Effects of RGP lens extended wear on glucose-lactate metabolism and stromal swelling in the rabbit cornea

To assess the chronic effects of rigid gas-permeable (RGP) contact lenses on corneal swelling and glucose-lactate metabolism in the rabbit cornea during 1 month of continuous extended wear and to establish the relationship between these effects and the oxygen transmissibility (Dk/L) of the test lens polymer.

Four RGP lenses of varying Dk/L were tested in 8 rabbits per test group (left eyes served as controls). After 7 days and 1 month extended wear, the concentrations of lactate and glucose in the corneal epithelium, stroma and aqueous humor were determined by enzyme assay; and epithelial and stromal ATP concentrations were separately measured by bioluminescence techniques. Corneal thickness was measured at a standard morning time by ultrasonic pachymetry before and after 1, 7, 15 days and 1 month extended wear.

After 7 days and 1 month extended wear, generalized decreases were found in aqueous humor lactate levels for all test lenses, while concomitant increased aqueous glucose concentrations were observed. Total epithelial lactate levels correlated inversely with decreasing Dk/L levels for lower oxygen transmissible lenses (R = 0.951, P = 0.0051); and remained unchanged after extended wear of the hyper-oxygen transmissible Dk/L 125 test lens. By contrast, stromal lactate levels consistently decreased at all time points measured forextended wear of all test lenses. As expected, both epithelial and stromal ATP concentrations simultaneously decreased in extended wear. Overnight corneal swelling values after 24 hours wear of Dk/L = 27, 43, 70 and 125 test lenses were increased by 9.8, 7.1, 5.5, and 5.2% while persistent (residual) stromal swelling after one month extended wear was 16.8, 10.1, 8.6, and 5.6% respectively, in excess of baseline values.

Chronic RGP contact-lens induced hypoxia is associated with altered glucose-lactate metabolism in the cornea and aqueous humor with excess production of increased levels of lactate in the epithelium for lower Dk/L test lenses, but decreased lactate concentration in the stroma and aqueous humor. Extended wear of the hyper-oxygen transmissible test lens (Dk/L = 125) however, produced no increase in epithelial lactate levels. Expected lens-induced decreases in epithelial and stromal ATP were not dependent on lens-oxygen transmissibility. Despite the persistence of lower than normal stromal levels of lactate during 1 month of extended wear for all test lenses, residual corneal swelling values remained consistently elevated above baseline values. Taken together, these data establish that increased stromal lactate accumulation cannot account for persistent stromal edema in chronic extended wear of RGP lenses; and that this effect appears to be independent of lens-oxygen transmissibility and may thus represent the prolonged mechanical effect of lens wear itself.