|The Merck Manual of Medical Information--Home Edition
|Section 20. Eye Disorders
Glaucoma is a disorder in which the pressure in the eyeball increases, damaging the optic nerve and causing a loss of vision.
Both the front (anterior) and back (posterior) chambers of the eye are filled with a thin fluid called the aqueous humor. Normally, the fluid is produced in the back (posterior) chamber, passes through the pupil into the front (anterior) chamber, and then drains from the eye through the outflow channels. If the flow of fluid is interrupted, usually by an obstruction that prevents the fluid from flowing out of the anterior chamber, pressure increases.
Usually, glaucoma has no known cause; however, it sometimes runs in families. If the outflow channels are open, the disorder is called open-angle glaucoma. If the channels are blocked by the iris, the disorder is called closed-angle glaucoma.
An ophthalmologist or optometrist can measure the pressure in the anterior chamber, called intraocular pressure or tension, by using a simple, painless procedure called tonometry. In general, measurements greater than 20 to 22 millimeters indicate increased pressure. Occasionally, glaucoma occurs when pressures are normal. Sometimes a series of measurements must be taken over time to determine that the problem is glaucoma. An examination with an ophthalmoscope (an instrument used to view the inside of the eye) may reveal visible changes in the optic nerve caused by glaucoma. Sometimes, the examiner uses a special lens to observe the outflow channels; this procedure is known as gonioscopy.
Glaucoma produces a loss of peripheral vision or blind spots in the visual field. To find out if such blind spots exist, an examiner asks the person to look straight ahead at a central point and indicate when light can be seen. The test may be done either using a screen and pointer or an automated device that uses spots of light.
In open-angle glaucoma, fluid drains too slowly from the anterior chamber. Pressure gradually rises--almost always in both eyes--causing optic nerve damage and a slow, progressive loss of vision. Vision loss begins at the edges of the visual field and, if not treated, eventually spreads to all parts of the visual field, ultimately causing blindness.
The most prevalent form of glaucoma, open-angle glaucoma is common after age 35 but occasionally occurs in children. The condition tends to run in families and is most common in people with diabetes or nearsightedness (myopia). Open-angle glaucoma develops more often and may be more severe in blacks than in whites.
Symptoms and Diagnosis
Initially, increased pressure in the eyes produces no symptoms. Later symptoms may include narrowing peripheral vision, mild headaches, and vague visual disturbances, such as seeing halos around electric lights or having difficulty adapting to darkness. Eventually, tunnel vision (an extreme narrowing of the visual fields that makes it difficult to see anything on either side when looking straight ahead) may develop.
Open-angle glaucoma may not cause any symptoms until irreversible damage has developed. Usually, the diagnosis is made by checking intraocular pressure. Therefore, every routine eye examination should include a test of intraocular pressure.
Treatment is more likely to be successful if started early. Once vision is greatly impaired, treatment may prevent further deterioration, but it usually can't restore vision completely.
Medicated eyedrops can usually control open-angle glaucoma. Typically, the first eyedrop medication prescribed is a beta-blocker--such as timolol, betaxolol, carteolol, levobunolol, or metipranolol--which probably decreases the production of fluid in the eye. Pilocarpine, which constricts the pupils and increases drainage from the anterior chamber, is also helpful. Other useful medications--such as epinephrine, dipivefrin, and carbachol--work either by improving outflow or decreasing fluid production. A carbonic anhydrase inhibitor, such as acetazolamide, can be taken by mouth, or dorzolamide can be used as eyedrops.
If medication can't control eye pressure or if side effects are intolerable, an eye surgeon can increase drainage from the anterior chamber by using laser therapy to create a hole in the iris or using surgery to cut out part of the iris.
Closed-angle glaucoma causes sudden attacks of increased pressure, usually in one eye. In people with this condition, the space between the cornea and iris where fluid filters out of the eye is narrower than normal. Anything that causes the pupil to dilate--dim lighting, eyedrops given to dilate the pupil before an eye examination, or certain oral or injected medications--can result in the iris blocking the fluid drainage. When fluid drainage is blocked, intraocular pressure suddenly increases.
An episode of acute closed-angle glaucoma produces sudden symptoms. It may produce a slight decrease in vision, colored halos around lights, and pain in the eye and head. These symptoms may last only a few hours before an extended attack occurs. The attack itself produces a rapid loss of vision and sudden, severe throbbing pain in the eye. Nausea and vomiting are common and may lead a doctor to think that the problem lies in the digestive system. The eyelid swells, and the eye gets watery and red. The pupil dilates and doesn't close normally in response to bright light.
Although most symptoms disappear after treatment with medication, attacks can recur. Each attack further reduces the field of vision.
Several medications can be used to quickly decrease pressure in the eye during an attack of acute closed-angle glaucoma. Drinking a prescribed mixture of glycerin and water can reduce increased pressure and stop an attack. Carbonic anhydrase inhibitors, such as acetazolamide, are also helpful if taken early in the attack. Pilocarpine eyedrops constrict the pupil, which in turn pulls on the iris, thus unblocking the outflow channels. Beta-blocker eyedrops are also used to control the pressure. After an attack, treatment usually continues with both eyedrops and several doses of a carbonic anhydrase inhibitor. In severe cases, mannitol is given intravenously to reduce pressure.
Laser therapy, which creates a hole in the iris to allow drainage, helps prevent further attacks and often cures the disorder permanently. If laser therapy doesn't resolve the problem, surgery is performed to create a hole in the iris. If both eyes have narrow outflow channels, they may both be treated, even if the attacks have affected only one of them.
Secondary glaucoma occurs because the eye has been damaged by an infection, inflammation, tumor, an enlarged cataract, or any eye disorder that interferes with fluid drainage from the anterior chamber. Inflammatory diseases, such as uveitis, are among the most common of these disorders. Other common causes include ophthalmic vein blockage, eye injury, eye surgery, and bleeding into the eye. Some medications, such as corticosteroids, can also increase pressure in the eye.
Treatment of secondary glaucoma depends on the cause. For example, when the cause is inflammation, corticosteroids are often used to decrease the inflammation along with medications that keep the pupils large. Sometimes surgery is necessary.