A Glaucoma Suspect is a person who has a nerve that looks abnormal. Glaucoma causes slow damage to the optic nerve, which changes the shape of the nerve where it enters the eye. Your optic nerve may be abnormal appearing because of early glaucoma, because of an old injury, or even because of developmental differences in the size and shape of the eye at birth. Most of the time glaucoma damage is caused by high pressure in the eye, but occasionally there can be damage even with normal pressures.
When your doctor sees an abnormal appearing optic nerve he needs to find out if it is abnormal because of: early glaucoma or from mildly abnormal nerve development. Glaucoma is diagnosed using three pieces of information: the appearance of the optic nerve, the eye pressure, and the results of specialized testing.
Optic nerve- the optic nerve is like a telephone cable that connects the eye and the brain. It has the appearance of a yellow-orange circle in the back of the eye. The circle actually looks more like a crater or donut in three dimensions. Glaucoma causes changes in the appearance of the optic nerve – the crater gets deeper with thinner walls, or the donut gets a much bigger hole and a thinner rim (see diagram to left). Advanced glaucoma is easy to recognize, but early glaucoma can be difficult to distinguish from the normal appearance because there is a lot of variation in the size and shape of the optic nerve from person-to-person. Subtle clues to early glaucoma include asymmetry between the appearance of the nerve in each eye (large donut-hole in one eye and normal size in the other), or vertical stretching of the “donut-hole”. Photos are useful to document the appearance of the optic disc. If glaucoma is present the appearance of the optic nerve will slowly change over time. By comparing photographs taken every few years, small changes in the appearance of the optic nerve can be noted, indicating that glaucoma is developing.
Eye pressure – the normal eye pressure ranges from 10-21. The risk of developing glaucoma increases if the pressure stays higher than 21 over many years. Some people with normal eye pressures can develop glaucoma (known as normal tension glaucoma). Regularly measuring eye pressures is very important to assess your risk of developing glaucoma. The thickness of the cornea (the clear front window of the eye) can affect the ability to accurately measure the eye pressure. If you have a thick cornea your eye pressure measurement will be artificially high, and if your cornea is thin your pressure measurement will be artificially low. Because of this a part of the glaucoma examination is the measurement of the corneal thickness, called corneal pachymetry.
Specialized Testing – Glaucoma causes loss of the peripheral vision in its early stages. This vision loss is usually asymptomatic (you don’t notice the vision changes in your day-to-day life). A visual field test evaluates the central and peripheral vision to see if any damage has occurred from glaucoma. If damage is present this confirms that there is glaucoma in the eye. As glaucoma gets worse, more and more of the visual field becomes damaged. Visual fields need to be tested regularly to see if changes are occurring that would confirm glaucoma was present and change your diagnosis from a suspect to definite glaucoma. A companion to the visual field test is a test called a OCT. The visual field test relies on your input – when you see the light you need to push the button. If you are tired or anxious the test results may not turn out very well. The OCT does not rely on your input, it uses light to make a very precise measurement of the nerve layers in the eye. Glaucoma causes these nerve layers to get thinner as the small nerves are damaged. The OCT test will also be done regularly to see if you are developing definite glaucoma.
Information from each of these three areas; the optic disc, the eye pressure, and specialized testing, helps your doctor to determine if you have glaucoma, and if the glaucoma is getting worse over the years. Several tests should be done early on to serve as a reference upon which all future tests can be compared. These tests will be repeated regularly to look for changes.