Glaucoma is usually, but not always, associated with elevated pressure in the eye (intraocular pressure). This pressure leads to damage to the eye (optic) nerve.
Actually, glaucoma is now considered a disease of the optic nerve (optic neuropathy) that causes a loss of vision, usually in both eyes (bilateral). This loss often begins with a subtle decrease in side (peripheral field) vision. If the glaucoma is not diagnosed and treated, it may progress to loss of central vision and blindness.
Worldwide, glaucoma is the leading cause of irreversible blindness. In fact, as many as 6 million individuals are blind in both eyes from this disease.
In the United States alone, according to one estimate, over 3 million people have glaucoma.
As many as half of the individuals with glaucoma, however, may not know that they have the disease. The reason they are unaware is that glaucoma initially causes no symptoms, and the loss of vision on the side (periphery) is hardly noticeable.