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E. Peli, A. R. Bowers, K. Keeney; Oblique Peripheral Prisms: A New Design for Optical Treatment of Hemianopia. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4733.
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To describe the development, implementation and initial testing of a novel form of (high power) prism correction for hemianopia. While keeping the prisms in the peripheral part of the spectacle lens, this design expands the vision centrally (near the horizontal meridian) and thus maintains single central binocular vision. The previous (horizontal) peripheral prism design placed two high power (40Δ, ~23°), 22 x 8 mm base-out prism segments at about 6 mm above and below the spectacle optical center on the side of the vision loss. This provided sections of field expansion above and below the horizontal meridian (each about 20° x 20° with a vertical gap between of about 40°), leaving the central area unaffected but also without expansion. The central area of the field is particularly important for driving, as the vertical field of view through the windshield is limited to about 25 to 30°.
Rotating both prism segments’ bases towards the horizontal meridian moves the field expansion area vertically and centrally, while the prism segments remain in the peripheral location (oblique prism).
The expansion of the field vertically and centrally increases by sin(a), where a is the angle of rotation, and is accompanied by a reduction in the lateral expansion (by 1-cos(a)). A rotation of 30° results in an 11° coverage of the vertical gap from each segment, and a reduction of only 3° in the lateral expansion. With the introduction of higher power PMMA Fresnel prism segments (57Δ, ~ 32°), we achieve a greater lateral expansion of the field and complete coverage of the vertical gap, that can be demonstrated with standard perimetry. The 40Δ press-on oblique prisms were evaluated and found effective in a (just completed) driving-with-hemianopia study (Tant, Vision 2008) and the PMMA 57Δ oblique prisms are being compared to horizontal prisms and to sham prisms in a multi-center clinical trial (NCT00494676).
A novel oblique design of peripheral prism treatment for hemianopia, which expands the field near the horizontal meridian while at the same time permitting single central binocular vision, has been successfully used when driving and walking.
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