PROSPECTIVE, RANDOMIZED COMPARISON OF SIMULTANEOUS AND SEQUENTIAL BILATERAL LASIK FOR THE CORRECTION OF MYOPIA* BY G. 0. Waring III, MD, FACS, FRCOPHTH, J. D. Carr, MD, MA, FRCOPHTH, R. D. Stulting, MD, PHD, AND K P. Thompson, MD ABSTRACT Purpose: To study the safety and efficacy of simultaneous bilateral laser in-situ keratomileusis (LASIK) Methods: Data were obtained from 254 consecutive patients that were randomized to simultaneous or sequential bilateral LASIK. Results: 146 patients were enrolled in the simultaneous group and 108 patients were enrolled in the sequential group. Mean follow-up was 10 months (range 6-18). There was no significant difference in intraoperative complication rate (p=0.34), loss of two or more lines of spectacle corrected visual acuity (p=0.9), or percentage of eyes within +/- 0.50D of intended (p=0.63) between simultaneous and sequential groups. Conclusion: The risk of performing bilateral simultaneous LASIK is not significantly different from that of sequential treatments. INTRODUCTION Simultaneous bilateral refractive surgical procedures are being performed increasingly frequently because the risk of complications is considered to be low and the risk of simultaneous complications in both eyes is even lower. There are no prospective studies and few case reports of the complications of bilateral simultaneous refractive surgery.'4 We therefore performed a prospective, randomized comparison of simultaneous and sequential bilateral laser in situ keratomileusis (LASIK) in order to study the safety and efficacy of these two treatment options. 'From the Department of Ophthalmology, Emory University School of Medicine, Atlanta. TR. AM. OPHTH. SOC. VOL. XCV, 1997
272 Waring et al PATIENTS AND METHODS In May 1995, a physician-sponsored investigational device exemption was conditionally approved by the US Food and Drug Administration (FDA) at the Emory Vision Correction Center in Atlanta for a study of LASIK utilizing the Summit OmniMed Excimer Laser (Summit Technology, Waltham, Mass) and the Chiron Automated Corneal Shaper (Chiron Vision Corp, Irvine, Calif). The study was based on experience with LASIK obtained by Waring and colleagues at the El-Maghraby Eye Hospital in Jeddah, Saudi Arabia.5 Among the objectives of the LASIK study were evaluation of the safety and efficacy of LASIK in treating myopia and comparison of the results of bilateral simultaneous versus sequential surgery (2 weeks apart) for patients who desired to have both eyes treated at the same time. We present here the results of the prospective, randomized comparison of simultaneous versus sequential bilateral LASIK. Patients were selected from the clinical practices of the 14 participating surgeons. Patients desiring refractive surgery were offered LASIK as an alternative to traditional surgical procedures for the management of their myopia. Three surgeons (G.O.W, R.D.S., and K.P.T.) had previous experience with lamellar corneal surgery. The 11 surgeons with no prior lamellar surgical experience received extensive didactic and practical instruction prior to and after commencement of the study. Inclusion criteria for the study were myopia between -2.00 and -30.00 D with less than 4.00 D of refractive astigmatism, stable refraction, no prior ocular surgery, normal corneal topography, normal ocular examination, and no evidence of ocular hypertension or glaucoma. Patients were 18 years of age or older and had no history of systemic collagen vascular disease; pregnancy and or use of systemic corticosteroids were contraindications to study enrollment. Soft contact lenses were removed at least 3 days, and hard contact lenses at least 2 weeks, before preoperative measurements were obtained. Patients with abnormal videokeratography were excluded unless prolonged cessation of contact lens wear restored corneal topography to normal. Patients requesting bilateral simultaneous surgery (about two thirds of patients) were randomly assigned to receive surgery either during the same surgical session (simultaneously) or sequentially at least 2 weeks apart. A random number table was used for treatment assignment. Patients who requested sequential surgery were not randomized and are not reported here. Patients satisfying the inclusion criteria for entry into the study received a full ophthalmic examination, which included manifest and cycloplegic refraction, distance and near visual acuity (with and without
