Glaucoma is a leading cause of blindness throughout the world. Its exact cause is not known. Glaucoma refers to a group of eye diseases that have common features. These features can include raised intra-ocular pressure, damage to the optic nerve and sight loss. There are several types of glaucoma, including primary open-angle, closed-angle, secondary, congenital and normal tension. The most common is primary open-angle glaucoma.