Comparison of Night Driving Performance after Wavefront-Guided and Conventional LASIK for Moderate Myopia

Comparison of Night Driving Performance after Wavefront-Guided and Conventional LASIK for Moderate Myopia Steve C. Schallhorn, MD, David J. Tanzer, MD, Sandor E. Kaupp, MS, Mitch Brown, OD, Stephanie E. Malady, BS Purpose: To compare preoperative and postoperative changes in simulated night driving performance after wavefront-guided (wlasik) and conventional LASIK (clasik) for the treatment of moderate myopia. Design: Retrospective, comparative study. Participants: All eyes of subjects with a preoperative manifest spherical equivalent (MSE) between 4.50 and 6.00 who underwent night driving simulator (NDS) testing from 2 clinical trials were entered into the study. This comprised 38 clasik patients (62 eyes; mean 5.46 diopters [D] MSE) and 21 wlasik patients (36 eyes, mean 5.20 D MSE). Methods: Patients records were reviewed from 2 LASIK clinical trials that had similar enrollment criteria. One trial treated patients with conventional LASIK using a bladed microkeratome (clasik) and the other treated with a wavefront-guided profile using a femtosecond laser (wlasik). In both trials, patients with moderate myopia were asked to participate in NDS testing. Main Outcome Measures: The detection and identification distances of road hazards were measured with and without a glare source before and 6 months after LASIK. Each eye was tested independently in bestcorrected trial frames by a masked operator. Results: In every category, there was a mean reduction in the preoperative to postoperative NDS performance after clasik (mean change, 21.3 to 27.9 ft, 6.5 to 8.5 m; 95% confidence interval [CI], 12.0 to 41.3 ft, 3.7 to 12.6 m); there was a corresponding mean improvement after wlasik (mean change 15.0 to 29.1 ft, 4.6 to 8.9 m; 95% CI, 8.3 to 41.5 ft, 2.5 to 12.6 m). Significant differences between clasik and wlasik NDS performance was observed in every category (P 0.;01, Tukey s honest significant difference for unequal numbers). A clinical relevant loss of NDS performance ( 0.5 seconds) was observed in 32% to 38% of clasik eyes for all tasks, whereas only 0% to 3% of eyes had this loss after wlasik. Between 2% and 7% of clasik eyes and 11% and 31% of eyes had a significant postoperative improvement in NDS performance in every task. Conclusions: Wavefront-guided LASIK to correct myopia combined with a femtosecond laser flap significantly improved mean night driving visual performance and was significantly better than clasik using a mechanical keratome. Financial Disclosure(s): Proprietary or commercial disclosures may be found after the references. Ophthalmology 2009;116:702 709 2009 by the American Academy of Ophthalmology. Night vision symptoms, such as glare, halos, and starburst, have been reported complaints after LASIK. 1 8 In a study by Bailey et al 3 of 604 previously myopic patients responding to a questionnaire, halos were reported by 30%, glare by 27%, and starbursts by 25%. These patients were treated by either conventional photorefractive keratectomy or conventional LASIK (clasik). However, it is often more useful to compare the preoperative to postoperative change in symptoms because glare and other visual phenomena have been reported by both spectacle and contact lens wearers. 9 In another study of conventional photorefractive keratectomy and clasik patients, Schein et al 4 found that 16.3% of subjects had a worsening of glare symptoms compared with their preoperative scores on the Refractive Status and Vision Profile questionnaire. Night driving problems have been a common complaint in those patients who also have night vision symptoms. 5 7 Tahzib et al 8 reported that 47.2% of patients experienced more glare from lights at night than before surgery, especially glare from oncoming headlights. Night driving was rated more difficult than before surgery by 39.4% of patients, whereas 59.3% had less difficulty driving at night. Schein et al. 4 found that 29.5% of subjects expressed a worsening of their ability to drive when compared with their preoperative scores. This occurred despite a good Snellen acuity outcome. It is unclear, however, if this was specifically for night driving. The observation that patients can have difficulty driving at night after LASIK should not be surprising. Night driving is among the most strenuous visual tasks commonly performed in our society. It is performed under low light conditions, has many potential glare sources, and requires a high level of visual performance to safely operate a vehicle. 702 2009 by the American Academy of Ophthalmology ISSN 0161-6420/09/$ see front matter Published by Elsevier Inc. doi:10.1016/j.ophtha.2008.12.038

