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Glaucoma

Screening and diagnosis

If your doctor suspects that you have glaucoma, he or she may perform a series of tests to detect any signs of damage. Tests include:

• Tonometry. Tonometry is a simple, painless procedure that measures your intraocular pressure. It is usually the initial screening test for glaucoma. Two common techniques are air-puff tonometry and applanation tonometry. Air-puff tonometry uses a puff of air to measure the amount of force needed to indent your cornea. An applanation tonometer is a sophisticated device that's usually fitted to a slit lamp. Slit lamps use an intense line of light — a slit — providing illumination of the cornea, iris, lens and anterior chamber, and allowing your doctor a good view of these structures. With tonometry, your doctor numbs your eyes with drops and has you sit at the slit lamp, where a small flat-tipped cone pushes lightly against your eyeball. The force required to flatten (applanate) a small area of your cornea translates into a measure of your intraocular pressure.

• Average normal eye pressures range from 10 to 21 or 22 millimeters of mercury (mm Hg), though most pressures are within 14 to 16 mm Hg. Doctors consider anyone with eye pressure greater than 23 mm Hg to be at risk of developing glaucoma and in need of careful monitoring for early signs of glaucoma.

• Test for optic nerve damage. To check the fibers in your optic nerve, your eye doctor uses an instrument called an ophthalmoscope or biomicroscope, which enables him or her to look directly through the pupil to the back of your eye. Your doctor may also use laser light and computers to create a three-dimensional image of your optic nerve. This can reveal slight changes that may indicate the beginnings of glaucoma.

• Visual field test. To check how your visual field has been affected by glaucoma, the doctor uses a perimetry test. One method, known as tangent screen perimetry, requires you to look at a screen with a target in the center. Your eye doctor or a technician manipulates a small object on a wand at different locations in your visual field. You indicate whenever you see the object come into view. By repeating this process over and over again, your entire visual field can be mapped.

• Pachymetry. Your eyes are numbed for this test, which uses an ultrasonic wave instrument to gauge the thickness of each cornea. The thickness of your corneas is an important factor for accurately diagnosing glaucoma. If you have thick corneas, your eye pressure reading may seem high even though you don't have glaucoma. Conversely, people with thin corneas can have low pressure readings, but have glaucoma.

• Other tests. To distinguish between open-angle glaucoma and angle-closure glaucoma, your eye doctor may use a technique called gonioscopy (go-ne-OS-kuh-pe), in which a special lens with an angled mirror is placed on the eye to inspect the drainage angle. Another test, tonography, can measure how fast fluid drains through the trabecular meshwork.

To establish a diagnosis of glaucoma, several factors must be present. These usually include elevated intraocular pressure, areas of vision loss and damage to the optic nerve. In glaucoma, the optic disk shows visible signs of damage. The optic disk is the area where all of the nerve fibers come together at the back of the eye before exiting the eyeball. An optic disk that has been affected by glaucoma appears indented, or excavated, as if someone scooped out part of the center of the disk. This condition is known as cupping. The normal contour and color of the disk may also be affected by the loss of nerve fibers.

 

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