The optic nerve in the human eye acts like an electric cable, and carries the images we see to the brain. Glaucoma is an eye disease that can damage this nerve, leading to irreversible vision loss. Eye injury, inflammation, tumors, cataracts, diabetes, or certain drugs such as steroids can all lead to some form of glaucoma.
There are two primary kinds of glaucoma. The most common is called primary open-angle glaucoma, and it is believed to be caused in part by pressure on the inner eye, the result of excess fluid. The eye has drainage canals that can become clogged over time. If that happens, extra fluid can't drain properly and builds up inside the eye, increasing pressure -- the same things that happen as a balloon fills with water. This type of glaucoma develops slowly, with no symptoms or early warning signs. It can usually be treated with medication if caught early enough.
The second kind is angle-closure glaucoma. Increased fluid pressure in the inner eye is also the cause of this type of glaucoma, except the drainage canals are blocked or covered over, causing a sudden, not a gradual, increase in eye pressure. This happens when the pupil enlarges too much or too quickly, as might happen when entering a dark room, causing the iris to bunch up and block the drainage canals. Doctors sometimes remove a small piece of the outer edge of the iris to unblock the canals so the excess fluid can drain. Symptoms of angle closure glaucoma include headaches, eye pain, nausea, blurred vision, and rainbow halos around lights at night.