Glaucoma | Description | Prevention | Causes and symptoms | Treatment
Definition
Glaucoma is a group of eye diseases that damage the optic nerve and gradually cause vision loss.
Description
The optic nerve, located at the back of the eye, consists of more than a million of nerve fibers that carry images from the retina to the brain. Glaucoma damages optic nerve fibers, with the result that blind spots develop in the visual field. If the entire nerve is destroyed, blindness results.
There are five types of glaucoma that all share as a common feature the damage that they inflict to the optic nerve:
· Primary open angle glaucoma (POAG). With POAG, the drainage canals of the eye become clogged over time. The inner eye pressure, also called intraocular pressure (IOP), increases because the correct amount of fluid cannot drain out of the eye. The entrances to the drainage canals are clear and the clogging occurs further inside the drainage canals. This type of glaucoma develops gradually over time and sometimes without apparent sight loss for many years.
· Closed angle glaucoma (CAG). CAG rarely occurs and is very different from POAG because the eye pressure usually increases very quickly due to clogged drainage canals. With CAG, the iris does not open as wide as it should. The outer edge of the iris bunches up over the drainage canals when the pupil enlarges too much or too quickly.
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Prevalence of open-angle glaucoma among adults 40 years and older in the United States |
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|
|
Glaucoma |
|
|
Years |
Persons |
(%) |
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source: Adapted from Archives of Ophthalmology, Vol. 122, |
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|
(Illustration by GGS Information Services. Cengage Learning, Gale.) |
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|
40–49 |
290,000 |
0.7% |
|
50–59 |
318,000 |
1.0% |
|
60–69 |
369,000 |
1.8% |
|
70–79 |
530,000 |
3.9% |
|
≥80 |
711,000 |
7.7% |
|
Total |
2,218,000 |
1.9% |
· Normal tension glaucoma (NTG). In NTG, the optic nerve is damaged even though the IOP is often in the normal range.
· Secondary glaucoma. This type of glaucoma can result from an eye injury, inflammation, tumor, or occur in advanced cases of cataract or diabetes. It can also be caused by drugs such as steroids.
· Pediatric glaucoma. This type of glaucoma includes glaucoma present at birth (congenital glaucoma), glaucoma that appears during the first three years (infantile glaucoma), or between age three and ten or young adult years (juvenile glaucoma), as well as all the secondary glaucomas occurring in these age groups.
Demographics
It is estimated that there are 60 million people with glaucoma worldwide with six million people becoming blind in both eyes as a result. According to the National Eye Institute (NEI), 50 million Americans are at risk for vision loss from glaucoma, a leading cause of blindness in the United States. People over the age of 60 are at increased risk for glaucoma, particularly Mexican Americans. Other groups at increased risk include African Americans over the age of 40 and people with a family history of glaucoma. In a study sponsored by the National Eye Institute (NEI), researchers at the Johns Hopkins University reported that glaucoma is three to four times more likely to occur in African Americans than in Whites. Glaucoma is also six times more likely to cause blindness in African Americans than in Whites. POAG is the most common form of glaucoma, affecting about three million Americans.
Causes and symptoms
The inner pressure of a healthy eye usually ranges from 12ȓ22 mm Hg. An OIP in this range is important for the eye to hold its shape and function properly. This optimal pressure is maintained by the fluids of the eye, of which there are two types. The first is the vitreous, the colorless gelatinous mass that fills the rear two-thirds of the eyeball between the lens and the retina. The second is theaqueous humor, the clear, watery fluid in the front of the eyeball. Aqueous humor is continuously produced before draining out of the eye. It nourishes the lens and the cornea while removing unwanted matter. In a healthy eye, aqueous humor is generated at the same rate as the eye drains fluid, and a normal OIP is maintained. The eye drains aqueous humor through a meshwork system (trabecular meshwork) located at the angle formed where the iris and the cornea meet, from which it passes into the bloodstream through a channel called “Schlemm's canal.” In people with glaucoma, the eye drainage mechanism becomes clogged, preventing the discharge of aqueous humor which then builds up, thus increasing the OIP.
Although glaucoma tends to run in families, a hereditary basis has not been firmly established. Some people are born with the iris too close to the drainage angle. In such eyes, the iris can be aspired into the drainage angle and block it completely. Since the fluid cannot exit the eye, pressure inside the eye builds up and causes glaucoma.
Most glaucoma cases do not have any symptoms. Half of all people with loss of vision resulting from glaucoma are not aware that they have glaucoma. By the time the condition is diagnosed, the eye damage is often severe. In some rare cases, a person can have an acute attack of glaucoma, where the eye becomes red and very painful. Nausea, vomiting and blurred vision may also occur.
