Glaucoma - Diagnosis

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

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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 the following: