ghost images (21,56)

* Loss of optical quality in paracentral and pe-
ripheral cornea (55)

* Visual distortion (27)

* Reduced BCVA (21,26,28)

* Diminished night vision (21,55)

C. ADVANTAGES

1. Rapid recovery. Usually within a week post-
operatively (Note: Second eye can be operated on
simultaneously, or within 1 week to 3 months.) (50)

2. Established safety.
* Years of clinical experience (21,65,66)
* Healed incision scars require special ophthalmic
instruments to view adequately (8)
* Minimal risk of vision loss from surgical com-
plications (8.3% mild and 0.3% severe) (67)

3. Procedures for astigmatism. There are
estimates that 63.2% to 73% of the U.S. population
has refractive astigmatism (5,6)

4, Cost. About $1,000 per eye (50)

D. DISADVANTAGES
1. Refractive
* Difficulties in contact lens fitting (8)

* Over correction may lead to early presbyopic
symptoms (Note: May result in premature need for
reading glasses.) (8)

* Irreversible (68)

* Residual refractive error (Note: About 33% of
RK patients require corrective lenses either full- or part-
time, e.g., reading glasses, night driving.) (69)

2. Limitations
* Limited to correction of myopia and low astig-
matism
* Risk of loss of vision increases with the num-
ber of incisions, intersecting incisions, and small optical
zones (70)

3. Predictability
+ Refractive outcome prediction is about + 2.00
D for 80-90% of eyes (71)
* Enhancement procedures are often required
(less effectiveness and predictability with repeated op-
erations) (21)

E. CERTIFICATION OF PILOT APPLICANTS
WITH RK
1. Certification process.

¢ Historical. Initially, airmen who had RK pro-
cedures were allowed to obtain Federal Aviation Ad-
ministration (FAA) medical certificates through the
waiver process. Applicants with RK were required to
submit a completed “Report of Eye Evaluation” (FAA
Form 8500-7) at least 6 months after their RK proce-
dure when applying for a FAA airman medical certifi-
cate. A follow-up second eye evaluation report was also
required 6 months later.

* Present. Presently, applicants with RK may
obtain medical certificates without a waiver by passing
the visual acuity standards for the class of medical
certificate applied. Airmen with a current medical cer-
tificate who have RK performed should discontinue
flying until an eye care specialist verifies that their vision
is stable and they have been released (72). Verification
can be obtained by submitting an eye evaluation report
(FAA Form 8500-7) completed by the surgeon who
performed the procedure or another eye care specialist
familiar with the applicant's ophthalmological history.
If no previous eye evaluation report (FAA Form 8500-
7) has been submitted by an airman with RK, one is
required at the time of certification renewal.


Technical Report Documentation Page

1. Report No. ‘2. Government Accession No. 3. Recipients Catalog No.

DOT/FAA/AM-98/25

@ Tile and Subiitle &. Report Date
The Aeromedical Certification of Photorefractive September 1998
Keratectomy in Civil Aviation: A Reference Guide 6. Performing Organization Code

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

Nakagawara, V.B., and Wood, K.J.

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, D.C. 20591 /4. Sponsoring Agency Code

15. Supplemental Notes

This work was performed under task AM-B-98-TOX-203

16. Abstract

The use of surgery to correct refractive errors continues to evolve at a significant pace. Radial keratotomy (RK), the first
widely accepted refractive surgical procedure, involves making radial incisions on the peripheral cornea. These incisions
weaken the cornea and allow intraocular pressure to push the peripheral cornea out, flattening the apex and reducing
refractive power. There are many disadvantages with RK that raise concerns regarding its use in the aviation
environment. These include: progressive hyperopic shifts, reduced corneal strength, fluctuation of vision, glare, poor
refractive predictability and altitude-induced corneal changes. In October 1995, the Food and Drug Administration
approved the use of the excimer laser to perform photorefractive keratectomy (PRK) to reshape the anterior curvature of
the cornea. Since that time, PRK has become the refractive surgical procedure of choice. It has been reported that for
low to moderate levels of myopia there is greater predictability, no fluctuation of vision or reduction in corneal strength,
and about 85% of patients have uncorrected visual acuity of 20/40 or better. As with RK, there are aspects of PRK that
raise concerns about its use in the aviation environment. Some of these include: night vision problems (e.g., glare, halos
around lights, haze, starbursts, and dim lighting difficulties), reduced contrast sensitivity, stability of refraction, reduced
best-corrected visual acuity, and induced anisometropia. Using a mathematical model, it was estimated that by the year
2000 there may be over 1,200 civil airmen who elect to have PRK performed. To provide the aeromedical community
with information to formulate administrative decisions and policies associated with this new refractive surgical
procedure, this paper reviews the results of clinical trials on PRK and discusses its applicability in aviation.

