Photorefractive keratectomy for residual myopia after radial keratotomy

Maurice E. John, MD, Eduardo Martines, MD, Tadeu Cvintal, MD

ABSTRACT

Purpose: To evaluate the results of photorefractive keratectomy (PRI<) to treat un­dercorrected radial keratotomy (RI<).

Setting: Instituto de Oftalmologia Tadeu Cvintal, Sao Paulo, Brazil.

Methods: A consecutive series of 28 eyes that had PRK to treat residual myopia after RK were studied. Refractive visual and safety data were collected and evaluated.

Results: One year after PRK, 75% of eyes had an uncorrected visual acuity of 20/25 or better and 85%, 20/40 or better. All but one case maintained or improved best corrected visual acuity; one case decreased from 20/25 to 20/30. At 1 year, 75% of eyes were within 0.50 diopter (0) of emmetropia and 90% were within 1.00 O. Only one case was more than 1.00 0 undercorrected (-1.125 0) at 1 year. Mean pre-RK myopia was -5.900 (range -2.00 to 11.80 0). Mean spherical equivalent improved from the residual postoperative level of -2.71 0 ± 0.86 (SO) before PRK to -0.21 ± 0.8601 to 3 months after PRK and to -0.40 ± 0.43 0 1 year after PRK.

Conclusion: Photorefractive keratectomy was efficacious in correcting residual my­opia after RK in a group of selected patients. J Cataract Refract Surg 1996; 22:901-905

A s a surgical procedure for correcting myopia, radial

keratotomy (RK) has been associated with signifi­

cant rates of undercorrection. For example, among the

385 patients in the benchmark PERK study,l an average

of28% (56% in the baseline subgroup with the highest

degree of myopia) remained myopic by more than

1.00 diopter (0) 4 years after surgery. Although reop­

eration was discouraged by protocol, by 4 years about

14% of cases had had enhancing procedures.

Current RK strategies usually emphasize a cautious

approach, avoiding overcorrection and titrating the pri-

From the John-Kenyon Eye Research Foundation, Jeffersonville, Indiana (John), and Instituto de Oftalmologia, Silo Paulo, Brazil (Martines, Cvintal).

mary surgical procedure with enhancing surgery. In a survey, members of the American Society of Cataract and Refractive Surgery reported enhancement rates of

19 to 29%.2 Most of the respondents had trained in the Casebeer system of RK. A study by Werblin and Staf­ford3 of outcomes using this system documented an en­hancement rate of 33%. Ultimately, 99% of the cases

analyzed (excluding amblyopes, those awaiting enhance­

ment, monovision targeted eyes, and vision the authors

deemed as discrepant with refraction) achieved an un­

corrected visual acuity of 20/40 or better following one

or more enhancements. Enhancements were necessary

in low and moderate as well as in higher myopes.

Reprint requests to Maurice E. John, MD, John-Kenyon Eye Center, 1305 Wall Street, Jeffersonville, Indiana 47130.

If undercorrection is the result of shallow incisions or an overly large optical zone, RK enhancement to treat

residual myopia is usually effective. If incisions are max­

imally deep, however, additional RK operations are less

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PRK AFTER RK FOR RESIDUAL MYOPIA

effective. For patients who have already received a max­

imum amount of RK surgery, RK enhancement is not

an option.

Recently, excimer laser photorefractive keratectomy

(PRK) has been used as an alternative to RK for modi­

£)ring anterior corneal curvature.4-

9 Although the fol­

low-up on this procedure is not as long as for RK, the

large U.S. Food and Drug Administration-regulated

clinical studies of excimer laser PRK have produced

promising results. These successes have prompted

the use of PRK as a secondary procedure to correct 'd 1 . aft RK10 - 15 . resl ua myopia er or penetratmg

keratoplasty. 13, 16, 17

In the study reported here, we used PRK to treat a

cohort of patients who were mildly to severely undercor­

rected after RK.

Subjects and Methods Twenty-eight eyes of 24 patients had routine, un­

eventful RK at least 6 months before having a secondary

PRK procedure for residual myopia. Mean myopia be­

fore RK was -5.91 D (range -2.00 to -11.75 D).

Mean residual myopia after RK was -2.70 D (range

-1.00 to -6.25 D), an average 46% undercorrection of

the pre-RK myopia level. The PRK was done using a Summit Technology

excimer laser at a constant energy density of

180 mJ/cm2, a 10 Hz repetition rate, and a 5 mm treat­

ment zone. The number of pulses per eye used to achieve the target dioptric correction was determined using

Summit Technology software. Topical anesthesia was applied to the cornea several

times before the procedure. With the patient fixating on the internal fixation target in the laser, practice applica­

tions were applied to the epithelium. The corneal epi­

thelium was removed with a #69 Beaver blade

centripetal to the RK scars to avoid opening the scars.