Schallhorn et al Night Driving Performance after LASIK for Moderate Myopia In an effort to reduce these symptoms and improve outcomes, there has been significant innovation and development in laser vision correction over the last 15 years. Optical zone sizes have increased from 4 mmto 6 mm. Transition zones were developed to create a blended area at the edge of the optical zone. 10 12 There has also been innovation in customized treatment profiles guided by wavefront aberrometry and topography as well as the development of aspheric profiles. 13 15 Wavefront-guided LASIK (wlasik) has been touted to be more technologically advanced, resulting in superior visual outcomes compared with clasik algorithms. In the studies that directly compared the 2 procedures, wlasik resulted in better mean postoperative contrast sensitivity. 16,17 This seems to be the case under photopic and mesopic testing conditions. Although the types of visual symptoms after wlasik are similar to clasik, symptoms seem to occur less frequently after wlasik. As reported by Lee et al 17 in a randomized clinical trial of 98 subjects, clasik had a higher percentage of patients (15.4%) who noted disturbing glare or halos symptoms than with wlasik (8.6%). The percentage of wlasik procedures is growing on a year-on-year basis and now represents 50% of all excimer laser procedures. However, the wavefront-guided procedure is more expensive and requires additional equipment and training. In addition, it remains unclear if there is a definitive benefit from wlasik compared with a conventional procedure when assessed with a task-specific measure, such as night driving performance. This study was conducted to evaluate LASIK-induced changes in night driving performance and compare conventional and wavefront-guided procedures for the treatment of moderate myopia. Materials and Methods Subjects Records were reviewed from 2 prospective clinical trials conducted at the Naval Medical Center, San Diego. The first was an evaluation of clasik to treat low to moderate myopia with or without astigmatism in an Institutional Review Board-approved study of 401 subjects (802 eyes) conducted between 2001 and 2002. The second trial was an evaluation of wlasik to treat low to moderate myopia with or without astigmatism in an Institutional Review Board-approved study of 100 subjects (200 eyes) conducted between 2005 and 2006. The enrollment criteria for both trials were identical with 2 exceptions. The preoperative refractive requirements were based on the manifest spherical equivalent (MSE) for the wlasik trial ( 1.0 to 6.0 diopters [D] MSE), whereas it was based on the manifest sphere for the clasik trial ( 1.0 to 6.0 D sphere). In both trials, these preoperative refractive enrollment requirements were based on the dominant eye. Both trials had a maximum preoperative cylinder of 3.0 D, as measured in minus notation. The other enrollment difference was that a high-quality, 6-mm undilated pupil wavefront capture suitable for treatment profile was required in the wlasik study. There were no pupil size limitations for the clasik trial. In both trials, participants who met the enrollment criteria and had moderate myopia were also asked to participate in a simulated night driving test with the Night Driving Simulator (NDS; Vision Sciences Research Corporation, San Ramon, CA). The refractive requirement for participation in NDS testing was based on the MSE of the dominant eye for the wlasik trial ( 4.5 to 6.0 D MSE) and the manifest sphere of the dominant eye for the clasik trial ( 4.5 to 6.0 D manifest sphere). A total of 21 patients in the wlasik group and 44 patients in the clasik group agreed to this additional testing. To account for the slight disparity in the preoperative refraction requirements for NDS testing in the 2 trials (MSE for wlasik and manifest sphere for clasik) and to match the 2 trials for comparison, only eyes with an MSE between 4.50 and 6.00 D were included in this study. This resulted in 36 eyes of 21 patients for wlasik and 62 eyes of 38 patients for clasik for the comparative study (Table 1). All participants in both trials underwent NDS testing preoperatively and at the 6-month postoperative examination. No patient was lost to follow-up. Operative Procedures