Sensitivity and specificity of posterior corneal elevation measured by Pentacam in discriminating keratoconus/subclinical keratoconus.

PURPOSE

To estimate the sensitivity and specificity of posterior elevation in discriminating keratoconus and subclinical keratoconus from normal corneas.

DESIGN

Prospective case-control study.

PARTICIPANTS

Seventy-five patients with keratoconus, 25 with subclinical keratoconus, and 64 refractive surgery candidates with normal corneas.

METHODS

In one eye of each patient, posterior corneal elevation was measured in the central 5 mm using the Pentacam rotating Scheimpflug camera (Oculus, Wetzlar, Germany). Posterior corneal elevation in keratoconus and subclinical keratoconus were compared with that in normal corneas in separate analyses. Receiver operating characteristic (ROC) curves were used to determine the test's overall predictive accuracy (area under the curve) and to identify optimal posterior corneal elevation cutoff points to maximize sensitivity and specificity in discriminating keratoconus and subclinical keratoconus from normal corneas. Logistic regression was used to support cutoff points identified through ROC curve analysis, and to check for model validity; model goodness-of-fit was estimated using r(2), and its internal validation was by bootstrapping analysis.

MAIN OUTCOME MEASURES

Posterior corneal elevation in keratoconus, subclinical keratoconus, and normal corneas.

RESULTS

Mean posterior corneal elevation was statistically higher in keratoconus (100.7+/-49.2 microm; P<0.001), and subclinical keratoconus (39.9+/-15.0 microm; P = 0.01) versus normal corneas (19.8+/-6.37 microm). ROC curve analyses showed high overall predictive accuracy of posterior elevation for both keratoconus and subclinical keratoconus (area under the curve 0.99 and 0.93, respectively). Optimal cutoff points were 35 microm for keratoconus and 29 microm for subclinical keratoconus. These values were associated with sensitivity and specificity of 97.3% and 96.9%, respectively, for keratoconus, and 68% and 90.8% for subclinical keratoconus. Similar cutoff points were obtained with logistic regression analysis (38 microm for keratoconus and 32 microm for subclinical keratoconus). The models showed good fit to the data, including after internal validation.

CONCLUSIONS

Posterior corneal elevation very effectively discriminates keratoconus from normal corneas. Its efficacy is lower for subclinical keratoconus, and thus data concerning posterior elevation should not be used alone to stratify patients with this condition.