Doctors don't know exactly how glaucoma damages the optic nerve. For many people, increased eye pressure seems to play an important role.
Your eye produces a watery fluid (aqueous humor), which goes into the eye and drains out. When your eye is healthy, the fluid drains through a mesh-like pathway and into the bloodstream. Aqueous fluid is produced by the ciliary body. It flows through the pupil and behind the clear cornea. Finally, it drains away through the trabecular meshwork.
For some people, fluid can't drain properly because of a faulty drainage system. Drainage that once worked well may gradually slow down as you get older. A sink that becomes clogged backs up with water. When there is no place for excess fluid to go, pressure inside the eye builds up.
This increased eye pressure may damage the optic nerve over time. Slowly, the nerve fibers that are essential for vision die.
For others, glaucoma damages the optic nerve without increased pressure. These people may be unusually sensitive even to normal levels of pressure. Their glaucoma may also be related to problems with blood flow in the eye. Doctors continue to study eye pressure and other possible causes of glaucoma.
Different people experience glaucoma differently. Usually, glaucoma affects side vision (peripheral vision) first. Late in the disease, glaucoma may cause "tunnel vision." In this condition, the person can only see straight ahead. That's why someone with glaucoma can have good straight ahead (central) vision. However, even central vision can be seriously damaged.