Screening subclinical keratoconus with Placido-based corneal indices

Abstract

Purpose: To assess in a sample of normal, keratoconic and keratoconus suspect eyes the performance of a set of new topographic indices computed directly from the digitized images of the Placido rings.

Methods: This comparative study comprised a total of 124 eyes of 106 patients from the ophthalmic clinics Vissum Alicante and Vissum Almería (Spain), in three groups: control group (50 eyes), keratoconus group (50 eyes) and keratoconus suspect group (24 eyes). In all cases, a comprehensive examination was performed including the corneal topography with a Placido-based CSO topography system. Clinical outcomes were compared among groups, along with the discriminating performance of the proposed irregularity indices.

Results: Significant differences at level 0.05 were found on the values of the indices among groups by means of Mann-Witney-Wilcoxon non-parametric test and Fisher’s exact test. Additional statistical methods, such as receiver operating characteristic analysis and K- fold cross-validation, confirmed the capability of the indices to discriminate between the three groups.

Conclusions: Direct analysis of the digitized images of the Placido mires projected on the cornea is a valid and effective tool for detection of corneal irregularities. Although based only on the data from the anterior surface of the cornea, the new indices performed well even when applied to the keratoconus suspect eyes. They have the advantage of simplicity of calculation combined with high sensitivity in corneal irregularity detection, and thus can be used as supplementary criteria for diagnosing and grading keratoconus that can be added to the current keratometric classifications.

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