SIMULTANEOUS AND SEQUENTIALBILATERALLASIK FOR THE CORRECTIONOF MYOPIA*
BYG. 0. Waring III, MD, FACS, FRCOPHTH, J. D. Carr, MD, MA, FRCOPHTH,
R. D. Stulting, MD,PHD, AND K P. Thompson,MD
ABSTRACT
Purpose: Tostudy the safetyand efficacyof simultaneous bilateral laser in-situkeratomileusis (LASIK)
Methods: Data were obtained from 254 consecutive patients thatwere randomized tosimultaneous or sequential bilateral LASIK.
Results: 146 patients were enrolledin 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 intraopera-tivecomplication rate(p=0.34), loss oftwoor morelines ofspectacle cor-rected 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 ofperformingbilateral simultaneous LASIKis not
significantlydifferent from that ofsequential treatments. INTRODUCTION
Simultaneousbilateral refractivesurgical proceduresarebeing performed increasingly frequentlybecause 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 fewcasereports of the com-plicationsof bilateral simultaneous refractivesurgery.'4 Wetherefore per-formed a prospective, randomized comparison of simultaneous and sequential bilateral laserinsitukeratomileusis (LASIK) inordertostudy
thesafety andefficacyof thesetwotreatmentoptions.
'FromtheDepartmentofOphthalmology,Emory UniversitySchool ofMedicine,Atlanta.
PATIENTSAND METHODS
In May 1995, aphysician-sponsored investigational device exemption was
conditionally approved by the US Food and Drug Administration (FDA) atthe Emory Vision Correction CenterinAtlanta for a study ofLASIK 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 inJeddah, 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
sequentialsurgery (2 weeks apart) for patients who desiredtohaveboth eyestreatedatthesametime.Wepresenthere the results ofthe prospec-tive, randomized comparison of simultaneous versus sequential bilateral LASIK.
Patients wereselected from theclinical practices of the14 participat-ingsurgeons. Patientsdesiringrefractive surgery were offered LASIK as an alternative to traditional surgicalprocedures for the management of their myopia. Three surgeons (G.O.W, R.D.S., and K.P.T.) had previous experiencewith lamellar cornealsurgery. The 11 surgeons withnoprior lamellar surgical experience received extensive didactic and practical instruction prior toand after commencement of the study.
Inclusion criteria for thestudyweremyopiabetween-2.00and -30.00 D with less than 4.00 D of refractive astigmatism, stable refraction, no prior ocular surgery, normal corneal topography, normal ocular examina-tion, andnoevidence of ocularhypertension orglaucoma. Patients were 18years of ageorolderand hadnohistory of systemic collagen vascular
disease;pregnancyandor useofsystemiccorticosteroidswere contraindi-cations tostudy enrollment. Soft contact lenseswere removed atleast3
days, and hard contactlenses atleast2weeks, before preoperative mea-surements wereobtained. Patientswith abnormalvideokeratographywere excluded unlessprolongedcessationofcontactlenswearrestored corneal
topographytonormal.
Patients requestingbilateral simultaneous surgery (abouttwo thirds ofpatients) wererandomly assignedtoreceive surgeryeitherduring the same surgical session (simultaneously) or sequentially at least 2 weeks apart. A random number table was used for treatment assignment. Patientswhorequested sequential surgerywerenotrandomizedandare notreported here.
