Accuracy of Scheimpflug-derived corneal biomechanical and tomographic indices for detecting subclinical and mild keratectasia in a South Asian population

Purpose: To test the predictive accuracy of the Belin-Ambrósio deviation index (BAD-D), the stiffness parameter A₁ (SPA₁), the Corvis biomechanical index (CBI), and the tomographic and biomechanical index (TBI) assessments for distinguishing subclinical and mild keratoconic eyes from normal eyes.

Setting: Medical Research Foundation, Sankara Nethralaya, Chennai, India.

Design: Retrospective case series.

Method: In this cross-sectional clinical study, the following 3 groups of eyes were analyzed: very asymmetric ectasia with normal topography (very asymmetric-normal topography), mild keratoconus, and normal control. All eyes had comprehensive assessment with corneal topography (TMS-IV), Scheimpflug tomography (Pentacam HR), and dynamic Scheimpflug biomechanical analysis (Corvis ST). The outcome measures were the BAD-D, SPA₁, CBI, and TBI. Receiver operating characteristic (ROC) curve analysis was performed to determine each parameter's predictive accuracy in distinguishing between eyes with subclinical or mild keratoconus and control eyes.

Results: The area under the curve (AUC) ROC values for the very asymmetric-normal topography and normal control comparison were 0.81 (BAD-D), 0.76 (SPA₁), 0.78 (CBI), and 0.90 (TBI). The TBI (using cutoff value 0.16) showed the highest diagnostic accuracy (85%), with 84% sensitivity and 86% specificity. The AUC ROC values for the mild keratoconus and normal control comparison were 0.998 (BAD-D), 0.91 (SPA₁), 0.97 (CBI), and 0.999 (TBI). The TBI (with a 0.63 cutoff) showed the highest accuracy (99.5%), with 99% sensitivity and 100% specificity. The TBI also showed the weakest correlation with mean keratometry, biomechanically corrected intraocular pressure, and central corneal thickness in normal eyes and keratoconic eyes.

Conclusions: The TBI best distinguished eyes with mild ectasia from normal eyes and had the weakest correlation with biomechanical confounding factors, reinforcing the hypothesis that the TBI represents corneal biomechanical susceptibility.