What is a Visual Field?
A visual field is an eye test which is used to discover a variety of eye diseases or central nervous system diseases. The test is designed to test the whole vision. It can test the central and peripheral portions of the vision. It can be used for detecting glaucoma, retinal diseases, strokes, tumors of the nervous system, and other neurologic diseases. Generally, the person stares at a central target and is presented a variety of stimuli or objects to see if the person can see them.
The visual field can be performed in a variety of ways. It can be simple counting fingers to the side, a simple screen on a wall, a manual machine, or a sophisticated computerized machine. These include:
- Finger counting
- Tangent screen
- Goldmann visual field (manual)
- Computerized visual field machines
- Confrontation Visual Field, Finger Counting, or Donder’s Test
This test is usually done by the nurse or certified ophthalmic technicians during a regular complete eye examination or other times if indicated. The examiner will have the patient cover one as the test is performed one eye at a time. You will be asked to stare at the examiners nose. The examiner will then hold up one, two, or the whole hand to see if the patient can count the number of fingers in their side vision. Another method is that the examiner may start out past a patient’s side and slowly bring the fingers toward the center of the vision and they tell the examiner when they can see it. Any defects will be noted in the chart and the eye doctor may repeat the test or order other tests to determine if there is disease that is causing the loss of side vision.
Perimetry is the medical term for the formal method for testing the visual field of a patient. The testing is performed using many techniques. In the advanced machine type visual fields, the patient is placed in front of a white bowl like sphere which encompasses one hundred eight degrees of peripheral vision. A light is shown on the white surface of the bowl and the patient responds when they see a light. The light maybe white or other colors depending on the eye doctors preference or the disease that they are trying to diagnose.
Why Perform the Test?
A visual field is one of the primary tests that is used to diagnose and monitor the status of glaucoma. If you have a neurologic disease, a visual field test may be indicated to help diagnose and monitor the disease. You are required to have a minimal amount of side vision to drive a car and your state may require that you be tested. Certain jobs may require a certain level of side vision such as a pilot, military, and commercial driving license. A visual field test is required if you are having trouble with drooping upper lids and wish to have insurance cover the procedure.
Types of Perimetry
A kinetic visual field uses a stimulus or light that moves usually from outside of a patient’s vision towards the central vision. When the patient first sees the stimulus they respond and the location is recorded. The stimulus is directed from many different directions of their side vision. This type of testing is performed for drivers tests and insurance approval for blepharoplasty of the upper eyelids.
A static Perimetry is performed by showing a light in specific locations around the field of vision. The lights may change in size, intensity, or color depending on the test parameters chosen by the eye doctor. There are varieties of methods of testing that the computer in the machine can use. Supra-threshold is more common with screening visual fields. An intensity of light that normally would be seen by a person of the age taking the test is shown. They either see the light or not and the results are recorded. This test is short and tells the doctor if they have a normal visual field. It is not very sensitive on picking up the early loss of vision from disease such as glaucoma. Threshold testing is much more sensitive but is a much longer test. The light stimulus is started at very low intensity and increased in that location randomly until the patient sees the light. Most doctors now use a faster method but still get good results with a fast threshold method. This takes only three to six minutes per eye to complete. A full threshold test could take as long twenty minutes per eye.
Types of Perimetry Tests
A black felt-like board or screen is attached to a wall. A black wand on which white or colored balls are attached to the end of the wand is used as the stimulus. The patient is placed in a chair at an exact distance from the screen and the end of the wand in moved from the periphery towards the center until the patient sees the ball. The location is recorded. This test is not used very often as the newer machines are more accurate.
Goldmann Visual Field
The Goldmann perimeter has been in use for many years. It is a white spherical bowl with an arm that passes behind the patient that shines a light onto the surface of the bowl. The patient is seated at the machine with one eye patched. The arm with the light is able to pass behind the patient and shine the light on the bowl. The light position is controlled by a technician running the test. This is a manual visual field. The light is moved from the periphery towards the center until the patient sees the light and the location is recorded. Static spots can also be shown as well. The size and intensity of the light can be varied.
Computerized or Automated Perimetry
There are several different automated visual machines on the market. The most commonly used machine is the Humphrey Field Analyzer. Other instruments are Oculus Easyfield and Octopus. These machines are controlled by computers. They can be programed to do a variety of visual field testing. They can use different color lights, sizes, and intensities. They can perform threshold, supra-threshold, and modified threshold tests. The patient is positioned in the machine with one eye patched and the computer program runs the test. They can be programed to perform just central field testing for the detection of macular disease or damage to the macula from medications like Plaquenil.
Visual field testing is a very important test of the vision system that used is used for detecting glaucoma damage or central nervous system disease. It is also used to monitor the status or progression of these diseases. It is used in certain businesses, jobs, and drivers testing to document the degree of a person’s side vision.