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 problems, such as overwearing syndrome and corneal clouding. Problems that still occur include hazy vision caused by mild corneal swelling (edema) or warping. Over time, corneal hypoxia may cause serious problems with the cornea.