Anatomical Considerations
Potentially Adverse Effects

Corneal Edema:
When the combination of forces which normally serve to dehydrate the cornea are overcome by forces which drive water into the cornea, corneal edema may result. This situation, which may be induced by contact lens wear when the corneal epithelium becomes deprived of its normal amount of oxygen, is known as corneal hypoxia. When PMMA lenses are worn, the amount of oxygen received by the cornea is dependent upon the adequate circulation of tears under the lens. An inadequate tear flow under the lens can result in corneal edema.

Corneal edema with PMMA lenses may be caused by the following factors:

Corneal edema with rigid gas-permeable lenses:

In rigid gas permeable lenses corneal edema can be caused by all of the factors as in PMMA lenses but only if there is inadequate transmission of oxygen through the lens. This can occur if the surface of the lens is coated extensively with deposits.

Corneal edema with soft lenses:

Any factor which limits the transmission of oxygen through the lens can result in corneal edema with soft contact lenses. Some of these factors might include:

The interrelationship of these three factors becomes especially important when fitting extended-wear lenses.

Prolonged corneal edema can result in excessive spectracle blur, limbal injection, and photophobia.