Patients satisfying the inclusion criteria for entry into the study
received a full ophthalmic examination, which included manifest and
correction), measurement of pupil diameter at 300 lux, slit-lamp microscopy of the anterior segment, measurementof 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 tosurgery.The preoperative consensus refraction was determinedby the surgeon on the basis of 2 manifest refractions performed on 2 different days and 1 cycloplegic refraction. This consensus refraction wasused 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 asdescribed in the user man-ual. At the beginning of each operating day, the laserwas calibratedby
performinganablationthrougha
100-pm-thick
pieceof wratten gel filter (Eastman Kodak Co, Rochester, NY). If thefilter perforated after 530 to 630pulses and thebreakthrough pattern metspecifications, the laser was considered calibrated. Two photorefractive keratectomy (PRK) ablation algorithms were programmed into the laser's computer:asingle-zone (6.0-mm) pattern and a 5.5-, 6.0-, and 6.5-mm single pass, multizone pattern. Themultizone ablation delivered 100% of the laser setting to the central 5.0mm,70%of the laser setting to the zone between 5.0mmand 6.0 mm, and 30% of the laser settingto the zone between 6.0 and6.5 mm. The SummitOmniMed/Apex laser alloweda maximum ablation of700pulses to prevent excessive depth of ablation in the central cornea. Thus, for desiredcorrectionsabove -18.1D, thediameters of the three zoneswere reducedinorder to avoiddeliveryof more than700pulses tothe central cornea. Thecornealflapwasapproximately160pm
thick, anda700-pulse laser ablation shouldproduce a central ablation depth of about 175pm,
assuminganablationrateperpulseof0.25
pm.
Thus,themaximum abla-tiondepthinthisstudywasapproximately335pm,
leaving approximately 215pm
ofresidual centralcornea(assumingacentralcornealthickness of 550pm).
TheChiron AutomatedCorneal Shaper (Chiron Vision Corp, Irvine,
Calif)wasusedtoperformlamellarkeratotomyusingaflat,nonadjustable
suction ring.Themicrokeratomewascleaned and assembledby technical personnel and testedbythe surgeon priortoeach operation. Accuracy of the desired correction was confirmedbythe laser operator and the sur-geon.
SURGICALTECHNIQUE
Primary Technique
Sedatives were not routinely used preoperatively. The operative eye(s) wereprepped and draped, and topical anesthesia was achieved using sev-eral drops of1.0% proparacaine. The cornea was focused and centered under the excimer laser withthe aidof the twohelium-neon laser beams. Fiduciary marks were made with arefractive keratotomy marker stained with methyleneblue dye. The suction ring was positioned on the cornea with slight nasal decentration; suction was maintained at or above 28 inch-es of mercuryby 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 andslid forward until the gearonthe microkeratome head encountered the unob-structed trackof thesuctionring,after which themicrokeratomewas acti-vated. 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 conjuncti-val surface using either smooth forceps or an iris sweep. Visible fluid was removed from the stromal bed with a merocel sponge before laser abla-tion. The two helium-neon laser aiming beams were focused on the corneal bed and centered. The patientwasthen askedtofixate the blink-ing greenlight, 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.
Eyeswith more than 1.0Dof astigmatism received arcuate transverse keratotomy (Arc-T), performed either outside the corneal flap (8.5-mm zone) orbeneath 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 thebed using the end of an irrigation cannula or an iris sweep and dried inposition for approximately 5 minutes without manipulation (dryrollertechnique). Before removalofthelid speculum, one drop ofa combination preparation oftobramycin and dexamethasone (0.1%) and onedrop of ketorolac wereplacedinthe eye. Theflap positionwas veri-fiedby slit-lampmicroscopy approximately 10minutes after surgery. The patientwasdischargedwithoutapatch.
Forsimultaneous bilateralprocedures, the right eyewastreated first. The lidspeculumremainedinpositioninthe righteyewhile the surgeon
performed LASIK on the left eyein the manneralready described. For
sequential procedures, the eye with the greater amount of myopiawas treated first.
Complications. An adverse reaction was defined as an unexpected event thatcouldpotentially reduce spectacle-corrected visual acuity.
PostoperativeMedications. 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 tobramycinand dexamethasone (0.1%)in theoperated eye 4 times daily for 1 week.Nonpreserved artificial tearswererecommended for mild irri-tation.
PostoperativeExaminations. Postoperative examinations were done at 24hours,2weeks, and 3,6,and 12 months. Onthe first postoperative day,
flap position, uncorrected visual acuity, and manifest refraction were recorded. Atall other testing intervals, uncorrected visual acuity, as well as manifest and cycloplegic refractions with visual acuity, were recorded. Glare andcontrastsensitivityweretestedinselectedsubsets of patients; videokeratographywasroutinely performedateach examination except on that of the first postoperative day. Contrast sensitivityand videokeratogra-phy data are not reported here.
