Screening Subclinical Keratoconus With Placido-Based Corneal Indices

Purpose. To assess in a sample of normal, keratoconic, and keratoconus (KC) 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 was composed of a total of 124 eyes of 106 patients from the ophthalmic clinics Vissum Alicante and Vissum Almería (Spain) divided into three groups: control group (50 eyes), KC group (50 eyes), and KC suspect group (24 eyes). In all cases, a comprehensive examination was performed, including the corneal topography with a Placidobased 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-Whitney-Wilcoxon nonparametric test and Fisher 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 KC 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 KC that can be added to the current keratometric classifications.

This study has been supported in part by the Thematic Network of the Cooperative Sanitary Research (RETIC) RD07/0062 from the Spanish Institute of Health “Carlos III.” AM-F and GCdL are partially supported by the Research Project FIS PI10/01843 from the Spanish Institute of Health “Carlos III.” AM-F and DR-L are also supported in part by the research group FQM-229 from Junta de Andalucía and by the project MTM2011-28952-C02-01 from the Ministry of Science and Innovation of Spain and the European Regional Development Fund. DR-L is partially supported by the FPU Program from the Ministry of Education of Spain. In addition, AM-F is partially supported by the Excellence Grant P09-FQM-4643 from Junta de Andalucía.