Placido-based indices of corneal irregularity

Abstract

Purpose: To construct a set of metrics computed from the raw keratoscopic data collected by any Placido corneal topographer. These indices that measure the irregularity of the anterior corneal surface are computed directly from the image of the mires reflected on the cornea, which is the most primary data possible. Besides the high sensitivity and specificity, this approach allows bypassing the surface or curvature reconstruction step that is currently performed by the software of any commercial Placido topographer.

Methods: Several basic indices are proposed in order to detect irregularities on the anterior surface of the cornea, via analyzing some geometric and mathematical properties of the mires. These individual primary indices are built in a natural and intuitive way directly from the displacement of the digitized images of the rings reflected on the cornea. Additionally, compound indices are proposed (such as the generalized linear model or the classification trees) by combining some of the so-called primary indices to improve their efficiency. The computed metrics were developed and tested for the CSO topography system (CSO, Firenze,Italy), but the methodology proposed here extends easily to any other commercial Placido disks topographer.

Results: The primary indices allow to discriminate, with excellent accuracy, between normal eyes and eyes with keratoconic corneas. Sensitivity and specificity of the primary indices is analyzed by using the ROC (receiver operating characteristic) curve methodology. Some combined indices are presented, and raise the efficiency to optimal.

Conclusions: All the primary indices proposed exhibit very good performance at discriminating between normal and irregular corneas. The combined indices accuracy is optimal, so avoiding their use in clinical practice as corneal markers of disease. All these indices are fast to compute and can be easily implemented in any corneal topography system.

Keywords: Corneal irregularities keratoconus irregularity index diagnosis corneal topography Placido disks