Typically, POAG has no symptoms in its early stages, and vision remains normal. Symptoms of CAG may include headaches , eye pain , nausea, rainbows around lights at night, and very blurred vision.
Diagnosis
Tests commonly used to establish diagnosis may include:
· Tonometry. A test using an instrument to measure the intraocular pressure. Drops are used to numb the eye and a special device is used to measure the eye's inner pressure.
· Ophthalmoscopy. Examination of the inside of the eye, especially the optic nerve, using a lighted instrument called an ophthalmoscope.
· Visual field test (Perimetry). During this test, the patient is asked to look straight ahead and indicate when a moving light passes the peripheral vision. The result is a map of the vision.
· Dilated eye exam. In this test, drops are placed in the eyes to dilate the pupils. A magnifying lens is then used to examine the retina and optic nerve for signs of damage and other eye problems.
· Gonioscopy. This test checks if the angle where the iris meets the cornea is open or closed, showing if either open angle or closed angle glaucoma is present.
· Scanning laser polarimetry (GDx). This test measures the thickness of the nerve fiber layer on the retinal surface just before the fibers pass over the optic nerve margin to form the optic nerve.
· Confocal laser ophthalmoscopy (Heidelberg Retinal Tomography or HRT II). This test scans the retinal surface and optic nerve with a laser to build a 3D image of the optic nerve including a contour outline of the optic cup. The nerve fiber layer thickness is also measured.
· Optical coherence tomography (OCT). This technique creates images using special light beams to obtain a contour map of the optic nerve, optic cup, and measure the retinal nerve fiber thickness.
QUESTIONS TO ASK YOUR DOCTOR
· How is glaucoma treated?
· How do I know if surgery is right for me?
· Is glaucoma surgery effective?
· Are there different types of glaucoma surgery?
· What are the risks of surgery?
· Can problems develop after surgery?
· What can I do to prevent deterioration of my vision?
Increased eye pressure is indicative of increased risk for glaucoma, but does not establish diagnosis, because a person only has glaucoma if the optic nerve is damaged. A measurement of eye pressure by tonometry is therefore not sufficient to detect glaucoma. Glaucoma is detected most often during an eye examination through a dilated eye exam. If tonometry reveals that the pressure in the eye is not in the normal range, and if the dilated eye exam shows that the optic nerve looks unusual, then perimetry, gonioscopy, GDx, HRT II or OCT tests can be performed to confirm diagnosis.
Treatment
There is no cure for glaucoma, but it can usually be controlled by eyedrops or pills, conventional surgery, or laser surgery. Eye care practitioners often recommend a combination of surgery and medication. The appropriate treatment depends upon the type of glaucoma diagnosed.
Nutrition/Dietetic concerns
According to the Glaucoma Research Foundation, the carotenoids lutein and zeaxanthin, the only carotenoids found in the eye and which are also antioxidants , may protect against cataracts and glaucoma. They are found in foods such as spinach, parsley, celery, broccoli, lettuce, green peas, pumpkin, Brussel sprouts, corn, green peppers, cucumbers and green olives.
Therapy
A number of medications can be prescribed to treat glaucoma. Typically, medications are intended to decrease the amount of fluid forming in the eye so as to reduce elevated IOP and prevent damage to the optic nerve.
Surgery is carried out to lower pressure when medications are not effective; however surgical approaches cannot reverse vision loss. Common procedures involve either laser treatment or making a cut in the eye to reduce the IOP. The type of surgery performed depends on the type and severity of the glaucoma and the general health of the eye. One option is laser surgery. In this type of operation, a tiny beam of light is used to make several small scars in the eye's trabecular meshwork. The scars then help increase the flow of fluid out of the eye. Another option is filtering microsurgery, which involves creating a drainage hole. The procedure is usually performed when laser surgery does not successfully lower eye pressure, or if it starts increasing again.
Prognosis
Vision loss from glaucoma is permanent. However, with early detection and treatment, the progression of visual loss can be slowed, or stopped, which lowers the risk of blindness. POAG usually responds well to medication, especially if caught early and treated.
Prevention
Early detection is vital to stopping the progress of glaucoma. Prompt treatment of glaucoma can prevent damage to the eye's nerve cells and prevent vision loss. Regular and comprehensive eye exams are recommended every two to four years after age 40, and every one to two years after age 64.
Caregiver concerns
Glaucoma blindness is expected to increase because of the increasing aging population. However, awareness about glaucoma in this age group is still lacking. Visual impairment by glaucomaamongst the elderly is also underrecognized because the elderly are not part of the active workforce. They also tend to be more reclusive, resigned to their condition, and with a tendency to delay seeking treatment.
