OCT corneal epithelial topographic asymmetry as a sensitive diagnostic tool for early and advancing keratoconus

Abstract

Anastasios John Kanellopoulos,1,2 George Asimellis1 1LaserVision.gr Eye Institute, Athens, Greece; 2Department of Ophthalmology, New York University Medical School, New York, NY, USA Purpose: To investigate epithelial thickness-distribution characteristics in a large group of keratoconic patients and their correlation to normal eyes employing anterior-segment optical coherence tomography (AS-OCT).Materials and methods: The study group (n=160 eyes) consisted of clinically diagnosed keratoconus eyes; the control group (n=160) consisted of nonkeratoconic eyes. Three separate, three-dimensional epithelial thickness maps were obtained employing AS-OCT, enabling investigation of the pupil center, average, mid-peripheral, superior, inferior, maximum, minimum, and topographic epithelial thickness variability. Intraindividual repeatability of measurements was assessed. We introduced correlation of the epithelial data via newly defined indices. The epithelial thickness indices were then correlated with two Scheimpflug imaging-derived AS-irregularity indices: the index of height decentration, and the index of surface variance highly sensitive to early and advancing keratoconus diagnosis as validation.Results: Intraindividual repeatability of epithelial thickness measurement in the keratoconic group was on average 1.67 µm. For the control group, repeatability was on average 1.13 µm. In the keratoconic group, pupil-center epithelial thickness was 51.75±7.02 µm, while maximum and minimum epithelial thickness were 63.54±8.85 µm and 40.73±8.51 µm. In the control group, epithelial thickness at the center was 52.54±3.23 µm, with maximum 55.33±3.27 µm and minimum 48.50±3.98 µm epithelial thickness. Topographic variability was 6.07±3.55 µm in the keratoconic group, while for the control group it was 1.59±0.79 µm. In keratoconus, topographic epithelial thickness change from normal, correlated tightly with the topometric asymmetry indices of IHD and ISV derived from Scheimpflug imaging.Conclusion: Simple, OCT-derived epithelial mapping, appears to have critical potential in early and advancing keratoconus diagnosis, confirmed with its correlation with established Scheimpflug-derived asymmetry topometric indices. Keywords: anterior-segment optical coherence tomography, early and advancing keratoconus, epithelial thickness imaging, novel corneal epithelial thickness-asymmetry indices, three-dimensional mapping, Scheimpflug imaging based Pentacam, topographic keratoconus grading, index of surface variance, index of height decentration