THE COLLABORATIVE LONGITUDINAL EVALUATION OF KERATOCONUS (CLEK) STUDY TOPOGRAPHY ANALYSIS GROUP

ABSTRACT: Purpose. To determine the test-retest reliability of three popular and commercially available Placido-ring videokeratography instruments in subjects with keratoconus. Methods. Nine subjects (16 eyes) with keratoconus of varying degrees of severity had up to four images per eye generated, in random order, from the EyeSys Model II, Dicon CT 200 and the Keratron Corneal Analyzer. Test-retest analyses for the images were sampled at four locations: 1.5 mm nasal, inferior, temporal, and superior from center. The average standard deviation of all points was used to determine the short-term variability of the measurements. Results. The short-term variability (in diopters) of the Dicon, EyeSys, and Keratron was 0.61 to 3.31 D, 0.94 to 1.51 D, and 0.58 to 2.85 D, respectively, for axial distance maps and 1.07 to 6.82 D, 0.79 to 1.77 D, and 1.23 to 3.03 D for tangential curvature maps. Conclusion. Results support the notion of a loss in repeatability for all three instruments when corneal irregularity is present, which reduces test-retest reliability. (Optom Vis Sci 2001;78:599–604) Key Words:corneal topography, precision, keratoconus, videokeratography Corneal topography is the single most informative measureof corneal regularity and curvature in the diagnosis andfollow-up of patients with keratoconus. Placido disk-based videokeratography is the most widely used type of corneal topo-graphic assessment of corneal curvature in keratoconus. Corneal irregularity has a significant effect on the optical performance o