Abstract
Purpose:
Tear neuropeptides CGRP and SP have distinct roles in corneal epithelial wound healing and probably also corneal nerve regeneration. This study aimed to investigate long-term effects of LASIK on tear concentrations of CGRP and SP and the relationship between these neuropeptides and corneal sub-basal reinnervation.
Methods:
A single visit cross-sectional study was performed. Twenty subjects who had bilateral myopic femtosecond LASIK 12 to 18 months previously (aged 26.8±5.1years, 10M:10F, -5.86±1.83D, superior hinge) and twenty controls (aged 22.0±4.3years, 12M:8F, -5.21±2.69D) were recruited. All subjects had healthy corneas with no ocular surface disease. Basal tears were collected and ELISAs were used to determine tear CGRP and SP concentrations. Central, temporal and superior corneal nerve images from the right eye were captured using a laser scanning confocal microscope (HRTII, RCM). Nerve density was calculated by manually tracing the total length of nerve fibres per mm2 using NeuronJ. Nerve tortuosity was assessed by a masked observer using a grading scale. Whitney U or Wilcoxon tests were used, as appropriate, for comparison between groups. Spearman’s ρ was used to examine correlations between variables. Significance was determined at a p value of 0.05.
Results:
Tear CGRP concentration post-LASIK was significantly higher than in the controls (P=0.001) but there was no difference in SP concentration between groups. Nerve density was lower (p=0.003) and nerve tortuosity was higher at the central cornea but not at periphery in the post-LASIK group. In the post-LASIK group higher CGRP concentration was associated with higher nerve density in the central cornea and with increased nerve tortuosity at all corneal regions (p<0.04). Higher SP concentration was only associated with reduced nerve density at the temporal cornea (ρ=-0.53, P=0.02). No associations between tear neuropeptide concentrations and nerve morphology were evident in the control group.
Conclusions:
CGRP seems to play the greater role in reinnervation at the late time point after LASIK. The higher CGRP concentration and the association with reinnervation in the post-LASIK group are likely to suggest CGRP is a marker for degree of reinnervation. A lower nerve density was associated with SP concentration. This study confirmed the role of tear neuropeptide on reinnervation after LASIK but a relationship with nerve morphology could not be found in the unoperated group.