June-July 2020

REFRACTIVE

Yes Connect
Corneal imaging for refractive surgery


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

For 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 biomechanical measurements can overlap between normal and keratoconic eyes and thus may not be the most reliable in detecting keratoconus. As such, he said he doesn’t use technologies that measure corneal biomechanics.
When asked about the different refractive procedures and their impact on corneal biomechanics, both Drs. Huang and Dupps said PRK and SMILE have a lesser effect on corneal biomechanical stability.
“SMILE I think carries some advantage over LASIK in terms of the structural preservation, and we know from the way it works it should be an advantage for most patients to not cut through those anterior structures,” Dr. Dupps said, adding that this is true in simulations as well.
He went on to say that ectasia risk is also lower with PRK.
“There is something about this tapered profile that’s done at the surface of the cornea that I think is friendlier from a structural standpoint vs. a flap that is being cut straight down through a lot of fibers and then you have the ablation,” Dr. Dupps said. “I have a higher percentage of PRK in my practice compared to some people. It’s about 35%.”
Dr. Dupps described a software program in testing called SpecifEye (not commercially available) that runs patient-specific simulations of the various refractive surgery procedures, giving a report on the stresses and strains, structural risk metric, and a refractive outcome.
Dr. Dupps said there has been an evolution in corneal imaging technologies. Fifteen years ago, he said, tomography was not the standard of care; most refractive surgery screening at that time included topography and ultrasound pachymetry at the center of the cornea. A decade ago, tomography took its place as the standard of care and anterior OCT has more recently become a thing, Dr. Dupps said.
“I think most young doctors won’t have been exposed to the epithelial mapping feature and how helpful that can be,” Dr. Dupps said.
While Dr. Dupps acknowledged that he has access to a range of available—and some not yet available—technologies to screen for refractive surgery, he stressed the importance of solid tomography, if little else. It’s important to learn how to use it, mostly by learning what normal, slightly abnormal, and frankly abnormal maps look like.
“The more cases you look at and see how people have interpreted those, the more comfortable you are going to be assessing new patients,” he said. “The other thing I would say is … when someone sets up the device, make sure you are there. … Some of these features you have to understand yourself to teach techs how to use them.”

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

References

1. Schallhorn JM, et al. Distinguishing between contact lens warpage and ectasia: usefulness of optical coherence tomography epithelial thickness mapping. J Cataract Refract Surg. 2017;43:60–66.
2. Moshirfar M, et al. Advances in biomechanical parameters for screening of refractive surgery candidates: A review of the literature, Part III. Med Hypothesis Discov Innov Ophthalmol. 2019;8:219–240.

Relevant disclosures

Dupps
: Alcon, Carl Zeiss Meditec, CorneaGen, Glaukos
Huang: Optovue

Contact

Dupps: bjdupps@outlook.com
Huang: huangd@ohsu.edu

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