Eye pressure: What is normal?
Normal eye pressure ranges from 10 to 22 millimeters of mercury (mm Hg). Anyone with eye pressure over 23 mm Hg is considered at risk of developing glaucoma and needs to be carefully monitored for early signs of glaucoma. People with intraocular pressure greater than 30 mm Hg are considered at high risk.
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Regular eye exams are the key to detecting glaucoma early enough for successful treatment. It's best to have routine eye checkups every 2 to 4 years after age 40 and every 1 to 2 years after age 65. If you're at increased risk, your doctor may recommend more frequent monitoring. If your doctor suspects that you have glaucoma, he or she may perform a series of tests on you to detect any signs of damage.
Tests to detect glaucoma
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. For this extremely accurate test, your doctor numbs your eyes with drops and has you sit at the slit lamp, where a small fiat-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 IOP.
Test for optic nerve damage. To check the fibers in your optic nerve, your eye doctor uses an instrument called an ophthalmoscope, 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 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, the doctor can map your entire visual field.
Other tests. To distinguish between open-angle glaucoma and angle-closure glaucoma, your eye doctor may use a technique called gonioscopy (goe-nee-OS-kuh-pee), in which a special lens is placed on the eye to inspect the drainage angle. Another test, tonography, can measure how fast fluid drains through the trabecular meshwork.
Diagnosing glaucoma
To receive a diagnosis of glaucoma, a person must exhibit several factors. These include an elevated lOP, areas of vision loss and damage to the optic nerve. In glaucoma, the optic disk will show visible
signs of damage. The optic disk is the area where all 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 be affected by the loss of nerve fibers.
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If your doctor determines that you have elevated IOP, an excavated optic disk and loss of visual field, you'll likely be treated for glaucoma. If you have only slightly elevated eye pressure, an undamaged optic nerve and no visual field loss, you may not need treatment but more frequent examinations may be advised to detect any future changes. If you have signs of optic nerve damage and visual field loss, even if your eye pressure is in the normal range, you may be treated to lower eye pressure further, which may help slow the progression of glaucoma. To find out more, you can check out Problems With Vision.