Comparison of Simultaneous and Sequential LASIKfor Myopia 273 correction), measurement of pupil diameter at 300 lux, slit-lamp microscopy of the anterior segment, measurement of intraocular pressure, computerized videokeratography, contrast sensitivity measurement, and evaluation of ocular dominance. Informed consent, approved by the Emory University Human Investigations Committee, was obtained prior to surgery. The preoperative consensus refraction was determined by the surgeon on the basis of 2 manifest refractions performed on 2 different days and 1 cycloplegic refraction. This consensus refraction was used to determine the laser setting by using a treatment nomogram based on results from a pilot series of 25 eyes followed for 2 weeks at the Emory Vision Correction Center. The Summit OmniMed/Apex excimer laser (Summit Technology, Waltham, Mass) was calibrated and operated as described in the user manual. At the beginning of each operating day, the laser was calibrated by performing an ablation through a 100-pm-thick piece of wratten gel filter (Eastman Kodak Co, Rochester, NY). If the filter perforated after 530 to 630 pulses and the breakthrough pattern met specifications, the laser was considered calibrated. Two photorefractive keratectomy (PRK) ablation algorithms were programmed into the laser's computer: a single-zone (6.0- mm) pattern and a 5.5-, 6.0-, and 6.5-mm single pass, multizone pattern. The multizone ablation delivered 100% of the laser setting to the central 5.0 mm, 70% of the laser setting to the zone between 5.0 mm and 6.0 mm, and 30% of the laser setting to the zone between 6.0 and 6.5 mm. The Summit OmniMed/Apex laser allowed a maximum ablation of 700 pulses to prevent excessive depth of ablation in the central cornea. Thus, for desired corrections above -18.1 D, the diameters of the three zones were reduced in order to avoid delivery of more than 700 pulses to the central cornea. The corneal flap was approximately 160 pm thick, and a 700-pulse laser ablation should produce a central ablation depth of about 175 pm, assuming an ablation rate per pulse of 0.25 pm. Thus, the maximum ablation depth in this study was approximately 335 pm, leaving approximately 215 pm of residual central cornea (assuming a central corneal thickness of 550 pm). The Chiron Automated Corneal Shaper (Chiron Vision Corp, Irvine, Calif) was used to perform lamellar keratotomy using a flat, nonadjustable suction ring. The microkeratome was cleaned and assembled by technical personnel and tested by the surgeon prior to each operation. Accuracy of the desired correction was confirmed by the laser operator and the surgeon.
274 Waring et al SURGICAL TECHNIQUE Primary Technique Sedatives were not routinely used preoperatively. The operative eye(s) were prepped and draped, and topical anesthesia was achieved using several drops of 1.0% proparacaine. The cornea was focused and centered under the excimer laser with the aid of the two helium-neon laser beams. Fiduciary marks were made with a refractive keratotomy marker stained with methylene blue dye. The suction ring was positioned on the cornea with slight nasal decentration; suction was maintained at or above 28 inches of mercury by the Chiron System suction pump. An applanation lens verified adequate intraocular pressure of approximately 65 mm Hg or greater. Several drops of balanced salt solution were placed on the cornea. The microkeratome head was engaged into the suction ring tracks and slid forward until the gear on the microkeratome head encountered the unobstructed track of the suction ring, after which the microkeratome was activated. The translation stopped when the stopper on the microkeratome head encountered the suction ring. The microkeratome was reversed and removed with the suction ring. The flap was reflected onto the conjunctival surface using either smooth forceps or an iris sweep. Visible fluid was removed from the stromal bed with a merocel sponge before laser ablation. The two helium-neon laser aiming beams were focused on the corneal bed and centered. The patient was then asked to fixate the blinking green light, which was coaxial with the excimer laser, and fixation was monitored by the surgeon. The laser was activated. Laser treatment was interrupted if loss of fixation occurred and resumed when the patient reestablished steady fixation. Eyes with more than 1.0 D of astigmatism received arcuate transverse keratotomy (Arc-T), performed either outside the corneal flap (8.5-mm zone) or beneath the corneal flap (6.0- or 7.0-mm zone) according to a treatment nomogram. Following the ablation (and Arc-T), the flap was rolled onto the bed using the end of an irrigation cannula or an iris sweep and dried in position for approximately 5 minutes without manipulation (dry roller technique). Before removal of the lid speculum, one drop of a combination preparation of tobramycin and dexamethasone (0.1%) and one drop of ketorolac were placed in the eye. The flap position was verified by slit-lamp microscopy approximately 10 minutes after surgery. The patient was discharged without a patch. For simultaneous bilateral procedures, the right eye was treated first. The lid speculum remained in position in the right eye while the surgeon performed LASIK on the left eye in the manner already described. For sequential procedures, the eye with the greater amount of myopia was treated first.
