The quantitative evaluation of total internal reflection in keratoconus

Abstract

Purpose: The phenomenon of apical total internal reflection in the cone of keratoconic patients has been described previously. This study was designed to assess the quantifiable nature of this sign and its ability to diagnose and monitor keratoconus.Method: A sample of 30 patients with varying severity of keratoconus was compared with a series of sex and age matched normal controls and selected cases of spherically equivalent myopia. Each patient had a comprehensive corneal assessment including pachymetry, corneal topography, and standardised oblique slit lamp photography. The slit lamp photographs were digitally analysed to assess the area of total internal reflections. The severity of keratoconus was graded with a six stage ranking system based on the clinical appearance.Results: The area of total internal reflection was compared to the ranking system of clinical grading. For moderate and marked cones, the area of total internal reflection correlates well with both the clinical appearance and corneal topography. For mild and borderline cases, the photographs show a less well defined area but this is nevertheless distinguishable from normals or spherically‐matched equivalents.Conclusions: The area of total internal reflection visible in a keratoconic cornea correlates with the established techniques of grading keratoconus. Imaging this phenomenon may form the basis of a quantitative clinical for this condition.