BOSTON--September 25, 2000, Harvard Medical School affiliate Schepens Eye Research Institute--High-powered prisms mounted atop eyeglass lenses appear to improve vision for people with a condition that leaves them with only half a field of vision in both eyes, a new study shows.
The prisms could offer an aid to mobility and navigation for the estimated 1 million stroke survivors in this country who experience some degree of vision loss after the stroke, said Eli Peli, O.D., Senior Scientist at The Schepens Eye Research Institute who did the study. About a third of the 3 million stroke victims in rehabilitation in this country have vision loss.
Peli believes that this new method will be helpful for a majority of people with hemianopic field loss. "At the moment, optical treatment remains the only viable option for these patients," he said. "While their disability may be relatively mild, safe mobility is not a minor issue. This method may help a lot of people regain some independence that might have been lost."
The research findings offer hope to 1 million people who have homonymous hemianopia, or loss of half the visual field on one side of both eyes. The condition frequently is caused by brain damage from stroke, head injury, or surgery to remove brain tumors.
The study was published in the journal Optometry and Vision Science (September 2000) and was authored by Peli, who also is Associate Professor of Ophthalmology, Harvard Medical School, and Adjunct Professor of Optometry and Visual Sciences, New England College of Optometry.
Since the vision loss is permanent in most cases, various optical aids and treatments, such as mirrors and prisms, have been used to help patients adapt to the vision loss and safely increase mobility. Some of these methods shift the field of view, rather than expand it, others cause 'diplopia' or double vision in the central field of vision, which often is bothersome and confusing to the patient.
Peli looked at the effect of high-powered elliptical prism segments on the upper and lower part of the glasses. The intent was to widen the patients' field of vision rather than shift it, and avoid central diplopia.
"Patients often complain of bumping into obstacles on the side of the vision loss, and getting bruises on their arms and legs," Peli said.
In designing the placement of the prisms, and figuring the power needed to do this, Peli studied reports of patients born with (congenital) hemianopia who develop exotropia, or divergent strabismus. In this condition, one eye deviates outward, so that the two eyes are not directed toward the same fixation point.
"The exotropia does result in an increased simultaneous field of view, but in order for it to be functional, the patient needs to develop what is called anomalous retinal correspondence, or ARC," Peli said. "For those of us with normal retinal correspondence, only one object is seen even though it is being viewed with two eyes. This is because we have one spot on each retina corresponding to the other where images are focused. So the message sent to the brain is 'there is one person standing in front of me.' Those who develop ARC, because of one deviating eye, have 're-mapped' the location on one retina where the image is fused with the image from the other retina, preventing double vision in the central field of view. It helps the patient adapt to the condition."
The design of the visual aids intentionally induced diplopia only in the peripheral vision, because the 12 patients in Peli's study, whose hemianopia was not congenital and vision loss was more recent, had not developed ARC.
"This is much less disturbing for the user than central double vision since peripheral physiological diplopia occurs in normal vision," Peli said. "It's rarely noticed because we don't pay any attention to it, but can be easily demonstrated."
Patients were instructed to look through the central part of the glasses and not through the prisms. "If the patient were to look through the prism, central double vision would result. When an object of interest is detected through the prism, in the patient's peripheral vision, the patient turns his or head, rather than look toward the object and then turn the head," Peli said. "It's a similar adaptation that many people make with bifocals."
Significant improvement occurred in all but one patient. In all but that one, the prisms added about 20 degrees to the visual field - most obstacles that one can run into are likely to be within this range from the midline. For two other patients, there were either difficulties with using the prisms or no benefit was derived from them, so they were removed. Except for those three patients, the others in the study were pleased with the effects of the correction. They reported rapid adaptation in about two to three weeks, and experienced significant improvement in avoiding obstacles. One young patient, not reported in the study, is now being considered by the Massachusetts Registry of Motor Vehicles for a driver's license learner's permit.