The impact of overnight orthokeratology on sub-basal nerve plexus morphology and corneal sensitivity

Abstract

Orthokeratology (OK) is a treatment for the temporary correction of myopia. Although there is considerable research on the efficacy and corneal response to OK, the effects of OK lens wear on corneal innervation is poorly understood. This thesis reports on the impact of OK on sub-basal nerve plexus (SBNP) morphology and corneal sensitivity in humans. Changes to SBNP during OK were assessed in studies using in vivo laser-scanning confocal microscopy (Heidelberg Retinal Tomographer with Rostock Corneal Module). Reductions in central corneal nerve fibre density (NFD) were observed compared to baseline and non-lens wearers following OK lens wear. Changes to temporal mid-peripheral corneal NFD were not observed, however, a clockwise rotation in the orientation of nerve fibres was observed compared to baseline after OK lens wear. Wide field montages of the SBNP in long-term OK lens wearers showed dramatic alterations to the nerve fibre pattern compared to non-lens wearers. Changes to corneal sensitivity during OK were assessed in studies using the Cochet Bonnet (COBO) and Non-Contact Corneal Aesthesiometers (NCCA). Using the COBO, central corneal sensation threshold (CST) increased compared to baseline and non-lens wearers following lens wear. Changes to temporal mid-peripheral corneal CST were not observed. No change was found in corneal sensitivity using the NCCA. Central corneal changes from baseline in SBNP morphology correlated with changes in corneal sensitivity after 1 month of OK lens wear. No association was found between these parameters at other corneal locations or lens wear periods, which suggest factors other than reduced sub-basal NFD contribute to decreases in corneal sensitivity during OK. OK lens-induced reduction in corneal sensitivity recovers with cessation of lens wear, returning to baseline shortly after lens wear discontinuation. Changes to central NFD did not recover fully after 3 months of lens wear discontinuation, suggesting that a longer non-lens wear period might be required for the return to baseline. The results of the research reported in this thesis shows that OK lens wear alters SBNP morphology and corneal sensitivity. The research also demonstrates that OK is a model for observing the rapid onset and recovery of SBNP morphology and corneal sensitivity changes in the healthy human eye.