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RL Woods, E Peli; Development Of A Novel Optical Aid For People With Severely Restricted Visual Fields . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3799.
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Purpose: A visual field of 10° or less impairs mobility. To assist in this situation Peli (1999, 2001) proposed a novel spectacle-based prism device that expands the visual field. Here we describe the development of the Trifield lens and pilot testing with two subjects. Methods: Two prisms, separated by a vertical junction (like a Franklin bifocal) are fitted apex to apex over the "worse" eye. The better eye has a conventional correction. The prism eye receives visual information shifted laterally from the direction of gaze by the prism. The direction of shift depends on the prism, and the prism is determined by the direction of gaze. Prism power must be sufficient to avoid diplopia. Phoria correction must be included in the prism power, such that the prism powers are often asymmetric. Results: Visual field expansion was demonstrated using perimetry. Fresnel prisms produced a reduction in vision of the two retinitis pigmentosa (RP) subjects that was greater than found for normally-sighted subjects. Collaboration with manufacturers now allows us to provide the prisms in conventional lenses. Failure to prescribe correct prism powers (field measurement or phoria) resulted in confusion due to diplopia. Even with correct prism power, subjects were unable to determine the direction of field expansion. In other words, they detected objects, but did not know on which side they were. We are testing tinted prisms to provide spectral information that may be associated with direction. Neither subject demonstrated adaptation of visual direction, a perceptual integration of the Trifield device, but the tints may help. Fitted clip-on sunglasses improved cosmesis and provided glare reduction required by people with RP. Conclusion: Trifield lenses provided some benefit to two subjects, by giving warning of nearby objects. However, neither subject demonstrated full adaptation to the field expansion with altered perception of visual direction. A larger study is about to commence to objectively evaluate the impact on walking, visual direction and quality of life.
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