Glaucoma - Risk Factors

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Tonometry and Pressure Tests

Doctors determine the intraocular pressure (IOP) of the aqueous humor inside the eye using a painless procedure called tonometry, which measures the force necessary to make an indentation in the eye. A tonometer (small smooth instrument) may be used. There are several methods and the doctor may apply anesthetic eye drops to first numb the eye:

• The applanation (Goldman) method, uses a blue-light filter and slit-lamp, which is moved forward toward the patient's face.
• Electronic indentation tonometry uses an electronic pen with a digital read-out.
• The noncontact approach does not use a tonometer. It applies a puff of air to measure the force needed to indent the eye.
• In the Schiotz method, the doctor presses very lightly against the eye with the tonometer. IOP is measured by the weight needed to flatten the cornea. This method is not considered as accurate as the others.

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In general, normal IOP is usually maintained at measurements of 10 - 21 mmHg. Intraocular pressure over 21 mmHg indicates a potential problem. The test is not completely accurate, however. Only about 10% of people with IOP levels of 21 - 30 mmHg 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.

Measurement of Cornea Thickness (Pachymetry)

Cornea thickness is an important indicator of disease progression in patients with elevated IOP. The doctor first applies numbing drops to the eye and then uses an ultrasonic wave instrument to measure cornea thickness.

Tests for Optic Nerve Damage

To check for damage in the optic nerve, the doctor first uses eye drops to dilate (widen) the pupils and then examines the eyes with a magnifying lens instrument such as an ophthalmoscope, which has a light on one end.

Damaged nerve fibers may be indicated by:

• An asymmetrical or elongated cupped optic nerve. (The cup of the optic disc is the center portion, which enlarges as nerve damage progresses.)
• The optic nerve color may be pale or an unhealthy-pink.