Researchers have designed new eyeglasses using high-power prisms to optimally expand the visual fields of patients with hemianopia — a condition in which the visual fields of both eyes are cut by half. The new designs address some limitations of existing prism correction available to this population.
Impairing either the left or right halves of the visual fields in both eyes, hemianopia is most commonly caused by stroke, brain tumours and head trauma. Hemianopia reduces the natural visual field of about 180° to a mere 90°. People with hemianopia have difficulty detecting hazards on their blind sides — leading to collisions, falls and other accidents.
One method of treatment for hemianopia is to expand the visual field with prisms mounted on or embedded in eyeglasses.
A research team led by professor Eli Peli from Harvard Medical School has been developing prism devices to expand the visual field for these patients for more than 15 years. The peripheral prism glasses — their most recent commercially available device, introduced in 2013 — have been shown to expand the visual fields of patients with hemianopia by as much as 30°, optically shifting objects from the blind side of the visual field to the seeing side.
With the goal of expanding the visual field on the blind side even farther, the researchers explored new optical techniques to create higher power image shifting devices designed to bend the light farther than the 30° limit of conventional prisms.
By embedding the current prism in a spectacle lens that has prismatic power in the opposite direction, the image shifting effect is increased by the summation of the power of both prism types. This design — described in the journal Optometry and Vision Science — allowed for up to 36° of expansion to the visual field on the patient’s blind side.
“The new optical devices can improve the functionality of the current prism devices used for visual field expansion and may find use in various other field expansion applications such as a mobility aid for patients with tunnel vision,” Peli said.