CORNEAL METABOLIC ACTIVITY IN HUMANS

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Contact lens wear is a safe and effective form of correction for refractive error. However, a significant number of people are unable to wear lenses because of chronic corneal edema. Previous studies have shown significant individual differences in the amount of corneal edema produced by contact lens-induced hypoxia. We will test the hypothesis that individual differences in corneal edema are associated with individual differences in metabolic activity of the cornea. In a large group of normal subjects we will measure corneal edema, oxygen consumption (Qc) and acid load produced by wearing contact lenses of high, medium and low oxygen transmissibility. Corneal edema will be determined by measuring corneal swelling following three hours of closed eye lens wear. Oxygen consumption will be determined by using a new non-invasive technique for measuring tear oxygen tension (PO2) under contact lenses of known oxygen transmissibility. Hypoxic acid load will be determined by measuring the change in corneal stromal pH during closed eye contact lens wear. Our hypothesis predicts that individuals with a relatively high Qc will have lower tear Po2 (i.e., relatively more hypoxia) for a given contact lens wearing condition. In order to maintain corneal energy levels, these individuals should have a greater dependence on glycolytic metabolism, which will lead to greater production of lactic acid. Thus, the hypothesis also predicts that individuals with high Qc will show the greatest drop in corneal pH. Since lactate is the osmotic agent for hypoxic corneal swelling, these individuals will show greater corneal swelling. An alternate hypothesis, that variability in corneal swelling is a function of the variability in endothelial function, will also be tested. If the primary hypothesis is accepted, then further studies will be initiated to determine if individuals with high metabolic demand have a greater propensity toward developing clinical complications while wearing contact lenses. If so, this could lead to the development of a provocative test for suitability of contact lens wear based on a quick and simple measure of corneal oxygen consumption.