17. Key Words 48. Distribution Statement
Aviation Vision; Refractive Surgery; Radial Document is available to the public through the
Keratotomy (RK); Photorefractive Keratectomy (PRK) 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 49

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



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28

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122.

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c) Painhasbeena major reason patients chose
to delay PRK or have RK to correct the second eye (239).
Individual perception of pain is highly variable, and
an objective analysis of pain may be complicated by
exposure to light, patching or non-patching, and prox-
imity of surgery to sleep periods (240).

* Corneal ulceration
a) Corneal ulcerations have been reported in
the early post-operative PRK period (168,210) which
may be related to the use of disposable soft contact lenses
to bandage the ablated cornea immediately after sur-

gery.

* Corneal sensitivity

a) Excimer laser PRK may sever corneal nerve
endings, resulting in transient corneal hyposensitivity.
Reduced corneal sensitivity can alter the corneal metabolic
process (114). After PRK decreased central corneal sensitiv-
ity has been measured, while the peripheral cornea (outside
the ablation zone) remained unaffected (241).

b) The corneal sensitivity recovery time for
low to moderate myopic patients is normally 4 to 6
weeks (242). However, at 3 months, 86% of highly
myopic patients reportedly still demonstrated reduced
sensitivity (100).

* Sterile corneal infiltrates

a) Sterile infiltrates normally develop during
the first several days after surgery (120).

b) Treatment with topical non-steroidal anti-
inflammatory agents and the use of therapeutic soft
contact lens may be predisposing factors for sterile
infiltrates. Lesions will typically disappear after 2 weeks
of corticosteroid treatment (100,243).

© Regression

a) Seldom reported after 3 months in low to
moderate myopia (114), myopic regression occurs more
frequently in higher corrections (118, 128,146,153,
171,196,201,244). Regression is associated with the
discontinued use of topical steroids (113), epithelial
hyperplasia, and redeposition of new collagen (196).
There also has been some thought that regression might
be caused by the slow distortion of existing tissue rather
than the growth of new tissue (206). There have been
reports that it may be exacerbated by exposure to UV
radiation (e.g., bright sunlight, and arc welding) after
treatment (95,245). Ithas been recommended thatall PRK

16

patients wear eye protection from UV (e.g., sunglasses that
block 100% UV tays, welding goggles) during post-
operative recovery (245).

b) Myopic regression may be influenced by
age, corneal steepness and hydration (88), and wound
healing response (246). It has occurred, in some rare
cases, up to 18 months after surgery (204,247).

c) Successful outcome of the first eye does not
guarantee the same result for second eye, as eyes may
heal differently (95).

d) Regression is a major cause of dissatisfac-
tion after PRK (171,172,176). Reoperation may be
required; however, an extended waiting period of 6 to
12 months is recommended (122).

* Over/Under correction

a) Normally, over correction and subsequent
regression occurs during the first 3 months after surgery
(117,125,165,171), which may interfere witha patient’s
ability to work (202). Refraction may remain unstable
for up to 1 year (153,164, 213,248). Over corrections
(196) also occur more frequently in patients 40 years or
older (164), steroid responders (164), multi-zone PRK
procedures (215), and with the use of non-steroidal anti-
inflammatory agents and bandage contact lenses (165).

b) There is increased risk of under correction
with higher refractive errors (102,105,148,173,
213,249).

c) The reoperation (enhancement) rate with
PRK has been reported to be 5-8% (compared to RK at
10-40%) (176,250).