RepeatedLASIK
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 theedge 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, exposingthestromal bed. The remainder of the procedure was the same as theprimaryLASIKprocedure. Arc-T wasdone aloneor in con-junctionwithLASIKforcorrectionofresidual astigmatism.
OutcomeMeasuresand StatisticalAnalysis
The mean difference between desired and observed postoperative spher-ical equivalent refraction was calculated for both groups. The sample meansweretestedfor statisticalsignificance by unpairedt-test. The
chi-squared test was used to test for statistically significant differences between the simultaneous and thesequentialgroupsby usingthe follow-ingoutcome measures:the percentage of eyes with postoperative spheri-calequivalent refractions within +/- 1.00 D and +/- 0.50 D ofintended,
the percentage of eyes with uncorrected visual acuity of better than or
equalto 20/40and 20/20, the percentage of eyeslosingtwoor morelines ofspectacle-correctedvisual acuity, and the percentage complicationrate. Pvalueswereconsideredstatistically significantwhen lessthanorequalto 0.05.
RESULTS
Fivehundredfiftyeight (558) eyes of 320 consecutive patients are the sub-ject 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 out-come because of intraoperative flap complications that precluded laser ablation. All eyes that were excluded from refractive outcome analysis because of aborted laser procedures followingintraoperativeflap compli-cationswereincludedin an intent-to-treatanalysis of procedure safety.
Of the 548 eyes with more than 6-month postoperative data, there were40 eyes (32sequential; 8 simultaneous) without data fromthe fellow eye. Each of31 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
simultaneouslytreatedpatients were excludedfrom analysis of refractive outcomebecausethey were treated sequentially followingflap complica-tionsduringLASIK inthe first eye.
Therefore, the analysis of procedure efficacy was based upon508eyes of254patients. Twohundred ninetytwo (292) eyes of146patients were randomized to receivesimultaneous 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
beingconsidered forrepeatedLASIKand/orArc-T. Themeanagewas 41 years(+/-8.5SD; range,18 to65)for the simultaneous group and41years (+/-8.9SD; range,21to60)for the sequential group. Mean preoperative spherical equivalent manifest refractionwas -7.2 D(+/-3.0SD; range,-2.6 to-21.75) forthe simultaneous group and-7.0 D (+/- 2.8SD; range, -2.1 to-16.8) for the sequential group. Therewas nostatisticallysignificant dif-ference between the preoperative characteristics of simultaneous and sequential groups.
One hundred eighty-two (62.3%) of292 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 and179(82.8 %) of216eyes inthesequential group achieved apostoperative spherical equivalent refraction within 1.00D of intended outcome (Table I). There was no statistically significant differ-encebetween thetwogroupsforthisoutcome measure.
Forthe simultaneous group, the mean refractive outcomewas -0.07 D +/-0.8 SD (range,-3.00 to3.25) with respecttothe intended
correc-tion. The mean refractive outcome forsequentially treated eyes was an undercorrection of 0.21 D+/-0.8SD(range, -4.00 to 2.75).This wasa sta-tistically significant difference (P=0.05;unpaired t-test). To clarify the rea-sonfor this significant differenceinrefractive outcome, amultiple linear regression analysis was performed. Test variables included eye, gender, Arc-Tperformed during LASIK, single or multizone ablation, occurrence of enhancement procedures, and protocol (simultaneous orsequential). The only significant variables that had an effect on the mean refractive outcomefor simultaneous andsequential groupswerearcuate transverse keratotomy performed at the time of LASIK and the use of either single-zone ormultizone 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 acuityworse than 20/20, and eyes intentionally undercorrectedto producemonovision (Fig 1). Eyes thatexited the study at12months post-operatively with residual myopia greater than one diopter, where the patientdid not desire enhancement, were also excluded. Three hundred eighty-three eyesarethesubject of thisanalysis of uncorrected visual acu-ity. Eighty-nine percent (n = 199) of simultaneously treated eyes and 92.5% (n = 148) of sequentially treated eyesachieved uncorrected visual acuitiesofbetter thanorequalto 20/40(P=0.37). Fifty-one percent (n = 113) ofsimultaneously treated eyes and40% (n=64)ofsequentially treat-ed eyes achievtreat-ed uncorrected visual acuities of better than or equal to 20/20(P=0.05; B =0.7).
