Evaluation of corneal elevation and thickness indices in pellucid marginal degeneration and keratoconus

Purpose: To determine and compare corneal elevation and thickness indices, thereby formulating a reliable index to distinguish eyes with pellucid marginal degeneration (PMD) from keratoconus eyes and normal (control) eyes.

Setting: LV Prasad Eye Institute, Hyderabad, India.

Design: Initial model-building retrospective study.

Methods: Anterior and posterior corneal elevations and thickness indices were obtained from the Orbscan IIz topographer. These values were analyzed and compared between PMD patients, keratoconus patients, and control subjects.

Results: Of the indices, the mean values of anterior elevation (AE), ratio of the AE to the anterior best-fit sphere, ratio of the average power values of nasal quadrant to the average power values of inferior quadrant, and difference between maximum keratometry (K) and minimum K were significantly different between the 3 groups (P<.05). The highest area under the receiver-operating-characteristic (AROC) curve in discriminating PMD from keratoconus was for asphericity (0.974; cutoff ≥-0.07; sensitivity 93.3%; specificity 90.6%) followed by the ratio of average power values of the nasal and temporal quadrants to the average power values of the inferior and superior quadrants (Avg NT((D))/IS((D))) (0.959; cutoff ≥1.005; sensitivity 96.7%; specificity 90.6%). The PMD index (AROC curve, 0.935), with a cutoff of 3.45 or higher had 90% sensitivity and 93.7% specificity to distinguish PMD from keratoconus and had 100% sensitivity and 100% specificity to distinguish PMD from control eyes, with a cutoff of 2.46 or higher (AROC curve, 1.000).

Conclusions: The PMD index appeared to be highly sensitive and specific for diagnosing PMD. Asphericity and Avg NT((D))/IS((D)) were clinically relevant in discriminating PMD from other groups.