Diagnosis of subclinical keratoconus using posterior elevation measured with 2 different methods.

PURPOSE: To compare the efficacy of posterior corneal elevation, measured by 2 methods, in discriminating subclinical keratoconus from normal corneas.
METHODS: In 30 consecutive patients with subclinical keratoconus, and 37 candidates for refractive surgery, posterior corneal elevation was measured using Pentacam's rotating Scheimpflug camera (Oculus, Wetzlar, Germany) with the standard method [maximal elevation above the best fit sphere (BFS)] and with the enhanced-BFS (E-BFS) method (difference in elevation measured above the E-BFS and that measured above the BFS). Using cutoff points selected a priori (≥ 29 and ≥ 12 µm, respectively, for the standard and E-BFS methods) the sensitivity, specificity, and overall accuracy, determined through the area under the receiver operating characteristic curves, were assessed for each method and then compared.
RESULTS: Mean posterior elevation values were statistically higher (P < 0.001) in corneas with subclinical keratoconus than in normal corneas, using either the standard (38 ± 15.9 µm vs. 20.3 ± 7.1 µm) or the E-BFS (15 ± 9.5 µm vs. 7.8 ± 5.5 µm) methods. Sensitivity and specificity rates were slightly higher with the standard method than with the E-BFS method (sensitivity, 73.3% vs. 60%; specificity, 86.5% vs. 83.8%), but neither difference was statistically significant (P > 0.05). The overall accuracy of the tests was similar (P = 0.19): the area under the curve was 0.80 for the standard and 0.72 for the E-BFS method.
CONCLUSIONS: Posterior corneal elevation measured using either standard or E-BFS method has similar efficacy in discriminating corneas with subclinical keratoconus from normal corneas. As a single parameter, posterior elevation can be considered quite effective with either method, but it cannot be used alone to identify patients with subclinical keratoconus.