After the procedure, the patient was examined with

a slitlamp before the speculum was removed. A topical

antibiotic/steroid ointment was instilled and a pressure

patch applied. Patients were supplied with neomycin,

polymyxin B sulfates, and dexamethasone (Maxitrol®)

solution and instructed to instill drops in the treated eye

four times a day for 4 weeks and then twice a day for

2 weeks.

Examinations were scheduled for 1 week, 1 and

3 months, and 1 year postoperatively. At all visits, visual

acuity, manifest refraction, keratometry, tonometry,

and slitlamp microscopy were performed. Postoperative

subepithelial corneal haze was graded on the following

scale: 0 = clear; 1 + = trace detectable on broad tangen­

tial illumination; 2 + = mild haze detectable with focal

illumination; 3+ = moderate haze partially obscuring

iris detail; 4+ = marked haze obscuring intraocular

structures.

Results Follow-up data were obtained for all 28 eyes 1 to

3 months after PRK and for 20 eyes (71 %) at 1 year.

Data on the 6 patients (8 eyes) who did not have a 1 year

follow-up are presented later in this section.

In the 1 year group, visual acuity improved in all

cases after PRK (Figure 1). At 1 year, 75% of patients

examined had an uncorrected visual acuity of 20/25 or

better and 85%, 20/40 or better (Figure 2). At the end of 1 year, all but one of these cases had

maintained or improved best corrected visual acuity

(Figure 3). In this case, corrected acuity decreased by

one Snellen line, from 20125 to 20/30, although uncor­

rected visual acuity improved from 20/400 to 20/70. The two cases in Figure 3 with a corrected visual acuity

of worse than 20/40 were amblyopic; however, PRKstill

improved their best corrected visual acuity. Twenty-six

percent of cases had improved best corrected visual acu-

20 25 30 40 50

Post 60 VAse 70 201 80

100 200 300 400 FC

(i) (i) 0 o 0 0 0 0 0 o ~/ 0 0

0 Better ~/ //

/

0

0

Worse

FC 400 300 200 100 80 70 60 50 40 30 25 20

Pre-PRK VAse 201

Figure 1. (John) Scatterplot of pre-PRK versus 1 year post-PRK

uncorrected visual acuities. The solid diagonal line represents no change in visual acuity. Points falling above the diagonal lines

show improvements in uncorrected visual acuity. There were no

decreases in uncorrected acuity.

902 J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996

PRK AFTER RK FOR RESIDUAL MYOPIA

20/30-40

20/50- 100

Figure 2. (John) Uncorrected visual acuity distribution (20 pa­tients) 1 year after PRK.

Post VAcc 201

20 25 30 40 50 60 70 80

100 200 300 400 FC

FC 400 300 200 100 80 70 60 50 40 30 25 20 Pre-PRK VAcc 201

Figure 3. (John) Scatterplot of pre-PRK versus 1 year post-PRK best corrected visual acuities. Points within the two diagonal dashed lines represent cases remaining within ::':: 1 line of pre-PRK acuity. Points above the diagonal lines had improved best cor­rected acuity.

ity, and 85% had a best corrected visual acuity of

20/25 or better (Figure 4). One year after PRK, 50% of eyes (10/20) were clear,

and 35% (7/20) had trace haze; only three patients had

Figure 4. (John) Best corrected visual acuity distribution (20 patients) 1 year after PRK.

mild to moderate haze (Figure 5). In one eye with mod­

erate haze, the haze resolved with subsequent photo­

therapeutic keratectomy.

As indicated by the mean spherical equivalents, the

surgical correction of residual myopia by PRK was sub­

stantial in magnitude and stable over time. The mean

spherical equivalent was -2.71 ± 1.190 before PRK,

-0.21 ± 0.86 0 1 to 3 months after PRK, and

-0.40 ± 0.43 0 1 year after PRK. At 1 year, 75% of

eyes were within 0.50 0 of emmetropia and 90%, within 1.00 D. One eye was more than 1.000 under­

corrected (-1.125 D).

Eyes were divided into two groups according to ini­

tial pre-RK myopia: more than 6.00 0 or less than

6.00 D. Mean residual myopia after RK was higher

(- 3.70 versus -2.20 D) in the nine eyes with a pre-RK

myopia greater than 6.00 0 (Table 1). However, after

PRK, the mean spherical equivalent was -0.54 0,

which indicates that a greater enhancement could be

achieved among the higher myopes, resulting in an

equivalent final outcome.

The mean of the average keratometry measurements

in these 20 patients decreased as a result of PRK, from

41.00 ± 1.49 0 before PRK to 38.64 ± 1.50 0 1 to

3 months after PRK. By 1 year, the mean average kera­

tometry had increased slightly to 39.23 ± 1.55 D.

Significant data for the procedures performed on six

patients who were not followed for 1 year are summa­

rized in Table 2. Six of the eight eyes had post-PRK

results essentially equivalent to those in eyes evaluated at

1 year. These six eyes were within 0.75 0 of emmetropia

at the final visit and had only trace to mild levels of haze.

