Inferior-Superior Epithelial Thickness Difference: Zone-Specific Analysis Across Topographic Asymmetry

To assess how OCT-derived epithelial thickness-difference metrics relate to corneal topographic inferior-superior (IS) asymmetry and to identify the most informative annular zones and axis-defined sector-pair metrics.

University of São Paulo.

Prospective, comparative observational study.

In this IRB-approved study of consecutive patients undergoing refractive surgery screening (regardless of ultimate eligibility), we analyzed 526 eyes. Axial IS values from Placido-based topography defined four strata (negative, 0.0-1.0, 1.0-1.4, and >1.4 D). Twelve epithelial thickness-difference metrics (including axis-defined sector-pair differences) were computed from 9-mm OCT maps across the 2-5 mm, 2-7 mm, and 2-9 mm annuli. Associations between continuous IS and each metric were tested using Pearson correlation and linear regression across the full spectrum and within analyses anchored at the clinically established IS = 1.4 D reference.

The central 2-5 mm annulus yielded the strongest associations with IS. Correlations were weak and slightly positive for IS ≤1.4 D but became stronger and consistently negative for IS >1.4 D, most prominently along the inferotemporal-superonasal (IT-SN) axis (r = -0.62; P < 0.0001). Mean epithelial differences were largely similar across strata up to 1.4 D, with clearer deviations in the >1.4 D stratum.

Using manually derived, zone- and axis-specific OCT epithelial thickness-difference metrics, we found a nonlinear association with topographic IS, with the most informative signal in the central 2-5 mm annulus and along the IT-SN axis. Clinically, central, axis-defined epithelial asymmetry may serve as an adjunct interpretive marker in higher-IS (or borderline) eyes, whereas low-IS eyes show weak and variable epithelial-IS coupling-potentially explaining the limited performance of epithelial metrics in low-asymmetry settings.