Ectasia is a recognized complication of LASIK in a minority of cases, but the degree of risk from pre-existing conditions remains largely unknown.
A preliminary analysis of a dataset containing more than 300,000 eyes treated with LASIK between 6 to 10 years ago is beginning to quantify some of the risks involved.
“This is the largest study of ectasia in a cohort of successive patients—361,848 eyes of 184,728 patients,” said Steven C. Schallhorn, MD, the initial principal investigator.
“Some of the early results are not surprising—specifically, that age and corneal shape are the dominant factors related to keratoconus after LASIK,” he said. “Other results were not quite what was expected.”
Dr. Schallhorn presented preliminary results on an ongoing analysis of a database created by Optical Express, the largest provider of refractive surgery in Europe. He launched the study as the chief medical director of Optical Express and oversaw the initial analysis. He currently maintains a private practice in San Diego, CA.
Refractive laser surgery is noted for its high rates of safety and patient satisfaction, but complications can, and do, arise. Within the entire cohort, ectasia was seen in 199 eyes of 146 patients during follow up, an incidence of about 0.055%.
Age and corneal shape before surgery have long been recognized as risk factors for ectasia. Because ectasia is such a rare complication, nearly all prior studies have been case-control.
Case-control studies can help to identify risk factors for rare conditions and infrequent complications, but cannot quantify risks, he noted.
“As clinicians, we want to know if a particular preoperative condition poses a risk for ectasia and, more importantly, the level of risk,” Dr. Schallhorn continued.
“If the chance of ectasia is 1:2,000 in the general population and a preoperative factor increases the risk to 1:1,900, it may be statistically significant but it is not very clinically relevant,” he said. “The goal of this study is to help quantify risk factors so that we can better select and counsel patients.”
Optical Express compiled a dataset of all patients treated with refractive laser surgery at its facilities between Jan. 3, 2007 and April 30, 2011. The company has a comprehensive electronic medical record of all patients from preoperative exams through surgery and long-term follow up.
The complete topography dataset is cumbersome to evaluate using traditional analysis techniques, so researchers randomly selected 3,700 eyes of 1,850 patients treated in 2008 who did not develop ectasia for this initial analysis. The analysis presumed that the corneal shape of this group was representative of the entire data set. Ongoing work will use more sophisticated data analysis techniques that are capable of evaluating the entire dataset.
This first analysis used demographic and preoperative data from the entire population and preoperative Pentacam topography from the 3,700 eyes. The mean time to the development of ectasia was 44.5 months. The mean age of patients in the entire cohort was 38.5 years, and the mean age of patients who developed ectasia was 29.2 years.
The preoperative sphere for the entire cohort was -1.94 and -2.88 D for patients who developed ectasia. Preoperative cylinder was -0.79 D for the entire cohort and -0.91 D for patients who developed ectasia.
Preoperative central corneal thickness for the entire cohort was 549 μm compared with 531 μm for ectasia patients. Corneal thickness was measured by the thinnest of the Pentacam and three ultrasonic pachymetry readings.
Topography showed clear distinctions between the entire cohort and ectasia patients. Patients who developed ectasia were more likely to have forme fruste keratoconus before surgery, inferior steeping, and a skewed radial axis.
However, in a large number of ectasia cases the topography was read as normal. Univariate analysis showed a number of variables to be significant in ectasia, such as age, refraction, flap type, and residual stromal bed.
Multivariate analysis identified the strongest predictors: age and corneal shape. In this analysis, age less than 30 years carried an odds ratio of 2.5 for ectasia and inferior steepening or skewed radial axis an odds ratio of 5.2. Combining either of those two irregular topographies with younger age upped the odds ratio for ectasia to 12.4.
Significantly, preoperative forme fruste keratoconus increased the chance of developing ectasia twenty-fold, and when combined with younger age, the odds ratio increased to 33.
“These results are fascinating, but the caution is that they are preliminary,” Dr. Schallhorn said. “Nevertheless, this is the kind of information we need as refractive surgeons to better understand keratoconus after LASIK. The risk may be small, but our preliminary analysis is showing great potential to quantify the level of risk.”