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

To compare the efficacy of posterior corneal elevation, measured by 2 methods, in discriminating subclinical keratoconus from normal corneas.

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.

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.

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.