High-Power Prismatic Devices for Oblique Peripheral Prisms

Abstract

PurposeHorizontal peripheral prisms for hemianopia provide field expansion above and below the horizontal meridian; however, there is a vertical gap leaving the central area (important for driving) without expansion. In the oblique design, tilting the bases of both prism segments toward the horizontal meridian moves the field expansion area vertically and centrally (closing the central gap) while the prisms remain in the peripheral location. However, tilting the prisms results also in a reduction of the lateral field expansion. Higher prism powers are needed to counter this effect.MethodsWe developed, implemented, and tested a series of designs aimed at increasing the prism power to reduce the central gap while maintaining wide lateral expansion. The designs included inserting the peripheral prisms into carrier lenses that included yoked prism in the opposite direction, combination of two Fresnel segments attached at the base and angled to each other (bi-part prisms), and creating Fresnel prism–like segments from nonparallel periscopic mirror pairs (reflective prisms).ResultsA modest increase in lateral power was achieved with yoked-prism carriers. Bi-part combination of 36Δ Fresnel segments provided high power with some reduction in image quality. Fresnel reflective prism segments have potential for high power with superior optical quality but may be limited in field extent or by interruptions of the expanded field. Extended apical scotomas, even with unilateral fitting, may limit the utility of very high power prisms. The high-power bi-part and reflective prisms enable a wider effective eye scanning range (more than 15 degrees) into the blind hemifield.ConclusionsConventional prisms of powers higher than the available 57Δ are limited by the binocular impact of a wider apical scotoma and a reduced effective eye scanning range to the blind side. The various designs that we developed may overcome these limitations and find use in various other field expansion applications.

Multiplexing Prisms for Field Expansion

“…9 shows binocular perimetry for a patient with severe retinitis pigmentosa and less than 10° of residual central vision. We fitted 40Δ outward prism serrations multiplexing prisms in the horizontal configuration (base out ) in the upper segment on the right lens and in the oblique configuration 15 (base out and down) in the upper segment of the left lens. Oblique prisms tilt the base of the prismatic elements, creating vertical as well as lateral prismatic effects, so that the view provided is closer to the horizontal midline, with slight loss of expansion extent.…”

“…Oblique prisms tilt the base of the prismatic elements, creating vertical as well as lateral prismatic effects, so that the view provided is closer to the horizontal midline, with slight loss of expansion extent. 15, 20, 21 However, the efficacy of the oblique configuration in this narrow area of central vision is questionable.…”

“…14, 15 This is particularly useful in the case of hemianopia, as one can start from the foveal line of sight that represents the most extreme ray that will fall on the functioning side of the retina after deflection by the prism. 15 Due to the reversibility of optics, the actual rays entering the prism from the object of regard follow the same paths.…”

The risk of pedestrian collisions with peripheral visual field loss

“…While that reach is suboptimal, it does cover a fairly high risk area in the distribution and thus may have a meaningful benefit. We have already designed and made prototypes of a few configurations of high-power prism devices that may reach closer to the 45° peak of the risk distribution (Peli, Bowers, Keeney, & Jung, 2016). …”

“…We know that visual field expansion via our peripheral prism glasses for hemianopia has been successful (Bowers, Keeney, & Peli, 2008, 2014; O'Neill et al, 2011), and our Peli and Jung (2016) paper identifies paths and reasons to believe that success can be achieved for expanding the visual fields prismatically for other situations, including severe PFL, meeting the challenges identified in Apfelbaum and Peli (2015). Finally, in Peli, Bowers, Keeney, and Jung (2016) we have identified a few options for developing higher power prisms that should provide more effective solutions to the mobility problem we have addressed here.…”