TABLEI. SUMMARYOF OUTCOME MEASURES AND STATISTICALSIGNIFICANCE AFTER BILATERALSIMULTANEOUS VSSEQUENTIALLASIK
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 ±0.50D 62.3%(n=182) 59.8%(n=128) P=0.63; 1=0.9 Loss 2lines 4.2%(n=13) 3.6%(n=9) P=0.9; 80.9 of spectacle-correctedacuity 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
TABLEII. MULTIPLELINEAR REGRESSION ANALYSIS OF REFRACTIVE OUTCOME AFTER
BILATERALLASIK (SIMULTANEOUSORSEQUENTIAL) 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 VARIABLESNOT IN THEEQUATION
VARIABLE BETA IN PARTIAL MINTOLER 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 transervekeratotomy;szlmz, single zone/multi-zone.
90 ,:.: _:... ii}~~~~~~~~~~~~~~~~~'~~~~~~; § " ' 4 .~...' . _ 100 90 80 70 80 02 7-X_~ Ra0l 7E0g 0 _ 0 0 0 0 0 0 0 0 A A A_ l_~ V...g__iAl_A Ag _ _ E _ | __2_FGURE |%40 S | _ 1 _
Cuuatv unorece viMsua ciydto simutaneous1 (SIM) an seunia SQ gL2rou
10 _ _ _
to_ _ E _ 0 Cg 0 _i
CD N CSC9 CDl CD CD 0 :
^ A lZ~~~~~~~~correcledYsual Acuit A A
FIGURE 1
Cumulative uncorrected visual acuitydata for simultaneous (SIM)andsequential (SEQ) groups.
Thirteen of 306 simultaneously treated eyes and 9 of 252 sequential-ly treated eyes lost two or more lines ofspectacle-corrected visual acuity (P=0.9) (Fig 2). Onesimultaneously treated eye and 2 sequentially treat-ed eyes lost2linesof spectacle-corrected visual acuity to worse than 20/40 (all3eyes saw 20/50).
Figs3and 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 postoperativecomplications in the simul-taneousgroup (3.9 %) comparedwith4postoperativecomplications (1.1 %)inthesequential group (P=0.02). Fig4shows that the more frequent occurrenceofepithelial ingrowth requiring flap revisioninthe simultane-ousgroup accounted for thisstatistically significant difference.
Threepatientsinthe simultaneous group had bilateralcomplications, all of which were postoperative. In 1 patient a marked inflammatory responsewas seeninthe stromal bed of both eyes at the 24-hour exami-nation. The corneal flap was raised and the bed cultured; there was no growth of any organism. Both eyes were treated with topical antibiotics andcorticosteroids and recovered withoutsequelae; thepresumedcause wasa toxicreaction tomaterial that may have been present on the
micro-139 140 120 100 te 8080 _ q 60 ; 40 20 0 -5 -4 -3 -2 -1 0 1 2 3 4 5
ChangeInlinesofSnellen visualacuity FIGURE 2
4 a3 a 0 2 E 4
a
S 0Intraoperativeflapcomplication FIGURE3
Intraoperativeflap complications for simultaneous (SIM) and sequential (SEQ) groups.
10 0 9 _ .;;5 _ S q-_ j .~~~~~~i_ _" 00 laE FG 4_ Postoperative In o u Postoperativecomplication FIGURE4
keratome blade.At3monthspostoperatively, the righteyehad
uncorrect-ed visualacuityof20/30,correctingto20/25with-0.75D/0.25 at50';the left eye had uncorrected visual acuity of20/25 correcting to 20/20 with -0.50/1.25at1750.
