A recent pilot study in Illinois suggests that the intraocular pressure spike associated with LASIK may cause nerve fiber loss. What makes the study especially interesting is that the use of brimonidine (Alphagan) appeared to prevent such damage in fellow eyes. Alphagan-manufacturer Allergan Pharmaceuticals is now funding a three-center study involving more than 2,000 patients to obtain more-definitive results.

“We’ve noticed, using the GDx Nerve Fiber Analyzer, that there is a dropout of between 5-15% of the total nerve fiber layer following LASIK in patients who have had the suction on the eye for 30 seconds or more,” says Edward Yavitz, M.D., of the University of Illinois College of Medicine. “I only ran a group of 20 patients. But we had 20 out of 20 whose one eye showed nerve fiber thinning at one month. The opposite eye, which was treated by Alphagan, showed no loss.”

To create a uniform flap in LASIK, surgeons apply a suction ring that raises IOP to about 80-85mm Hg. The suction time—which varies from about 15-45 seconds depending on surgical skill and the type of microkeratome—may be a determinant of potential NFL thinning. Dr. Yavitz has found no nerve fiber loss (without Alphagan) in patients for whom the suction time was 20 seconds or less. In a separate study of 60 patients who received no Alphagan and had LASIK with suction times less than 30 seconds, New York surgeon Sanjeev Nath, M.D., likewise measured no significant nerve fiber loss on GDx.

Perhaps the tool matters as well. A microkeratome housing an integrated suction ring may allow for a quicker pass than a less-wieldy two-piece unit. Dr. Yavitz and the other investigators in the multi-center study plan to use various microkeratomes to gauge the effect of different suction times.

Skeptics initially questioned whether Dr. Yavitz’s findings might simply be artifacts reflecting postoperative changes in the cornea, a medium of the polarization measured by the GDx. (The system features a corneal compensator.) Dr. Yavitz refutes the skepticism by noting the absence of nerve damage in the Alphagan-treated eyes. The larger study aims to reproduce his findings using the same protocol as the pilot series: Alphagan tid in one eye and placebo in the other for three days pre-op and one week post-op, with GDx measurements preoperatively and one month after surgery. Dr. Yavitz hopes to complete the study within about a year.

In the meantime, should you warn patients about a possible risk of nerve fiber layer loss in LASIK? Dr. Yavitz thinks we should wait and see. After all, his was merely a small series of 20 patients. And remember, the GDx changes appeared only in cases where the suction time was 30 seconds or more. Besides, says Dr. Yavitz, the 5-15% NFL dropout measured in his patients may be clinically insignificant. The GDx measurements showed that, despite the loss, their NFL thicknesses remained within normal limits. “Since this is a patient population of high myopia, which is more prone to developing glaucoma later in life, we don’t like to see any nerve fiber layer loss at this early age,” Dr. Yavitz says.

Studies suggest that patients can sacrifice 30-50% of their nerve fiber layer before the onset of visual field loss. Even with that leeway, TLC’s J. James Thimons, O.D., considers the extent of NFL loss in the Illinois study cause for concern: “It means you’ve eaten up a third of the available nerve fiber layer before they show a field loss, according to some of the studies. That’s a huge change in the normal anatomy and physiology of the eye.”