Several preoperative features have been identified as important risk factors for ectasia after laser refractive surgery, particularly in patients undergoing LASIK for high myopia or myopic astigmatism. This complication can occur in the absence of any of these findings, however.
"There is good consensus that preoperative corneal topographic abnormalities, corneal thickness, residual stromal bed thickness, high myopia and myopic astigmatism, and patient age are important risk factors for ectasia. However, this event is probably less iatrogenic than we think," said Dr. Netto, Department of Ophthalmology, University of São Paulo, Brazil.
"It appears some patients are genetically predisposed to develop ectasia post-LASIK even in the absence of any identifiable risk factors and or even without undergoing laser refractive surgery," he said. "Therefore, when ectasia occurs postoperatively, perhaps refractive surgeons do not always need to feel guilty."
The first lesson was that ectasia is likely to evolve in patients with subclinical keratoconus. Of the 15 cases of ectasia in the series, three (20%) involved eyes determined to have subclinical keratoconus preoperatively.
The second lesson was that ectasia can develop after LASIK for hyperopia. This situation was represented in one of the 15 eyes, a patient who had preoperative refractions of +2.5 and +3.25 D and normal corneal thickness.
"The literature also contains some reports of ectasia developing after hyperopic LASIK," he said.
The third lesson was that ectasia can develop after PRK for low myopia. Dr. Netto's series of ectasia cases also included a single eye with this history and no evidence of suspicious topography preoperatively, and he noted the similar reports in the literature.
Nine eyes in the series were the basis for a fourth lesson, which was that ectasia can develop in eyes without any evident risk factor.
"This situation has also been previously reported and so was not too surprising," Dr. Netto said. "What was more surprising was that ectasia did not develop in a number of eyes that had several preoperative risk factors and long-term follow-up extending up to 10 years postoperatively."
He noted that Perry Binder, MD, also reported in the Journal of Cataract and Refractive Surgery (2007;33:1530-1538) absence of ectasia development in eyes with multiple risk factors. This experience suggests that certain features, which are not yet identified, may be useful for predicting risk.
Recognizing the limitations of current screening approaches, Dr. Netto suggested that in addition to evaluating patients for accepted probable risk factors, including taking particular care in examining the preoperative topography, other possible predisposing features may be sought in the history-taking or using new technologies.
"A review of personal files reveals additional potential risk factors for post-LASIK ectasia that include a habit of eye rubbing, family history of keratoconus, increased level of higher-order aberrations, best-corrected visual acuity less than 20/25, and topographic and refractive instability," he said.
Additional diagnostic measurements that may be obtained include posterior corneal surface and corneal thickness maps, Dr. Netto said. Also, corneal biomechanical properties can be evaluated by measuring corneal hysteresis and the corneal resistance factor, he added.
"However, further study is needed to establish the utility of these newer assessments as ectasia screening tools," Dr. Netto said.
Meanwhile, topography and ultrasound pachymetry remain the mainstays for preoperative screening of at-risk patients. Findings of inferior-superior asymmetry more than 1.4 D, a central keratometry value greater than 47.2 D, non-orthogonal astigmatism, and significant asymmetry of topographic patterns between eyes are considered topographic risk factors for post-LASIK ectasia. Regarding pachymetry, although surgeons should review preoperative corneal thickness and predicted residual stromal bed thickness, it is important to be aware that no "magic numbers" exist for these parameters that would identify patients who can safely undergo LASIK.
"Management decisions should be guided by considering these data together with the preoperative refraction, topographic findings, and other clinical features," he concluded.