Hypoxia occurs when the cornea does not get enough oxygen. It is the most common complication of contact lens wear, especially extended-wear lenses.

The cornea has no blood supply of its own, so it gets oxygen only from tears and directly from the atmosphere. A contact lens reduces the oxygen supply to the cornea, making the cornea swell. Wearing contacts overnight further decreases the amount of oxygen that gets to the cornea.

Gas-permeable hard contact lenses have almost eliminated some once-common problems, such as overwearing syndrome and corneal clouding; still-common problems include hazy vision caused by mild corneal swelling (edema) or warping. Over time, corneal hypoxia may cause serious problems with the cornea.