Videokeratographic Indices to Aid in Screening for Keratoconus

Abstract

ABSTRACT PURPOSE: To design a computer software program with quantitative indices to aid in videokeratographic screening for keratoconus. METHODS: A computer software program was designed to calculate three indices quantifying the keratoconus phenotype from videokeratographics generated by the Topographic Modeling System: Central K (central steepening); I-S values (reflecting inferior-superior dioptric asymmetry); and SRAX (relative skewing of the steepest radial axes above and below the horizontal meridian). One hundred ninety-five normal and 40 keratoconus patients who had not previously worn contact lenses were analyzed with the software. These 235 patients were then studied with combinations of indices to determine how best to distinguish keratoconus from normals by videokeratographic. RESULTS: Central K greater than 47.20 diopters (D) and I-S value greater than 1.20 D detected 39 of 40 (98%) keratoconus patients, with 10 of 195 (5%) normals identified as keratoconus. By adding the SRAX index of greater than 21° to these indices, 39 of 40 (98%) keratoconus patients were identified, but only 1 of 195 (0.5%) normals was identified as keratoconus. CONCLUSIONS: This new index, which quantifies irregular astigmatism in keratoconus, increases the specificity of previously reported quantitative descriptors of keratoconus without decreasing their sensitivity. True sensitivity and specificity of these indices for identifying keratoconus must be verified on a separate patient population. [J Refract Surg. 1995;11:371-379.]