How Glaucoma is Diagnosed
Early or mild glaucoma has no symptoms which makes it impossible determine a glaucoma diagnosis without a doctor. The eye is an amazing optical device, and the whole visual system, including both eyes and the brain, is even more remarkable. When glaucoma damage starts to injure the small optic nerve fibers, very small areas of the vision are affected. These small areas tend to be in the peripheral vision, so they are not very noticeable. There are other reasons that early glaucoma damage has no symptoms. One is that glaucoma damage happens very slowly, and it isn’t easy to notice if your vision fades away very slowly. The other reason that often there are no symptoms with early glaucoma is that we are lucky to have two eyes that see the same things – the visual areas each eye sees overlap. Even if one eye loses 50% of its vision, the overall vision won’t change if the other eye is healthy.
Because glaucoma has no symptoms, the only way that you can be correctly diagnosed with glaucoma is with a thorough eye exam by an optometrist or ophthalmologist. This is why it is recommended that people at risk for glaucoma get an eye exam at least yearly. If you wait until you have symptoms before you come in for your eye exam, it will be too late and severe irreversible glaucoma damage may have already developed.
Correctly diagnosing glaucoma takes a little detective work by your eye doctor. One clue that glaucoma may be developing is an increased eye pressure. Just like your medical doctor takes your pulse and blood pressure at each visit, your eye doctor measures your eye pressure to look for glaucoma. Some people develop glaucoma even with a normal eye pressure, but usually an elevated eye pressure is what causes the nerve damage. A normal eye pressure is 20 or less. Many people with glaucoma have eye pressures between 22 and 30.
Since the eye pressure is measured indirectly, by gently pushing against the cornea, a thick or thin cornea can lead to incorrect eye pressure readings. Corneal Pachymetry is a test that measures the thickness of the cornea and allows your doctor to verify the accuracy of your eye pressure readings. If the cornea is thicker than normal, a few points can be subtracted from the reading. If the cornea is thinner than normal a few points will need to be added to the eye pressure.
The next clue used to diagnose glaucoma is the appearance of the optic nerve. When glaucoma causes damage to the nerve there are characteristic changes to the nerve appearance. A healthy nerve looks like a small orange circle inside the back of the eye. The nerve looks a little like a donut with a small depression in the middle like a donut hole. When nerve damage develops the donut hole enlarges. This is also referred to as increased “cupping”. During an eye exam your doctor will look at the nerve to see if there are any signs of nerve damage developing. This is easiest to do when the pupil is dilated. Early damage may be a little tricky to identify as there is a lot of variability in the appearance of normal nerves from different individuals. In this case a doctor may miss the nerve damage if he isn’t paying close attention during the eye exam.
The last clues that an eye doctor may use to help to diagnose glaucoma are tests that are done in the office. One of these tests is the visual field test. With this test, small lights or patterns are flashed in areas of the peripheral vision. When the patient sees the light, they respond by pushing a button to tell the computer that the light was seen. Some lights are dim and others are bright. By flashing many lights, the vision is mapped. This map can then be compared to a “normal” group of patients. If glaucoma damage has started to develop, the visual field map will appear abnormal. The visual field test usually takes about 5 minutes for each eye.
The other type of test done to detect glaucoma is a measure of the structure of the nerve and nerve fibers inside the eye. There are one million nerve fibers that travel within the optic nerve. When the nerve reaches the eye these nerves enter the back of the eye and then spread out covering the inside of the eye like a smooth thin carpet. If glaucoma damage develops, the “carpet” of nerve layers inside the eye will change. A thinner carpet is a sign of glaucoma developing, especially if the thinning carpet occurs in certain areas of the retina. The OCT and GDx tests measure the nerve layers inside of the eye and compare the thickness to a group of normal eyes. If your scan shows thinning compared to a normal group there is a high likelihood that glaucoma is developing.
In summary, as part of a thorough yearly eye exam your eye doctor will do some detective work to see if there are signs of glaucoma developing. By measuring the eye pressure, looking carefully at your optic nerve, and doing glaucoma tests, your doctor can determine if glaucoma damage is developing. Since glaucoma has no symptoms an eye exam needs to be done at least yearly.