The exact cause of glaucoma isn't known. Experts think that increased pressure in the eye (intraocular pressure) may cause the nerve damage in many cases. But some people who have glaucoma have normal eye pressure.

Get more information on eye anatomy and function.

Open-angle glaucoma

In open-angle glaucoma (OAG), fluid in the eye (aqueous humor) doesn't drain well. When this happens, the fluid builds up. This buildup increases the intraocular pressure (IOP) and may damage the optic nerve.

Up to half of the people with OAG don't have higher-than-normal IOP. This is called normal- or low-tension glaucoma.

Closed-angle glaucoma

In closed-angle glaucoma (CAG), fluid can't drain because the drainage angle is blocked. This may occur when:

• The colored part of the eye (iris) and the lens block the movement of fluid between the chambers of the eye. The blockage of fluid causes pressure to build up in the eye and makes the iris press on the eye's drainage system (trabecular meshwork). The increased pressure can cause damage to the optic nerve, leading to vision loss and possible blindness.
• You have a defect in your iris or another problem that causes the iris to fall forward and block the drainage angle.
• You have scar tissue between the iris and the cornea, and it blocks the eye's drainage system.

Congenital and infantile glaucoma

Glaucoma that is present at birth (congenital glaucoma) or that develops in the first few years of life (infantile glaucoma) is often caused by certain birth defects. A birth defect may occur because of an infection in the mother during pregnancy, such as rubella, or because of an inherited condition such as neurofibromatosis.

Secondary glaucoma

Some people get glaucoma after an eye injury or after eye surgery. A cataract and some medicines (corticosteroids) that are used to treat other diseases may also cause glaucoma. Glaucoma caused in these ways is called secondary glaucoma.