Eyeworld

30 | EYEWORLD | JUNE/JULY 2020 R YES CONNECT EFRACTIVE by Liz Hillman Editorial Co-Director Screening patients for refractive surgery is a critical step in ensuring both the safety and success of the procedure. In this month's YES Connect column, we asked two experts in this area to share their tips on using corneal tomography and OCT to evaluate refractive surgery patients. Much of what I use to screen patients I have learned from these two amazing individuals, and I hope that you will find their insights useful. —Julie Schallhorn, MD YES Connect Co-Editor F or refractive surgery, the preop eval- uation 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 Sys- tems), 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 steepen- ing 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 epithe- lial deformation, such as contact-lens related warpage, dry eye, and epithelial basement mem- brane dystrophy. 1 Dr. Huang stressed the importance of learning how to interpret OCT epithelial and pachymetry maps. "In keratoconus, there is coincident epithe- lial 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 nor- mal." 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 epitheli- um 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 topogra- phy-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. Corneal imaging for refractive surgery About the doctors William Dupps, MD, PhD Professor of ophthalmology Cleveland Clinic Lerner College of Medicine Cleveland, Ohio David Huang, MD, PhD Martha and Eddie Peterson Professor of Ophthalmology Casey Eye Institute Oregon Health & Science University Portland, Oregon

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