Central epithelial thickening
Because the natural shape of the cornea is altered by the laser during post-refractive surgery, the epithelium acts as a filler and tries to cover the areas where the curvature has been changed. Essentially, it tries to bring the cornea back into its original shape as a natural and compensatory mechanism.
Numerous reports have been published describing central epithelial thickening following central tissue removal by myopic excimer laser ablations, which has been related to myopic regression.4,5 During myopic corrections, the tissue is removed from the central part of the cornea, which flattens postoperatively because of the treatment.
However, since the corneal epithelium can alter its thickness profile and either partially or totally mask the presence of an irregular underlying stromal surface, it shows a tendency toward proliferation in the central zone in its attempt to compensate for the central flattening.
My colleagues and I conducted a study of epithelial thickness changes following small incision lenticule extraction (SMILE) for myopia. We observed a statistically significant epithelial thickness increase in the central zone (6.83% for low, 9.26% for moderate and 12.7% for high myopia, P values <0.05 for all groups) and superior zone (3.98% for low, 7.82% for moderate and 9.87% for high myopia) across all the three groups, which correlated positively with the degree of myopia corrected (r2=0.723 for central zone, r2=0.585 for superior zone, P<0.001 for both zones) at 3 months’ follow-up.
Four eyes of two patients with high myopia (SE >-8D) had regression due to significant epithelial thickening. Therefore, we concluded that significant epithelial thickening may occur following SMILE, especially in high myopia, leading to regression and changes in refraction. The study of the epithelial thickness profile may differentiate between regression and under correction seen in the postoperative period.6
It may also be an indispensible tool to diagnose early ectasia prior to LASIK or SMILE and differentiate it from regression. In the former scenario, the epithelium would show thinning over the steepest portion of the cornea, while in the latter scenario it would show thickening in the central zone corresponding to the flattened area post-myopic laser refractive surgery.
The changes after hyperopic LASIK, however, follow an opposite trend. Three-dimensional high-resolution ultrasound mapping of epithelial thickness profile after LASIK for hyperopia demonstrated thinner epithelium centrally and thicker epithelium paracentrally.7
Presumably, the paracentral epithelial thickening compensated in part for the stromal tissue removed by the hyperopic ablation, whereas the central epithelial thinning compensated for the localised increase in corneal curvature. A hyperopic ablation is designed to steepen the central cornea, which bears a similarity to the morphology of a keratoconic cornea, so the epithelial thickness profile after hyperopic ablation is expected to follow a similar pattern to that seen in keratoconus.
Reinstein et al found that the epithelial thickness profile across the central 8-mm diameter follows an epithelial doughnut pattern characterised by epithelial thinning over the cone surrounded by an annulus of epithelial thickening.8
This degree of epithelial thickening was significantly greater than the central epithelial thickening reported after myopic LASIK. Reinstein et al previously reported a maximum of 18 μm epithelial thickening after a −13.50 D myopic ablation.
This could be explained by the epithelial response according to the profile of tissue removed; hyperopic ablations induce a more abrupt change in curvature as the ablation must fit into half the cornea compared with myopic ablations where the change in curvature is more gradual across the diameter of the cornea. The epithelium appears to compensate more for a localised stromal irregularity.