Influence of the Reference Surface Shape for Discriminating Between Normal Corneas, Subclinical Keratoconus, and Keratoconus

Abstract

To compare the discriminating ability of corneal elevation generated by a dual Scheimpflug analyzer calculated with different reference surfaces for distinguishing normal corneas from those with keratoconus and subclinical keratoconus. A total of 391 eyes of 208 patients were prospectively enrolled in the study and divided into three groups: 167 eyes of 113 patients with keratoconus, 47 contralateral topographically normal eyes of patients with clinically evident keratoconus in the fellow eye, and 177 eyes of 95 refractive surgery candidates with normal corneas. All eyes were measured with a dual Scheimpflug analyzer (GALILEI Analyzer; Ziemer Ophthalmic Systems AG, Port, Switzerland). Maximum elevation values were recorded within the central 5-mm diameter in both anterior and posterior elevation maps. Discriminating ability of corneal elevation measurements obtained by best-fit toric and aspheric (BFTA) and best-fit sphere (BFS) reference surfaces were compared by receiver operator characteristic (ROC) curves. ROC curve analysis showed that corneal elevation measured by BFTA had a significantly better ability than with BFS for distinguishing normal corneas from those with keratoconus and forme fruste keratoconus (P = .01). Posterior elevation measured by BFTA had a significantly higher predictive accuracy for forme fruste keratoconus than anterior elevation with an area under ROC curves of 0.88 and 0.80, respectively (P = .01). The sensitivity and specificity achieved with the maximum posterior elevation for detecting keratoconus and forme fruste keratoconus were 99% and 99% for keratoconus and 82% and 80% for forme fruste keratoconus with the cut-off value at 16 and 13 μm, respectively. The ability to discriminate between normal cornea and forme fruste keratoconus with elevation parameters was significantly improved by using BFTA instead of BFS reference surface.