* Corticosteroids

a) Corticosteroid treatment has been used to
treat post-operative pain, photophobia (117), corneal
haze (114,251), and to reduce myopic regression after
PRK (251).

b) The ocular response to steroids suggests
ongoing corneal healing (176). At 1 year post-operative,
there is reportedly no statistical difference between PRK
patients with < 6.00 D of myopia who received corticos-
teroids and those who did not (171,252). However,
Seiler et al. (101) reported that discontinuation of
corticosteroids caused an increase in corneal haze (145)
and a myopic shift of 0.50 to 1.00 D within days,
combined with a subsequent drift of 0.25 to 0.50 D over
1 month (103). (Note: Emmetropiais normally restored
by reinitiating corticosteroids within the first weeks
after myopic regression (204).) In patients who develop


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.


C. SURGICAL PROCEDURES INVOLVING Complications: Over and under correction

THE CORNEA (7,10,331), corneal perforation during lathing (7,10),
epithelialization of the interface (7,10), irregular astig-
KERATOMILEUSIS matism (7,10,11,12,13,330,331), loss of BCVA (330),

Introduced by Barraquer in the mid-1960s (7,328), infection, corneal opacities (9), healing problems (14),
over 10,000 procedures on myopes have been per- glare (11,13), foreign bodies in interface (13), and
formed in Colombia (329). Introduced in the United endothelial cell loss (13).

States in 1979 (328), nearly 2,000 procedures have been

performed in the United States (329). The procedure is Advantages: It can correct very high myopic (i.e.,
unique in that it was the first time a part of the body was 16.0 - 18.0 D (8)) and hyperopic refractive errors
removed, modified, and returned to its original location (7,328) even with large anisometropia (8).

(9). Itconsists of removing the anterior two-thirds of the

cornea (lamellar keratectomy). The excised corneal tis- Disadvantages: Complexity of procedure, sophisti-
sue is then frozen and cryolathed on the stromal side cated equipment (7), cost (12), long post-surgical care
according toa computer program that has calculated the (9), reduced quality of vision (9), unpredictability (13),
corneal curvature for the new corneal power, and the and disk displacement (332).

lathed autograft is then resutured to the corneal bed
(7,8) (see Figure 9). There are currently studies under-
way using a nonfreeze (thought to avoid corneal damage
due to freezing (12)), nonsuture automated lamellar
keratectomy (ALK) (11,330).

KERATOMILEUSIS

Lamellar Keratectomy

Lenticule
ae ae,

Corneal Profile

Myopic Keratomileusis Hyperopic Keratomileusis
Myopic Lenticule results Hyperopic Lenticule results
in corneal flattening in corneal steepening

FIGURE9

A-2


10.

ll.

12.

13.

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LASER-ASSISTED IN SITU KERATOMILEUSIS (LASIK)

'| Laser Pulses,

Hinge

Corneal Flap |.

Ablation Zone

Intrastromal Tissue

FIGURE 7

Recently, the Ophthalmic Devices Panel of the
FDA’s Medical Devices Advisory Committee recom-
mended the agency grant its first approval for the
marketing of LASIK for the correction of myopia from
1D to 15 D (315). Currently, surgeons can recommend
and perform LASIK on a patient, if in their medical
judgment, it is the best procedure to correct their
particular refractive error. However, surgeons cannot
actively market or promote the LASIK procedure (316).
It is anticipated that more surgeons will be perform-
ing LASIK in the future as they obtain the extensive
surgical skill.

Second generation excimer lasers are being used
in foreign countries and include delivery methods
that allow scanning ablation, multi-zone/multi-pass
techniques, and increased beam diameters. Scanning
lasers may produce more uniform ablations with fewer
central islands. Multi-zone/multi-pass laser techniques
are being used that may result in better healing and faster
recovery. There have been more reported complications
(e.g., halos) associated with smaller zones, so many
foreign countries are now using a 6.5-mm ablation
zone. Second generation lasers in U.S. clinical trials

19

include head and eye auto tracking, which may improve
predictability. Solid-state, third generation lasers may
allow ablations at 213-nm, rather than 193-nm used by
the excimer laser (317).

