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Hemianopia

What is Hemianopia?

Hemianopia is defined as a visual field defect that respects the horizontal or vertical meridian or loss of vision in the visual field. The kind of hemianopia typically describes the location of the visual field defect and depends on the site of the lesion along the visual pathway. Diseases of the retina or choroid do not respect the horizontal or vertical midlines while diseases of the optic nerve head respect the horizontal meridian (altitudinal defects) and disorders at the chiasm and post-chiasmal visual pathways respect the vertical meridian (hemianopic defects). A hemianopia can be in both eyes; congruous (equal in both eyes) or incongruous (asymmetric between the two eyes). Homonymous hemianopia shows the visual field defect on the same side in each eye. Heteronymous hemianopia shows the visual field loss on the opposite side in each eye. Bi-temporal and bi-nasal hemianopias are forms of heteronymous hemianopia.

What causes Hemianopia?

Hemianopias are strongly related to trauma/injury, organic lesions, and cerebrovascular diseases. Multiple strokes or lesions can cause compound patterns of visual field loss or impairments. For example, a unilateral neurological event in area V1 will likely give rise to a homonymous hemianopia and if bilateral symmetrical damage, to cortical blindness.

The prevalence of homonymous hemianopia is highest in patients with lesions or cerebrovascular accidents (CVA) in the occipital lobe, parietal radiations and with middle or posterior cerebral artery infarcts than in other areas in the brain. Homonymous hemianopia visual field disorders were found in about 20 to 30 percent of all patients in neurologic rehabilitation centers. The most common type of visual field loss was found to be complete (54 percent) homonymous hemianopia where the entire half of the field is affected with no macular sparing and partial (19.5 percent) homonymous hemianopia where the field loss between the two eyes is asymmetric with macular sparing.

How does Hemianopia affect vision?

Hemianopia can cause many negative visual disturbances. The main complaint, however, is often vague or the individual may not be symptomatic at all. This is predominantly true if there is a coexisting visuospatial neglect from a right hemispheric lesion and the person has anosognosia until he/she is diagnosed with hemianopia. After which, the signs and symptoms may include one or more of the following: seeing half of an image such as one half of the TV or one’s face in the mirror, difficulty finding the start of a line or seeing ahead in a line of text when reading, failing to see things in the missing field and bumping into things in it, thinking other people are bumping into them or objects suddenly appearing out of nowhere and visual midline shift symptoms such as thinking the floors/walls are tilted and tilting the body to compensate makes mobility and navigation very difficult, especially in unfamiliar or crowded environments.

Visual field defects can be assessed via numerous visual field testing methods: confrontation, tangent screen, Goldman or automated perimetry. These tests all document the field loss based on the patient’s perspective. Many hemianopic/altitudinal defects are readily picked up by these techniques. Those patients with concurrent neglect, on the other hand, require the extinction confrontation field testing. This is to elicit a phenomenon in which a single target presented in the left or right field will be seen while one of the simultaneously presented targets in the left and right field will not be seen. The tangent screen may be useful to see if there is tunnel vision or field constriction with increasing testing distance in functional vision impairments. Computerized visual field testing has become the gold standard and is especially helpful in monitoring progression or improvements over time.

Total recovery from hemianopia rarely happens and the average partial recovery of 5 degrees happens in about 10 to 20 percent of patients in the first 2 to 3 months following the insult with vision gradually returning in phases beginning with light perception to motion to form to color to stereopsis. The approaches to treating hemianopia involve visual scanning, special optical aids and vision restorative therapy. They are all coping strategies to compensate for, and to recover as much as possible, the field loss.

Visual scanning training reminds and encourages the person to be aware of their blind side to increase the looking there in a systematic way. For example in reading, people with left hemianopia would read the word as a whole at the left side of the page and people with right hemianopia would make a saccade to the end before reading it. Over time, it has been shown that the speed and accuracy of the saccadic scanning to find targets improved and the visual search area expanded., Other options include a marker or line/edge guides and rotating the reading material 90 degrees to read up and down instead of left to right.

Treatments for Hemianopia

Optical aids such as Peli or Gottlieb prism or an adjustable mirror can be installed or fitted by a skilled professional on either one or both eyeglass lens(es) to shift or reflect the image of objects from the blind field into the unimpaired field to act as a signal to look into the prism/mirror or towards the affected side.

Computer-based Vision Restoration Therapy (VRT) is also available to widen the field of view and the company NovaVision claims in one clinical study of 302 patients, over 70 percent showed notable improvements, independent of injury onset.

Of course, adjunct vision therapy and/or neuro-optometric rehabilitation to redevelop new connections via neural plasticity or to enhance visual efficiency and visual processing skills are also necessary to help increase the flexibility of the visual process for safer maneuverability at home, work, and play.