Glaucoma occurs when increased pressure within the eye causes harm to the optic nerve, which carries visual information from the eye to the brain. A common symptom is the loss of side, or peripheral, vision. Those at risk of developing glaucoma include African Americans, diabetics, people over age 40, and people with a family history of the disease. If detected and treated in its early stages, glaucoma can be controlled and vision can be saved.

 In its early stages, glaucoma is symptomless. The gradual increase of pressure inside the eye does not cause any pain or discomfort. As the disease progresses, however, vision begins to deteriorate. The deterioration usually begins with the peripheral vision—sight at the outer edges of the visual field. If glaucoma is left untreated, the field of vision continues to shrink until a person becomes blind.

 Glaucoma can be detected before vision loss occurs by a tonometry test, which is a simple, painless part of a routine eye exam. An instrument called a tonometer blows a puff of air into the eye to measure the pressure inside the eye. Some tonometers measure pressure by means of a small plastic prism that is pressed lightly against the surface of the eye. Tests to measure peripheral vision help detect vision loss due to glaucoma. Finally, an instrument called an ophthalmoscope permits examination of the inside of the eye to detect damage to the optic nerve.