Glaucoma Channel

Overview

Glaucoma, the second-leading cause of adult blindness in the United States after age-related macular degeneration, often results from intraocular pressure (fluid pressure within the eye, or IOP) that is too high for the optic nerve to tolerate. About 2.2 million Americans age 40 and over have glaucoma, and, because the condition does not cause symptoms in its early stages, half of them do not know it. Another 5 million to 10 million people are at increased risk for the disorder.

Open-angle glaucoma, which accounts for 90 percent of all glaucoma cases in the United States, progresses slowly and produces no obvious symptoms until its late stages. Closed-angle glaucoma occurs most often in people of Chinese descent and some other Asian groups. The distinction between open-angle and closed-angle glaucoma is made by examining the front part of the eye to check the angle where the iris meets the cornea. Both types of glaucoma can lead to blindness by damaging the optic nerve. Even people with normal IOP can suffer damage to the optic nerve-in fact, 25 percent to 30 percent of people with glaucoma do not have elevated IOP levels.

Causes

Each day, the eye produces about 1 teaspoon of aqueous humor -a clear fluid that provides nutrients to, and carries waste products away from, the lens and cornea. Ordinarily, fluid production and drainage are in balance, and intraocular pressure (IOP) is between 12 and 22 mm Hg (millimeters of mercury, the same units of measurement used for blood pressure). In people with open-angle glaucoma who have higher-than-normal IOP levels, ophthalmologists suspect that a partial blockage traps the aqueous humor. Exactly how this happens is unclear. As more aqueous humor is produced than is removed, the blockage causes an increase in IOP. When IOP remains elevated or continues to rise, fibers in the optic nerve are compressed and eventually die, leading to a gradual loss of vision .

In some people, even a normal level of IOP is sufficient to contribute to optic nerve damage. These individuals may have thinner-than-normal corneas that make IOP measurements appear to be lower than they actually are. The American Academy of Ophthalmology now recommends that people with other risk factors for glaucoma (for example, advanced age, elevated IOP, and African-American ancestry) have their corneal thickness measured.

Closed-angle glaucoma is caused by a blockage of aqueous humor at the pupil, leading to a bowing forward of the iris. The blockage of the outflow of aqueous humor results in a sudden increase in IOP that can lead to severe, permanent vision damage within one or two days if not treated.

Need-to-know anatomy

Aqueous humor -a clear fluid that provides nutrients to, and carries waste products away from, the lens and cornea-flows from behind the iris through the pupil and into the front chamber of the eye; it then drains from the eye through a spongy network of connective tissue called the trabecular meshwork , where it ultimately enters the bloodstream. An alternate drainage system, the uveoscleral pathway , is located behind the trabecular meshwork.

Ordinarily, fluid production and drainage are in balance. In people with open-angle glaucoma who have higher-than-normal IOP levels, ophthalmologists suspect that a partial blockage of the trabecular meshwork traps the aqueous humor. As more aqueous humor is produced than is removed, the blockage causes an increase in IOP. Fibers in the optic nerve are compressed and eventually die, leading to a gradual loss of vision.

In some people, even a normal level of IOP is sufficient to contribute to optic nerve damage. These individuals may have thinner-than-normal corneas that make IOP measurements appear to be lower than they actually are.

Closed-angle glaucoma is caused by a blockage of aqueous humor at the pupil, leading to a bowing forward of the iris that prevents aqueous humor from reaching the trabecular meshwork. The blockage of the outflow of aqueous humor results in a sudden increase in IOP that can lead to severe, permanent vision damage within one or two days if not treated.

Risk factors

The risk of glaucoma varies with race and age . The condition is more common in blacks and Hispanics than in whites, and it occurs more frequently with increasing age. In one study, the prevalence of glaucoma was about 1 percent in people ages 40 to 49; in those over age 70, it reached 10 percent in whites and 20 percent in blacks. Heredity may play a role in glaucoma risk; mutations causing glaucoma have been identified.

Inhaled corticosteroids-commonly used to treat asthma-and nasal sprays containing corticosteroids appear to raise the risk of elevated intraocular pressure (IOP) and open-angle glaucoma, possibly by inhibiting the drainage of aqueous humor. Oral corticosteroids may have the same effect. People who must use corticosteroids should have their IOP and vision monitored regularly.

In some people, even a normal level of IOP is sufficient to contribute to optic nerve damage. These individuals may have thinner-than-normal corneas that make IOP measurements appear to be lower than they actually are. The American Academy of Ophthalmology now recommends that people with other risk factors for glaucoma have their corneal thickness measured.