Visual Field Examination: Perimetry
The visual field is the entire area one can see. It includes central and peripheral (side) vision. A visual field test can detect problems with vision in any part of the visual field. Changes in the visual field may be difficult to notice since both eyes are generally used at the same time. One eye can sometimes compensate for some vision loss in the other. A problem may not be detected until each eye is tested separately.
The visual field test provides information that no other test can. It is used to detect many diseases, such as glaucoma or retinitis pigmentosa, which affect the eye, optic nerve, and brain. It can also help diagnose neurological diseases, brain tumors, strokes, and other conditions. Visual field testing helps diagnose the disease and can follow the progress of the disease and its treatment.
This test is usually conducted by a technician or doctor who is trained to help you get the most accurate reading possible. While you are seated comfortably, during a visual field test, one eye is temporarily patched while the other eye is being tested. You are asked to look straight ahead at a fixed spot and watch for targets to appear in your field of vision. You will be instructed to press a buzzer when you become aware of a small light to one side, within your peripheral field. This test usually takes only a few minutes for each eye. It is VERY important that you keep your eyes focused on the central target throughout this examination so the technician can get an accurate reading of your complete visual field. To get the best results, you must be alert and concentrate on the central target. Once the results from this test are printed out by the computer, your eye doctor can easily detect blind spots in your visual field.
If a field test reveals an abnormal, the doctor may retest certain areas of your visual field using a computer. The printed results of this test will help the doctor verify the first test was correct. To confirm a blind spot, the computer may recheck an area several times. It is best to relax and do your best.
These images, showing the sequence of visual fields of a right eye, demonstrate what can happen if glaucoma is left untreated. The lightly shaded areas are normal; the dark areas show blind spots. These "blind spots" are often called visual field defects, and may be quite mild (light grey) or very severe (black). Although the sequence shown here would often take many years (even decades) to develop the end result is permanent loss of vision. This can only be prevented with early detection and treatment.
Tips for Field Testing.
Some patients find it difficult to maintain their focus on the central target during field testing. Here are a few tips. If you're having difficulty, to let the field technician know. The technician or the doctor is there to help.
- You may ask the technician to stop the test for a short break, if necessary. The visual fields test normally takes 20-30 minutes to complete.
- Try to book the test during the hours when you feel most alert during the day.
- Discuss the difficulties with your doctor. It may be possible to use a different field test in the future.