The other2patientsfrom the simultaneousgroupwith bilateral
com-plicationshadpostoperativeflapslippage,partialin3eyesandtotal
dislo-cation in 1.All 4flapswererepositionedonthe firstpostoperativeday.At
6monthspostoperatively, the firstpatienthad uncorrected visualacuityof 20/30 OD and20/25 OS, each withaone-line loss of spectacle-corrected
visualacuity.Theotherpatienthad uncorrected visualacuityof20/40 OD and20/20OS;therighteyehadaone-line loss ofspectacle-corrected
visu-alacuity.
Teneyeshadnolaser ablation because ofintraoperativeflap
compli-cations,4fromthe simultaneous group(cases 1through 4) and6fromthe sequential group (cases 5 through 10) (Table III). Only2eyes from the
simultaneous group lost two or more lines ofspectacle-corrected visual
acuity;1 eyelosttwolinesto20/25,and the other lost threelinesto20/50.
TABLEIII. SPECTACLE-CORRECTEDVISUAL ACUITY DATA FOR TENEYES THAT HAD
INTRAOPERAIVEFLAP COMPLICATIONS AND NO LASER ABLATION
CASE PREOPERATIVE FINAL CHANGE INTRAOPERATIVEFLAP
SCVA SCVA IN COMPLICATION
SCVA (LINES) SIMULTANEOUS EYES 1 20/20 20/20 0 Incomplete flap 2 20/20 20/20 0 Incompleteflap 3 20/16 20/25 -2 Button-holeinflap 4 20/25 20/50 -3 Freeflap SEQUENTIALEYES 5 20/20 20/16 +1 Freeflap 6 20/20 20/20 0 Freeflap 7 20/20 20/20 0 Bilaminarflap 8 20/20 20/16 +1 Incomplete flap 9 20/16 20/16 0 Incomplete flap 10 20/20 20/20 0 Incomplete flap
DISCUSSION
Complications are an inevitablefeature of every ocular surgical procedure currently performed. Ineach case, the likelihood ofcomplicationsis com-pared with the likely benefit of the particular surgical procedure. Ocular surgical procedures, such as cataract surgery, aretypicallyperformedon oneeye at a timebecause of thepossibility of potentially blinding bilater-al ocular complications, such as bacterial endophthalmitis or intraocular hemorrhage, when both eyes aretreated simultaneously. Refractive surgi-calprocedures, such as refractivekeratotomy, PRK, and LASIK, are elec-tiveprocedures andtherefore require ahighstandard ofsafety.
We think ourstudy is the first to compare the safety and efficacy of simultaneous and sequential surgeryina prospective, randomized man-ner. Wefound noclinically important differences between the two groups. There was no difference inthe percentageof eyes achieving a postopera-tivespherical equivalent refraction within 0.50 D or 1.00 D of intended. Interestingly, the meanrefractive outcomefor simultaneous and sequen-tial groupswasstatisticallydifferent,with the simultaneous group achiev-ing a postoperative spherical equivalent refraction closer to intended by 0.14 D.To clarify this finding, a multiplelinearregressionanalysis of the difference between intended and observed postoperative refraction showed that the difference between simultaneous and sequentialgroups could beexplained by the presence ofArc-Tprocedures and the differ-ences inproportions ofsingle-zone and multizone ablations between the twogroups(Table II). The effect of theprotocol (simultaneousor sequen-tial)wasstill of borderlinesignificance(P =0.07).
There was nostatisticallysignificant difference between the percent-ageof simultaneous andsequential eyes with uncorrected visual acuity of better thanorequalto 20/40.Therewashowever, asignificantdifference inpercentageof eyes withuncorrected visual acuity of better than or equal to20/20,albeitwith low statistical power (30%). If this differenceistruly
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 eyesin the sequential group with residual myopia. which did notundergo enhancement, between 3 and 12months after the primary procedure. This residual myopia would decrease the number of eyes
achieving20/20uncorrected visual acuity.