Comparison of Simultaneous and Sequential LASIKfor Myopia 275 Complications. An adverse reaction was defined as an unexpected event that could potentially reduce spectacle-corrected visual acuity. Postoperative Medications. No postoperative medications were used after discharge on the day of surgery. Patients were instructed to sleep with a metal shield over the eye and, on the first postoperative day, to begin placing one drop of a combination preparation containing tobramycin and dexamethasone (0.1%) in the operated eye 4 times daily for 1 week. Nonpreserved artificial tears were recommended for mild irritation. Postoperative Examinations. Postoperative examinations were done at 24 hours, 2 weeks, and 3, 6, and 12 months. On the first postoperative day, flap position, uncorrected visual acuity, and manifest refraction were recorded. At all other testing intervals, uncorrected visual acuity, as well as manifest and cycloplegic refractions with visual acuity, were recorded. Glare and contrast sensitivity were tested in selected subsets of patients; videokeratography was routinely performed at each examination except on that of the first postoperative day. Contrast sensitivity and videokeratography data are not reported here. Repeated LASIK Repeated LASIK was performed for the correction of residual myopia or astigmatism after the 3-month examination. The surgeon used a Sinskey hook to elevate the edge of the flap that had been created at the time of primary LASIK. The flap was folded back with blunt forceps or an iris sweep, exposing the stromal bed. The remainder of the procedure was the same as the primary LASIK procedure. Arc-T was done alone or in conjunction with LASIK for correction of residual astigmatism. Outcome Measures and Statistical Analysis The mean difference between desired and observed postoperative spherical equivalent refraction was calculated for both groups. The sample means were tested for statistical significance by unpaired t-test. The chisquared test was used to test for statistically significant differences between the simultaneous and the sequential groups by using the following outcome measures: the percentage of eyes with postoperative spherical equivalent refractions within +/- 1.00 D and +/- 0.50 D of intended, the percentage of eyes with uncorrected visual acuity of better than or equal to 20/40 and 20/20, the percentage of eyes losing two or more lines of spectacle-corrected visual acuity, and the percentage complication rate. P values were considered statistically significant when less than or equal to 0.05.
