There are three possible mechanisms underlying the apparent reduction in central nerve density and interconnections in the OK lens wearers: diversion of nerves from their usual trajectory, increased distance between nerves, or a loss of nerves, such as occurs in ocular disease.
4–14 First, from the nerve map, it appears that the paths of the nerves that originate from the outer limits were diverted from their normal trajectory, towards a new path within the mid-periphery. Hence, the nerves normally seen in the center appear to have been redistributed towards the mid-periphery. Second, the reshaping of the corneal profile causes compression of the epithelial cells centrally,
24,27 thereby increasing the en face surface area of each cell. Since fibers in the SBNP are thought to mainly run between neighboring epithelial cells,
18 the flattening of cells may increase the separation of nerves, thereby giving the impression of reduced nerve density. Third, the reduction in the nerve density centrally may be the result of nerve loss, which should have implications for the sensitivity of the cornea, particularly in relation to its regional variations. Hiraoka et al.
39 found an overall reduction in corneal sensitivity after 3 months of OK treatment. In that study, measurements are taken at five locations: at the corneal apex, and at 2 mm from the limbus in the superior, inferior, nasal, and temporal corneal locations, and no regional variations in sensitivity were found at baseline or at the 3-month visit. However, sensitivity was measured using the Cochet-Bonnet aesthesiometer with the thicker 0.12-mm diameter nylon monofilament, which has been demonstrated to have a truncated stimulus intensity range.
40 Therefore subtle differences across the cornea may not have been detected. A change in sensitivity would be expected with such marked changes in morphology as have been observed in this present study, as many studies involving subjects with eye
4–5,7,11–12,14 and systemic disease,
12–13 as well as following corneal surgery,
16–17 show a relationship between changes in nerve morphology and corneal sensitivity. In addition, a reduction in nerve density may also have implications on corneal wound healing
41–42 and the immune response to corneal infections.
15 Hence, the clinical implications of restructuring the SBNP on corneal sensitivity as well as the long-term health of human eyes treated with OK require further investigation.