Glaucoma

Definition

Glaucoma is a group of eye disorders that results in vision loss due to a failure to maintain the normal fluid balance within the eye. If fluid pressure builds up, then damage to the optic nerve occurs, leading to vision loss. If detected in its early stages, vision loss can be prevented through the use of medications or surgical procedures that restore the proper fluid drainage of the eye.

Description

Vision is an important and complex special sense by which the qualities of an object, such as color, shape, and size, are perceived through the detection of light. Light that bounces off an object first passes through the cornea (outer layer) of the eye and then through the pupil and the lens to project onto a layer of cells on the back of the eye called the retina. When the retina is stimulated by light, signals pass through the optic nerve to the brain, resulting in a visual image of an object.

The front chamber of the eye is bathed in a liquid called the aqueous humor. This liquid is produced by a nearby structure called the ciliary body and is moved out of the eye into the bloodstream by a system of drainage canals known as the trabecular meshwork. The proper amount of fluid within the chamber is maintained by a balance between fluid production by the ciliary body and fluid drainage through the trabecular meshwork. When fluid accumulates in the front chamber, either because of an overproduction of fluid or because of a failure of the normal drainage routes, fluid pressure builds up within the eye. Over time, this increased fluid pressure causes damage to the optic nerve, resulting in progressive visual impairment. The condition of increased eye fluid pressure leading to vision loss is known as glaucoma.

Glaucoma is actually a group of many different eye disorders and can manifest alone or as a sign of more than 60 different diseases, or even in a healthy person who has experienced an injury to the eye. Physicians classify glaucoma by the type of abnormality in the drainage system. When the drainage passage is narrowed, but still open, it is termed open-angle glaucoma. If the drainage passage is completely blocked, it is termed closed-angle glaucoma. Glaucoma can also be classified by the age of the affected individual: infantile or congenital glaucoma affects infants at birth or children up to three years old, juvenile glaucoma affects individuals from 3–30 years old, and adult glaucoma affects people greater than 30 years old.

Types of glaucoma and related genetic information

Genetic profile

There are different forms of glaucoma that either occur alone or as the result of a genetic abnormality. In some cases, specific genetic abnormalities have been identified, while in other forms, the cause is unknown. The known types of glaucoma and the corresponding

genetic defect include forms of glaucoma are not inherited and thus are not represented in the table.

Glaucoma can be inherited in either an autosomal recessive or an autosomal dominant fashion. In autosomal recessive inheritance, two abnormal genes are needed to display the disease. A person who carries one abnormal gene does not display the disease, and is called a carrier. A carrier has a 50% chance of transmitting the gene to a child, who must inherit one abnormal gene from each parent to display the disease. Alternatively, in autosomal dominant inheritance, only one abnormal gene is needed to display the disease, and the chance of passing the gene and the disease to offspring is 50%.

Research is ongoing concerning the heritability of genetic risk factors for primary open-angle glaucoma. This type of glaucoma is particularly troublesome since it is common, progressive, and one of the leading causes of blindness around the world. A major study done in Wisconsin, the Beaver Dam Eye Study investigated the family aggregation and heritability of risk factors of primary open-angle glaucoma among 5,924 participants. The researchers found that there are strong genetic familial effects on risk factors for open-angle glaucoma. The researchers noted that there was a strong and consistent relationship between intraocular pressure and optic cup measurements in siblings, parents, and children, with a lack of correlation of those measurements in spouses.

The genetics of juvenile open-angle glaucoma have been widely studied. Research done in the late 1990s identified a trabecular meshwork-induced glucocorticoid TIGR gene strongly associated with juvenile open-angle glaucoma. Further research identified a protein associated with the cytoskeleton of the retina that was coded for by the myocilin gene. While the exact function of the protein coded for the myocilin gene is unknown, studies have shown a high rate of mutations in this gene in patients with juvenile open-angle glaucoma.

Demographics

Glaucoma is the leading cause of preventable blindness in the United States, affecting more than two million Americans, and is the third leading cause of blindness worldwide. The prevalence of glaucoma increases with age, but the eye condition can also be present in infants and young children. The adult types of open-angle glaucoma account for the majority (70%) of glaucoma cases, while the infantile and juvenile types of glaucoma are relatively uncommon.

The types and rates of glaucoma are not distributed equally among different ethnic groups. For example, the prevalence of glaucoma in Caucasians over 70 years old is 3.5%, while the prevalence in African Americans is 12%. Also, the primary closed-angle type of glaucoma is much more common in people of Asian or Inuit descent. Apart from ethnicity, risk factors for the development of glaucoma include elevated eye pressure, increasing age, diabetes , and presence of glaucoma in a family member.

Signs and symptoms

In the adult and juvenile forms of open-angle glaucoma, vision loss begins at the periphery (outer edges) of the visual field, resulting in tunnel vision. Because the visual loss in not in the individual's central vision, they may not notice this change. However, if the glaucoma is left untreated, loss of vision progresses and the central vision is often affected, sometimes resulting in blindness. The average time from development of high eye fluid pressures to the appearance of visual loss is 18 years in the adult form, but much shorter in the juvenile form.

