Night-driving performance improves after wavefront-guided LASIK

March 1, 2008

Changes in night driving performance after LASIK were compared among patients treated with an all-laser wavefront-guided technique versus a group undergoing conventional ablation with a mechanical microkeratome-created flap. The results show significant differences favoring the wavefront-guided procedure.

New Orleans-Wavefront-guided LASIK offers a marked improvement in functional vision compared with conventional LASIK as measured by postoperative change in night-driving performance, according to results from retrospective analyses presented by Capt. (retired) Steven C. Schallhorn, MD, at the annual meeting of the American Academy of Ophthalmology.

In a refractive surgery free paper session, Dr. Schallhorn reported findings from evaluations conducted using a night driving simulator in 21 subjects who had undergone wavefront-guided LASIK and 38 subjects who had conventional LASIK. All of the patients had a preoperative SE in the range of -4.0 to -6.0 D and performed the night driving test preoperatively and at 6 months after surgery. The simulation required that they both detect and identify hazards under glare and no glare conditions. The analyses were based on changes in detection and identification distances before and after surgery.

The results showed that average night driving performance was reduced after conventional LASIK, whereas it improved significantly among those who underwent the wavefront-guided procedure. Not only were significant differences found between the treatment groups comparing average changes in detection and identification distances, but "startling differences" were found favoring the wavefront-guided treatment in analyses of the proportions of patients who had clinically relevant improvement and worsening, Dr. Schallhorn said. He is in private practice in San Diego and is medical director of Optical Express.

"Although this study is subject to the limitations of a retrospective analysis, its strength is that it assessed outcomes with a performance-based task rather than just evaluating functional vision based on contrast sensitivity testing. The combination of wavefront-guided treatment with a femtosecond laser-created flap is the first refractive surgery procedure for which we have documented an improvement in night driving performance, and the compelling data from these analyses provided the basis for the decision to authorize wavefront-guided, all-laser LASIK in U.S. Navy aviators," Dr. Schallhorn said.

Patients included in the analyses participated in two prospective studies that were undertaken at the Naval Medical Center in San Diego. At the time those trials were conducted, Dr. Schallhorn was director of refractive surgery.

The wavefront-guided LASIK study was performed between 2003 and 2005. The ablation was performed with an excimer laser (VISX S4, Advanced Medical Optics [AMO]) using a 6.0-mm optical zone and 8.0-mm transition zone with Fourier analysis but no iris registration. All flaps were created with a femtosecond laser (IntraLase, AMO) with an intended thickness of 100 µm.

The conventional LASIK study was performed between 1999 and 2002. It randomly assigned patients to treatment with one of four excimer lasers (LADARVision 4000, Alcon Laboratories; Technolas 217, Bausch & Lomb; EC-5000, Nidek; or VISX S3, AMO). The ablations had a 6.5-mm optical zone and 8.0-mm transition zone, and flaps were created using a microkeratome (Hansatome, Bausch & Lomb) with a 160-µm plate.

"This was a costly and time-consuming study, and the results were overwhelmingly in favor of the all-laser, wavefront-guided LASIK procedure. It is unknown whether the results would be similar comparing custom LASIK with LASIK performed using an aspheric ablation profile. It would be difficult and expensive to conduct a prospective, randomized study of that kind. However, it would be interesting to see how patients who undergo a wavefront-optimized procedure compare with a group treated with a wavefront-guided technique," Dr. Schallhorn noted.

Comparisons of the wavefront-guided and conventional groups showed that they were well matched on mean age, preoperative mean spherical equivalent, cylinder, and mesopic pupil size. The 6-month refractive outcomes showed that both treatments performed well in reaching the target of emmetropia, although the customized treatment had better predictability and was associated with better uncorrected visual acuity and contrast sensitivity outcomes. No eyes in either group lost two or more lines of best-corrected visual acuity.

Night driving performance

The assessment of night-driving performance simulated driving 55 mph on a rural road. Testing was performed in each eye independently and with best spectacle correction to eliminate any effect of spectacle blur. Subjects were instructed to push a button when they saw a road hazard (e.g., pedestrian, traffic sign, business sign) and when they could identify the hazard. Testing with a glare source replicated the situation of a car driving 50 m behind with low-beam headlights shining in the rearview mirror.

The technician assisting subjects with the testing was masked to the surgery performed. All subjects underwent initial testing to ensure that their reaction times and detection times reached a plateau, and at both visits, the testing involved 144 threshold measurements per subject (72 identification tests, 72 detection tests). Results for the identification tests and detection tests were pooled for the analyses comparing the two surgical procedures based on results of multivariate analyses that showed no significant differences in the change in performance from pre- to postoperatively with regard to type of conventional laser, road hazard, or eye tested (right versus left).

When members of the conventional LASIK group were tested without glare, they had an average 18-foot worsening in the distance needed to detect a road hazard after surgery and an average 23-foot worsening in the identification distance. In the wavefront-guided group, subjects, on average, could detect a road hazard 20 feet earlier after surgery compared with before, and identification was achieved 31 feet earlier, on average. The differences between groups in their changes in performance after surgery were statistically significant (Figure 1).

For testing with glare, the results were relatively similar in the wavefront group to those measured without glare; the conventional group showed slightly greater worsening of performance after surgery under the glare conditions. Again, all differences between the wavefront-guided and conventional groups were statistically significant.

National Highway Traffic and Safety Commission criteria were used to define a significant loss and gain of night driving performance. Using a change of greater than 0.5 seconds in reaction time, which is equivalent to a change of more than 44 feet when driving at 55 mph, 38% to 42% of eyes in the conventional group had a significant loss of night driving performance for all four tasks analyzed (detection and identification with and without glare). No eyes in the wavefront-guided group experienced a significant loss during testing of detection with glare. For the remaining three tasks, only 3% of eyes in the wavefront-guided group demonstrated a significant worsening of performance.

When considering the proportion of eyes that experienced a significant improvement, 6% to 13% of patients in the conventional group showed significantly better performance after compared with before surgery, whereas 18% to 46% of eyes performed significantly better after customized wavefront-guided surgery.OT