Diagnosing glaucoma involves more than testing eye pressure. One type of glaucoma, Normal Tension Glaucoma, occurs even when eye pressure is within normal limits and in that type of glaucoma eye pressure must be lowered below normal levels. Cornea thickness also matters. A thin cornea can show artificially low eye pressure readings whereas a thick cornea may show a higher eye pressure than actually exists.
To diagnose glaucoma, your eye pressure, the shape and color of the optic nerve, field of vision, angle where your iris meets the cornea, and your corneal thickness should all be evaluated.
Inner eye pressure. This is measured with a tonometer. A small amount of pressure is applied to the eye with a thin device or with a warm puff of air. The normal range of eye pressure is 12 to 22 mm Hg. Some people can have glaucoma when eye pressure is within the normal range because eye pressure is unique to each person. Eye pressure, regardless of how it is measured, is only a risk factor of glaucoma and is not definitive of the diagnosis.
Shape and color of the optic nerve. Using an ophthalmoscope the doctor looks through the pupil of your eye to view your eye’s interior. Dilating the pupils opens them wider for a better look inside the interior of the eye. If the optic nerve looks unusual or your eye pressure is elevated your doctor may ask you to have the following additional glaucoma tests.
Field of vision. The test that measures your field of vision is called perimetry. It produces a map of your complete field of vision. This test helps your doctor determine if your vision has been affected by glaucoma. The test measures the total area where objects can be seen with your peripheral (side) vision. During the test you will look straight ahead and indicate when you see moving lights pass your peripheral vision. If glaucoma is diagnosed you will usually repeat this test once or twice a year to monitor your vision.
Angle where the iris meets the cornea. This exam is done using a goniolens and the exam is called gonioscopy. The goniolens is a type of hand-held contact lens and is placed on the eye. The lens has a mirror that shows whether the drainage angle that lies between your iris and cornea is blocked or open.
Thickness of the cornea. The measurement of your cornea thickness is done with an ultrasound device called a pachymeter. It only takes about a minute to measure the corneal thickness of each eye. The measurement of corneal thickness gives your doctor a better understanding of what your eye pressure readings indicate. Eye pressure may be underestimated in patients with thin corneas and overestimated in patients with thick corneas. In addition, corneal thickness measurements help determine the correct type of glaucoma you have and which treatment will be the most effective for you.
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