Inferior-superior epithelial thickness difference: zone-specific analysis across topographic asymmetry

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

University of São Paulo, Sao Paulo, Brazil.

Prospective, comparative observational study.

In this Institutional Review Board-approved study of consecutive patients undergoing refractive surgery screening (regardless of ultimate eligibility), 526 eyes were analyzed. Axial I-S values from Placido-based topography defined 4 strata (negative, 0.0-1.0, 1.0-1.4, and >1.4 diopters [D]). 12 epithelial thickness-difference metrics (including axis-defined sector-pair differences) were computed from 9 mm OCT maps across the 2 to 5 mm, 2 to 7 mm, and 2 to 9 mm annuli. Associations between continuous I-S and each metric were tested using Pearson correlation and linear regression across the full spectrum and within analyses anchored at the clinically established I-S = 1.4 D reference.

The central 2 to 5 mm annulus yielded the strongest associations with I-S. Correlations were weak and slightly positive for I-S ≤1.4 D but became stronger and consistently negative for I-S >1.4 D, most prominently along the inferotemporal-superonasal (IT-SN) axis ( r = -0.62; P < .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-specific and axis-specific OCT epithelial thickness-difference metrics, a nonlinear association with topographic I-S was found, with the most informative signal in the central 2 to 5 mm annulus and along the IT-SN axis. Clinically, central, axis-defined epithelial asymmetry may serve as an adjunct interpretive marker in higher-I-S (or borderline) eyes, whereas low-I-S eyes show weak and variable epithelial-I-S coupling-potentially explaining the limited performance of epithelial metrics in low-asymmetry settings.