Differential Diagnosis Of Keratoconus And Pellucid Marginal Degeneration By Topographic And Pachymetry Patterns And Indices

1. Carlos G. Arce, MD^1,2
2. Paulo Schor, MD³

¹Associate Volunteer Ophthalmologist, Ocular Bioengineer and Refractive Surgery Sectors, Department of Ophthalmology, Paulista School of Medicine, Federal University of São Paulo, São Paulo, Brazil
²Medical Director – Galilei R&D Consultant, Ziemer Ophthalmic Systems AG, Port, Switzerland
³Professor and Chief of Ocular Bioengineer and Refractive Surgery Sectors, Department of Ophthalmology, Paulista School of Medicine, Federal University of São Paulo, São Paulo, Brazil

Purpose
Compare curvature, elevation and pachymetry map patterns, shape and wavefront indices of KC and PMD

Methods
Dual Scheimpflug analysis was performed in eyes with KC and PMD

Results
Typical KC has with-the-rule symmetric or asymmetric vertical steeper bow-tie on curvature maps. Negative vertical coma increases (<-0.40 µm) and spherical aberration becomes less positive (SA<0.15 µm) or negative due to change from prolate ellipsoid to parabolic/hyperbolic shape and Є2>+1.0. Thinnest corneal point (TCP) dislocates temporal inferior and becomes thinner increasing the thickness corneal profile. Asymmetric KC with oblique astigmatism, oblate superior and prolate inferior segments may have Є2<+1, however preserving the asymmetric change of aspheric curvature on best fit toric aspheric (BFTA) elevation maps following the steeper axis of astigmatism. Typical PMD has against-the-rule horizontal steeper bow-tie, incomplete or complete clamp-like pattern on curvature maps with a flatter drop-like thinnest zone just at center of both arms above the steeper area, and inferior hot zone with increased positive elevation in best fit sphere (BFS) maps. The asymmetric shape of superior and inferior corneal hemispheres induces an Є2 index closer to zero, negative vertical coma (<-0.40 µm), positive SA>0.30 µm, and increased trefoil or quatrefoil (>±0.30 µm). Pachymetry shows a TCP dislocated temporal inferior with thinner or normal central cornea. Typically the asymmetric change of aspheric curvature on BFTA maps follows the flatter axis of astigmatism.

Conclusions
Typical KC and PMD seem to have distinct features in curvature, elevation, thickness, shape, and aberration patterns that help the differential diagnosis.