AUG 2013

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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Page 33 of 66

August 2013 EW CORNEA 31 Ectasia screening remains center of attention for refractive surgeons by Michelle Dalton EyeWorld Contributing Writer A spate of new studies brings attention back to this potentially devastating complication P ost-LASIK ectasia is rather uncommon (with estimates of the incidence well under 1%), but it can be a devastating complication, which may explain why so much effort has been put into trying to identify and weigh various risk factors. In 2008, the Ectasia Risk Study Score (ERSS) was published1 and validated, yet the debate continues. The ERSS identified "multiple variables in a weighted fashion to improve screening strategies beyond topographic pattern and residual stromal bed thickness," wrote Renato Ambrosio Jr., MD, and J. Bradley Randleman, MD,2 but still generated 8% false negatives and 4% false positives in the original study populations. The ERSS (also known as Randleman's Risk Score System, or RRSS) uses a combination of preoperative central corneal thickness, axial curvature maps of the front corneal surface, degree of myopia, the anticipated residual stromal bed, and the patient's age to assess risk. The original ERSS acknowledged cutoff values for central corneal thickness and residual stromal bed are not absolute and should be analyzed on a case-by-case basis along with other factors. There are, however, "other factors that may play a role in ectasia risk," such as posterior float/elevation or pachymetric profile analysis as seen in the Pentacam (Oculus, Wetzlar, Germany) software, said Colin Chan, MBBS(Hons), FRANZO, conjoint associate professor, UNSW and Vision Eye Institute, Australia. In a recent study,3 Dr. Chan and colleagues found some eyes still had apparently normal topography, even after considering the additional topographic measures. "Other factors, genetic and environmental, must be considered. Family history we believe is important and the ideal refractive screening for a young adult would include topography of both parents. Eye rubbing may also be important as it causes chronic biomechanical stress," he said, especially when a patient has a borderline risk factor score. Surgeons "must be aware of the risk assessment tools for prediction of ectasia when choosing the procedures they recommend their patients undergo," said Elad Moisseiev, MD, Department of Ophthalmology, Tel Aviv Sourasky Medical Center, Israel. "A potentially high risk for ectasia should outweigh any benefits of LASIK." And surgeons should remember that even without any risk factors, a patient may develop ectasia. "This could result in tear and perforation into the anterior chamber. Also, it will now be possible to identify intraoperatively the presence of a type-2 bubble, which occurs in approximately 20% of cases," Prof. Dua said, although in the paper the sample size was relatively small. He believes the experiments his group performed will help to not only identify a type-2 bubble, but also to alert surgeons to take precautions to avoid its tearing or bursting. In the lab, the group was able to blow a bubble, create the type-1 bubble, deflate it and "punch out" a 7 mm disc comprising endothelium, Descemet's membrane and Dua's layer, which Prof. Dua said is much easier than separating just the Descement's membrane and endothelium in the operation of Descemet's membrane endothelial keratoplasty. There is a "possible implication" in acute hydrops of keratoconus, as the condition "might be due to a tear in both the Descemet's membrane and the Dua's layer in the setting of abnormal stromal collagen," Prof. Dua said, who re-emphasized that at the present time, those are just theories. EW Shifting surgical preferences Dr. Moisseiev and colleagues found that the increased awareness of ectasia has also shifted the bulk of laser vision correction from LASIK to surface ablation,4 particularly in cases where the ectasia risk is at a risk score of 2 or more. A younger age, increased myopia, and lower residual stromal bed and corneal thickness measurements were all found to be significantly associated with the preference for surface ablation over LASIK, he said. "I do not think central corneal thickness is the most important factor driving the decision, and I personally have no specific cutoff value for it," he said. "It should be noted New continued from page 30 said. The most commonly performed Descemet's baring technique is the big bubble technique, where it was hitherto believed that Descemet's is separated from the corneal stroma via an injection of air. "We also know that the type-1 bubble is the more frequent and preferred one wherein the separation occurs between the deep stroma and Dua's layer, leaving Dua's layer behind together with the recipient Descemet's membrane and endothelium. This leaves behind a much stronger eye compared to a penetrating keratoplasty even though the layer is only 10-15 microns thick." Plus, because the type-1 bubble has a limit of about 8.5 mm in diameter, a DALK procedure with a larger diameter might cause issues as the cuts would involve the stroma where it had not yet separated from Dua's layer. Reference 1. Dua HS, Faraj LA, Lowe J. Human Corneal Anatomy Redefined: A Novel Pre-Descemet's Layer (Dua's Layer). Ophthalmology. 2013 May 25. pii: S0161-6420(13)00020-1. doi: 10.1016/j.ophtha.2013.01.018. [Epub ahead of print]. Editors' note: Prof. Dua has no financial interests related to this article. Contact information Dua: harminder.dua@nottingham.ac.uk that corneal topography is actually the most important factor to consider for risk of ectasia." Prof. Chan also agrees with the ERSS—"abnormal topography is the most important risk factor for postLASIK ectasia despite the fact that ectasia can occur even with normal topography. A family history of keratoconus is also an important exclusion factor for LASIK," he said. In his practice, patients with inferior steepening of more than 1.5 D are usually offered surface ablation instead of LASIK, "but if there are more suspicious signs of forme fruste keratoconus such as irregular astigmatism, elevated and decentered posterior float and thin pachymetry, then we recommend against even a surface ablation." Prof. Chan believes that "ultimately, other biomolecular markers for ectasia screening need to be developed." Drs. Ambrosio and Randleman said aspects such as "regional and relational corneal thickness metrics" may be able to provide more information than central corneal thickness readings alone. They wrote, "well-established Placido-based criteria" will remain an integral part of the screening process until technology render those criteria outdated. EW References 1. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115:37-50. 2. Ambrosio R, Randleman JB. Screening for ectasia risk: What are we screening for and when should we screen for it? J Refract Surg. 2013;29(4):230-2. 3. Chan CC, Hodge C, Sutton G. External analysis of the Randleman Ectasia Risk Factor Score System: a review of 36 cases of postLASIK ectasia. Clin Experiment Ophthalmol. 2010;38:335-40. 4. Moisseiev E, Sela T, Minkev L, Varssano D. Increased preference of surface ablation over laser in situ keratomileusis between 20082011 is correlated to risk of ectasia. Clin Ophthalmol. 2013:793-8. Contact information Chan: colin.chan@vgaustralia.com Moisseiev: elad_moi@netvision.net.il

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