The front of the eye is a D-shaped chamber in which the lens and iris (the coloured part) make up the straight wall, while the cornea (the surface of the eye) forms the curved wall. A gland behind the upper eyelid fills this chamber with a clear liquid (aqueous humour) that supplies the front of the eye with oxygen and nutrients and keeps it inflated. A steady supply of liquid is produced, and it drains out through a mesh of tiny holes behind the lower eyelid, called the trabecular meshwork.
In glaucoma, the liquid is produced normally but the trabecular meshwork can't drain it due to clogging or some other reason. Liquid pressure builds up in the eye, pressing on the optic nerve (the nerve that links the eye to the brain). The nerve cells are then slowly strangled of blood, eventually dying. The outer nerves fail first, so vision loss tends to start at the edges, progressing to "tunnel vision" and blindness. Many people don't notice this at first, and there's usually no pain, so glaucoma can be quite advanced before it's detected. The US Glaucoma Foundation estimates that only 50% people with glaucoma are aware of the disease.
Types of glaucoma are classified by what's stopping the liquid from draining:
Primary open-angle glaucoma (POAG) is the most common form of glaucoma in North America, affecting about 1 in 100 people. People of African descent are especially susceptible. It normally strikes after the age of 50. The trabecular meshwork looks fine on examination but doesn't drain properly. Some researchers believe this is because aging makes the cells less efficient. Others suspect a drainage problem under the eye, or another defect. In any case, pressure builds up and the optic nerve starts to fail. Blind spots eventually appear in the peripheral vision, and later in the central "seeing" area. This damage can't be repaired. POAG gets steadily worse over time if it isn't treated.
Angle-closure glaucoma is most common in people of Asian or Inuit descent, and in women and farsighted people. It can be chronic or intermittent. Since the trabecular meshwork is located in the angle of the D between the iris and the cornea, anything that swells the iris or pushes it forward can close that angle, blocking the meshwork. Certain diseases, such as diabetes or uveitis (inflammatory condition of the eye), can cause the iris to be pushed forward. Aging also tends to thicken the lens and iris, blocking liquid flow between the front and rear chambers of the eye. Pressure builds in the rear chamber (the main part of the eyeball), pushing the iris forward and closing the trabecular meshwork.
In normal tension glaucoma, the pressure in the eye is in the normal range, yet the optic nerve is still damaged. It's believed that poor blood flow to the optic nerve makes the nerves more vulnerable. Pressures just slightly above average can therefore damage the eye.
Pigmentary glaucoma mostly affects younger nearsighted people. The iris, which is concave in myopia, rubs against the pigment layer that holds the eye's colour. Flakes of pigment rub off and clog the trabecular meshwork, raising pressure. Exfoliation syndrome, most common in Caucasians aged 50 and older, is similar. Flaky white material appears on the lens before falling off to clog the meshwork. Exfoliation syndrome doesn't necessarily result in glaucoma, but it makes it six times more likely.
Traumatic glaucoma is the result of eye injury, often appearing years after the event.