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Hypoxia, Lack of Oxygen

Oxygen needed by the cornea diffuses from the tears on the corneal surface. Contact lenses create a barrier that reduces the amount of available oxygen. Contact lens wear (especially with a closed lid during sleep) can cause acute hypoxia. If mild, hypoxia produces epithelial edema and temporary blurred vision. If severe, it can cause epithelial cell death.

Contact lens users who over-wear their lenses or fail to replace their lenses may also have chronic or long term hypoxia. Epithelial microcysts produce a mild decrease in vision. It takes several weeks for the microcysts to disappear after dicontinu7ation of the contact lenses. Also, swelling of the cornea may cause temporary or even permanent changes of prescription, usually increased nearsightedness.

With more severe chronic corneal hypoxia there may be neovascularization. Neovascularization is new blood vessel growth. If less than 2 mm it is not visually significant and generally is well tolerated but is a sign of hypoxia and may be a harbinger of more significant problems. Deep stromal neovascularization can occur with significant abuse of contact lenses. Chronic hypoxia can cause decreased corneal sensitivity. That may be the reason why some patients experience decreased comfort with more oxygen-permeable contact lenses.

Chronic hypoxia also leads to thinning of the epithelium, decreased epithelial shedding, increased cell size, and increased binding of bacteria to the corneal surface. The thinner epithelium poses less of a barrier to bacterial penetration. The reduced shedding of epithelial cells allows the attached bacteria to remain on the eye for longer periods of time. The increased binding of bacteria enables greater numbers of bacteria to attach to the epithelial surface. Thus, over-wear of contact lenses greatly increases the risk of eye infections.