Variability in hypoxia-induced corneal swelling is associated with variability in corneal metabolism and endothelial function.

Abstract

To determine if the variability in contact lens-induced corneal swelling is associated with variability in corneal oxygen consumption (Q(C)) or corneal endothelial function. Corneal swelling was induced in 30 non-contact lens wearers by 2 hours of closed-eye contact lens wear, using thick (oxygen transmission [Dk/t] = 4.0 x 10-9) and thin (Dk/t = 12 x 10-9) hydrogel lenses of identical design. Following the induction of swelling, corneal thickness was continually measured by pachymetry until open-eye steady-state (OESS) thickness was achieved. The percentage of recovery per hour was calculated as a measure of endothelial function. Tear oxygen tension (PO(2)) beneath the hydrogels was measured in the open eye and after 5 minutes of eye closure to obtain estimates of Q(C). A change in corneal pH during eye closure while wearing the hydrogels was used as a measure of hypoxic acidosis. Associations between corneal swelling and endothelial cell density or corneal epithelial thickness were also tested. There were modest but significant (P < 0.05) correlations between thick-lens corneal swelling and thick-lens closed-eye PO(2) (r = -0.36); thin lens corneal swelling and thin-lens closed-eye and open-eye PO(2) (r = -0.40 and -0.39, respectively). Corneal swelling also increases with increasing Q(C) (r = + 0.29 to + 0.33). Corneal swelling was associated with the decrease in pH during lens wear (r = + 0.30 and + 0.20 for thick and thin lenses, respectively). Thick- and thin-lens corneal swelling was significantly associated with percentage of recovery per hour (r = -0.40 and -0.34, respectively). Multiple regression analysis of corneal swelling with PO(2) and percentage of recovery per hour suggested an additive effect, however the effects were not significant. There was a modest inverse association between corneal swelling and OESS, however there were no associations between corneal swelling and endothelial cell density or epithelial thickness. The variability in contact lens-induced corneal swelling is associated with both corneal metabolic activity (Q(C) and pH) and endothelial function (percentage of recovery per hour). Our interpretation is that individuals with larger Q(C) produce more lactic acid (i.e., more swelling) whereas stronger endothelial function resists swelling. The modest correlations, however, suggest that other factors also are involved in explaining the phenomenon of corneal swelling.