Glaucoma is not a single eye disorder but a group of disorders that affect the eye. The optic nerve is made up of 1 million nerve fibers. It carries visual images from the retina, the layer of light-sensitive tissue at the back of the eyeball, to the brain. Untreated glaucoma may lead to vision loss or even complete blindness. In most cases of glaucoma, the optic nerve is damaged by increased fluid pressure inside the eyeball. However, reduced blood supply to the optic nerve, caused by the increased pressure, can also be a factor.
Glaucoma is often categorized as either primary or secondary. Primary glaucoma refers to glaucoma that is not triggered by an injury or other medical condition. It accounts for about 90 percent of cases. There are several major types of primary glaucoma:
- Open-angle glaucoma. This term refers to the angle in the front portion of the eye where the cornea meets the iris. In normal circumstances, fluid flows in and out of the front of the eye through a meshwork of tissue at the angle that acts like a drain. In open-angle glaucoma, the angle remains open and fluid continues to pass through the meshwork, but not quickly enough. As a result, fluid builds up inside the eye, and the fluid pressure may rise high enough to damage the optic nerve. Open-angle glaucoma is the most common type of primary glaucoma.
- Closed-angle glaucoma. Closed-angle glaucoma is a disorder in which the angle between the iris and the cornea is blocked, usually because the iris becomes swollen from pressure and moves forward to touch the meshwork directly, thus preventing fluid from draining out of the eye. Closed-angle glaucoma can develop either gradually or suddenly. Acute closed-angle glaucoma is a medical emergency.
- Congenital glaucoma. This is a type of glaucoma that is present at birth and is usually noticeable within the first year of life. It is more common in boys than in girls. In congenital glaucoma, there is a defect in the structure of the baby 's eye that slows down the normal drainage of fluid.
- Normal-pressure glaucoma. Also called normal-tension glaucoma, this is a type of primary glaucoma in which the optic nerve is damaged even though the fluid pressure within the eye is within normal range. Doctors do not yet understand the causes of this type of glaucoma, although one theory suggests that the optic nerve is damaged by normal fluid pressure because its blood supply has been reduced.
- Pigmentary glaucoma. This is a type of glaucoma that develops when pigment granules from the iris flake off and block the drainage meshwork in the angle between the iris and cornea.
About 10 percent of cases of glaucoma are secondary, meaning that they develop as a result of injury to the eye or another disease, most commonly diabetes, leukemia, or sickle cell anemia. Secondary glaucoma may result from a blow to the eye, a tumor, cataract surgery, or the use of corticosteroid medications.
People 's experience of glaucoma varies considerably depending on which type they have and whether it is chronic (developing slowly) or acute (sudden onset). In addition, glaucoma can affect both eyes or only one.
Children with congenital glaucoma are usually diagnosed within the first few months after birth because their eyes look cloudy, are unusually sensitive to light, and secrete large amounts of tears.
Chronic open-angle glaucoma, the single most common type, develops over a period of years and is related to the aging of the drainage meshwork in the angle between the iris and cornea. In the early stages, patients with this type of glaucoma may have no symptoms at all. Gradually, however, they find it more difficult to see objects to the side or on the edges of their visual field. They may also develop tunnel vision, in which they can see only objects straight in front of them.
Closed-angle glaucoma can come on suddenly, often in dim light, with eye pain, reddening of the eye, a sudden severe headache, and nausea and vomiting. The person may also see colored or rainbow-like halos around lights.
Glaucoma is largely an eye disorder of adults. Congenital glaucoma is rare and childhood glaucoma is also unusual. Glaucoma in middle-aged adults, however, is a common eye disorder and the second leading cause of blindness in the United States. According to Prevent Blindness America, more than 2.2 million Americans over age forty have open-angle glaucoma, but only half of them know that they have it. In North America and Europe, glaucoma affects one person in every two hundred aged fifty or younger, but one in ten over the age of eighty.
When to See the Doctor
Some types of glaucoma develop gradually and can usually be detected during routine periodic eye examinations. Acute closed-angle glaucoma, however, is a medical emergency. Anyone with any of the following symptoms should see an eye doctor at once:
- Blurred vision
- Severe pain in the affected eye or eyes
- Redness in the eye
- Sudden violent headache
- Abdominal cramping
- Seeing rainbow-like halos around lights
Emergency treatment is needed because this type of glaucoma can cause permanent blindness in as little as one or two days.
Some people are at increased risk of glaucoma:
- Those with increased fluid pressure in the eye, sometimes called ocular hypertension. A high level of fluid pressure inside the eye is the greatest single risk factor for glaucoma. However, as noted earlier, some people develop the disorder even though their eye fluid pressure is normal.
- Age. The risk of glaucoma increases over age sixty for most Americans; it rises over age forty for African Americans.
- Race and ethnicity. African Americans have six to eight times the risk of glaucoma as Caucasians. Mexican Americans over the age of sixty are also at increased risk. Asian Americans and Alaskan Inuit have a higher than average risk of acute closed-angle glaucoma, and Japanese Americans have an increased risk of normal-pressure glaucoma. The reasons for these differences are not yet understood.
- Family history of glaucoma.
- Certain diseases or conditions, including high blood pressure, diabetes, heart disease, hypothyroidism, and sickle cell anemia.
- Myopia (nearsightedness).
- A history of injury to the eye, inflammation of the eye, tumors in the eye, or cataract surgery.
