Glaucoma is a slow progressive disease of the optic nerve. The optic nerve is important as it carries all the information about what you see to your brain. Glaucoma is often called the “sneak thief of sight” as you do not notice symptoms until the condition is very advanced.
Glaucoma occurs when the intraocular pressure within the eye is high enough to cause damage to the structure of the optic nerve. Sometimes this is due to elevated eye pressure, or sometimes it is because the nerve is weak and is damaged at normal eye pressures. It causes a loss of peripheral vision.
To test for glaucoma your optometrist will measure the eye pressure, look at and photograph the optic nerve, and perform a peripheral vision test and OCT laser scanning. Although there is no cure for glaucoma the condition can be controlled with pressure lowering eye drops or surgery

Watch the video below by the Southwestern Eye Center. Title: Glaucoma: Causes, Prevention and Treatment