A quantitative corneal topography index for detection of keratoconus

Detection of keratoconus has become a critical issue in patients who are potential candidates for refractive surgical procedures. In eyes with early stages of keratoconus, slit-lamp corneal changes are either absent or too subtle for detection, and keratometry may be normal. Videokeratography systems have greater sensitivity for detection of such cases, but interpretation of the topographic map is sometimes difficult, especially when pathologies with similar topographic patterns are suspected. We introduce an index using corneal topography for detection of keratoconus.

From March 1994 to December 1995, a total of 513 unoperated eyes, with no pathology other than keratoconus, from 283 consecutive patients were categorized into three groups based on slit-lamp examination and corneal topographic findings. Those with clinically obvious keratoconus were placed in the first group (53 eyes [10.3%]), those with keratoconus suspect comprised the second group (17 eyes [3.3%]), and the third group was that of normal eyes (443 eyes [86.4%]). In all videokeratographs of these eyes, the highest rate of corneal power changes (in diopters per millimeter) away from the apex (the point of maximum power) to the periphery was measured along its semimeridian. This index, called the "highest rate of steepening (HRS)", was statistically analyzed in the three groups.

The mean +/- SD of highest rate of steepening was 3.60 +/- 1.70 D/mm (range 1.59 to 7.53 D/mm) in the clinically obvious keratoconus group, 1.59 +/- 0.21 D/mm (range 1.35 to 1.98 D/mm) in the keratoconus suspect group, and 0.72 +/- 0.31 D/mm (range 0.18 to 1.63 D/mm) in normal eyes. There was a highly statistically significant difference in the means of highest rate of steepening in the three groups (p < 0.0001). A small overlap in the highest rate of steepening ranges was shown among the three groups. Considering the first and second groups as cases of keratoconus, and 1.40 D/mm as the optimum cut-off value for highest rate of steepening, a sensitivity of 95.7% (67 out of 70 cases were detected), a specificity of 96.4% (16 false-positive cases), and an accuracy of 96.3% was obtained.

The highest rate of steepening may be a useful measure for keratoconus screening with videokeratography. Validation is needed to evaluate its accuracy.