Corneal nerve morphology and sensitivity changes after ultraviolet A/riboflavin treatment

Collagen crosslinking induced by riboflavin and ultraviolet A irradiation (UVAR) has recently been introduced as a clinical treatment to halt or reverse the progression of keratoconus. We investigated changes in corneal sensitivity and nerve morphology as part of a comprehensive safety evaluation of this treatment. Fifty-four New Zealand white rabbits were divided into three experimental groups: UVAR with deepithelialization, UVAR without deepithelialization, and deepithelialization alone. Corneal sensitivity was measured with a Cochet-Bonnet esthesiometer before treatment and 3, 7, 14, 30, 90, and 180 days after treatment. Corneal nerve morphology was evaluated using acetylcholinesterase histochemistry staining. We found that corneal sensitivity in the center of the treated area was significantly reduced 3 days after UVAR with deepithelialization treatment compared with the corneal sensitivity of the control eye but gradually recovered to normal levels at 90 days. Corneal sensitivity after deepithelialization treatment was significantly lower than control corneal sensitivity at 3 days but was significantly higher after 30 days of recovery compared with the corneal sensitivity after UVAR with deepithelialization. Corneal sensitivity after UVAR without deepithelialization treatment had significantly decreased at 7 days compared with control corneal sensitivity but was not significantly different from control values at other measurement times. In parallel with these functional alterations, corneal nerve degeneration was visible in the treatment area by 3 days; by 7 days there was a significant decrease in nerve density. Corneal nerve sprouts were identified from neighboring non-injured nerve fibers 7 days after treatment; by 90 days, excessively regenerating nerves were observed throughout the anterior stroma. The density of corneal nerve fibers appeared normal by 180 days. Ultraviolet A/riboflavin with deepithelialization treatment resulted in corneal nerve fiber damage and subsequent regeneration in the treatment area, simultaneously accompanied by the reduction and recovery of corneal sensitivity.