As part of NAVH's Educational Series we are pleased to provide this excerpt from our article "ANATOMY OF THE EYE IN GLAUCOMA."
GLAUCOMA - THE SNEAK THIEF OF SIGHT
Glaucoma is the second leading cause of blindness in the United States. It is THE leading cause of blindness among African-Americans, and the leading cause of PREVENTABLE blindness in the U.S. Glaucoma may occur at any age but happens with increasing frequency during advancing years. Approximately 2% of the population over the age of 40 is affected. It's estimated that over 2 million Americans have glaucoma, and more than half of them are not aware they have the disease.
THE EYE'S ANATOMY: HOW THE EYE WORKS
Your eye is like a camera. Light enters the clear dome in the front of the eye (cornea), passes through the focusing part (lens), and comes to rest on the "film" or recording part (retina). The nerve attached to the eye carries these recorded pictures "back to the brain" and WE SEE.
Eyes must stay inflated and round, just like a basketball. To stay inflated, the eye constantly makes fluid (aqueous humor) which circulates inside the front of the eye (anterior chamber), and leaves the eye by an intricate drain (trabecular meshwork). The balance between how much fluid is made and how much fluid leaves the eye determines how inflated the eye is (intraocular pressure). If all is working well, the fluid made inside the eye equals the amount of fluid leaving the eye, and the intraocular pressure is normal. If fluid cannot leave the eye properly, it builds up inside and the eye's pressure rises. This is the basic fact of glaucoma: high pressure inside the eye damages the eye's delicate optic nerve.
The optic nerve is the structure made up of over one million fibers that allow images detected by the eye to be transmitted to the brain.
The actual damage that occurs may be by direct pressure on the optic nerve, or by pressure on the blood vessels that supply nourishment to the nerve.
WHAT IS GLAUCOMA?
Glaucoma is an ocular disorder which embodies a group of conditions that have in common optic nerve damage, predominantly as a result of elevated intraocular pressure (IOP). When there is such pressure on the fragile optic nerve, which is attached to the eye's rear surface, defects in peripheral vision occur. Although the eye continues to produce fluid to keep itself inflated, the drains that allow fluid out are not working sufficiently.
A recent study, conducted by the Baltimore Eye Survey of 5,500 randomly selected people, showed that glaucoma was the third leading cause of legal blindness among Caucasians, and the second most common cause among African Americans, who are four times more likely to have the disease. Only macular degeneration among Caucasians and cataracts overall cause more legal blindness.
Although, as recently as a decade ago, diagnosis and treatment were based on measuring the intraocular pressure, recent discoveries indicate that, used alone, such analysis misses half of early glaucoma cases.
Patients who exhibit consistently elevated intraocular pressures (higher than 21 mmHg), without evidence of optic nerve or visual field damage, are called ocular hypertensives. Studies have shown that 1-3% of these patients, when untreated, will develop damage each year. The indications to treat ocular hypertension vary, and considerations include: the level of intraocular pressure, the presence of risk factors for the development of glaucoma, and a determination of how the risks of treatment compare with the risks of observation alone. Another term for this condition is glaucoma suspect.
The term glaucoma suspect usually applies to patients who demonstrate normal visual fields and intraocular pressure, but have optic nerves that are suspicious for damage from glaucoma. After the possibility of glaucoma is ruled out, these patients are usually monitored for any change in their optic nerve, visual field, or intraocular pressure.
Patients who manifest optic nerve and visual field damage identical to that seen in glaucoma, without detection of an elevated intraocular pressure, are assigned the diagnosis of normal tension glaucoma. Although some clinicians believe that this disease is related to insufficient blood supply to the optic nerve, no definitive mechanism for this disease has been determined. A large battery of tests may be employed in evaluating a patient with normal tension glaucoma, including CAT and MRI scanning of the brain and optic nerve, and evaluation of ocular blood supply. The determination of which tests are indicated is made by the ophthalmologist on the basis of the patient's individual presentation.
In the early stages of glaucoma, a person's peripheral vision is affected. The ability to see straight ahead (central vision) does not change until much later.
THIS IS AN EXCERPT!
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This excerpt is made possible through the cooperation of the following members of the National Association for Visually Handicapped's Medical and Technical Advisory Boards.
This publication was written by ANDREW M. PRINCE, M.D. and GREGORY K. HARMON, M.D, members of the NAVH Medical Advisory Board. In addition to Drs. Prince and Harmon, we wish to express deep appreciation to the following who reviewed the copy: Andrew S. Farber, M.D., James McGroarty, M.D., Joseph B. Walsh, M.D., and Frank J. Weinstock, M.D.
Copyright ©1993 by NAVH. All Rights Reserved. 1st Printing 1993, Revised 1996.