Comparison of Epithelial Thickness Mapping in Normal Corneas with Different Types of Astigmatism

Corneal epithelial thickness mapping has a wide range of applications including screening for keratoconus, screening for anterior basement membrane dystrophy, and assessment of dry eye. It also plays an important role in corneal laser refractive surgery. These all require an understanding of the epithelial thickness profile in a normal cornea as a reference.

This study aimed to compare corneal epithelial thickness along different meridians in normal eyes with different types of corneal astigmatism.

In this cross-sectional study, corneal topography and epithelial thickness mapping by anterior segment optical coherence tomography was performed for 154 normal eyes. Corneal astigmatism was classified based on the orientation of the steepest corneal meridian (with-the-rule [WTR], against-the-rule [ATR], and oblique) and the amount (low, moderate, and high). On a 9-mm epithelial thickness map, the thickness along the horizontal (180°), vertical (90°), and diagonal (45 and 135°) meridians was calculated.

With-the-rule, oblique, and ATR astigmatisms were observed in 40, 31, and 29% of the eyes, respectively. In all types of astigmatism, the mean epithelial thickness was greater along the 180° meridian and thinner along the 90° meridian. There was no statistically significant difference in mean thickness along different meridians for WTR (P = .24) and oblique (P = .46) astigmatism, whereas the difference along the 180 and 90° meridians (P = .003) for ATR astigmatism was statistically significant. Also, there was a statistically significant difference in thickness in different types of astigmatism separately for each meridian (P < .05) so that the highest and lowest thicknesses were seen for ATR and oblique astigmatisms, respectively. The epithelial thickness showed no statistical difference in various amounts of astigmatism along different meridians.

Thicker epithelium along the horizontal meridian in ATR may point to the surface regularizing functions of the epithelium to create a regular and smooth surface, but thickness assessment along the different meridians in oblique and WTR showed no curvature-dependent changes.