276 Waring et al RESULTS Five hundred fifty eight (558) eyes of 320 consecutive patients are the subject of this prospective, randomized comparison of simultaneous and sequential bilateral LASIK. Four simultaneously treated eyes and 6 sequentially treated eyes were excluded from the analysis of refractive outcome because of intraoperative flap complications that precluded laser ablation. All eyes that were excluded from refractive outcome analysis because of aborted laser procedures following intraoperative flap complications were included in an intent-to-treat analysis of procedure safety. Of the 548 eyes with more than 6-month postoperative data, there were 40 eyes (32 sequential; 8 simultaneous) without data from the fellow eye. Each of 31 sequentially treated patients had less than 6 months of data from one eye; the second eye of one sequentially treated patient had no laser procedure following an intraoperative flap complication. Eight simultaneously treated patients were excluded from analysis of refractive outcome because they were treated sequentially following flap complications during LASIK in the first eye. Therefore, the analysis of procedure efficacy was based upon 508 eyes of 254 patients. Two hundred ninety two (292) eyes of 146 patients were randomized to receive simultaneous bilateral LASIK, and 216 eyes of 108 patients were randomized to receive sequential bilateral LASIK. All primary surgical procedures were performed between June 30, 1995, and August 21, 1997. Patients were followed for 3 months before being considered for repeated LASIK and/or Arc-T. The mean age was 41 years (+/- 8.5 SD; range, 18 to 65) for the simultaneous group and 41 years (+/- 8.9 SD; range, 21 to 60) for the sequential group. Mean preoperative spherical equivalent manifest refraction was -7.2 D (+/- 3.0 SD; range, -2.6 to -21.75) for the simultaneous group and -7.0 D (+/- 2.8 SD; range, -2.1 to -16.8) for the sequential group. There was no statistically significant difference between the preoperative characteristics of simultaneous and sequential groups. One hundred eighty-two (62.3%) of 292 eyes in the simultaneous groups and 128 (59.3%) of 216 eyes in the sequential group achieved a postoperative spherical equivalent manifest refraction within 0.50D of intended outcome. Two hundred fifty-one (86.0 %) of 292 eyes in the simultaneous group and 179 (82.8 %) of 216 eyes in the sequential group achieved a postoperative spherical equivalent refraction within 1.00D of intended outcome (Table I). There was no statistically significant difference between the two groups for this outcome measure. For the simultaneous group, the mean refractive outcome was -0.07 D +/- 0.8 SD (range, - 3.00 to 3.25) with respect to the intended correc-
Comparison of Simultaneous and Sequential LASIKfor Myopia 277 tion. The mean refractive outcome for sequentially treated eyes was an undercorrection of 0.21 D +/- 0.8 SD (range, -4.00 to 2.75). This was a statistically significant difference (P=0.05; unpaired t-test). To clarify the reason for this significant difference in refractive outcome, a multiple linear regression analysis was performed. Test variables included eye, gender, Arc-T performed during LASIK, single or multizone ablation, occurrence of enhancement procedures, and protocol (simultaneous or sequential). The only significant variables that had an effect on the mean refractive outcome for simultaneous and sequential groups were arcuate transverse keratotomy performed at the time of LASIK and the use of either singlezone or multizone laser ablation patterns (Table II). The uncorrected visual acuity at the most recent visit after LASIK (including enhancements) for simultaneous and sequential groups was evaluated after exclusion of eyes with preoperative spectacle-corrected visual acuity worse than 20/20, and eyes intentionally undercorrected to produce monovision (Fig 1). Eyes that exited the study at 12 months postoperatively with residual myopia greater than one diopter, where the patient did not desire enhancement, were also excluded. Three hundred eighty-three eyes are the subject of