Post-LASIK night-driving visual performance shows changes

January 1, 2009

Wavefront-guided LASIK was associated with better night-driving performance than conventional LASIK in two studies conducted in a night-driving simulator. The better outcomes following wavefront-guided procedures could be attributable to fewer higher-order aberrations and use of a femtosecond laser.

Key Points

The significant difference between the two types of treatment results in part from less induction of higher-order aberrations associated with wavefront-guided procedures but also may have been influenced by femtosecond laser use, said Capt. Steven C. Schallhorn, MD, at the annual meeting of the American Academy of Ophthalmology (AAO).

Dr. Schallhorn, a refractive surgeon in private practice in San Diego and medical director for Optical Express, also reported that night-driving performance correlates with night-driving symptoms, whereas contrast sensitivity correlates with identification tasks but not with detection tasks. He presented findings from two related clinical trials at the meeting.

In the two trials, 59 subjects (98 eyes) underwent night-driving testing before and 6 months after LASIK. One trial involved 21 subjects who underwent wavefront-guided femtosecond LASIK using proprietary systems (VISX S4 CustomVue and IntraLase, Advanced Medical Optics); the other trial included 38 subjects whose conventional LASIK procedures were performed with a mechanical microkeratome (Hansatome, Bausch & Lomb). All patients in the latter group were treated with a 6.5-mm optical zone and an 8.0-mm transition zone. They were randomly assigned to one of four conventional LASIK platforms (LADARVision 4000, Alcon Laboratories; 217, Bausch & Lomb; EC-5000, Nidek; or Star S3, VISX).

The average patient age was 33 years, and mean preoperative myopia was –5.36 D (range, –4.50 to –6.0 D). The mean preoperative cylinder was –0.73 D (range, 0.00 to –2.50 D), and the mean mesopic pupil diameter was 6.3 mm (range, 4.0 to 8.0 mm). Contrast acuity testing was performed on all subjects using a backlit 5% contrast acuity chart, and the U.S. Navy visual symptom questionnaire was administered.

Postoperatively, the results of the refractive procedures were good, as expected, Dr. Schallhorn said, with 20/20 uncorrected visual acuity or better being achieved in 83% of subjects. A slight overall loss of contrast sensitivity was reported (≥2 lines, 18%).

Simulator evaluation

For the night-driving evaluation, subjects were tested in a simulator under conditions equivalent to driving on a rural road at night at a speed of 55 miles per hour. Each eye was tested independently before and after surgery in best-corrected spectacle lenses to eliminate possible effects of spectacle blur.

Both detection and identification tasks were given to the patients, and these tasks were performed with and without a glare source, which consisted of low-beam headlights 50 m behind the subject. Investigators recorded 144 threshold measurements per eye after a training period in which the baseline was established. The observer was masked with reference to test conditions.

"In the analysis, we found no differences in the change in performance of each individual with respect to the type of conventional laser used, or the road hazard that was tested, or the eye that was tested, whether it be the left or the right," Dr. Schallhorn said. "So we pooled the data for the analysis into a detection task, with or without a glare source, and an identification task, again with or without the glare source."

Overall, a slight decrease was seen in night-driving performance for detection and identification tasks, both with and without glare, of 5 to 11 feet. Significant variation existed among the patients, however.

The next goal for investigators was to determine predictive factors for the changes in visual performance that had occurred, he said. The analysis of variance examined the influence of demographics (age and gender), preoperative factors (refraction and low-light pupil diameter), treatment (conventional and wavefront-guided), and postoperative factors (refraction, change in best-corrected visual acuity and contrast acuity, glare, and halo symptoms).

Ablation profile

"We found that the ablation profile was very significantly related to the change in performance," Dr. Schallhorn said. "In fact, that was the dominant factor, explaining most of the variability that we saw for detection with and without glare (p < 0.0001). Patients' change in their glare symptoms at night was also predictive, as well as a change in halos. That helped us to validate the night-driving simulator; patients who had complaints were more likely to have worse night driving. Conversely, it also helps to validate the questionnaire."

Change in contrast had a small influence over performance on detection tasks, whereas in identification tasks it emerged as a significant factor (p < 0.0051 without glare, p < 0.0011 with glare). Patients who had the worst contrast sensitivity had the worst results. This is understandable, because contrast sensitivity is important to identification tasks such as reading signs at night, he said.

Other important factors affecting performance on identification tasks were the ablation profile, which was the dominant factor, and change in night-vision symptoms.

The influence of the ablation profile was analyzed using a scale in which a half-second decrease was correlated with a significant decrease in night-driving performance. About 30% of eyes treated with a conventional profile experienced a significant decrease versus 0% to 3% of those who had undergone wavefront-guided treatment.

The low-light pupil size was not a significant factor in night-driving performance, nor were age or preoperative refraction. The relatively young mean age of the study population (33 years) and the relatively tight preoperative refraction were important limitations of the study, Dr. Schallhorn concluded.

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