Ophthalmology => Glaucoma
Glaucoma, family of eye diseases characterized by increased pressure within the eye and resulting loss of vision. Although the vision loss caused by glaucoma is irreversible, medication or surgery can usually control the pressure within the eye and slow or halt the progression of the disease. If glaucoma is diagnosed in its early stages, it can be treated effectively, and vision can usually be preserved.
Glaucoma occurs in two main varieties, known as chronic simple and acute glaucoma. In chronic simple glaucoma, pressure within the eye rises gradually, and vision loss progresses over a period of years. In acute glaucoma, the pressure inside the eye rises suddenly and immediate medical treatment is necessary to preserve vision. Acute glaucoma is also known as narrow-angle or angle-closure glaucoma, because the angle between the cornea (the transparent layer of tissue at the front of the eye) and the iris (the colored part of the eye) becomes smaller than normal. Chronic simple glaucoma is also known as open-angle glaucoma because the angle between the cornea and iris remains normal. This article focuses on chronic simple glaucoma, which accounts for 95 percent of all cases of glaucoma.
Worldwide, approximately 5.2 million people are blind due to glaucoma, making this condition the third leading cause of blindness. More than 50,000 people in the United States alone have lost significant vision due to glaucoma, and the condition is the leading cause of preventable blindness in the nation. More than two million people are currently being treated for glaucoma, and an estimated one million additional cases of the disease are undiagnosed.
The front part of the eye is filled with a watery fluid, known as the aqueous humor. This fluid helps the eye maintain its shape and delivers oxygen and nutrients to the cornea and the lens, the eye structure that refracts light to form images. The aqueous humor is produced by the ciliary body, a small gland located just behind the lens. The fluid percolates through the pupil and circulates through the front chamber of the eye. It then drains away through a network of tiny channels, called the trabecular meshwork, located at the front of the eye where the cornea and iris meet.
When the aqueous humor does not drain properly, the fluid backs up, causing the pressure inside the eye to increase, and glaucoma develops. The increased pressure inside the eye compresses and damages the optic nerve, the bundle of nerve cells that transmit visual information from the eye to the brain. This damage to the optic nerve results in vision loss. In chronic simple glaucoma, the aqueous humor drains through the trabecular meshwork more slowly than normal, much as a sink empties more slowly when the drain is clogged.
Age is a primary risk factor for glaucoma: the condition strikes 1.5 percent of people over age 40, and 15 to 20 percent of people over age 70. The disease tends to run in families, although it is not inherited in a regular, predictable pattern. Glaucoma affects three times as many blacks as whites, and blacks may develop glaucoma at a younger age and suffer more severe vision loss than whites. Other factors that increase the risk of glaucoma include diabetes, nearsightedness, high blood pressure, and long-term use of cortisone or other steroid medication. Although risk factors for the disease are well understood, the cause of glaucoma remains unknown.
SYMPTOMS AND DIAGNOSIS
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 .
Most cases of glaucoma can be controlled with medication in the form of eyedrops or pills. These medications reduce the pressure inside the eye either by decreasing the amount of aqueous humor produced or by increasing the rate at which the fluid drains from the eye.
When the pressure inside the eye cannot be controlled by medication, surgery may be necessary. In laser surgery, a high-energy laser beam is aimed at the eye to help stretch the trabecular meshwork and make it easier for fluid to flow out of the eye. Sometimes patients who already have had laser surgery develop increased pressure inside the eye again. At that point, conventional surgery is often performed to create a new channel through which fluid can leave the eye. A valve may be surgically implanted in the eye to help control the pressure.
Acute glaucoma is most common in Asians and in people who are farsighted. It tends to run in families and can develop at any age. An acute attack can be brought on by entering a darkened room, stress, and certain medications. The pressure inside the eye increases because the iris is suddenly pushed down over the trabecular meshwork, much as a stopper is put in a drain. The pressure increases very rapidly- within a matter of a few hours. Symptoms develop suddenly and include severe headache, nausea, sensitivity to light, redness of the eyes, blurred vision, and seeing rainbowlike halos around lights. The pressure inside the eye must be lowered within 24 hours to prevent severe vision loss. Medications to decrease the production of aqueous humor and to constrict the pupil are used to treat acute glaucoma. Laser surgery is another common treatment. A test called gonioscopy can be performed as part of an eye exam to predict a person's chance of having an acute attack.