Human corneal oxygen demands at superior, central, and inferior sites.

BACKGROUND: Oxygen depletion of the superior and central cornea in response to standard conditions is of clinical interest because of pathological conditions, usually resulting from the hypoxia of contact lens wear, that affect these areas. A study was designed to determine if there are differences in oxygen depletion at these two sites. METHODS: Oxygen depletion times from a standard reservoir following corneal exposure to three different oxygenated environments (20.9%, 5%, and 0% oxygen) were assessed micropolarographically at central, superior, and inferior corneal surface positions on both corneas of six healthy young adult human subjects. RESULTS: Based on 48 measurements for each corneal position at each oxygen percentage (432 total measurements), the superior cornea 1.0 mm from the limbus was found to have longer oxygen depletion times (slower oxygen uptake rates) than the central or inferior corneal locations. The differences reduced to nearly zero during the course of the initial 1.5-day testing procedure, but were re-established months later when testing was resumed. CONCLUSIONS: There is a small reversible increase in superior corneal oxygen depletion time (a decrease of the oxygen uptake rate) due to the chronic hypoxia that results from the presence of an overlying superior eyelid or other physiological factors.

Areal differences in oxygen supply to a cornea wearing an optically powered hydrogel contact lens.

If there is little lateral diffusion of oxygen in an optically powered hydrogel contact lens, then point-to-point differences in the thickness of the lens can result in different areas of the cornea receiving different amounts of oxygen. Specifically, areas under the thicker portions of the lens will receive less oxygen. Cells in these oxygen-deprived areas may produce metabolic products that spread laterally in the cornea resulting in cornea-wide edema. The fact that most of a contact lens transmits adequate oxygen to the cornea does not lead to satisfactory lens performance, if some part of the lens is too thick. We address the problem of lateral diffusion of oxygen in a contact lens and describe the methods for quantifying the point-to-point thickness of optically powered hydrogel lenses. Areal differences in oxygen supply to the cornea are found to be almost totally dependent on optical power; water content and lens design have far less effect.

Entoptic phenomena and reproducibility of corneal striae following contact lens wear.

Vertical corneal striae distributed across the posterior cornea are one of the objective signs of clinically unacceptable corneal swelling (greater than 6%) resulting from contact lens wear. This study reports that corneal striae are repeatable both in configuration and location with different levels of hypoxia. In most instances entoptic phenomena result from the presence of these lines. The results suggest that the healthy, avascular, transparent cornea has certain localised areas in its anatomical structure which may give rise to bundles of collagen fibres being made visible objectively and subjectively during conditions of corneal swelling.

Occlusion of the cornea with contact lens and anterior chamber lactate levels.

Rabbits (n = 11) were fitted with large PMMA lenses to completely occlude the cornea and the anterior chamber lactate measured. The corneal epithelium was examined by light microscopy. The time period was 12 h. The results were compared with other authors' finding. There was a similarity of findings. The levels of lactate were found to have increased and the epithelium though edematous not stripped off. An individual response appeared to be the reason for the large variation in the individual findings.

Epithelial morphological response to soft hydrogel contact lenses.

Hydrogel (HEMA) contact lenses of increased thickness were worn by owl monkeys (Aotus tivergatus) under open and closed eye conditions to study ultrastructurally the corneal epithelial response to increased stress. The 0.07 mm thick lens with a calculated Dk/L of 4.2-2.2 caused an epithelial thinning without oedema. This thinning was due to a loss of superficial cells and a flattening of the remaining ones. Factors in the mechanism of epithelial thinning without oedema may include anoxia, trauma, lid pressure, and lens weight. The 0.4 mm thick lens offers negligible gas transmission in the closed eye, and after 48 hours much of the epithelium was stripped, while oedema and degenerative cytoplasmic changes were prominent in areas of surviving cells. In such areas the thickness of the epithelium was reduced to one or two layers of cells. The basement membrane was in all experiments unharmed by hydrogel contact lens wear. The epithelial innervation was maintained in all corneas except those of the closed eye, and finding correlates well with the good corneal touch threshold in soft contact lens wearers.