Glaucoma is a group of eye disorders that leads to progressive damage to the optic nerve, and it is characterized by loss of nerve tissue resulting in loss of vision. The optic nerve is a bundle of over one million individual nerve fibers and it transmits the visual signals from the eye to the brain. The most common form of glaucoma, primary open-angle glaucoma, is associated with an increase in the fluid pressure inside the eye. This increase in pressure can cause progressive damage to the optic nerve and loss of nerve fibers. Vision loss may result. Advanced glaucoma may even cause blindness. Not everyone with high eye pressure will develop glaucoma, and many people with normal eye pressure will develop glaucoma. When the pressure inside an eye is too high for that particular optic nerve, whatever that pressure measurement may be, glaucoma will develop. Glaucoma is the second leading cause of blindness in the U.S.