|
CONVENTIONAL LASIK can significantly degrade night driving visual performance, a US clinical study confirms.Speaking at the XXI Congress of the European Society for Cataract and Refractive Surgery, Steve C. Schallhorn MD presented a study of 105 consecutive patients who underwent bilateral LASIK for moderate to high myopia (range -3.63 to -11.25 D, mean -6.32 D). The study specifically compared patients' responses to night-time driving conditions before and after surgery.
The night driving simulator tested the distance needed to detect and identify three target objects - a business sign, a traffic sign, and a pedestrian - when driving 55 mph on a rural road with low-beam headlights. The testing was done with patients at their best-corrected acuity in trial frames, and each eye was tested independently. For each of the detection and identification tasks, six thresholds were measured with and without a glare source. The results showed that for all 12 test conditions patients needed to be significantly closer after surgery than before to be able to detect or identify the target objects. The researchers converted the decrease in performance distance post-LASIK into recognition time based on the travelling speed. The detection time for some targets would be prolonged by one second or longer in more than 40% of patients, reported Dr. Schallhorn.
There are some limitations to this study because it was done with monocular testing and with the patient wearing trial frames. However, from these data, we still conclude that we need to develop refractive surgery techniques that do not induce aberrations affecting night driving performance. These findings may be another reason for performing wavefront-guided surgery, and we are currently investigating that possibility in an ongoing study of patients undergoing custom cornea ablations." he conventional LASIK procedures in the present trial were performed with a large ablation zone, typically 6.5 mm with a transition zone. Patients were not excluded because of pupil size, which exceeded 8.0 mm in some eyes. The six-month refractive and UCVA outcomes for the cohort of 105 patients were good, especially considering the relatively high levels of myopia treated. The mean postoperative MSE was -0.21 D (range, +0.87 to -2.62 D) and 73% of patients could see 20/20 or better and 95% achieved UCVA of 20/40 or better. Best-corrected visual acuity improved after surgery by one line or more in about 30% of eyes, and only a few patients had a loss of two or more lines. The night driving simulator results, both with and without glare, and before and after surgery showed that patients were able to both detect and identify the traffic sign earliest (at the farthest distance), followed next by the business sign and then the pedestrian. Preoperatively, the mean distances for detection of the three objects with or without glare ranged from 730 to 876 feet; the distances at which the objects were identified ranged between 611 and 802 feet, respectively. At six months after surgery, the mean distance for performing each of the 12 tasks was reduced by between 14 and 41 feet (1.8% to 6.5%).r. Schallhorn and colleagues also used regression statistical methods to try to identify variables that influenced night driving vision performance. Included in those analyses were a number of preoperative characteristics and results from 5% contrast acuity testing as well as from a subjective vision quality questionnaire. They identified statistically significant correlations between reduced detection distance and higher preoperative MSE, worse halo complaints at six months, and lower levels of 5% contrast acuity at six months. Pupil diameter had no significant predictive value. "The absence of a significant relationship between worsening of night driving performance and pupil size is consistent with other studies we have done showing pupil size is not a major factor in quality of night vision after LASIK. Our finding that higher levels of preoperative myopia explain some of the variance in vision performance is understandable, recognising that the amount of higher-order aberration induced after conventional LASIK, especially spherical aberration, increases with increasing amount of correction," Dr. Schallhorn said. The data were not analysed with patients stratified by age, although Dr. Schallhorn observed the population was a relatively young group, clustered predominantly in their mid 30s and with the oldest patient only around 45 years old.teven Schallhorn |