KEY TERMS
Antioxidant —Any substance that reduces damage due to reactive oxygen such as that caused by free radicals.
Aqueous humor —The clear, watery fluid in the front of the eyeball.
Carotenoids —Red to yellow pigments responsible for the characteristic colour of many plant organs or fruits, such as tomatoes, carrots, etc.
Cornea —Transparent front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power.
Intraocular pressure (IOP) —The inner pressure of the eye. Normal intraocular pressure usually ranges from 12–22 mm Hg, although people with relatively low pressures can still have glaucoma.
Iris —Pigmented tissue lying behind the cornea that gives color to the eye and controls amount of light entering the eye by varying the size of the pupil.
Lens —Transparent, biconvex crystalline tissue that helps bring rays of light to a focus on the retina.
Peripheral vision —The seeing of objects displaced from the primary line of site and outside of the central visual field.
Pupil —Variable—sized black circular opening in the center of the iris that regulates the amount of light that enters the eye.
Retina —Light—sensitive tissue at the back of the eye.
Trabecular meshwork —The series of canals or tubes behind the iris that filters the aqueous humor and allows it to drain into the bloodstream.
Vitreous —Transparent, colorless gelatinous mass that fills the rear two—thirds of the eyeball, between the lens and the retina.
Resources
BOOKS
Gupta, Deepack. Glaucoma Diagnosis and Management. Philadelphia, PA: Lippincott Williams & Wilkins, 2004.
Galloway, Nicholas R. Common Eye Diseases and their Management. 3rd ed., New York, NY: Springer, 2005.
Harmon, Gregory K., and Nancy Intrator. What Your Doctor May Not Tell You About Glaucoma: The Essential Treatments and Advances That Could Save Your Sight. Lebanon, IN: Grand Central Publishing, 2004.
Mayo Clinic. Mayo Clinic On Vision And Eye Health: Practical Answers on Glaucoma, Cataracts, Macular Degeneration & Other Conditions. Scottsdale, AZ: Mayo Clinic Trade Paper, 2002.
Netland, Peter, editor. Glaucoma Medical Therapy: Principles and Management. Oxford, UK: Oxford University Press, 2007.
Trope, Graham E. Glaucoma: A Patient's Guide to the Disease. Toronto, ON: University of Toronto Press, 2004.
PERIODICALS
de Voogd, S., et al. “Incidence of open-angle glaucoma in a general elderly population: the Rotterdam Study.” Ophthalmology 112, no. 9 (September 2005):1487–1493.
Gupta, N., and Y. H. Yücel. “What changes can we expect in the brain of glaucoma patients?”Survey of Ophthalmology 52, suppl. 2 (November 2007): S122–S126.
Kanner, E., and J. C. Tsai. “Glaucoma medications: use and safety in the elderly population.” Drugs and Aging 23, no. 4 (2006): 321–332.
OTHER
Cyclophotocoagulation. Foundation of the American Academy of Ophthalmology, Information Page. (March 08, 2008)http://www.eyecareamerica.org/eyecare/treatment/cyclophotocoagulation/index.cfm.
Glaucoma. American Academy of Family Physicians, Patient Information Page (March 08, 2008)http://familydoctor.org/online/famdocen/home/seniors/common-older/216.printerview.html.
Glaucoma. Foundation of the American Academy of Ophthalmology, Information Page. (March 08, 2008) http://www.eyecareamerica.org/eyecare/conditions/glaucoma/index.cfm.
Glaucoma. Mayo Foundation for Medical Education and Research, Fact Sheet (March 08, 2008) http://www.mayoclinic.com/print/glaucoma/DS00283/DSECTION=all&METHOD=print.
Glaucoma. National Eye Institute, Information Page (March 08, 2008)http://www.nei.nih.gov/health/glaucoma.
ORGANIZATIONS
American Academy of Ophthalmology (AAO), P.O. Box 7424, San Francisco, CA, 94120-7424, (415) 561-8500, (415) 561-8500, patientinfo@aao.org, http://www.aao.org.
EyeCare America, 655 Beach St., San Francisco, CA, 94109 1336, (877) 887-6327, (800) 222-3937,http://www.eyecareamerica.org/eyecare.
Glaucoma Foundation, 80 Maiden Lane, Suite 700, New York, NY, 10038, (212) 285-0080, info@glaucomafoundation.org, http://www.glaucomafoundation.org.
National Eye Institute, 2020 Vision Place, Bethesda, MD, 20892-3655, (301) 496-5248, 2020@nei.nih.gov, http://www.nei.nih.gov.
Monique Laberge Ph.D.
