Glaucoma is a group of diseases in which eye structures are damaged.  There are two main types of glaucoma:  Open-angle glaucoma is the most common type, and aging is a high risk factor. When a person ages, the eye's drainage apparatus for the fluid inside eyes may be dysfunctional over time. This causes the intraocular fluid pressure to increase gradually inside eye, thereby damaging the optic nerve and impairing one's eye sight. With narrow-angle (or angle-closure) glaucoma, the eye pressure is normal until the drainage (angle for fluid ) becomes suddenly blocked. The intraocular pressure then rises abruptly to dangerous levels. When this occurs, immediate treatment of the high pressure is necessary to prevent blindness.