Investigating the topographic effect of epithelium in myopic eyes with subtle topographic preoperative abnormalities

Purpose: To explore the topographic impact of the epithelium in Keratoconus Suspected (KCS) and in “Normal” Placido classified corneas. Setting: Rothschild Foundation, Paris, France. Design: Prospective interventional case series. Methods: Anterior corneal specular Placido topography using OPD-Scan® II (NIDEK, Gammagori, Japan) was performed in 97 eyes of 67 patients undergoing PRK for myopia, before and after epithelium removal. The differences in axial keratometry, asphericity and astigmatism were computed. Results: After epithelial peeling, some Placido-normal classified corneas became KCS. Therefore, we have subdivided this group into two groups: one of normal classified corneas which stayed normal after epithelium removal (Group NN), and one of corneas that became KCS classified (Group NK).The mean difference in axial mean keratometry in the third central millimeter rings was +0.50 ± 0.24 D, 0.69 ± 0.31 D and 0.49 ± 0.35 D and the mean difference in the magnitude of epithelial induced astigmatism in the first central millimetre ring was 0.37 D x 89° (positive cylinder), 0.54 D 86° and 0.52 D 86° respectively in Group NN, NK and KK (KCS corneas that stayed KCS). These differences were significant (p< 0.0001). Preoperative keratometry was the only predictive factor differentiating Group NN from NK (p<0.001). Conclusions: The epithelial layer tended to reduce the magnitude of the Bowman layer’s astigmatism, prolateness and keratometry, more importantly in Group NK. In KK group we found a similar trend as in normal eyes (Group NN). The epithelium would be able to mask Bowman layer’s irregularities until a certain degree of severity.