EyeWorld India September 2020 Issue

REFRACTIVE EWAP SEPTEMBER 2020 35 F or refractive surgery, the preop evaluation for candidacy is almost more important than the procedure itself. A large part of this evaluation includes corneal imaging. William Dupps, MD, PhD, said the “single most important technology” for refractive surgery evaluation is tomography. Dr. Dupps said he has access to the Pentacam (Oculus) and Galilei (Ziemer Ophthalmic Systems), calling the Scheimpflug imaging systems very similar. Tomography, refractive error, and best corrected visual acuity can determine whether most of his patients are a refractive surgery candidate, Dr. Dupps said. “I will open up the Pentacam and go through it very carefully to look for any signs of corneal irregularity, especially focal steepening and elevation of the front of the cornea,” he said. “Then I will focus on looking for similar colocalizing features on the posterior corneal maps.” David Huang, MD, PhD, similarly first looks at the topography map, specifically scanning for signs of forme fruste keratoconus (inferior steeping or a skewed axis). From there, if the topography is suspicious, he said OCT can help distinguish between keratoconus and topographic distortions due to primary epithelial deformation, such as contact-lens related warpage, dry eye, and epithelial basement membrane dystrophy. 1 Dr. Huang stressed the importance of learning how to interpret OCT epithelial and pachymetry maps. “In keratoconus, there is coincident epithelial and pachymetric thinning at the location of focal steepening. In contrast, primary epithelial deformation is often characterized by epithelial thickening in steep areas and thinning in flat areas,” he said. “Finally, I calculate the expected residual stromal bed thickness both in micron terms and as a fraction of preoperative corneal thickness to make sure both are within acceptable limits,” Dr. Huang said. Dr. Dupps said he gets an OCT on every patient he sees, finding it to be a “useful way to take a patient who might look a little suspicious in terms of if they have a small area of inferior steeping but the posterior cornea looks normal.” More specifically, Dr. Dupps said OCT’s epithelium mapping feature allows him to look at whether there is thinning of the epithelium over a steep spot of the cornea (which could indicate keratoconus). Conversely, the epithelium could just be thicker, causing the curvature feature on the surface that isn’t really corneal ectasia. This type of epithelial irregularity, he said, could be addressed with PRK or topography-guided LASIK. “We have a lot of split decisions that will often fall on one side or the other by adding the OCT epithelial mapping,” Dr. Dupps said. Other tools that Dr. Dupps uses in these split decisions are of interest but not necessarily available yet in the United States. He said the Corvis ST (Oculus) is CE marked in Europe but being beta tested in the U.S. This device gives information about the biomechanical response to the cornea. “We can start to sift these corneas into structurally robust corneas vs. those that might be at higher risk for corneal ectasia,” Dr. Dupps said. The Ocular Response Analyzer (Reichert) also provides information on corneal biomechanics. “There is significant interest in characterizing corneal biomechanical properties in the hopes of advancing screening methods for refractive surgery candidates,” Moshifar et al. wrote in a 2019 literature review. 2 Based on this review, the authors concluded that “it is warranted to consider in vivo mechanical assessment as an appropriate approach for screening of corneal ectasia” because “changes in biomechanical properties may occur before disease becomes apparent via tomography or topography.” The study authors also wrote that this review showed several clinical applications for biomechanical parameters, including assisting in screening of surgical candidates and tracking postop changes. Dr. Huang cautioned that Corneal imaging for refractive surgery by Liz Hillman Editorial Co-Director This article originally appeared in the June/July 2020 issue of EyeWorld . It has been slightly modified and appears here with permission from the ASCRS Ophthalmic Services Corp. Contact information Dupps: bjdupps@outlook.com Huang: huangd@ohsu.edu