Glaucoma.

Glaucoma is a group of diseases characterized by high pressure within the eye, damage to the optic nerve, and loss of peripheral vision. It is a leading cause of blindness in the United States. Although there is no cure, there are several treatment options, including medication, laser surgery, and traditional surgery, that can control the condition.
An Introduction to Glaucoma
Glaucoma is a group of diseases sharing certain features, commonly including high pressure within the eye (intraocular pressure), damage to the optic nerve, loss of peripheral (side) vision, and possibly blindness.

Nearly 3 million people have glaucoma. Approximately 80,000 Americans become blind each year because of it. The condition causes another 900,000 people to lose partial vision.

However, with early treatment, you may be able to protect your eyes against serious vision loss.

Understanding the Eye
The cornea is the clear outer covering of the eye. Separating it from the iris (the colored part) is the anterior chamber, a space filled and inflated by aqueous humor. This clear fluid (unrelated to the tears which bathe the outside surface of the cornea) starts in the ciliary body just behind the iris. It circulates in the anterior chamber, nourishing the eye's delicate tissue and keeping it from collapsing. To maintain equilibrium, the aqueous humor drains through a porous tissue in the angle in front of the iris, where it meets the cornea, called the trabecular meshwork.

If the aqueous humor cannot drain properly, either because the drainage canals become clogged (as in open angle glaucoma) or because the iris is pushing against the cornea (as in angle closure glaucoma), it backs up, putting pressure on the gel in the vitreous cavity at the center of the eye. Eventually, the building pressure affects the delicate optic nerve at the rear of the eye. Since the optic nerve transmits visual images to the brain, any damage to it reduces vision.

The optic nerve is a bundle of more than one million nerve fibers. It connects the retina to the brain. The retina is the light-sensitive tissue at the back of the eye. A healthy optic nerve is necessary for good vision.