V. AEROMEDICAL ISSUES

A. CERTIFICATION OF PILOT APPLICANTS

Airmen with PRK have applied for and received
FAA medical certificates prior to the October 1995
FDA approval of the Summit excimer laser. Patients
went to foreign countries, primarily Canadaand Mexico,
for this procedure. Before FDA approval, it was esti-
mated that about 60% of the PRK patients in Canadian
clinics were U.S. citizens.

* Applicants with PRK and other refractive
surgical procedures are being medically certified with-
out a waiver.

* PRK applicants are assigned a FAA-specific
eye pathology code (130) with a surgery prefix (5) on
their medical records to identify them as having refrac-
tive surgical procedures.


prevalence of myopiaas average family income rises. For
the total U.S. population, the rates of myopia increased
from 18% to 36% as family income increased (5).

B. HYPEROPIA

Hyperopia, or farsightedness, isa condition in which
parallel rays of light from an object being viewed come
to focus at a point behind the retina (see Figure 1). The
lens of the eye lacks sufficient converging power. Usu-
ally, the chief cause of hyperopia is a shortening of the
anteroposterior axis of the eye (axial hyperopia) (i.e., eye
is smaller than normal). Another cause of hyperopia is
when the front surface of the cornea or lens has less
curvature than normal (curvature hyperopia). At birth,
most eyes are hyperopic (about 2.00 or 3.00 D). With
aging, the eye normally lengthens. (Note: Each mm that
the eye is too short is equal to 3.00 D of hyperopic
refractive change (4).) Statistics are vague on the preva-
lence of hyperopia, since many studies incorrectly incor-
porate presbyopia, which also requires plus power lenses,
as part of the total percentage of hyperopia in the
population.

C. ASTIGMATISM

Astigmatism, or blurred vision at all distances, is a
condition in which rays of light from an object being
viewed are not refracted equally in all directions, so that
a point focus on the retina is not attained. The most

common type is regular astigmatism, which can be
subdivided into: simple, one focal point always falls on
the retina while the other focal point is behind (simple
hyperopic) or in front (simple myopic) of the retina (see
Figure 2); compound, light rays are refracted so that
both focal points lie either behind (compound hyper-
opic) or in front (compound myopic) of the retina (see
Figure 3); and, mixed, one focal point lies behind and
the other focal point lies in front of the retina (see Figure
3). Astigmatism is normally the result of the radius of
curvature of the cornea not being equal in all directions.
Although at birth the cornea is usually spherical, by 4
years of age the cornea shape changes. With-the-rule
astigmatism occurs as the horizontal axis of the cor-
nea steepens with age (horizontal light rays refracted
more sharply than vertical rays) (4). When the verti-
cal axis of the cornea is refracted more steeply than
the horizontal axis, the result is against-the-rule astig-
matism. Irregular astigmatism most often occurs if
the cornea has been damaged by trauma, inflamma-
tion, scar tissue, or developmental anomalies. This
cannot be completely corrected by ophthalmic spec-
tacle lenses, due to the irregular corneal surface and
the lack of any geometric form. Approximately 63.2%
of the population has astigmatism; of those, 59.8%
have astigmatism < 3.00 D (48% < 1.00 D, 8.8%
1.10 D - 2.00 D, 3% 2.10 D - 3.00 D) and 3.4% have
astigmatism > 3.00 D (6).

SIMPLE HYPEROPIC
ASTIGMATISM

Light Rays

Light Rays

SIMPLE MYOPIC
ASTIGMATISM

FIGURE 2


KERATOPHAKIA

Introduced by Barraquer in 1980 (7), it consists of
adonor corneal tissue to correct aphakic refractive errors
by placing an intralamellar plus-powered lenticule in
the host cornea. A microkeratome is used to make a
lamellar section in the host cornea. The pre-lathed
donor lenticule is then centered in the intralamellar
space, and the disc is resutured over the lenticule (7) (see
Figure 10).

Complications: Induced astigmatism and loss of
best-corrected acuity (7).

