Glaucoma is a progressive loss of vision from damage to the optic nerve, a set of nerves that runs from the retina (the membrane at the back of the eyeball) to the brain. The damage usually occurs because the watery fluid of the eyeball (the aqueous humor) does not circulate properly through various parts of the eye, and the fluid (and pressure) builds up in the eyeball. Eventually, this high intraocular pressure (IOP) kills cells and nerve fibers and can lead to blindness. In fact, glaucoma is the second-leading cause of blindness in the United States, the leading cause among African-Americans, and a major source of vision problems in the elderly. Over the course of 20 years, three-fourths of those with glaucoma may become blind. Little information is available, however, about the natural course of the disease if you have mild visual defects that are revealed by routine screening tests.

Source:http://www.realage.com/check-your-health/disease-prevention/glaucoma