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Glaucoma

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of glaucoma.

Diagnosis

A diagnosis of glaucoma no longer simply relies on the presence of pressure within the eye. It requires that there be optic nerve damage or a strong suggestion of damage, which can be clearly seen during a dilated eye examination of the optic nerve. In general, the hallmark sign of this condition is a loss of peripheral vision. With peripheral vision loss, a person can see in front of him- or herself but has lost the vision to the side.

Because chronic glaucoma has no warning symptoms, half of its victims are unaware that they have the condition. Early diagnosis, however, is the key to successful treatment of glaucoma. One study reported that the more years since the last visit to an eye professional, the greater the risk for having visual loss.

Recommendations for Glaucoma Screening Everyone over age 65 and African Americans over 40 years old should have periodic eye exams, including tests for glaucoma, every other year. African Americans between ages 20 - 39 should have eye examinations every 3 - 5 years. Other people at higher risk (people with diabetes, history of eye injuries, a family history of glaucoma, or those taking corticosteroid medications) should have eye examinations every year after age 35. People with known glaucoma should have frequent examinations to check peripheral vision and to be sure treatment is maintaining a safe eye pressure. After such examinations, the ophthalmologist will assess current treatment and make necessary adjustments.

Tonometry and Pressure Tests

Doctors determine the intraocular pressure (IOP) of the aqueous humor inside the eye using tonometry , which measures the force necessary to make an indentation in the eye. There are several methods:

• In the Schiotz method, the doctor first anesthetizes the eye with drops, then presses very lightly against it with tonometer , a tiny smooth instrument that is used to measure the pressure.
• In the applanation method, the doctor touches a strip of orange-dyed paper to the side of the eye. The stain helps with the examination and rinses out with tearing. The doctor uses a slit-lamp, which is moved forward toward the patient's face until the tonometer touches the eye.
• The noncontact approach applies a puff of air and measures the force needed to indent the eye.

Attempting to close the eyelids during the test can increase eye pressure and produce errors in the results.

In general, normal IOP is usually maintained at measurements of 10 - 20 mm Hg. Glaucoma pressure over 21 mm Hg indicates a potential problem. The test is not completely accurate, however. Only about 10% of people with IOP levels between 21 - 30 mm Hg will actually develop glaucoma and optic nerve damage. On the other hand, many people with glaucoma have normal pressure, at least for part of the time.

Changes in posture may also affect IOP. A 2006 study indicated that IOP increases during sleep or when a person is lying down. As IOP tests are generally given in a doctor’s office when a patient is sitting up, they may not provide a completely accurate evaluation of eye pressure.

Measurement of Cornea Thickness

The cornea thickness may be an important indicator of disease progression in patients with elevated IOP. According to some research, patients with thinner corneas have a significant risk for developing damage from glaucoma, while those with thicker corneas have a low risk.

Tests for Optic Nerve Damage

In order to determine early damage in the optic nerve, a number of diagnostic instruments have been developed to assess the nerve fiber layers at the back of the eye (the fundus) and to check for optic disk cupping. (The cup of the optic disc is the center portion, which enlarges as nerve damage progresses.) The two most common procedures for identifying nerve damage are ophthalmoscopy and fundus photography . Other instruments have been developed, including those that use laser technology and computers, but none have proved to be infallible. No test has proven to be completely accurate, however, and none is routinely performed by all eye professionals.

In order to be accurate, the tests require a skilled professional and there are certain common factors:

• The pupils must also be widely dilated using eye drops before the procedure.
• Even mild cataracts and a slightly less-than-optimally dilated pupil can degrade the results. Such conditions are common in elderly people, who are the most likely to develop glaucoma.
• If the back of the eye is lightly pigmented (colored), the area under observation is less distinct.
• If the glaucoma is diffuse and there is a generalized loss of nerve fiber (which occurs in half of patients), it is more difficult to detect than if the glaucoma is more localized.

