Hemianopia is a blindness in one half of the visual field due to damage of the optic pathways in the brain. This damage can result from brain injuries caused by stroke, tumor or trauma. A patient with hemianopia may be unaware of what he or she cannot see and may frequently bump into walls, trip over objects or walk into people on the side in which the visual field is missing.
Prism eyeglasses were invented by Dr. Eli Peli of the Schepens Eye Research Institute, an affiliate of Harvard Medical School, to assist patients with hemianopia. Dr. Peli attached small high power prisms on the top and bottom of one spectacle lens, leaving the center of the lens untouched. The prisms pull in images missing from the visual field above and below the line of sight on the side of the vision loss. The prisms alert the patient to the presence of a potential obstacle, so that the patient can then move his/her head and eyes to examine the prism-captured image directly through the clear center of the lens.
In the trial, 32 of 43 participants (74%) who were fitted with prism glasses continued wearing the glasses at week six; at 12 months, 20 (47%) were still wearing the spectacles eight hours daily and rating them as “very helpful” for obstacle avoidance.
A larger study is currently underway to evaluate a newer model of the eyeglasses.
SOURCE: “Community-Based Trial of a Peripheral Prism Visual Field Expansion Device for Hemianopia”, Bowers, et al, Archives of Ophthalmology, 2008, vol. 126, no5, pp. 657-664.