A visual field test (perimetry) maps a patient’s entire field of vision, including central and peripheral areas. This procedure precisely measures how wide an area the eye can see while focusing on a fixed point. This mapping helps eye care professionals identify blind spots or areas of diminished sight that a person may not recognize in daily life.
The Role of Corrective Lenses During Testing
For the most common type of automated perimetry, such as the test conducted on a Humphrey Field Analyzer, you will typically be asked to remove your own eyeglasses. This is because the frames of personal glasses can physically block parts of the peripheral vision. Blocking the vision leads to false results that incorrectly indicate a vision problem.
Instead of personal glasses, the technician will use special trial lenses inserted into a holder on the testing machine. These lenses match your current distance vision prescription, ensuring your vision is optimally corrected for the test. This corrects any existing refractive error so the test accurately measures the sensitivity of your retina and optic nerve.
If you wear contact lenses, you are usually permitted to keep them in during the examination. Contact lenses correct vision directly on the surface of the eye and do not obstruct peripheral vision like eyeglass frames do. Always follow the specific guidance provided by the technician, as the protocol may vary depending on the equipment and the purpose of your test.
Understanding the Purpose of Visual Field Tests
Eye care professionals perform the visual field test to detect and monitor subtle changes in vision that indicate underlying health conditions. The test is important for managing diseases that cause a gradual loss of side vision, which patients often do not notice until the damage is advanced. The results provide a detailed, objective map of visual sensitivity across the entire visual field.
The test is primarily used to diagnose and track the progression of glaucoma, which damages the optic nerve and causes peripheral vision loss. It is also an important tool for identifying optic nerve damage from other causes, such as poor circulation or inflammation. Visual field testing can also help diagnose neurological conditions, including strokes or tumors, that affect the visual pathways in the brain.
What to Expect During the Examination
The visual field test is a non-invasive procedure performed using a bowl-shaped instrument called a perimeter. You will place your chin on a rest and your forehead against a bar to keep your head still throughout the procedure. To ensure accuracy, the test is performed monocularly, meaning one eye is covered with a patch while the other eye is being examined.
During the test, you will be instructed to look steadily at a central fixation light inside the bowl. While maintaining your gaze on this central point, dim lights of varying brightness will flash randomly in your side vision. You will hold a handheld response button and press it every time you see a light flash. The machine tests your visual threshold, meaning you are not expected to see every light, and the entire process generally takes between three to eight minutes per eye.