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Jae-Hyun Jung, Eli Peli; No useful field expansion for hemianopia or acquire monocular vision with full-field prisms. Invest. Ophthalmol. Vis. Sci. 2018;59(9):639.
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© ARVO (1962-2015); The Authors (2016-present)
Full-field (full or full-diameter) prisms which fill the entire spectacle eye wire have been considered field expansion devices in acquired monocular vision (AMV), monocularly or homonymous hemianopia (HH), binocularly as yoked prisms. Due to the edge thickness, weight, and image quality, only low power prisms (<15Δ) have been used. Full-field prisms shift the seeing field toward the blind (base) side. However, foveal fixation on an object of interest through a full-field prism requires head and/or eye rotation away from the blind side which negates the field shift into the blind side. Higher effective prism power at a high angle of incidence, nasally, may contribute to field expansion in AMV. We examined field expansion with full-field prisms using calculated ray tracing and perimetric measurements.
We fit meniscus (+4.00 base curves) and flat (back surface parallel to the frame) full-field 7Δ and 12Δ yoked prisms on dress frames and measured the effect using Goldmann perimetry in HH and AMV. The subjects were allowed to turn their heads freely to fixate. We confirmed the fixation on the perimeter target by mapping the blind spot. We compared the perimetry to ray tracing calculations (Zemax).
In the ray tracing, the rated prism power was in effect at the primary gaze for all prisms, and the meniscus prisms maintained almost constant power at all eccentricities. The calculated effective prism power of the flat prisms in the periphery of AMV (nasal eye wire at 40°) was 4°-9° higher than the rated power. The subjects’ heads and eyes turned away from the blind side, which negated the field shift into the blind side. It was confirmed perimetrically by an increase of the far seeing temporal field (not covered by the prisms). In HH, there was no difference in the perimetry results on the blind side with any of the prisms. In AMV, the nasal field was slightly shifted (~5°) into the blind side with the flat prisms, but not with the meniscus prisms. Apical scotomas were recorded in the temporal periphery (~40°) in both HH and AMV.
Full-field prisms are not an effective field expansion device for HH or AMV due to inevitable fixation shift. There is potential for a small shift with the flat full-field prism in AMV, but such lenses cannot incorporate refractive correction. Further, in considering the apical scotoma, even that shift provides a mere field substitution at best.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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