Boston: Scientists have developed new eyeglasses which use high-power prisms to help 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 tumors and head trauma, researchers said.
Hemianopia reduces the natural visual field of about 180 degrees to a mere 90 degrees. People with hemianopia have difficulty detecting hazards on their blind sides, leading to collisions, falls and other accidents, they said.
One method of treating hemianopia is to expand the visual field with prisms mounted on or embedded in eyeglasses. Scientists from Schepens Eye Research Institute and Harvard Medical School in the US have been developing prism devices to expand the visual field for these patients for over 15 years.
Their most recent commercially available device introduced in 2013, the peripheral prism glasses, has been shown to expand the visual fields of patients with hemianopia by as much as 30 degrees, 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, researchers explored new optical techniques to create higher power image shifting devices designed to bend light farther than the 30-degree limit of conventional prisms.
In conventional prisms, increasing the angle eventually results in the light bending back into the prism, trapped by what is called “total internal reflection,” researchers said. “It is not just that we need a device with a higher angle of light shifting to let them see farther. We also want the new devices to provide the additional range of vision when the patient scans their eyes in both directions,” said Eli Peli from Schepens Eye Research Institute.
“The current prism devices support such flexibility only when scanning into the seeing side,” said Peli. Researchers introduced three new high-power prism concept devices – Yoked Prisms in the Carrier Lens, Bi-Part Double Fresnel Prism and Mirror-Based Periscopic Prism.
The first approach allows for up to 36 degrees of expansion to the visual field on the patient’s blind side. The second device allows for up to 43 degrees of expansion to the visual field on the patient’s blind side and an increase to 14 degrees scanning range into the blind side. The third approach – not yet fully manufactured – uses a pair of angled mirrors to deflect the image from the blind side to the seeing side – not unlike prism correction.
“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,” said Peli. The findings were published in the journal Optometry and Vision Science.