Uncorrected visual acuity improved substantially, and

Trace (3 Cases) Mild-Mod

Figure 5. (John) Distribution of haze scores (20 patients) 1 year after PRK.

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PRK AFTER RK FOR RESIDUAL MYOPIA

Table 1. Change in spherical equivalent by pre-RK myopia level.*

Pre-RK Myopia s6.oo 0 (n = 16) Pre-RK Myopia >6.00 0 (n = 9)

Mean Range Percentage

Within 1.00 ot Mean Range Percentage

Within 1.00 ot

Spherical equivalent (0) Pre-RK Pre-PRK Final post-PRK

-4.54 -2.21 -0.45

*In three cases, pre-RK myopia unknown tOf emmetropia *Two cases

-2.00 to -5.75 -3.50 to -1.00 -2.10 to +0.50

Table 2. Final visit results in eyes with less than 1 year follow-up.

-8.33 -11.80 to -6.30 -3.70 -6.30 to -1.80 -0.54 -1.10to +0.20

o o

89

Preoperative Postoperative Months

Patient Postop Haze Score SE (0) UCVA BCVA SE (0) UCVA BCVA

1* 4 -2.000 20/100 20/20 0.125 20/30 20/20 1t 4 -2.250 20/400 20/20 -0.500 20/25 20/20 2 -2.250 20/400 20/20 0.250 20/25 20/20 3 4 -5.625 CF 20/25 -0.250 20/25 20/25 4 3 2

1 2 2

-2.750 20/400 20/30 -0.250 20/70 20/25 5 8 -3.500 CF 20/30 -0.750 20/70 20/25 6* 8 -1.250 20/30 20/20 -2.125 20/50 20/40 6t 4 -2.125 20/60 20/25 -1.125 20/70 20/40

SE = spherical equivalent; UCVA = uncorrected visual acuity; BCVA = best corrected visual acuity; CF = finger counting *First eye tSecond eye

best corrected visual acuity was maintained within one line of the preoperative values in the six eyes.

One patient lost to follow-up had bilateral PRK (patient 6, Table 1). Eight months after PRK, the first operated eye had a postoperative refractive astigmatism of more than 2.00 D (refraction -1.00 -2.25 X 60), which was four times the preoperative refraction (-1.00 -0.50 X 120). Uncorrected visual acuitywors­ened as a result of the myopic astigmatism. The second operated eye was left with a spherical equivalent of -1.125 D at the last visit (4 months after surgery), which was less than 50% of the desired correction. Un­

corrected visual acuity decreased from 20/60 to 20/70 and best corrected visual acuity, from 20/25 to 20/40. The patient moved and was lost to follow-up.

Discussion Although many undercorrected RK patients have

RK enhancements with good results, additional inci­sional procedures may not be indicated in many cases.

Patients who had high preoperative myopia who have already received the maximum amount ofRK surgery or

patients who did not respond well to RK may benefit from a different surgical approach to reduce their resid­

ual myopia. This results of this study of a limited number of

cases receiving PRK to treat residual myopia after RK shows significant promise. The results certainly support the general efficacy of the procedure. Residual myopia after RK was predictably and substantially reduced by the secondary PRK procedure regardless of the amount of myopia after RK.

Overall, 26 of 28 eyes (93%) had significant in­

creases in visual acuity and significant decreases in my­opia. Although 8 of those eyes involved 6 patients who

did not have a 1 year follow-up examination, results from the other 20 eyes showed no significant changes

between 1 to 3 months and 1 year postoperatively. Thus, it is unlikely that any significant variations would have been observed in these 8 eyes had they been examined at 1 year. The patient with bilateral PRK over RK (patient

904 J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996

PRK AFTER RK FOR RESIDUAL MYOPIA

6, Table 2) did not achieve good results because of the

high level of induced astigmatism. Minimal haze was noted after the secondary PRK

procedure in these patients. At 1 year, 85% had no or trace levels of haze.

The results of this study corroborate the good re­sults reported by Durrie and coauthors l4 in a series of 96 eyes receiving PRK to correct residual myopia after RK. In that study, only a third of the cases were followed for 1 year, but visual results were similar. Other re-

10-13 15 b dIe . hI' . d ports ' were ase on on y a few cases WIt Imlte

follow-up, but results were also encouraging. Gimbel and coauthors reported on a group of 15 undercorrected RK cases treated with PRK ("Enhancement Excimer La­

ser Treatment After Previous PRK or RK," presented at the 3rd American-International Congress on Cataract, IOL and Refractive Surgery, Seattle, Washington, May

1993). Although visual acuity improved in all eyes, the secondary PRK was less predictable than the results re­

ported here. General predictability appears to be well supported

by the results from this study. At the last visit, 71 % of all cases were within 0.50 D of emmetropia and 86%, within 1.00 D. There were four overcorrections of less

than 0.50 D. Based on our experience, the postoperative course of

PRK after RK seems to be similar to that of primary PRK. Overall, our results suggest that PRK is a promis­ing option for selected patients with residual myopia after RK.

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