Visual Field Testing
A perimeter is used to test your visual field. Your visual field includes both your central and side vision. During this test, while you are seated comfortably, you will be asked to look straight ahead at a central fixation target. You will be instructed to press a button when you become aware of a small shimmering light anywhere within your peripheral field. It’s important that you keep your eyes focused on the central target throughout this examination so that an accurate reading of your visual field is obtained.
This test can detect vision loss, even if the loss is far to the side or quite subtle. Even a large and profound vision loss in one eye’s visual field can go unnoticed by a patient because the visual field of the opposite eye overlaps it to a great extent. One eye simply “fill in” the area of vision loss present in the opposite eye. This is why the side-vision test is always administered to each eye separately.
The nerve pathways from the retina to the part of the brain where vision is interpreted are very uniform from person to person. Because of this uniformity, visual field analysis has a great deal of diagnostic value. Glaucomatous visual field loss usually follows a particular pattern. Other diagnostic patterns of visual field loss can indicate retinal detachment and neurological diseases, adding significantly to the value of this test. The nerve pathways for vision are long and cross over many brain structures on their route from the eye to the very back of the brain (visual cortex). Neurologists make great use of visual fields to help them determine where in the brain a tumor or other pathology is located.
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