There are many types of glaucoma. Most types are associated with high eye pressure (intraocular pressure). Other types, known as "low tension" or "normal tension" glaucoma are not associated with high eye pressure. The most common type of glaucoma associated with elevated pressure is called "primary open angle glaucoma" or POAG. In this case, the elevated eye pressure causes damage to the optic nerve, which is an important structure that carries visual information from the eye to the brain. As nerve damage progresses, vision is irreversibly lost. In the vast majority of cases, there is no eye pain or other symptoms to indicate a problem. Therefore, most people with undiagnosed glaucoma do not realize something is wrong until the advanced stages of the disease. The lack of symptoms has earned glaucoma the reputation of being the "silent thief of sight." Early detection and treatment are critical to preserving sight. The best way to detect glaucoma is through a complete eye examination.