Predicting Ectasia After LASIK
Perhaps the greatest ongoing challenge faced by refractive surgeons is identifying patients who are at risk of developing ectasia after LASIK. Of the various postoperative complications associated with keratorefractive surgery, ectasia appears to be the easiest to avoid with proper preoperative screening. After 10 years of trying to identify the risk factors for its development, however, we refractive surgeons still have only incomplete knowledge of ectasia's etiology.
Researchers have attempted to reduce the incidence of ectasia by designing scoring systems that calculate an individual patient's risk of the complication. This article describes the evolution of the Ectasia Risk Factor Score System and discusses how shifting the focus from corneal thickness to topography might improve surgeons' ability to identify patients at risk for postoperative ectasia.
ECTASIA AND THE INTERNET
The first published reports of a link between LASIK and ectasia focused on two areas of risk: (1) altering the corneas of patients with forme fruste keratoconus and (2) performing deep treatments that leave an excessively thin residual stromal bed.1-3 Despite the appearance of these articles in peer-reviewed journals, however, surgeons participating in peer-to-peer discussion forums on the Internet were slow to recognize ectasia as a real pathological entity. By 2000, the same online forums showed that surgeons were aware of ectasia but also that they had a poor understanding of its etiology and risk factors.
In May 2001, R. Doyle Stulting, MD, PhD, Professor of Ophthalmology at the Emory Eye Center in Atlanta, proposed on an Internet discussion group that preexisting forme fruste keratoconus was a significant risk factor for postoperative ectasia. His comment was based on an assessment of nine patients with subclinical keratoconus who progressed to ectasia after LASIK. He noted, however, that he lacked additional data to definitively support a link between the two conditions. For example, he was not aware of any statistics showing the percentage of patients who had stable outcomes after LASIK despite having forme fruste keratoconus. He also did not have a formal method that would allow him to discriminate between topographic images of normal, abnormal, and at-risk corneas. The first article to compare Dr. Stulting's original cases with control eyes that did not become ectatic after LASIK also confirmed that patients whose topographies showed signs of forme fruste keratoconus had an increased risk of developing ectasia after LASIK.4
THE ECTASIA RISK FACTOR SCORE SYSTEM
By 2008, J. Bradley Randleman, MD; R. Doyle Stulting, MD; and their colleagues at the Emory Eye Center had accumulated enough data to publish the Ectasia Risk Factor Score System5 (Tables 1 and 2).
Their landmark study compared the characteristics of eyes with ectatic corneas seen at Emory and those described in the peer-reviewed literature with a group of control eyes that experienced a normal course after LASIK. In addition to providing key insights into the factors that contribute to ectasia, Randleman et al revealed that the average time to this progressive condition's presentation was 12 months and that 90 of cases occurred by 36 months postoperatively. A closer look at the criteria this system uses to calculate the risk of ectasia, however, suggests that further refinements are necessary.
POTENTIALLY CONFOUNDING FACTORS
Sub-Bowman's Keratomileusis and Femtosecond Lasers
Ninety-five percent of the cases that Randleman et al used to develop the Ectasia Risk Factor Score System involved eyes that had undergone LASIK prior to 2003. Since then, Daniel Durrie, MD, and Stephen Slade, MD, have developed sub-Bowman's keratomileusis (SBK), a procedure that uses the Intralase FS femtosecond laser (Advanced Medical Optics, Inc., Santa Ana, CA) to customize the parameters of the corneal flap for individual patients. The thin, planar flaps created during SBK appear to be more biomechanically stable than those used for traditional LASIK.6,7 Because only a few cases of ectasia have been reported to date in eyes with flaps created by the Intralase FS, it is unclear whether the Ectasia Risk Factor Score System will be an effective tool for evaluating patients undergoing thin-flap LASIK with femtosecond lasers.8
Corneal Topography
According to the Ectasia Risk Factor Score System, a 25-year-old patient whose central cornea measures 510 ?m has an increased risk of developing ectasia after LASIK, even if his topography appears normal. In a study of 9,816 myopic eyes that underwent LASIK by Perry Binder, MD, however, none with a risk factor score of 5 or more and a normal topography became ectatic after LASIK.9 It is our opinion that further work is required to determine how other parameters affect the risk of ectasia for eyes that do not have topographic abnormalities.
Preoperative Pachymetry
The idea that patients who have thin corneas but no other risk factors for ectasia have an increased risk of developing the complication and should therefore avoid LASIK is not supported by the peer-reviewed literature.10 It is also refuted by three studies that failed to identify a link between low central corneal thickness (CCT) and ectasia.11-13 (See CCT as an Independent Risk Factor for a description of how the Ectasia Risk Factor Score System uses CCT to screen?patients for LASIK.)