All procedures were performed in accordance with the Professional Use Information of the applicable laser platform and manufacturers recommendations for the keratome. All eyes were targeted for emmetropia. Table 1. Enrollment and Preoperative Refractive Criteria of the 2 Clinical Trials and the Night Driving Simulator Study clasik wlasik Clinical trial Total enrollment 401 patients/802 eyes 100 patients/200 eyes Refractive enrollment criteria (dominant eye) 1.0 to 6.0 D sphere; Up to 3.0 D cylinder* 1.0 to 6.0 D MSE; up to 3.0 D cylinder* Refractive requirement for NDS testing (dominant eye) 4.50 to 6.0 D sphere 4.50 to 6.0 D MSE Total subjects/eyes tested with NDS 44/88 21/42 MSE of NDS subjects (both eyes) 5.74 D ( 3.50 to 8.00D) 5.07 D ( 3.38 to 6.13) NDS study Refractive requirement for study (either eye) 4.50 to 6.0 D MSE 4.50 to 6.0 D MSE Subjects/eyes 38/62 21/36 NDS study MSE 5.46 D ( 4.50 to 6.00) 5.20 D ( 4.50 to 6.00) clasik conventional LASIK; D diopters; MSE manifest spherical equivalent; NDS night driving simulator; wlasik wavefront-guided LASIK. *Cylinder measured in minus power. 703

Ophthalmology Volume 116, Number 4, April 2009 The wlasik group underwent a wavefront-guided procedure using the VISX WaveScan coupled with the Star S4 excimer laser (CustomVue, Advanced Medical Optics [AMO], Santa Anna, CA). The wavefront analyzer software employed Fourier wavefront reconstruction, but iris registration was not available. The optical zone diameter for all treatments was 6 mm (minor axis of the ablation for myopic/astigmatic corrections) with an overall ablation diameter of 8 mm. The LASIK flap was created with a femtosecond laser (15 khz; IntraLase FS Laser, AMO) and the programmed thickness was 100 m with a diameter of 9.1 mm. The clasik group had a conventional ablation profile with 1 of 4 excimer lasers (10 subjects/17 eyes treated with the Alcon LadarVision 4000 [Alcon Laboratories, Ft Worth, TX], 10 subjects/15 eyes with the Bausch & Lomb Technolas 217 [Bausch & Lomb, Rochester, New York, NY], 6 subjects/11 eyes with the Nidek EC-5000 [Nidek, Aichi, Japan], and 12 subjects/19 eyes with the AMO VISX Star S-3 [AMO]). Manufacturers guidelines were followed for all aspects of the procedure, including calibration and operating parameters. The optical zone was 6.5 mm for all treatments and the transition zone varied by laser type. The LASIK flap was created with a Hansatome (Bausch & Lomb) using a 9.5-mm ring and the 160- m plate. Examinations All patients were evaluated 1 day, 1 week, and 1, 3, and 6 months after surgery. Contrast sensitivity was measured with a backlit 5% contrast acuity chart with logarithmically arranged letters in a darkened room. The low-light pupil diameter was measured in a darkened room with an infrared pupillometer. All subjects underwent testing with a NDS preoperatively and 6 months postoperatively. There were no adverse events, complications or retreatments. Night Driving Simulation The NDS utilizes video- and computer-generated images of night driving scenes and provides a 90 field of view and 20/30 resolution. This allows for independent presentation of the driving scene and targets. The 6 identification targets were 2 green text signs ( For Businesses Take Left [or Right] Exit ), 2 yellow warning traffic signs (indicating a 3- or 4-way intersection ahead, black or T on yellow background), and either a left- or right-facing pedestrian in light-colored clothing seeming to move right or left. The road signs and pedestrian hazards were tested with and without rearview mirror glare (40 lux). All eyes underwent the same testing protocol by 1 technician. The technician was masked as to the laser and procedure. At every examination, each eye was tested independently (monocular) using best-corrected trial frames. The test sequence consisted of a 55 mph (89 km/h) nighttime rural driving scene with simulated low beam headlights (Fig 1). The subject was given a training sequence to familiarize them with the targets and tested for reaction time (6 times for type of road hazard). After the training cycle, each target was presented in random sequence. The subject s response to detection froze the driving scene and the target disappeared from view. Upon correct verbal detection, the driving scene was continued and the target reappeared until it was properly identified. Responses were recorded in terms of detection and identification distances in feet. Improper responses caused the scene to continue until correct identification was achieved. Randomly placed blanks were inserted during the testing to ensure vigilance on the part of the subject. Six threshold measurements were made for each of 24 conditions (6 targets, with and without glare for both detection and identification) yielding 144 total measurements per eye. Figure 1. Night driving simulator. The subject is in trial frames looking onto a rural night driving scene moving at 55 mph (89 km/h). A traffic sign is present on the right side of the road. 704

