Glaucoma – What to Expect in a Visual Field Test

Visual field testing is done for a number of different reasons but most frequently to test or manage patients who have glaucoma. So just what is this test that your Eye Doctor wants you to have? It may be referred to by different names; automated perimetry, visual field test, threshold test, SITA (slang for Swedish Interactive Thresholding Algorithm) or various other terms. The original testing method was called a tangent screen which is a black square of material hanging on the wall. The Goldmann Bowl Perimeter was next and it was a manual version something like the automated devices your eye doctor uses today. Your sensitivity to light on the retina (the light sensitive layer lining the inside of the eye) resembles a topographical map of a hill. The most sensitive point is in the center like the high point on a hill and the farther you move away from this central vision point the less sensitive the tissue is to light (like the lower elevations on a topographical map). The central high point corresponds to the macula, which is why macular degeneration only has to compromise a small area to have devastating effects on vision. A visual field test measures the eyes sensitivity out into the peripheral parts of the retina. Most eye exams only test the small central peak of the hill. The visual field test measure not only the peripheral retina but to some extent the pathway of the eye nerves through the brain.

Your visual field test will be either a screening test or a threshold test. Screening test are used by an Optometrist to check for eye diseases and vision problems in a method similar to how your family doctor would do a single blood pressure reading to screen for high blood pressure. If your family doctor was more concerned they would want multiple readings and more refined testing methods, sometimes a cardiac stress test to see how your heart and blood pressure processes under different conditions with a much higher number of data points. Likewise, a threshold visual field test done by your eye doctor is much more detailed testing. Since the hill of vision shrinks with age, a threshold test will compare your results to an age matched population sample without glaucoma and other eye diseases. The eye test will yield a detailed statistical analysis.

You can expect to take the test to take about 3 to 5 minutes for each eye in most cases. The vision test is standardized for eyes that are dilated so you will need to plan on 30 to 60 minutes in the office even though the visual testing itself is fairly brief. For the actual visual field test you will be listed with a half of a bowl like instrument in front of you. If you wear contact lenses they will need to be removed. The visual field instruments calculate a different prescription compensating for the distance you are viewing. Prescription lenses are placed in a holder in front of your eyes to substitute for your contact lenses or eyeglasses. One eye will be covered and you will need to fixate on a central light through the testing. A device resembling a computer mouse will be held in one hand and you simply click it each time you think you see a small point of light flash anywhere in the bowl. The flashes are very brief and may be close to the point your eye is fixing on or very far off in the peripheral field of sight. The brightness of the flash of light also varies from very dim to very visible. In our Eye Center we typically start the testing sequence in demonstration mode to familiarizeize patients with the process.

Many people worry about their responses; if they clicked too late, clicked accidentally, and are uncomfortable with the dimness of some light points just on the edge of visual perception. This should not be a concern. Visual field testing is still sophisticated today and measures your mistakes, retests them, and gives an index telling us how much variation there was in your eye exam. If the reliability is too low we repeat the test on another day and almost always find the reliability of the eye exam goes up to acceptable levels. There are even adjustments made by the software to factor out the fact that you may have cataracts or other conditions that are not in the retina or visual pathway through the brain. With most visual field instruments you can pause the test yourself by holding down the clicker. That can give you a chance to relax, change position, or ask a question to clarify what you responses should be. We usually start on a demonstration mode to familiarizeize patients with the process and change over to actual testing once patients appear to be comfortable. You can always request a demonstration period first.

Beside glaucoma, a visual field test can be used to diagnose pituitary tumors, neurological problems, and unexplained loss of sight and headaches.

One last point- if you live in area like Fort Collins the Colorado sunshine is extremely bright after your eyes are dilated. Be sure you leave with temporary sunglasses if you do not already have quality sunwear.