this analysis of uncorrected visual acuity. Eighty-nine percent (n = 199) of simultaneously treated eyes and 92.5% (n = 148) of sequentially treated eyes achieved uncorrected visual acuities of better than or equal to 20/40 (P=0.37). Fifty-one percent (n = 113) of simultaneously treated eyes and 40% (n = 64) of sequentially treated eyes achieved uncorrected visual acuities of better than or equal to 20/20 (P=0.05; B = 0.7). TABLE I. SUMMARY OF OUTCOME MEASURES AND STATISTICAL SIGNIFICANCE AFTER BILATERAL SIMULTANEOUS VS SEQUENTIAL LASIK OUTCOME SIMULTANEOUS SEQUENTL LASIK STATISTICAL MEASURE IASIK SIGNIFICANCE UCVA>20/40 89.2% (n=199) 92.5% (n=148) P=0.37; 8=0.8 UCVA>20/20 50.7% (n=113) 39.5% (n=64) P=0.05; B=0.7 ±1.00 D 86.0% (n=251) 82.8% (n=179) P=0.55; B=0.9 ±0.50 D 62.3% (n=182) 59.8% (n=128) P=0.63; 1=0.9 Loss 2 lines 4.2% (n=13) 3.6% (n=9) P=0.9; 80.9 of spectaclecorrected acuity Intraoperative 4.2% (n=6) 2.7% (n=9) P=0.34; B=0.9 complications Postoperative 3.9% (n=18) 1.1% (n=4) P=0.02; B=0.6 complications
278 Waring et al TABLE II. MULTIPLE LINEAR REGRESSION ANALYSIS OF REFRACTIVE OUTCOME AFTER BILATERAL LASIK (SIMULTANEOUS OR SEQUENTIAL) SIGNICANT VARIABLES VARIABLE B SE B BETA T SIG T Arc-T.171297.071872.105001 2.383.0175 SZ/MZ.220787.072700.133796 3.037.0025 (Constant) -.339673.065919-5.153.0000 VARIABLES NOT IN THE EQUATION VARIABLE BETA IN PARTIAL MIN TOLER T SIG T Enhance -.040751 -.034845.711688 -.783.4341 D Eye -.009786 -.009915.992577 -.223.8239 Protocol.079801.080689.988604 1.817.0697 Gender -.025106 -.025412.990353 -.571.5685 Arc-T, arcuate transerve keratotomy; szlmz, single zone/multi-zone. 100 90 9 0,:.: _:... ii} ~~~~~~~~~~~~~~~~~ '~~~~~~ ; " ' 4.~... '. _ 80 70 0 0 0 0 0 0 0 %40 S _ 1 _ 80 02 7-X _~ Ra 0l 7E 0g 0 _ 0 gl2roua A A_ l_~ V... g ial _A Ag 10 _ E _ 2_FGURE E _ g _ to 0 C 0 i Cuuatv unorece vimsua ciydto simutaneous1 (SIM) an seunia SQ CD N CSC9 CDl CD CD 0 : ^ A lz~~~~~~~~correcled Ysual Acuit A A FIGURE 1 Cumulative uncorrected visual acuity data for simultaneous (SIM) and sequential (SEQ) groups.
Comparison of Simultaneous and Sequential LASIKfor Myopia 279 Thirteen of 306 simultaneously treated eyes and 9 of 252 sequentially treated eyes lost two or more lines of spectacle-corrected visual acuity (P = 0.9) (Fig 2). One simultaneously treated eye and 2 sequentially treated eyes lost 2 lines of spectacle-corrected visual acuity to worse than 20/40 (all 3 eyes saw 20/50). Figs 3 and 4 show intraoperative and postoperative complication data for simultaneous and sequential groups. The simultaneous group had 6 intraoperative flap complications in 460 procedures (1.3 %), compared with 9 flap complications in 372 procedures (2.4 %) for the sequential group (P = 0.34). There were 18 postoperative complications in the simultaneous group (3.9 %) compared with 4 postoperative complications (1.1 %) in the sequential group (P = 0.02). Fig 4 shows that the more frequent occurrence of epithelial ingrowth requiring flap revision in the simultaneous group accounted for this statistically significant difference. Three patients in the simultaneous group had bilateral complications, all of which were postoperative. In 1 patient a marked inflammatory response was seen in the stromal bed of both eyes at the 24-hour examination. The corneal flap was raised and the bed cultured; there was no growth of any organism. Both eyes were treated with topical antibiotics and corticosteroids and recovered without sequelae; the presumed cause was a toxic reaction to material that may have been present on the micro- 140 120 100 te 80 80 _ q 60 ; 40 20 139 0-5 -4-3 -2-1 0 1 2 3 4 5 Change In lines of Snellen visual acuity FIGURE 2 Change in lines of Snellen visual acuity for simultaneous (SIM) and sequential (SEQ) groups.
280 Waring et al 4 4 a3 a 0 2 E a S 0 Intraoperative flap complication FIGURE 3 Intraoperative flap complications for simultaneous (SIM) and sequential (SEQ) groups. 0 9 _ 10.;;5 _ S q-_ j.~~~~~~i " 00 lae FG 4_ Postoperative In o u Postoperative complication FIGURE 4 Postoperative flap complications for simultaneous (SIM) and sequential (SEQ) groups.