Schallhorn et al Night Driving Performance after LASIK for Moderate Myopia Table 2. Demographics, Preoperative, and 6-Month Clinical Data Data Analysis clasik wlasik Eyes (n) 62 36 Subjects (n) 38 21 Age (yrs) Mean 34.1 31.4 Range 22 51 24 42 Gender (%) Male 79 87 Female 21 13 Low light pupil (mm) 6.1 6.6 Range 4.0 8.0 5.8 7.7 Preoperative MSE (D) 5.46 5.20 Range 4.50 to 6.00 4.50 to 6.00 Preoperative cylinder (D) 0.69 0.81 Range 0.00 to 2.50 0.00 to 2.50 6-month MSE (D) 0.13 0.55 0.21 0.30 Range 1.25 to 1.00 1.00 to 0.38 6-month cylinder (D) 0.53 0.35 0.42 0.23 Range 0.00 to 1.75 0.00 to 1.00 20/20 UCVA (%) 79 91 Mean change BCVA (logmar) 0.02 0.05 0.03 0.05 Range 0.12 to 0.14 0.04 to 0.10 Loss 2 lines BCVA (%) 0 0 Mean change 5% CA (logmar) 0.08 0.12 0.01 0.09 95% CI 0.05 to 0.11 0.04 to 0.02 Loss 2 lines 5% CA (%) 25 5 BCVA best-corrected visual acuity; CA contrast acuity; CI confidence interval; clasik conventional LASIK; D diopters; LogMAR logarithm of the minimum angle of resolution; MSE manifest spherical equivalent; UCVA uncorrected visual acuity; wlasik wavefront-guided LASIK. For each eye, the preoperative to postoperative difference in responses (feet) for each NDS road hazard (business sign, traffic sign, and pedestrian hazard) was calculated for detection (no glare), identification (no glare), detection with rearview mirror glare, and identification with glare. There were no differences in the preoperative to postoperative change in performance with regard to road hazard and eye tested (P 0.75 and 0.13, respectively; multivariate tests of significance Wilks ). Therefore, the data were pooled for analysis, yielding a detection and identification index (with and without glare) for each subject. In addition, a multivariate analysis of variance determined that the type of laser system used for the clasik treatments did not predict change in NDS performance (detection P 0.264, identification P 0.663, detection with glare P 0.414, identification with glare P 0.681). Thus, the results for all 4 conventional excimer lasers were pooled into a clasik group. The change index was converted from feet into seconds using the simulator speed of 55 mph (80.66 feet/second). This represents the gain or loss in recognition time related to the surgery. A difference of 0.5 seconds was deemed significant based on studies conducted through the National Highway Traffic Safety Administration for a single attention task. 18 Results The average preoperative MSE in the wlasik group was 5.20 D (range, 4.50 to 6.00 D), the average cylinder was 0.81 D (range, 0.00 to 2.50 D; Table 2). Six months postoperatively, the mean MSE was 0.21 0.30 D (range, 0.38 to 1.00 D) and 91% achieved 20/20 uncorrected visual acuity. No eye had a loss of 1 lines of best-corrected visual acuity. Two eyes (5%) had a loss of 2 lines of contrast acuity; no eye lost 2 lines. The average preoperative MSE in the clasik group was 5.46 D (range, 4.50 to 6.00 D), the average cylinder was 0.69 D (range, 0.00 to 2.50 D). At 6 months postoperative, the mean MSE was 0.13 0.55 D (range, 1.00 to 1.25 D) and 79% achieved 20/20 uncorrected visual acuity. Three eyes (5%) had a loss of 1 line of best-corrected visual acuity; no eye lost 1 line. Eleven eyes (19%) had a loss of 2 lines of contrast acuity and 4 eyes (7%) had a loss of 2 lines. Night Driving Simulation Results There was a significant increase (improvement) in mean postoperative NDS performance after wlasik for all indices (detection and identification index, with or without glare; t test; P 0.001; Tables 3 and 4). On average, subjects could detect and identify targets farther away after surgery ranging from 15.0 to 29.1 ft (4.57 8.87 m). In contrast, clasik had a mean significant decrease in postoperative performance for all indices ranging from 21.3 to 27.9 ft ( 6.49 to 8.50 m). There was a significant difference in the change of performance between conventional and wlasik for all indices (P 0.001; Tukey s honest significant difference