Differentiation Between Pellucid-Like Keratoconus and Pellucid Marginal Degeneration Using Swept-Source Optical Coherence Tomography.

Purpose: The distinction between pellucid-like keratoconus (PLK) and pellucid marginal corneal degeneration (PMD) based on tomographic examinations is difficult. In this study, corneal tomographic and biomechanical parameters, after classifying PMD and PLK using swept-source optical coherence tomography, were analyzed.

Methods: Diagnoses of PLK and PMD were made using SS-OCT imaging, two groups were formed: PLK (n = 30) and PMD (n = 10). Corneal thickness was measured at the 5, 6, and 7 o'clock positions centrally, superiorly, and inferiorly with SS-OCT. Tomographic, densitometric, and biomechanical parameters were investigated using Scheimpflug-based methods.

Results: Patients with PMD were significantly older than patients with PLK (55.2 [95% CI, 47.0-63.3] vs. 42.0 years [95% CI, 38.2-46.0]; P = 0.002). The median Kmax value did not differ significantly between PMD and PLK (54.7 D [95% CI, 48.0-62.8] vs. 52.0 D [95% CI, 49.4-53.9]; P = 0.5). Astigmatism was significantly higher in the PMD group than in the PLK group (median 9.0 D [6.1-11.5] D vs. 3.7 [2.4-4.6] D; P < 0.001). The biomechanical parameters did not differ between PMD and PLK (DA Ratio max 2 mm, integrated radius, stiffness parameter at first applanation, and Corvis Biomechanical Index (CBI); each P > 0.05). There were significant differences in the corneal thickness ratio inferior to the center between the two groups (-23.7% [-31.5 to (-15.9)] versus 2.4% [-1.2 to 5.9], P < 0.001) and at 6 o'clock (-29% [-38.7 to -19.29] versus 0.18% [-4.62 to 5, P < 0.001) and 7 o'clock (-24.5% [-34.2 to -14.7] versus -2.0% [-6.65 to 2.73], P < 0.001).

Conclusions: Neither Scheimpflug-based tomographic nor biomechanical parameters can differentiate between PLK and PMD. SS-OCT can help with this differentiation because of the typical thinning of corneal pachymetry near the limbus.