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N. C. Ross, A. R. Bowers, E. Peli; Consideration of Optical Scotomas in Designing Visual Field Expansion Devices. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4734.
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Prisms are used in rehabilitation of hemianopia to expand or relocate the visual field (VF). In addition to shifting images to a functional part of the VF, they also create a scotoma in visual space at the prism apex, which could compromise areas of remaining vision. We investigated whether prism apical scotomas (PS) impacted binocular viewing for configurations of high-power, unilateral peripheral prisms (Peli, OVS 2000), and examined the correspondence between calculated and measured PS size and position.
Patients with hemianopia (n=4) wore habitual peripheral prism glasses with unilateral Fresnel prism (upper and lower) segments of 57Δ or 40Δ placed base-out on the side of the VF defect. Normal vision (NV) subjects (n=4) wore spectacles with 40Δ press-on Fresnel peripheral prism segments. A dichoptic perimeter was used to test each eye independently under binocular viewing conditions. PSs were mapped using an 8x8 mm square bipolar (checkerboard) target of 95% contrast at 1m.
As expected, when the target was presented only to the eye wearing the prisms, a PS was mapped for all NV and hemianopic subjects. When the target was presented to both eyes (normal binocular viewing), no part of the PS could be mapped for those wearing 40Δ prisms; however, for the one patient with 57Δ prisms, part of the PS (10o lateral extent) was mapped in the binocular view. The difference between measured and calculated lateral PS extent was similar for patients (n = 3) and NV subjects wearing 40Δ prisms (3o (SD 2o); 14% difference), reflecting the error inherent in VF plotting. However, the difference between measured and calculated PS position was greater (patients: 6o (SD 5o); NV: 5o (SD 3o)), as it includes errors in measurements of prism position and distance from the cornea, as well as VF plotting errors.
Though higher prism powers give greater VF expansion, the accompanying PS is larger and could impact central areas of the VF in binocular viewing (as for the patient using 57Δ prisms). This effect will be more dramatic for bilateral prism designs, and may affect centrally positioned prism designs due to suppression. Calculations of PS position can provide a guide to the expected position, but should be verified with VF plots.
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