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SCIENCE / MEDICINE : Annual Checkups Are a Key to Detecting Glaucoma

June 18, 1990| Reprinted from the July issue of The Johns Hopkins Medical Letter. and

"I haven't felt a thing, felt it at all."

--President George Bush, after he learned he was at risk for glaucoma.

If George Bush hadn't been President--and under the watchful care of his own personal physician--he might very well have not bothered to see an ophthalmologist since he hadn't "felt a thing." Instead of discovering he had ocular hypertension (elevated pressure within the eye that can lead to glaucoma) before it caused any detectable symptoms, Bush might have become one of the statistics that has made glaucoma the leading cause of blindness in America.

An estimated 8 million Americans are at risk of vision damage from glaucoma. Almost a quarter-million have already lost sight in one eye, and another 70,000 are completely blind, because of the disease. Just what causes this devastating eye disorder is still not known, despite extensive research--but researchers do know that the greatest risk factor for glaucoma is simply getting older. The disease occurs most often in those over 40, and affects 3% of those 65 and over. This risk is compounded by a family history of the condition, diabetes, myopia (nearsightedness), and among blacks.

It is not until extensive--and irreversible--damage has occurred that glaucoma causes noticeable symptoms. In its earliest stages, as Bush reported, afflicted people do not feel or notice anything unusual. But during this symptom-free time, a lot is going on within the eye that, if left untreated, could lead to blindness. Fortunately, an ophthalmologist can detect early changes before sufferers are aware of them--which is why annual visits are so important after the age of 40.

The lens and cornea of the eye are the only parts of the body with no blood supply. Constantly flowing between the lens and the cornea is a clear fluid called the aqueous humor, which carries nutrients and washes away waste products When all is working well, this wash of fluid is carefully calibrated by the body so that a constant pressure is maintained in the eyeball. About one teaspoon of aqueous humor is produced each day, and the fluid is replaced every two hours.

In open-angle glaucoma (the most common type, for which Bush is at risk), drainage of the aqueous humor is partially blocked or impaired, so that this equilibrium is shifted. Because of the anatomy of the eye, the aqueous humor cannot simply flow out, like tears. Instead, the surplus builds up in the eyeball, and pressure within it rises (ocular hypertension). If pressure is elevated long enough, cells in the optic nerve are compressed and can die, resulting in vision loss. At this point, the condition has progressed to glaucoma. (Closed-angle glaucoma is an acute, emergency condition caused by sudden structural changes in the eye, and requires immediate surgery.)

Initially, increased pressure in the eye leads to a decrease in peripheral (out-of-the-corner-of-the-eye) vision. This loss is not detectable in its early stages; on a standard eye test, a person could have 20/20 vision and still have glaucoma. A special test that measures pressure within the eye, called tonometry, is necessary--and is best done by an ophthalmologist every year. To do the procedure, an ophthalmologist applies an anesthetic to the cornea and places a special instrument there momentarily to measure the force necessary to indent a small area of the cornea. In addition, the doctor examines the optic disc (the visible portion of the optic nerve) for evidence of damage. If either of these tests indicates the possibility of glaucoma, the patient will then need a visual field test, which can detect specific areas of vision loss. The results of these three exams are weighed to determine if the patient needs treatment.

In a 24-hour period, normal eye pressure may vary, ranging between 12 and 18 millimeters of mercury (mm Hg); pressures 21 mm Hg or higher are generally considered abnormal. However, there is some debate about when pressure is high enough to warrant treatment. Abnormal pressures do not necessarily mean that nerve cells have been damaged. The pressure at which, say, Bush would suffer damage may be different from that at which, say, Soviet President Mikhail Gorbachev would be affected. At the very least, closer monitoring is in order if eye pressure is elevated.

The first line of treatment for glaucoma, when detected at an early stage, is medicated eyedrops. The drops relieve pressure in one of two ways: by enabling more aqueous humor to leave the eye, or by decreasing the production of aqueous humor. Glaucoma eyedrops are medication, just like pills, and can cause side effects. They should be included when giving a doctor or pharmacist a list of the medications a person uses.

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