Efficacy of axial and tangential corneal topography maps in detecting subclinical keratoconus

Abstract

To investigate and compare the efficacy of axial and tangential corneal topography maps obtained using the Orbscan IIz scanning-slit device to distinguish differences in eyes with subclinical keratoconus from normal eyes. LV Prasad Eye Institute, Hyderabad, India. Retrospective cross-sectional study. Axial and tangential maps of the anterior and posterior corneal surfaces were obtained in eyes of patients with subclinical keratoconus (normal fellow eye of an eye with clinically diagnosed keratoconus). The following measurements were obtained from each map and compared between the 2 groups: location of the corneal apex in millimeters relative to the corneal vertex, dioptric power at the corneal apex and corneal vertex, corneal irregularity indices at 3.0 mm and 5.0 mm, anterior-posterior apex ratio, and inferior-superior dioptric asymmetry value. Maps were obtained in 71 eyes (71 patients) with subclinical keratoconus and 76 normal eyes (76 patients). The tangential posterior apex curvature was significantly steeper than the axial posterior apex curvature in both groups (P < .001). The anterior-posterior apex ratio on the tangential map demonstrated the highest area under the receiver operating characteristic (ROC) curve of 0.992 (cutoff ≥-6.97, sensitivity 98.6%, specificity 98.7%) followed by the posterior irregularity index in the 5.0 mm zone with an area under the ROC curve of 0.938 (cutoff ≥0.4, sensitivity 98.6%, specificity 84.2%) in differentiating subclinical keratoconus eyes from normal eyes. The anterior-posterior apex ratio on the tangential map and 5.0 mm irregularity index of the posterior corneal surface were highly sensitive and specific for early detection of subclinical keratoconus. Tangential corneal topography maps should be considered in preoperative subclinical keratoconus screening. No author has a financial or proprietary interest in any material or method mentioned.