The brain organizes visual information by combining what is seen by both eyes into 'visual fields.' Because of the arrangement of these anatomical pathways, the type of vision loss that occurs with diseases in the brain is different from the vision loss that occurs with a problem in one eye. When a problem in the brain affects vision, a person loses vision within the corresponding part of the visual field—which means that both of the eyes will have difficulty seeing in a particular direction. It is common for a patient with a visual field deficit to mistakenly assume that the problem is in one eye. It is important to recognize that the same visual deficit is present in both eyes, meaning that the problem is actually in the brain’s map of the visual fields.
Often a patient will be aware of an abnormal visual field, especially if the change is sudden. In some cases, however, the change is gradual, and it can be difficult to notice the loss of vision. Symptoms of a visual field deficit might include bumping into objects on the affected side. Importantly, car accidents may occur because of the decreased peripheral vision. There can be trouble reading, especially when the enlarged blind spot reaches the center of the visual field. While a small visual field deficit might not be very noticeable, a large visual field deficit can clearly have a profound effect on a person’s life.
The doctor will check the visual fields for each eye separately. First, this will be done by asking the patient to describe any missing or blurry areas, for example when looking at the doctor’s face. Then the patient may be asked to count fingers presented in different parts of the visual field. Finally, more thorough techniques can be used to test the visual fields; one common method is an automated test in which the patient pushes a button every time a tiny flash of light is seen. Using the results of these different tests, the doctor can better understand where a problem exists in the visual pathways of the eyes and brain.
There is some controversy about specific treatments for a visual field deficit. There can often be some spontaneous recovery, even without specific treatments or therapies. Many patients gradually develop strategies to compensate for the change in their vision, by moving their head and eyes more frequently to the affected side to check the enlarged “blind spot.” Other practical strategies can also be very helpful. For example, if reading is difficult,
then a bright colored marker on the left margin can help guide the eye movements to the correct starting point.
Most specific training programs that intended to expand the visual field are not supported by strong scientific studies. Of course, these products are probably not harmful—except for their financial costs, which are not typically covered by health insurance plans because of their unproven efficacy.
Other potential treatments for a visual field deficit aim to improve a patient’s use of intact vision, rather than specifically trying to restore the vision that is missing. Some low-vision opticians will try special prisms on eyeglasses to artificially expand the visual field toward the affected side. This approach has mixed results, but some patients find it very useful. A commercially-available training program is available that attempts to improve one’s ability to search the visual environment using eye movements. Some patients find this type of formal practice to be helpful.
Depending on its size and location, a visual field deficit can greatly affect one’s ability to drive a car safely. A reduced visual field may endanger the driver as well as other motorists and pedestrians. Most states have laws specifying the exact minimum vision required to maintain a driver’s license. For example, in Massachusetts a driver is required to have 120 degrees of vision across the middle of the visual field (in addition to having a minimum of 20/40 visual acuity). When the visual field is compromised to less than 120 degrees, it is no longer safe to drive.
Safe driving also depends on other cognitive functions, such as attention and memory. The decision to continue driving after a neurological injury can sometimes be complex. For this reason, many rehabilitation centers offer driving simulators to carefully assess one’s ability to drive safely.
The prognosis of a visual field deficit depends upon the exact cause of the problem. Following most strokes, although some recovery is possible, there are usually permanent deficits. When a visual field deficit is due to a brain tumor, the recovery depends on the amount of initial visual loss and the extent of surgery that might be performed. Visual field deficits that result from other processes, such as inflammation or swelling, might recover spontaneously or with medications (such as corticosteroids).
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