Placido-based indices of corneal irregularity.

Purpose: To construct a set of indices that measure the irregularity of the anterior corneal surface, computed directly from the image of the Placido disks reflected on the cornea. 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 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 directly from the displacement of the digitized images of the rings reflected on the cornea. In addition, compound metrics are proposed (such as the generalized linear model or the classification trees) by combining some of the 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 discriminating, with excellent accuracy, between normal eyes and eyes with keratoconic corneas. Sensitivity and specificity of the primary indices is analyzed by using the receiver operating characteristic (ROC) curve methodology. Some combined indices are presented, which raise the efficiency to optimal.
Conclusions: All the primary indices proposed in this work exhibit very good performance in discriminating between normal and irregular corneas. The accuracy of the combined indices is optimal within the test group (perfect classification), allowing their use in clinical practice as corneal markers of a disease. All these indices are fast to compute and can be easily implemented in any corneal topography system.