Distribution of Anterior and Posterior Corneal Astigmatism in Eyes With Keratoconus

PURPOSE: To investigate the magnitude, with-the-rule (WTR) or against-the-rule (ATR) orientation, and vector components (Jackson astigmatic vectors [J(0) and J(45)] and blurring strength) of the anterior and posterior corneal astigmatism (ACA and PCA) in patients with keratoconus (KC) in a retrospective study, and to try to find suitable cutoff points for ACA and PCA in an attempt to discriminate KC from normal corneas. DESIGN: Retrospective age-and sex-matched case control study. METHODS: Using the Pentacam images, the aforementioned parameters were compared between 1273 patients with KC and 1035 normal participants. RESULTS: The mean magnitude of the ACA and PCA was 4.49 +/- 2.16 diopter (D) and 0.90 +/- 0.43 D, respectively. The dominant astigmatism orientation of the ACA was ATR in KC patients and WTR in normal participants (P < .001), while for the PCA it was WTR in KC patients and ATR in normal participants (P < .001). There was a significant agreement between the axis orientations of ACA and PCA in KC patients (kappa = 0.077, P < .001), but not in the normal group (P = .626). ACA and PCA magnitude, M, J(0), J(45), and blur significantly increased by increasing KC severity. There was a trend for increasing anterior ATR and posterior WTR, and decreasing oblique astigmatism on both corneal surfaces by increasing the KC severity according to the Amsler-Krumeich classification. A cutoff value of 1.8 D for ACA had 90.2% sensitivity and specificity, and that of 0.4 D for PCA had 89.5% sensitivity and 85.0% specificity for discriminating KC from normal corneas. CONCLUSION: Our findings can help clinicians in the diagnosis of KC and lens manufacturers in designing suitable contact or intraocular lenses. (C) 2016 Elsevier Inc. All rights reserved.