Comparison of Simultaneous and Sequential LASIKfor Myopia 281 keratome blade. At 3 months postoperatively, the right eye had uncorrected visual acuity of 20/30, correcting to 20/25 with -0.75D/0.25 at 50'; the left eye had uncorrected visual acuity of 20/25 correcting to 20/20 with -0.50/1.25 at 1750. The other 2 patients from the simultaneous group with bilateral complications had postoperative flap slippage, partial in 3 eyes and total dislocation in 1. All 4 flaps were repositioned on the first postoperative day. At 6 months postoperatively, the first patient had uncorrected visual acuity of 20/30 OD and 20/25 OS, each with a one-line loss of spectacle-corrected visual acuity. The other patient had uncorrected visual acuity of 20/40 OD and 20/20 OS; the right eye had a one-line loss of spectacle-corrected visual acuity. Ten eyes had no laser ablation because of intraoperative flap complications, 4 from the simultaneous group (cases 1 through 4) and 6 from the sequential group (cases 5 through 10) (Table III). Only 2 eyes from the simultaneous group lost two or more lines of spectacle-corrected visual acuity; 1 eye lost two lines to 20/25, and the other lost three lines to 20/50. TABLE III. SPECTACLE-CORRECTED VISUAL ACUITY DATA FOR TEN EYES THAT HAD INTRAOPERAIVE FLAP COMPLICATIONS AND NO LASER ABLATION CASE PREOPERATIVE FINAL CHANGE INTRAOPERATIVE FLAP SCVA SCVA IN COMPLICATION SCVA (LINES) SIMULTANEOUS EYES 1 20/20 20/20 0 Incomplete flap 2 20/20 20/20 0 Incomplete flap 3 20/16 20/25-2 Button-hole in flap 4 20/25 20/50-3 Free flap SEQUENTIAL EYES 5 20/20 20/16 +1 Free flap 6 20/20 20/20 0 Free flap 7 20/20 20/20 0 Bilaminar flap 8 20/20 20/16 +1 Incomplete flap 9 20/16 20/16 0 Incomplete flap 10 20/20 20/20 0 Incomplete flap SCVA, spectacle corrected visual acuity.
282 Waring et al DISCUSSION Complications are an inevitable feature of every ocular surgical procedure currently performed. In each case, the likelihood of complications is compared with the likely benefit of the particular surgical procedure. Ocular surgical procedures, such as cataract surgery, are typically performed on one eye at a time because of the possibility of potentially blinding bilateral ocular complications, such as bacterial endophthalmitis or intraocular hemorrhage, when both eyes are treated simultaneously. Refractive surgical procedures, such as refractive keratotomy, PRK, and LASIK, are elective procedures and therefore require a high standard of safety. We think our study is the first to compare the safety and efficacy of simultaneous and sequential surgery in a prospective, randomized manner. We found no clinically important differences between the two groups. There was no difference in the percentage of eyes achieving a postoperative spherical equivalent refraction within 0.50 D or 1.00 D of intended. Interestingly, the mean refractive outcome for simultaneous and sequential groups was statistically different, with the simultaneous group achieving a postoperative spherical equivalent refraction closer to intended by 0.14 D. To clarify this finding, a multiple linear regression analysis of the difference between intended and observed postoperative refraction showed that the difference between simultaneous and sequential groups could be explained by the presence of Arc-T procedures and the differences in proportions of single-zone and multizone ablations between the two groups (Table II). The effect of the protocol (simultaneous or sequential) was still of borderline significance (P = 0.07). There was no statistically significant difference between the percentage of simultaneous and sequential eyes with uncorrected visual acuity of better than or equal to 20/40. There was however, a significant difference in percentage of eyes with uncorrected visual acuity of better than or equal to 20/20, albeit with low statistical power (30%). If this difference is truly significant, there are two explanations for this finding. First, the mean refractive outcome for sequentially treated eyes was a slightly larger undercorrection compared with the simultaneous group. Second, there were more eyes in the sequential group with residual myopia. which did not undergo enhancement, between 3 and 12 months after the primary procedure. This residual myopia would decrease the number of eyes achieving 20/20 uncorrected visual acuity. The analysis of safety showed that there was no significant difference in the percentage of eyes losing two or more lines of spectacle-corrected visual acuity. There was no significant difference in the occurrence of intraoperative complications between the two groups. However, there were significantly more postoperative complications in the simultaneous