for unequal numbers). To more fully assess the implications of the NDS results, the performance was converted from the preoperative to postoperative change in distance (feet) to the equivalent difference in seconds while traveling at 55 mph. A change of 0.5 seconds (40 feet) was deemed significant. Using this criteria, 32% to 38% of subjects after clasik had a decline in performance (detection/identification occurring 0.5 seconds later postoperatively compared with preoperatively), whereas between 0% and 3% of wlasik patients were in the same category (Table 5). In a similar fashion, the per- Procedure Table 3. Preoperative to Postoperative Change in Night Driving Simulation without Glare Mean SD Range 95% CI P-Value* Mean SD Range 95% CI P-Value clasik 21.3 37 90 to 59 30.1 to 12.0 0.0001 24.8 43 131 to 115 34.6 to 13.3 0.0001 wlasik 18.9 21 37 to 58 12.0 to 25.8 29.1 37 57 to 122 16.7 to 41.5 CI confidence interval; clasik conventional LASIK; SD standard deviation; wlasik wavefront-guided LASIK. A negative number indicates that targets had to be closer after surgery (postoperative performance worse than preoperative). *Two-sample t test; where appropriate a Satterthwaite adjustment was used for unequal variances. 705

Procedure Ophthalmology Volume 116, Number 4, April 2009 Table 4. Preoperative to Postoperative Change in Night Driving Simulation with Glare Preoperative to Postoperative Change Mean SD Range 95% CI P-value* Mean SD Range 95% CI clasik 26.0 42 181 to 61 36.6 to 15.4 0.0001 27.9 54 188 to 139 41.3 to 14.4 0.0001 wlasik 15.0 21 30 to 64 8.3 to 21.8 27.7 36 35 to 107 13.7 to 38.8 P-value CI confidence interval; clasik conventional LASIK; wlasik wavefront-guided LASIK; SD standard deviation. A negative number indicates that targets had to be closer after surgery (postoperative performance worse than preoperative). *Two-sample t test; where appropriate, a Satterthwaite adjustment was used for unequal variances. centage of eyes with an improvement in performance (detection/ identify after surgery occurring 0.5 seconds sooner postoperatively than preoperatively) was calculated using the same criteria (Table 6). Between 11% and 31% of subjects had an improvement in NDS performance after wlasik, whereas only 2% to 7% of subjects in the clasik group were in this category. A multivariate analysis of variance was performed to determine whether demographic (age and mesopic pupil) variables and treatment type (wavefront-guided versus conventional) could predict the change in NDS performance. The only significant factor was the type of treatment (conventional versus wavefront guided; Table 7). Although the results of all 4 conventional lasers were pooled for the clasik analysis (see Methods), a separate analysis was conducted of the NDS performance for each conventional laser and compared with wlasik. Every conventional laser had a mean reduction in postoperative NDS performance for all indices (detection, identification, with and without glare), ranging from 10.4 to 35.6 ft ( 3.17 to 10.85 m; Tables 8 and 9). In addition, every conventional laser NDS index was significantly worse than the corresponding wlasik results (P 0.042 to P 0.0001; Dunnett s test for significance). Discussion A carefully constructed, performance-based task can accurately assess real-life visual performance. The results have intrinsic meaning and, by definition, can relate directly to a task performance. This is different from other outcome measures, such as uncorrected visual acuity or best-corrected visual acuity, which are commonly used to assess LASIK. Those clinical parameters, although very useful, have limitations when it comes to understanding the nature Table 5. Percentage of Subjects who Had a Significant Decrement in Night Driving Simulation Performance after LASIK Procedure Without Glare With Glare clasik 36% 32% 38% 33% 95% CI 24% 47% 21% 44% 26% 53% 21% 45% wlasik 0% 3% 0% 0% 95% CI 0% 8% and severity of vision complaints, such as difficulty with night driving. One of the strengths of this study is that it used a performance-based task to assess operative outcomes. The task was carefully selected because night driving difficulty has been a frequently reported complaint reported after LASIK. 