A Study of Some Indicators of Corneal Metabolic Activity in Periods of Contact Lens Wear

Abstract

The study describes a series of experiments that improve existing techniques and develop new techniques for the determination and monitoring of contact lens wear effects on the cornea. Three main aspects are studied: (1) Oxygen requirement (2) Carbon dioxide production (3) Lactic acid production and dynamics. Experimental topics include: oxygen consumption and carbon dioxide efflux rates from the corneal epithelial surface, lactic acid production and dynamics across the limiting layers in normal and anoxic cornea, lactic acid dynamics in a soft lens polymer and the use of soft lenses as a sample collection medium. In all the experiments, attention is paid to produce techniques suitable for use in the clinical environment. The experimental data indicates that oxygen consumption rate is a reliable indicator of corneal metabolic activity and that there is no significant diurnal variation. However, measurement of carbon dioxide efflux rate is unsuitable for use as a diagnostic test due to the insensitivity and bulk of the electrode. Lactic acid concentrations in all tissues of the anterior segment of the eye, with the exception of the stroma plus endothelium, increase after eight hours of epithelial anoxia, thus measurement of corneal lactic acid concentration can be used to monitor corneal metabolic activity. However, the only sample suitable for in vivo use is the tears. In tears collected by capillary tube, measurement of lactic acid concentration using thin layer chromatography is insufficiently accurate and enzymatic assay provides only qualitative information. Hydrated Sauflon 85 is freely permeable to lactic acid and is suitable for use as a collecting medium for the lactic acid component of tears. Lactic acid collected from tears using Sauflon 85 soft contact lenses shows an increase in tear fluid lactic acid after eight hours epithelial anoxia.