Advantages: Corrections of > + 10.0 D (7) are
possible.

Disadvantages: Limited supply of donor corneas
and synthetic materials not being well tolerated (7,14).

EPIKERATOPHAKIA

Developed by Kauffman at Louisiana State Univer-
sity, it was originally based on the classic Barraquer
procedures of keratophakia and keratomileuses, but it is
simpler and safer (7). The procedure consists of lyo-
philized (freeze-dried) lenticules pre-lathed to correct a
particular refractive error. A small epithelial defect is
made in the peripheral cornea and a shallow annular
keratotomy is performed. The lenticule is rehydrated
and the edge of the lens is sutured into the annular
keratotomy. After the surgery, the peripheral host epi-
thelium slowly covers the anterior surface of the donor
lens (7) (see Figure 11).

Complications: Traumatic separation of the graft
in the early healing period, neovascularization, chronic
epithelial defects, deposits at the cornea interface, infec-
tion (7), and reduced contrast sensitivity (15).

KERATOPHAKIA

Lamellar Keratectomy

< ~

Plus-powered Lenticule

EPIKERATOPHAKIA

Donor Lenticule

—

Cornea

Small Epithelial Defects

FIGURE 10

A-3

FIGURE 11


INTRASTROMAL CORNEAL RING

A plastic piece is implanted in the mid-peripheral
stromal channel, through asmall corneal incision, avoid-
ing direct surgical intervention in the central cornea (see
Figure 13). Changes in corneal curvature are achieved
by varying the thickness of the ring, with increased
corneal flattening resulting from increased thickness
(18). This procedure is under investigation by the FDA.

Complications: Transient elevated IOP due to cor-
ticosteroid use and peripheral haze in the intrastromal
channel (18).

Advantages: The procedure is reversible by remov-
ing the ring, which results in minimal visual effects since
it lies outside the central corneal zone (18,19).

Disadvantages: No known disadvantages are re-
ported, but risks common to corneal surgery exist.

INTRASTROMAL CORNEAL RING INSERTION

FIGURE 13

‘WUS. GOVERNMENT PRINTING OFFICE: 1998 - 660-088/80010


RADIAL KERATOTOMY (RK)

Iris Radial

Side View

incisions

Front View

FIGURE 4

reducing its refractive power (8). There are three surgical
variables: 1) diameter of the clear zone (smaller zone =
greater flattening), 2) number of incisions (4 incisions
are equivalent to 75% of the effect achieved with 8
incisions), and 3) depth of incisions (deeper = greater
flattening) (21,22). There is a greater effect from RK
with age (.75 - 1.00 D/decade of life) (21,22). RK
candidates should have stable non-progressive myopia,
normal corneas, and be free of systemic disease that
might influence corneal healing (21).

B. COMPLICATIONS
1. Anesthesia.
* Optic nerve damage
* Perforation of the globe
(Note: These complications have been reduced with
the use of topical anesthesia (8,23)).

2. Operative.
* Corneal perforations (8,24,25)
* Decentered clear zone (21,26,27)
* Incisions across the visual axis
* Monocular diplopia (26,28)

3. Intraocular and eyelid complications.

* Endophthalmitis (8,25)

* Cataract (8,25)

* Traumatic rupture of keratotomy scars (29,30,
31,32,33,34)

* Ptosis (25)

* Inferior rectus palsy (25) (Note: This complica-
tion has been reduced with the use of topical anesthesia.)

* Retrobulbar hemorrhage (25) (Note: Thiscom-
plication has been reduced with the use of topical
anesthesia.)

© Iritis (25)

* Traumatic hyphema (8)

¢ Epithelial implantation resulting in cysts (8,35)

4. Corneal.
* Bacterial keratitis (36,37,38,39,40,41,42)
* Herpes keratitis (25) (Note: Reactivation is
normally due to steroid use.)
* Stromal keratitis (25)
+ Fungal keratitis (43)
* Epithelial erosions (8,25)
¢ Epithelial ingrowth (8)
+ Endothelial cell loss (8,44)
+ Delayed wound healing (25,27,29,45)