Hemianopia is a vision defect where half of the field of vision of the eye is affected by blindness or reduced vision. One eye or both may be affected by this partial blindness. The condition is usually caused by damage to the nervous pathways in the brain that control vision. Optical pathway damage causing this type of visual impairment may be caused by physical damage to the brain, strokes, or brain tumors.
In optometry, the field of vision in each eye is considered to have two halves, the temporal half and the nasal half. If the visual field of the eye is considered to be vertically divided into two halves, then the temporal half of the field of vision is the outer half. The nasal half of the field of vision is the inner half.
Where both eyes are affected, there are two main variations of hemianopic disorders. In the homonymous variant of this disorder, the patient’s vision is affected in the opposite half of each eye, so that the nasal half is affected in one eye, and the temporal half in the other. In the heteronymous variation, vision is impaired in either the nasal sides of both eyes, known as binasal hemianopia, or the temporal sides of both eyes, which is termed bitemporal hemianopia.
Hemianopia causes are usually serious brain problems such as strokes caused by brain hemorrhages, lesions of the optical pathways, or brain tumors pressing against the optical nerves. There are other causes that are less serious, however, and these may result in only temporary loss of vision. Some migraine sufferers may endure temporary bouts of hemianopia, either during a migraine episode, or immediately prior to a migraine. It may also occur temporarily in some patients that experience a transient episode of very high blood pressure, such as may occur in eclampsia, a complication of pregnancy.
Depending on the root cause, hemianopia may recede spontaneously, as sometimes occurs in stroke patients. If it persists for more than six months in such patients, however, then it is likely to be a permanent condition. Treatment may include rehabilitation training to assist sufferers in adapting to their limited vision. If such training is successful, then sufferers may even be able to drive. Sometimes, specially designed glasses are a successful way of ameliorating the condition. These kinds of glasses may use prisms or mirrors to expand the visual field of the wearer.