Topography of the corneal epithelium and Bowman layer in low to moderately myopic eyes

Purpose: To compare the epithelium (air-tear film) and Bowman layer's specular topographies in patients having low to moderate myopia corrected by photorefractive keratectomy (PRK).

Setting: Rothschild Foundation, Paris, France.

Design: Prospective interventional case series.

Methods: Anterior corneal specular Placido topography using the OPD-Scan II Placido-based topographer was performed in eyes of patients having PRK for myopia before and after epithelial removal. The differences in axial keratometry (K), asphericity (Q value), astigmatism magnitude (toricity), and axis were computed in the 1st, 3rd, and 5th central corneal millimeter zones.

Results: The study comprised 51 patients (90 eyes). The mean difference in axial mean K was 0.56 diopter (D) ± 0.26 (SD), 0.56 ± 0.27 D, and 0.56 ± 0.24 D in the 1st, 3rd, and 5th central millimeter rings, respectively. The mean difference in the magnitude of epithelium-induced astigmatism was 0.42 ± 0.43 D × 90, 0.41 ± 1.60 D × 78, and 0.02 ± 1.82 D × 83 (positive cylinder) in the 1st, 3rd, and 5th central millimeter rings, respectively. Corneal astigmatism shifted toward an increased with-the-rule orientation after epithelial removal. The mean difference in asphericity (over the central 6.0 mm zone) was -0.07 ± 0.21. These differences were significant (P < .0001) and independent of age, sex, central pachymetry, and spherical equivalent.

Conclusions: In low to moderately myopic eyes, the topography of Bowman layer was significantly steeper than that of the epithelium. The epithelial layer tended to reduce the magnitude of Bowman layer's astigmatism and prolateness.

Financial disclosure: None of the authors has a financial or proprietary interest in any material or method mentioned.