5 7 To our knowledge, this is the first study in which a night task performance was prospectively assessed before and after a refractive surgery procedure. However, a NDS was used in a previous study 19 conducted as part of the United States Food and Drug Administration clinical study for the Array multifocal intraocular lens (AMO). Thirty-three bilateral multifocal intraocular lens patients were compared with 33 bilateral monofocal intraocular lens patients with the NDS. This study demonstrated the validity of using the driving simulator to assess visual function after surgery. The NDS utilized in this study had many unique properties that made it a suitable tool to assess night driving performance. No driving skills were required. The subjects were seated and observed the driving scenes. This eliminated the important confounding variable of subject-tosubject differences in the cognitive skills of driving. Only the visual elements needed to safely navigate at night were measured. A training period and assessment of reaction time was made before each trial. This ensured that optimal performance was achieved every time a subject was tested. The scenarios and hazards were based on actual driving conditions. Both detection and identification were measured. For instance, the detection task was to identify a potential pedestrian hazard. The identification task was to determine if the pedestrian was walking into the street (imminent hazard) or away from it (not a hazard). Finally, a glare source was Table 6. Percentage of Subjects who Had a Significant Improvement in Night Driving Simulation Performance after LASIK Procedure Without Glare With Glare clasik 3% 2% 3% 7% 95% CI 0% 8% 0% 5% 0% 8% 0% 13% wlasik 11% 31% 11% 31% 95% CI 1% 21% 16% 46% 1% 21% 16% 46% CI confidence interval; clasik conventional LASIK; wlasik wavefront-guided LASIK. CI confidence interval; clasik conventional LASIK; wlasik wavefront-guided LASIK. 706

Schallhorn et al Night Driving Performance after LASIK for Moderate Myopia Table 7. Results of Multivariate Analysis of Variance for Preoperative Variables to Predict Change in Night Driving Simulator Performance with Glare with Glare Age P 0.325 0.616 0.890 0.545 Mesopic pupil 0.074 0.492 0.104 0.746 diameter Type of treatment 0.000 0.000 0.000 0.000 Wavefront-guided versus conventional LASIK. added that simulated the headlights in a rear view mirror. All of these design elements strengthen the usefulness and applicability of the NDS. As a primary outcome of this study, clasik was shown to reduce mean night driving performance and wlasik showed a mean improvement. In addition, there was a significant difference between the 2 procedures. When the data were analyzed in terms of criteria from previous studies conducted by the National Highway Traffic Safety Administration, clasik resulted in a large proportion of eyes with a measurable loss of night driving visual performance. Between 32% and 38% of eyes had a loss after clasik in contrast to wlasik where the loss was 0% to 3%. Similarly, the percentage of eyes with a measurable gain in performance was much higher in wlasik than in clasik. There is a growing awareness that higher order aberrations (HOAs) can have a negative influence on the quality of vision, especially under low light conditions. Visual symptoms of glare, halos, and starburst have been correlated to HOAs. 20 Many studies have shown that wlasik results in less induction of HOAs than clasik, especially spherical aberration. 16,21 25 The difference in induced HOA between procedures may be directly related to the observed difference in NDS performance. Mean contrast sensitivity can be reduced after clasik, 26 whereas wlasik either does not change or improves mean contrast sensitivity. 27,28 Corroborating these reports, a difference in contrast acuity was noted in this study with a loss of 2 lines in 25% of eyes in clasik versus 5% in wlasik. This difference in contrast sensitivity may also help to explain the favorable NDS results of wlasik compared with clasik. It is important to appreciate that the 2 procedures did not just differ by their ablation profile (clasik vs wlasik), but also by the method of flap creation (mechanical keratome vs femtosecond laser). There is evidence of improved outcomes when using a femtosecond laser to create the flap. 29 31 It is impossible to separate out the contribution of the ablation profile from the method of flap creation in the NDS results. The improved night driving performance results of wlasik may be due to the combination of 2 technologies: wavefront-guided ablation profile and femtosecond flap creation. The implications of this study are important. Night driving is perhaps the most visually demanding common task in a modern society. It has