In contrast to the adult and juvenile forms, congenital or infantile open-angle glaucoma is noted at birth or within the first three years of life. Symptoms include cloudy corneas, excessive tearing, and sensitivity to light. Because the eye is very flexible in infants, increased fluid pressure may cause bulging of the eye (buphthalmos, or ox eye). Children with glaucoma in only one eye are usually diagnosed earlier because a difference in eye size can be noticed. When the disorder affects both eyes, many parents view the large eyes as attractive and do not seek help until other symptoms develop, delaying the diagnosis.

With closed-angle glaucoma, symptoms come on suddenly. People may experience blurred vision, severe pain, headache, sensitivity to light, and nausea. The development of this type of glaucoma is an emergency and requires immediate treatment.

Diagnosis

The diagnosis of glaucoma may be suggested by certain physical findings, especially in infants, but is confirmed by tests with special instruments. Parents may bring their young infant to a physician if they notice changes in the eye shape and size, signs of infantile glaucoma. In adults, who do not show obvious signs of glaucoma, the condition is frequently detected by routine screening eye exams and other tests.

Using an ophthalmoscope (a hand-held or machine-mounted instrument with a light source), a physician or optometrist will look through the pupil to the back of the eye. There, they may detect characteristic changes in the region where the optic nerve meets the eye, called the optic disk.

In another portion of a routine eye exam, an ophthalmologist or optometrist will measure the fluid pressure of the eye through the use of a special instrument called a tonometer. The test is painless and involves brief contact of a small probe with the surface of the eye. Presence of elevated pressure (more than 21 mm Hg) means that a person is at risk for glaucoma.

Once high pressures or changes in the optic disk are noted, an ophthalmologist can also use a gonioscope (small lens with a reflecting mirror) to inspect the drainage passageways of the eye and determine if they are blocked. Visual field tests (in which individuals indicate whether they can see small flashing lights that are directed in different spots of their visual field) are used as a final indicator for the presence of glaucoma or a measurement of how far glaucoma-related visual loss has progressed.

Treatment and management

Although there is no treatment for the optic nerve injury and vision loss caused by glaucoma, it is possible to prevent further visual loss by lowering eye fluid pressure. In the adult, this is primarily achieved through medications. Medications can reduce eye fluid pressure by either decreasing fluid production or by increasing fluid drainage from the eye, and can be taken orally (by mouth) or applied to the eye through drops. The names of different classes of medications used to treat glaucoma include beta-blockers, alpha agonists, carbonic anhydrase inhibitors, and prostaglandin analogues.

For infantile glaucoma, the treatment is primarily surgical. Laser surgery or microsurgery to open the drainage canals can be effective in increasing drainage of eye fluid. Other types of surgery can be performed to reduce the amount of fluid production. Many children require several operations to lower or maintain their eye fluid pressures adequately, and long-term treatment with medications may be necessary. For closed-angle glaucoma, immediate hospitalization and treatment with medication is required. Once the person's condition has been stabilized, laser surgery is used to create a passageway for fluid drainage.

All individuals with glaucoma should see an ophthalmologist regularly to evaluate progress of the condition and whether it is being adequately treated. Beginning at the age of 40, all people should receive regular screening exams to detect early signs of glaucoma. People with a family history of glaucoma or with diabetes should receive these screening tests beginning in young adulthood.

Prognosis

Since even small amounts of vision loss due to glaucoma cannot be reversed, early detection of the condition through regular eye examinations is critical. If glaucoma is detected early, lifelong medical treatment can halt the progress of the disease and result in relatively normal vision. If left undiagnosed or untreated, many people with glaucoma will progress to blindness.

Closed-angle glaucoma is an emergency and the prognosis depends on how quickly medical attention is obtained and the severity of the attack. If left untreated, the condition can quickly lead to total vision loss in the affected eye.

Resources

BOOKS

Marks, E., and R. Mountauredes. Coping With Glaucoma. Garden City Park, NY: Avery Publishing Group, 1997.

Trope, G. E. Glaucoma: A Patient's Guide to the Disease. Toronto: University of Toronto Press, 1996.

PERIODICALS

Coleman, A. L. "Glaucoma." Lancet 354 (November 1999): 1803–1810.

Klein, B. E. K., R. Klein, and K. E. Lee. "Heritability of Risk Factors for Primary Open-angle Glaucoma: The Beaver Eye Dam Study." Investigative Ophthalmology and Visual Science 45 (January 2004): 59–62.

Migdal, C. "Glaucoma Medical Treatment: Philosophy, Principles and Management." Eye 14 (June 2000): 515–518.

Satoko, S., P. R. Lichiter, A. T. Johnson, et al. "Age-dependent Prevalence of Mutations at the GLCIA Locus in Primary Open-angle Glaucoma." American Journal of Opthalmology 130 (2000): 165–177.

ORGANIZATIONS

Glaucoma Foundation. 33 Maiden Lane, New York, NY 10038. (800) 452-8266. (April 19, 2005.) <http://www.glaucoma-foundation.org>.

Glaucoma Research Foundation. 200 Pine St., Suite 200, San Francisco, CA 94104. (800) 826-6693.

WEBSITES

"Glaucoma." Online Mendelian Inheritance in Man. National Center for Biotechnology Information, National Library of Medicine. Building 38A, Room 8N805, Bethesda, MD 20894. (April 19, 2005.) <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM&cmd=search&term=glaucoma>.

Glaucoma Resources on the Internet. (April 19, 2005.) <http://www.healthcyclopedia.com/glaucoma.html>.

Oren Traub, MD, PhD

Edward R. Rosick, DO, MPH, MS