In addition, a retrospective review of a database of ectatic eyes maintained by one of the investigators (Dr. Trattler) showed that only 10 of 94 eyes with preoperative CCTs of 500 ?m or less developed this complication after LASIK.14 Of the 10 affected eyes, eight also had topographic abnormalities. The only ectatic eyes that had a thin cornea preoperatively but no suspicious topographic findings had -9.00 D of myopia, were not evaluated with intraoperative pachymetry, and underwent LASIK only 1 week after the discontinuation of rigid gas-permeable contact lenses.
Intuitively, one might think that thinner corneas are inherently weaker than thicker ones; however, this correlation has not been proven. The peer-reviewed literature does not describe an increased risk of keratoconus in older individuals (corneal tissue typically becomes thinner and stiffer with age) or in population groups that have thinner-than-average corneas (ie, blacks)
Intraoperative Pachymetry
When the researchers at Emory designed the Ectasia Risk Factor Score System, only 12 of the cases they analyzed included intraoperative pachymetric measurements of the corneal flap. They therefore had to estimate the thickness of the residual stromal bed for the remaining 88.
Although the Ectasia Risk Factor Score System assumes that the surgeons who treated the study population consistently preserved the targeted residual stromal bed, research has now demonstrated that mechanical microkeratomes can create flaps that are thicker than expected.15 If a percentage of the eyes analyzed by Randleman et al had excessively thick flaps and, consequently, unexpectedly thin residual stromal beds, their preoperative characteristics (ie, CCT) might not be reliable indicators of risk for ectasia (see Case Study for a clinical example of this phenomenon). For example, we know that corneas become stiffer with age. A young individual who has thin corneas preoperatively would have a higher risk of developing ectasia after LASIK if his corneal flap is thicker than expected versus an older individual whose thicker, stiffer cornea potentially provides more biomechanical stability, even in the presence of a thin residual stromal bed.
EVOLVING UTILITY
The introduction of the Ectasia Risk Factor Score System is an important step toward expanding refractive surgeons' knowledge of ectasia, but its utility can be improved. Of all the risk indicators associated with post-LASIK ectasia, topographic abnormalities appear most consistently. A recent comparison by Randleman et al of new cases of post-LASIK ectasia (n = 50) with a control group of normal eyes (n = 50) showed that 72 of the ectatic eyes had significant preoperative corneal topographic abnormalities that would place them in the moderate- or high-risk group, independent of other preoperative findings.16
In contrast, a small percentage of eyes in the same study (8.8 from patients younger than 30 years, 5.1 with CCTs of less than 481 ?m, and 13.3 with more than -14.00 D of myopia preoperatively) had scores indicating a moderate or high risk of ectasia.
These findings suggest that corneal topography could be more predictive of ectasia than many of the criteria used by currently available risk calculators. If that is the case, topography should play a greater role in scoring systems used to screen patients for LASIK.
We also believe that refractive surgeons should not declare patients ineligible for LASIK based solely on the results of the Ectasia Risk Factor Score System. Although we agree that patients with topographic findings consistent with forme fruste keratoconus, keratoconus, or pellucid marginal degeneration should not undergo LASIK, we strongly disagree with the score system's assertion that LASIK is automatically contraindicated in a 25-year-old patient with -2.00 D of myopia, normal topography, and a CCT of 510 ?m.
CONCLUSION
As we refractive surgeons develop strategies for preventing ectasia, we should focus on improving our ability to interpret corneal topography. Although this imaging modality has become a standard part of the pre-LASIK workup, we may be missing important clues that can help us identify patients at risk of ectasia. Additional research may also help us identify critical "cut-offs" in eyes with normal-to-borderline topography and thus improve the accuracy of our predictive models.
Perry S. Binder, MD, practices at the Gordon Binder Vision Institute in San Diego. He is a consultant to Advanced Medical Optics, Inc.Dr. Binder may be reached at (858) 455-6800; garrett23@aol.com.
William B. Trattler, MD, is Director of Cornea at the Center for Excellence in Eye Care in Miami and is a volunteer assistant professor of ophthalmology at Bascom Palmer Eye Institute, also in Miami. He has received research grants from, is a consultant to, and is on the speakers' board of Advanced Medical Optics, Inc. Dr. Trattler may be reached at (305) 598-2020; wtrattler@earthlink.net.
Ready to Claim Your Credits?
You have attempts to pass this post-test. Take your time and review carefully before submitting.
Good luck!