Theanalysisofsafetyshowed that therewas nosignificant difference
inthe percentage of eyes losingtwo or morelines ofspectacle-corrected
visual acuity. There was no significant difference in the occurrence of intraoperative complications between the two groups. However, there were significantlymorepostoperative complications in the simultaneous
group than in the sequential group, although the statistical powerof this observation was low at40 %(Fig 4).The main difference in postoperative complications between simultaneous and sequential groups was in the more frequent occurrence ofepithelial ingrowth that required flap revi-sionin 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 microkeratomeblade had already been used on the first eye. Inourstudy, there was no predilection for the second of two simul-taneously treated eyes to develop epithelial ingrowth. Moreover, a previ-ous multivariate analysis of risk factors for epithelial ingrowth that required flap revision identified microperforation at the time of simulta-neous Arc-T and postoperative epithelial defects as the only significant riskfactors.6
The key issueinthis comparison of simultaneous bilateralLASIKwas the occurrence of bilateralcomplications. Thesedeveloped in3 patients postoperatively. The mostsignificant was the patient with bilateral sterile toxic keratitis at 24 hours; cultureswere negative and the patient recov-ered with use oftopical antibiotics and corticosteroids with good visual outcome.
The other 2patients with bilateral complications had postoperative
flapslippage, necessitating flap repositioningat 24hours. Theoccurrence of postoperativeflapslippagehas become lessfrequent during thecourse ofour LASIKstudy (datanotshown), probably owingtoimprovementin thetechniqueforreplacing the cornealflap atthe end of theprocedure.
Inthe early part of the study, it ispossible thatexcessiveirrigation of bal-ancedsalt solution beneath the cornealflap may have allowed greater flap edema and consequent poorer apposition of theflap edgetothe underly-ingcornealbed,predisposingtoflapslippage.
Onelimitationofourstudywasthat all enrolled eyes didnotreach the 12-month postoperative interval. It isunlikely, however, that further fol-low-up would change the resultsof our study, because postoperative
com-plications invariably occur during the initial few days to weeks. In addi-tion, the postoperative spherical equivalent refraction does not fluctuate significantly after the third postoperative month.7 Asecond limitation of ourstudywasthat therewas alack of statistical powertoallow confidence inacceptingstatistical differences between simultaneous and sequential groups.Alarger samplesizewould haveprovided greater statistical power.
CONCLUSION
Wehavedemonstrated that the risk of performing simultaneous bilateral laserinsitukeratomileusis(LASIK)is notsignificantly different from that
of sequential treatments. The simultaneous bilateral complications all occurred in theearly postoperative period and were associated with good outcome.
REFERENCES
1. ElkinsBS,ClinchTE.Complications of bilateral photorefractive keratectomy for high myopia.(Letter) ArchOphthalmol1997;115:420-421.
2. KligerCK,MaloneyRK.Keratitisas acomplication of bilateral simultaneous radial ker-atotomy.(Letter) Am J Ophthalmol 1994;118:680-681.
3. RashidEA,Waring III GO.Complicationsofradial andtransversekeratotomy. Surv
Ophthalnol1989;34:73-106.
4. StultingRD, BalchK, Carr JD, etal. Complicationsoflaserin-situkeratomileusis.
InvestOphthalmolVisSci1997;38:S231.
5. SalahT,Waring GO III, ElMaghrabyA, etal. Excimerlaserinsitu keratomileusis underacornealflapformyopiaof2 to 20diopters.AmJOphthalmol1996;121:143-155. 6. Carr JD, Nardone R, Stulting RD, et al. Risk factors for epithelial ingrowth after
LASIK. InvestOphthalmolVisSci 1997;38(4):S232.
7. GuellJL,MullerA. Laserinsitukeratomileusis (LASIK) formyopiafrom-7 to-18
