Purpose: To define an unconventional diagnostic factor for keratoconus and to assess the correlation between anterior and posterior corneal surfaces based on vectorial astigmatic analyses.
Design: Observational descriptive comparative cross sectional study
Method: This study included two hundred and forty-four eyes of 244 patients divided into groups; normal corneas, or controls (C, n [100]), fruste (FFKc, n [28]) and manifest keratoconus (Kc, n [116]). Full Ophthalmic examination was performed. All candidates were examined using a rotating Scheimpflug corneal tomographer (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) to obtain corneal measurements. Astigmatic vector analyses were carried out according to the method proposed by Thibos.
Results: The area under receiver operating characteristic curve (AUC) for posterior corneal APV between normal and manifest keratoconus was 0.73 (95% confidence interval): 0.66 - 0.80. By using ROC curve Sensitivity, Specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy at cutoff 0.30 were (65.0%, 80.0%, 78.9%, 66.1% and 73.1% respectively). As regard posterior corneal Blur; the AUC between normal and manifest keratoconus was 0.92 (95% confidence interval): 0.88 - 0.96. By using ROC curve Sensitivity, Specificity, PPV, NPV and accuracy at cutoff 6.65 were (85.3%, 89.0%, 90.0%, 84.0% and 86.1%) respectively.
Conclusion: Vector analysis of posterior corneal astigmatism; APV and Blur, is a simple, friendly, objective and supplementary approach in the differentiation of normal from manifest keratoconus. Astigmatisms of anterior and posterior corneal surfaces are correlated, particularly among keratoconic corneas.