We have discussed previously that glaucoma is a disease that damages a layer of the retina called the nerve fiber layer. The optic nerve can be compared to a cable carrying many many little "wires." In our case, these "wires" are the nerve fibers that come together to actually form the optic nerve. From the eye, the optic nerve carries the visual information to the brain where is is interpreted as sight. In that sense, the optic nerve is like a cable connecting a video camera to the monitor. If the cable is defective, even though the monitor is OK, you won't get a good picture.
Each fiber in the optic nerve carries visual information from a specific part of the retina to a specific location in the brain. We can, therefore, create a "map" of the visual world. Specific defects in the retina or optic nerve cause defects in our visual map or, more correctly, visual field. The visual field is the total expanse of all vision from one side to the other and from top to bottom. In the very center, namely the spot onto which you are directing all of your focus attention, is called "fixation." Every spot that is not central fixation becomes peripheral, or side vision. Some are very close to the center while others are very far away. The further away from fixation we get, the less clear the vision and the less sensitive the retina becomes.
If you are directing all your attention to a small spot directly ahead of you and you do not move your eye off that spot, you can usually see someone approaching you from the side. Perhaps you see that person's body moving or just a hand moving. Whichever it is, that represents side, or peripheral vision. Something as large as a body is more easily seen that a small hand. Similarly, you may be looking straight ahead while a light flashes off to the side. A bright light is more easily seen than a dim one. Glaucoma causes loss of peripheral sensitivity.
In order to measure this sensitivity, we use a highly technical computerized instrument called a visual field analyzer. It is pictured below:
The
Humphrey Visual Field Analyzer . . . . . .
The patient sits with his or her head on a chin rest which holds the head still. The eye not being tested is covered. The patient is then directed to look straight ahead into the machine. It looks like a large white bowl. In reality, it is a screen. During the exam, the instrument projects a tiny spot of light onto the screen in the periphery of the vision. The patient simply pushes a button when the light is seen, and it is recorded. The light is initially very very dim, and its intensity is gradually increased until it becomes visible. The computer in the instrument then compares the light sensitivity level to that which it should be in a person with no disease. Reduced levels of sensitivity or total loss of some peripheral vision is a hallmark of glaucoma damage.
It is, in fact, the ability to maintain full peripheral, or side vision that we wish to accomplish in treating glaucoma. This visual field analysis is really the only measurement that can actually show on paper exactly how well your visual system is functioning. All other measurements , important as they are in telling us if our treatment is working, do not indicate how you, the patient, are really functioning. This is why the field exam is so important. If you can go your entire life without losing any peripheral vision from glaucoma, then our management is successful. Changes that may occur in the visual field pattern over time might be an indication of ongoing glaucomatous loss, and if these are occurring, it might be necessary to change your glaucoma medication.
Unrelated to glaucoma, you should know that there are many neurologic problems that affect the peripheral vision. Loss of side vision from a stroke, for example, is quite common. This often occurs in the periphery of both eyes. Loss of visual field in one may may occur from retinal detachment, hemorrhage, tumors and other retinal disorders. We will perform this examination for many different reasons. It is painless and perfectly safe. Please don't hesitate to ask us about it.
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