- Long-term use of corticosteroid medications.
The most important cause of glaucoma is increased fluid pressure within the eye resulting from overly slow drainage of fluid through the meshwork in the angle between the iris and cornea or complete blockage of the angle. This buildup of fluid pressure damages the optic nerve. In some cases inadequate blood supply to the optic nerve is also a factor.
There is some evidence that genetic factors are also involved in glaucoma because the disorder is known to run in some families. Several genetic mutations have been linked to primary open-angle glaucoma, but no single gene has been shown to cause the disorder.
The symptoms of the various types of glaucoma were described earlier.
The diagnosis of chronic primary glaucoma is usually made during a routine eye examination by an ophthalmologist, who is a doctor specializing in the diagnosis and treatment of eye disorders. A complete eye examination, depending on the individual patient 's history and risk factors, will include most or all of the following tests:
- Dilation of the eye. The doctor will dilate the pupil of the eye by placing drops in the eyes that keep the iris from narrowing the pupil when the doctor shines a bright light directly into the eye. The doctor will then be able to see directly to the back of the eye to check for damage to the optic nerve.
- Tonometry. This is a test for measuring the level of fluid pressure inside the eye. It can be performed either by resting an instrument briefly on the surface of an anesthetized eye, or by blowing a puff of air onto the surface of the eye while the patient 's chin is held steady. The fluid pressure is estimated by measuring the response of the eye to the puff of air.
- Tests of peripheral vision. Since open-angle glaucoma often starts with gradual loss of side vision, doctors may measure whether such loss has occurred by asking patients to look at a set of blinking lights and indicate when they can see the lights. The patients ' answers allow doctors to map how much, if any, of the patients ' visual field has been lost.
- Tests to measure the thickness of the cornea. This test is done to rule out the possibility that a cornea that is either unusually thick or unusually thin is affecting the measurement of the fluid pressure inside the eye.
- Gonioscopy. This is a test than uses a gonioscope, an instrument with a mirror as well as a light source. It allows the doctor to tell whether the angle between the iris and the cornea is open or closed.
A patient with acute closed-angle glaucoma will usually be treated by an ophthalmologist in a hospital emergency department. Emergency treatment consists of medications to quickly reduce the fluid pressure inside the eye and laser surgery to cut a drainage hole in the iris.
Glaucoma can be treated with medications, surgery, or both. The medications used include special eye drops or oral medications. In some cases the patient may be asked to use more than one type of eye drop. There are several different types of drugs that may be prescribed; some work to lower the fluid pressure inside the eye by decreasing the amount of fluid produced. Other drugs work by increasing the outflow of fluid. All of these medications have side effects, however, and must be used exactly according to the doctor 's instructions.
Surgery is usually used to treat congenital glaucoma and acute closed-angle glaucoma. The procedure used to treat closed-angle glaucoma is
called an iridotomy. The surgeon uses a laser to cut a hole in the iris to relieve the increased pressure inside the eye.
Surgery can also be performed to treat open-angle glaucoma. There are two major types of laser surgery that can be done. One technique involves using the laser to open clogged portions of the drainage network. Another approach is called filtering surgery. The surgeon uses the laser to cut a small hole in the sclera (white part of the eyeball) and remove a small portion of the clogged meshwork. The extra fluid can then leave the eye through the hole in the sclera.
Children with glaucoma and adults with secondary glaucoma can be treated with drainage implant surgery. The implant is a small silicone tube that the doctor inserts inside the eye to help drain the excess fluid.
The prognosis of glaucoma depends on its type and the stage at which it is diagnosed. Vision that has been lost to any type of glaucoma cannot be restored. Without treatment, acute glaucoma results in permanent vision loss within days. Untreated chronic glaucoma can progress to blindness within several years.
The best prevention for glaucoma is regular eye exams. Anyone over age eighteen should be screened periodically for glaucoma. The schedule of exe examinations recommended by the National Eye Institute (NEI) is as follows:
- Eighteen to sixty years of age: Every two years.
- Over sixty: Every year.
- One or more risk factors other than age: Every year before and after age sixty.
Other preventive steps that people can take include getting treatment (special eye drops) if diagnosed with high fluid pressure inside the eye; controlling one 's weight; and getting treatment for high blood pressure.
Glaucoma is likely to become more common in the general population because of the growing number of people over age fifty. Present research is focused on identifying more genes associated with glaucoma; developing better treatments for high fluid pressure in the eye before it leads
to glaucoma; and comparing the effectiveness of the various surgical treatments for glaucoma.
SEE ALSO Cataracts; Diabetes; Hypertension; Leukemia; Myopia; Sickle cell anemia
WORDS TO KNOW
Cornea: The transparent front part of the eye where light enters the eye.
Iris: The circular colored structure at the front of the eyeball that controls the amount of light entering the eye by changing the size of the pupil.
Ophthalmologist: A doctor who specializes in diagnosing and treating eye disorders and can perform eye surgery.
Pupil: The circular opening in the center of the iris.
Retina: The light-sensitive layer of tissue at the back of the eyeball.
Sclera: The opaque white portion of the eyeball.
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Glaucoma Consultants. Glaucoma Animations. Available online at http://www.glaucomaconsultants.org/patient.php#animations (accessed September 1, 2008). This is a set of nine animations about the various types of glaucoma and treatments for the disorder. Each animation takes about a minute to play.
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