Post-LASIK ectasia possible with no known risk factors

"Careful preoperative evaluation to screen out inappropriate candidates is critical for avoiding ectasia post-LASIK, but we also encourage surgeons to consider routinely measuring intraoperative stromal bed pachymetry," said Dr. Klein, clinical instructor, Institute of Ophthalmology and Visual Science, New Jersey Medical School, Newark. "However, even in the eye that appears to have no risk factors, ectasia can occur."

Dr. Klein together with Randy J. Epstein, MD, J. Bradley Randleman, MD, and R. Doyle Stulting, MD, PhD, collected a series of cases of post-LASIK ectasia that developed in the absence of risk factors. Over a 7-month period in 2003, they solicited cases through three Internet news groups.

The risk factor exclusion criteria consisted of: calculated residual stromal bed <250 μm, preoperative central pachymetry <500 μm, any keratometry reading >47.2 D, I - S value >1.4, attempted initial correction > –12 D, Orbscan II posterior float >50 μm (when available), > two re-treatments, and surgical/flap complications.

"We also went back to the literature to find cases of ectasia that met our study criteria and even when limiting our review to eyes with a residual stromal bed >250 μm, it was very difficult to find cases that would satisfy our rigorous exclusion criteria," Dr. Klein said. "Most of the cases in the literature had topographies that we found to be suspicious when evaluated very carefully or were lacking other data that are important for determining risk."

Three hypotheses

Dr. Klein proposed three hypotheses to account for why patients might develop ectasia despite having no apparent risk factors. One possibility is that the residual stromal bed was inadequate due to creation of an excessively thick flap and/or excessive ablation.

"Intraoperative data from stromal bed pachymetry were only available for one of the nine eyes in our series and so we did not know flap thickness for most of the cases," he noted.

Dr. Klein also proposed that these patients may have had preoperative topographic abnormalities that were undetected because the machines were not sensitive enough or not used correctly. Supporting that possibility is the fact that the submitted scans for some of the patients were of limited detail, making I - S calculations difficult.

According to Dr. Klein, a third possibility is "most tantalizing," and that is that these patients have biomechanically unstable corneas with no preoperative abnormalities detectable using widely available technology.

"I like to think of these cases as a forme fruste keratoconus and that they are unable to withstand the insult of LASIK," Dr. Klein said.

Of interest with respect to that concept, in an analysis of the demographic and operative data for the series of eight cases, age emerged as a potentially remarkable factor for the group. The eight patients in the series were found to be significantly younger than the mean age reported in a previously published series [Randleman, et al. Ophthalmology 2000;110:267-275] for both patients who developed ectasia and control groups.

"Perhaps some of these patients might have eventually developed keratoconus even if they had not undergone LASIK, but the surgery accelerated the onset," Dr. Klein explained. "However, our data suggest that the risk of ectasia in the absence of known risk factors may be higher in younger patients."