In a normal eye a fluid called the aqueous humor constantly flows through a small chamber at the front of the eyeball, bathing the lens and iris. As new fluid is produced, excess fluid drains out of the eye into the blood stream through a complex drainage system.
Glaucoma occurs when the drainage outlets become blocked, causing fluid pressure within to build.
The chamber becomes enlarged, pressing on the larger vitreous cavity behind. This in turn bulges against and pinches off the small blood vessels and nerve fibers at the back of the eyeball. The compressed tissues eventually die, and the sight in the affected eye is progressively damaged. A recent theory holds that the damage to nerves is caused by the chemical glutamate, which may be released from nerves damaged by pressure. Glaucoma can also be caused by disease or injury to the eye, or be a congenital condition, occurring at birth.