If IOP is low or normal and tests report optic nerve damage and peripheral visual loss, doctors should also check for other conditions before starting any treatment for glaucoma. Such problems include steroid use, anemia, and previous hemorrhage or severe low blood pressure.

Ophthalmoscopy. The eye professional (or even a primary care doctor) uses an ophthalmoscope to peer through the pupil directly at the optic nerve. The examiner can then check the shape and color of the nerve fibers to evaluate whether they have been damaged by the high pressure of glaucoma. Damaged nerve fibers may be indicated by:

• An asymmetrical or elongated cupped optic nerve
• The optic nerve color may be pale or an unhealthy-pink

If results show no optic nerve damage in patients who have mild elevations in pressure, the ophthalmologist may want to retest frequently but delay drug treatment, unless the patient has significant risk factors.

Fundus Photography. Fundus photography may be used to take pictures of the optic nerve and can reveal changes years in advance of vision loss. It is an unpleasant procedure requiring drops and a bright flash. This procedure has the same limitations as ophthalmoscopy.

Laser Polarimetry. Polarimetry uses laser technology to scan the eye and does not require any response from the patient. It is reported to be able to measure nerve fiber thickness in the eye and so be able to reveal early signs of deterioration. Preliminary studies have indicated that it has a diagnostic accuracy of over 90% for both confirming and ruling out glaucoma. One study, however, reported that laser polarimetry was sensitive enough to detect glaucoma in only up to 57% of patients with early glaucoma, 71% of those with moderate disease, and 81% of those with severe glaucoma. More research is needed.

Other Devices. Computer-assisted devices, such as the confocal scanning laser ophthalmoscope, are now available that may be useful for evaluating the retinal nerve layer. Another instrument, the optical coherence tomograph, measures the echo time delay of light that is scattered back from different layers in the retina. The value of these tests has not yet been determined.

Perimetry and Other Tests of the Visual Fields

If there is indication of optic nerve damage, the eye professional will conduct tests of the visual fields (the areas that the patient can see). In most people with glaucoma, the first areas to become noticeably impaired are the peripheral visual fields (areas of sight that are not directly in front of a person but more to the sides).

Standard Perimetry Tests. Perimetry tests are used to check peripheral vision. One variation of this test is as follows:

• A person sits closely facing a large computer-like monitor.
• Small bright white lights flicker on and off hundreds of times, at different places on the screen, while the patient clicks a button whenever one of the lights is seen.
• The machine prints out a report that maps any blanked-out areas in the person’s vision.

The test is complex and lengthy; elderly people and those with short attention spans may be inappropriate candidates. Other perimetry tests, some requiring less time to administer and some using "virtual reality" techniques, are currently being developed.

Other Tests. Other visual field tests are being developed that can detect abnormalities years before they can be detected by standard perimetry. Experts recommend some of these tests in selected patients with suspected glaucoma.

For example, a screening test called frequency doubling technology (FDT) checks for changes in particular cells in the retina that are indications of early glaucoma. It takes less than a minute to perform.

Another test called short wave automated perimetry (SWAP) uses colors (blue-on-yellow) and also detects very early abnormalities in the visual field. Testing time is longer than with FDT, however, and the presence of certain types of cataracts can interfere with its accuracy.

Markers

ELAM-1. Endothelial leukocyte cell adhesion molecule 1 (ELAM-1) is a molecule that has been found in glaucoma but not in healthy eyes. This molecule may prove to be a "marker" and its presence may be helpful in diagnosing glaucoma.

Test for Acute Closed-Angle Glaucoma

A simple test using a penlight helps determine the risk for acute closed-angle glaucoma. A beam of light is directed from the side of the face toward the patient's iris. If no shadow appears on the nose, then most likely the angle is wide enough to dilate. Using an instrument called a gonioscope, ophthalmologists can also inspect the front of the eyes and assess the drainage angle between the cornea and the iris and the channels in the trabecular meshwork. This test can differentiate between closed- and open-angle glaucoma.

• Review Date: 3/3/2007
• Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.

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