both social and safety implications for anyone who drives a vehicle. Not only was there a significant difference between 2 different ablation profiles, but the clasik procedure reduced mean night driving visual performance whereas the wlasik procedure improved it. This alone should increase the demand for wavefront-guided treatments over conventional modalities. However, the testing in this study was monocular. In real life, both eyes are used for a task such as night driving. Binocular testing was not performed. Thus, it is unknown if a clasik procedure would result in a mean loss of night driving performance under binocular conditions. Nonetheless, based on the monocular results, binocular wlasik night driving visual performance would likely be improved over clasik. LASIK has not been previously authorized for aviators in the US Navy and US Air Force because of concern about the postoperative quality of vision. The results of this study were instrumental in the decision to proceed with LASIK in aviators and astronauts. All branches of the US military have now embraced laser vision correction as a way to improve performance. It is notable that age and low-light pupil size were not associated with NDS performance. The population was relatively young, however, and no subject was 51 years old. Thus, it is not known how LASIK affects the night driving performance of older patients. The fact that the low-light pupil size was not related to NDS performance should not be a surprise. Most studies have not shown an association Laser Table 8. Preoperative to Postoperative Change in Night Driving Simulation for Each Conventional Laser Type without Glare Mean SD 95% CI P-Value* Mean SD 95% CI P-Value A 10.4 31 25.7 to 5.0 0.0095 13.1 26 33.1 to 6.8 0.0032 B 25.9 45 42.3 to 9.6 0.0001 25.7 49 46.9 to 4.4 0.0002 N 34.5 30 53.6 to 15.3 0.0001 32.8 54 57.6 to 8.0 0.0001 V 19.9 38 34.5 to 5.4 0.0002 30.0 44 48.9 to 11.2 0.0001 CI confidence interval; SD standard deviation. A negative number indicates that targets had to be closer after surgery (postoperative performance worse than preoperative). *The Dunnett test for difference with respect to wavefront-guided LASIK. Laser A Alcon LadarVision 4000; Laser B Bausch & Lomb Technolas 217; Laser N Nidek EC-5000; Laser V AMO VISX Star S-3. 707

Laser Ophthalmology Volume 116, Number 4, April 2009 Table 9. Preoperative to Postoperative Change in Night Driving Simulation for Each Conventional Laser Type with Glare Mean SD 95% CI P-Value* Mean SD 95% CI Laser A 11.5 29 28.8 to 5.7 0.042 13.8 38 37.0 to 9.5 0.025 Laser B 33.8 41 52.1 to 15.4 0.0001 35.6 48 59.3 to 9.8 0.0004 Laser N 33.3 39 54.7 to 11.9 0.0007 35.5 62 64.4 to 6.6 0.0016 Laser V 28.7 54 45.5 to 12.0 0.0002 31.1 65 53.7 to 8.5 0.0004 P-Value CI 95% confidence interval; SD standard deviation. Negative number indicates that targets had to be closer after surgery (postoperative performance worse than preoperative). *The Dunnett test for difference with respect to wavefront-guided LASIK. Laser A Alcon LadarVision 4000; Laser B Bausch & Lomb Technolas 217; Laser N Nidek EC-5000; Laser V AMO VISX Star S-3. between pupil size and quality of vision complaints after LASIK. 6,7 This study confirms that observation with an objective night task performance. The most important limitation of this study was that the comparison between wlasik and clasik was retrospective. Subjects with moderate myopia were asked to participate in NDS testing. This preoperative patient selfselection may introduce bias. It is possible that patients who were selectively comfortable, or not comfortable, with driving at night or confident, or not confident, in their abilities elected to undergo this additional testing. This could then be expected to skew the preoperative outcomes of the NDS testing. However, because the method of patient selection for NDS testing was the same, this potential bias would have been equally likely to occur with the clasik clinical trial as the wlasik trial. In addition, both trials were prospective. Patient selection for NDS testing was done before operative intervention and would not account for the preoperative to postoperative change in NDS performance. To minimize bias in the selection of subjects for the comparative study, only eyes with a preoperative MSE between 4.50 and 6.00 D were utilized. Beside the necessity for a 6-mm wavefront capture for wlasik, this accounted for the only other difference in the subject enrollment criteria of the 2 clinical trials (MSE vs manifest sphere). In addition, both clinical trials evaluated NDS performance using an identical testing protocol. The same NDS technician conducted all examinations and was masked to the procedure. This technician was not involved in any other examinations. All NDS subjects of both clinical trials were present for the 6-month examination, eliminating follow-up bias. All clinical examinations were conducted by clinicians who were masked to the procedure and not members of the surgical team. The only preoperative difference between the NDS wlasik and clasik cohorts was that the pupil size was larger in the wlasik group. This was because of the requirement for a 6-mm wavefront pupil capture. Nonetheless, pupil size was not found to be a predictor of NDS performance. Another possible limitation was that the clasik participants had treatment with 1 of 4 excimer lasers. A multivariate analysis of variance analysis determined that the type of conventional laser system did not predict the change in NDS performance. This allowed us to pool the clasik results. In addition, all conventional laser systems had a mean reduction in NDS performance and they were all significantly worse than wlasik. This is in contrast with wlasik, for which there was a mean improvement in postoperative performance. Thus, independent of the conventional laser platform, NDS performance was significantly better with wlasik than with clasik. The study only evaluated the treatment of moderate myopia and myopia with astigmatism. No conclusions can be drawn regarding NDS performance for the treatment of hyperopia. It is also important to appreciate another limitation, namely, that the subjects in this study where active-duty military personnel. Their general health was excellent, with no chronic or debilitating medical conditions. In addition, they were relatively young and predominantly male. The study population represents what many would consider ideal candidates for LASIK. There are several salient conclusions that can be drawn from this study. First, the added expense of wlasik to correct myopia and myopia with astigmatism is clearly offset by a significant improvement in night driving performance over a conventional procedure in ideal LASIK candidates. Second, the advantage of wlasik in night visual performance will likely reduce the number of patients who have postoperative difficulty driving at night compared with clasik. This should be of value to patients who undergo LASIK, refractive surgery practices, and society in general. Patients who suffer night visual symptoms can be dissatisfied, require much more clinical attention, expensive diagnostic workups, and can be difficult to treat. Third, wlasik improved mean visual performance compared with spectacles. The ramifications of providing a procedure that improves night visual performance over spectacles has not been explored, but may herald a new appreciation and direction for refractive surgery. References 1. Bailey MD, Zadnik K. Outcomes of LASIK for myopia with FDA-approved lasers. Cornea 2007;26:246 54. 2. Jabbur NS, Sakatani K, O Brien TP. Survey of complications and recommendations for management in dissatisfied patients seeking a consultation after refractive surgery. J Cataract Refract Surg 2004;30:1867 74. 708

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Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes: potential impact on wavefront-guided laser in situ keratomileusis. J Cataract Refract Surg 2005;31:97 105. 30. Waheed S, Chalita MR, Xu M, Krueger RR. Flap-induced and laser-induced ocular aberrations in a two-step LASIK procedure. J Refract Surg 2005;21:346 52. 31. Medeiros FW, Stapleton WM, Hammel J, et al. Wavefront analysis comparison of LASIK outcomes with the femtosecond laser and mechanical microkeratomes. J Refract Surg 2007;23:880 7. Footnotes and Financial Disclosures Originally received: May 30, 2008. Final revision: November 19, 2008. Accepted: December 15, 2008. Manuscript no. 2008-664. The Navy Refractive Surgery Center, Naval Medical Center, San Diego, California. Financial Disclosure(s): S. Schallhorn is a consultant for Advanced Medical Optics and AcuFocus. No other authors has any proprietary or commercial interest in any materials discussed in this article. Funded by the Commander Naval Air Forces but had no role in the design or conduct of this research. Correspondence: Steve Schallhorn, 11730 Caminito Prenticia, San Diego, CA 92131; E-mail: scschallhorn@yahoo.com. 709