Comparison of Simultaneous and Sequential LASIKfor Myopia 283 group than in the sequential group, although the statistical power of this observation was low at 40 % (Fig 4). The main difference in postoperative complications between simultaneous and sequential groups was in the more frequent occurrence of epithelial ingrowth that required flap revision in the simultaneously treated eyes. One might hypothesize that the second treated eye (left eye in our study), in a bilateral simultaneous LASIK case, might be more susceptible to epithelial ingrowth beneath the flap because the microkeratome blade had already been used on the first eye. In our study, there was no predilection for the second of two simultaneously treated eyes to develop epithelial ingrowth. Moreover, a previous multivariate analysis of risk factors for epithelial ingrowth that required flap revision identified microperforation at the time of simultaneous Arc-T and postoperative epithelial defects as the only significant risk factors.6 The key issue in this comparison of simultaneous bilateral LASIK was the occurrence of bilateral complications. These developed in 3 patients postoperatively. The most significant was the patient with bilateral sterile toxic keratitis at 24 hours; cultures were negative and the patient recovered with use of topical antibiotics and corticosteroids with good visual outcome. The other 2 patients with bilateral complications had postoperative flap slippage, necessitating flap repositioning at 24 hours. The occurrence of postoperative flap slippage has become less frequent during the course of our LASIK study (data not shown), probably owing to improvement in the technique for replacing the corneal flap at the end of the procedure. In the early part of the study, it is possible that excessive irrigation of balanced salt solution beneath the corneal flap may have allowed greater flap edema and consequent poorer apposition of the flap edge to the underlying corneal bed, predisposing to flap slippage. One limitation of our study was that all enrolled eyes did not reach the 12-month postoperative interval. It is unlikely, however, that further follow-up would change the results of our study, because postoperative complications invariably occur during the initial few days to weeks. In addition, the postoperative spherical equivalent refraction does not fluctuate significantly after the third postoperative month.7 A second limitation of our study was that there was a lack of statistical power to allow confidence in accepting statistical differences between simultaneous and sequential groups. A larger sample size would have provided greater statistical power. CONCLUSION We have demonstrated that the risk of performing simultaneous bilateral laser in situ keratomileusis (LASIK) is not significantly different from that
284 Waring et al of sequential treatments. The simultaneous bilateral complications all occurred in the early postoperative period and were associated with good outcome. REFERENCES 1. Elkins BS, Clinch TE. Complications of bilateral photorefractive keratectomy for high myopia. (Letter) Arch Ophthalmol 1997;115 :420-421. 2. Kliger CK, Maloney RK. Keratitis as a complication ofbilateral simultaneous radial keratotomy. (Letter) Am J Ophthalmol 1994;118:680-681. 3. Rashid EA, Waring III GO. Complications of radial and transverse keratotomy. Surv Ophthalnol 1989;34:73-106. 4. Stulting RD, Balch K, Carr JD, et al. Complications of laser in-situ keratomileusis. Invest Ophthalmol Vis Sci 1997;38:S231. 5. Salah T, Waring GO III, El Maghraby A, et al. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996;121:143-155. 6. Carr JD, Nardone R, Stulting RD, et al. Risk factors for epithelial ingrowth after LASIK. Invest Ophthalmol Vis Sci 1997;38(4):S232. 7. Guell JL, Muller A. Laser in situ keratomileusis (LASIK) for myopia from -7 to - 18 diopters. J Refract Surg 1996;12:222-228.