January 2021 Image of the Month

Image of the Month spotlights diagnostic images captured by corneal specialists. Some include case studies, while others include captions.

Epithelial Mapping for Corneal Ectasia

By Brian Shafer, MD

BACKGROUND

Imagine this: A patient in his early 20s presents for a refractive surgery evaluation. As part of the evaluation, the patient undergoes a manifest refraction, cycloplegic refraction, tomography, OSDI, and aberrometry. Results show subtle posterior elevation with otherwise normal anterior corneal curvature, normal Belin/Ambrósio analysis and normal D value. How can we determine with confidence whether to proceed with keratorefractive surgery? The answer is by using epithelial mapping.

EPITHELIUM OVERVIEW

The corneal epithelium is a dynamic layer that remodels in response to underlying stromal irregularity. For example, following myopic ablations in LASIK or PRK, the epithelium often compensates by thickening over the ablation zone. In corneal ectasia, the weakened collagen leads to stromal thinning with early breaks in Bowman’s Layer and late breaks in Descemet’s membrane, or hydrops. In a cornea that has early ectasia, or forme fruste keratoconus, the topography may look entirely normal, despite a developing cone. To compensate for the ectasia, the epithelium develops central thinning with an annulus of thickened epithelium overlying the cone. As a result, axial topography appears deceivingly regular.

ENTER EPITHELIAL MAPPING

First performed with ultrasound, epithelial mapping is now readily performed with OCT-based systems. On average, epithelial thickness is around 53 μm. A normal epithelial map shows even thickness across the entire cornea with less than a 5 μm difference from the center to the midperiphery. The compensatory apical thinning and annular thickening seen in keratoconus can be detected reliably and early using epithelial mapping.

THE YOUNG PATIENT

After the patient’s irregular tomography, many surgeons would be at a crossroads. On the one hand, a seemingly normal topography would make it reasonable to wait 6 to 12 months to re-evaluate and see whether there are any signs of ectasia progression. On the other hand, if epithelial mapping shows a greater than 5 μm difference in thickness, particularly with a thinner area that has surrounding thickening overlying the region of posterior elevation, holding off on refractive surgery would make sense. Instead, such a patient may benefit from long-term monitoring for ectasia and, ultimately, corneal cross-linking.

IMAGE GLOSSARY

1. Anterior segment OCT in a patient with keratoconus demonstrating thinning of the epithelium overlying the area of ectasia with surrounding thickening of epithelium.
2. Epithelial map with thin central area (red) and annular thickening surrounding the region of thinning (green, blue)

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