No Useful Field Expansion with Full-field Prisms

Abstract

Significance: Full-field prisms that fill the entire spectacle eye wire have been considered as field expansion devices for homonymous hemianopia (HH) and acquired monocular vision (AMV). Although the full-field prism is used for addressing binocular dysfunction and for prism adaptation training after brain injury as treatment for spatial hemineglect, we show that the full-field prism for field expansion does not effectively expand the visual field in either HH or AMV.

Purpose: Full-field prisms may shift a portion of the blind side to the residual seeing 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, thus negating the shift of the field toward the blind side.

Methods: We fit meniscus and flat full-field 7Δ and 12Δ yoked prisms and conducted Goldmann perimetry in HH and AMV. We compared the perimetry results with ray tracing calculations.

Results: The rated prism power was in effect at the primary position of gaze for all prisms, and the meniscus prisms maintained almost constant power at all eccentricities. To fixate on the perimetry target, the subjects needed to turn their head and/or eyes away from the blind side, which negated the field shift into the blind side. In HH, there was no difference in the perimetry results on the blind side with any of the prisms. In AMV, the lower nasal field of view was slightly shifted into the blind side with the flat prisms, but not with the meniscus prisms.

Conclusions: Full-field prisms are not an effective field expansion device owing to the inevitable fixation shift. There is potential for a small field shift with the flat full-field prism in AMV, but such lenses cannot incorporate refractive correction. Furthermore, in considering the apical scotoma, the shift provides a mere field substitution at best.

Figure 1.

Schematic illustration (right eye with base-in prisms) of full-field prism configurations. (A) Meniscus full-field prisms mounted with the bevel positioned at the front surface for the best cosmetics. (B) Flat full-field prisms with the bevel positioned at the back surface for mounting to the frame (outward prism serration). (C) Flat prisms mounted with bevel positioned at the front surface (eyeward prism serration). Due to the difference in back surface shape (curved, flat or slanted), the angle of incidence, i (from the normal to the back surface, see the inset), varies with prism configuration and direction. The smaller angles of incidence at the base end (blue dashed lines) in (A) and (C) result in lower effective prism powers than in (B).

Figure 2.

Fixation shift through full-field prisms. For simplicity, we illustrate right eye only with a base-in full-field prism. Note that we assume the flat spectacle frame (orange solid line) is orthogonal to the head direction, and the fronto-parallel plane is orthogonal to the line of sight to the fixation target (green cross mark) with no prism. The solid lines indicate the actual ray path from the fixation target to the retina (through the full-field prism in B-D), and the dashed lines show the apparent path. (A) When a patient with left homonymous hemianopia or right acquired monocular vision (right seeing eye) fixates on a far fixation target with the eye at the primary position of gaze, the fovea aims at the fixation target. (B) The image of the fixation target through the full-field prism is shifted toward the apex of the full-field prism (see dashed blue line and apparent cross image shifted from the fixation direction). The fixation target is now imaged off the fovea. The patient may rotate (C) the eyes and/or (D) the head and eyes together away from the blind side to foveate on the fixation target through the full-field prism, which may negate the field-of-view shift towards the base (red solid line).

Figure 3.

Pictures of full-field prism configurations (12Δ yoked prisms). (A) Meniscus full-field prisms mounted with the bevel positioned at the front surface. (B) Flat full-field prisms with bevel positioned at the back surface to mount to the frame (outward prism serration). (C) Flat prisms mounted with bevel positioned at the front surface (eyeward prism serration). Note that the prisms were mounted in a special frame with very narrow eyewire to reduce the thickness of the prism base.

Figure 4.

Simulated optical ray tracing in full-field prisms and calculated effective prism power variation. (A) Meniscus, (B) flat outward (OPS), and (C) eyeward prism serrations (EPS) full-field prisms (12Δ). Colored ray tracings indicate visual eccentricities from −60° (left, base side) to 50° (right, apex side) at 10° intervals. The angle of incidence and range of visual eccentricities covered by the full-field prisms vary between the configurations due to the different back surfaces. Graphs show the effective prism power variation of (D) 7Δ and (E) 12Δ in each configuration as a function of visual eccentricity. The effective prism power in flat outward prism serration full-field prisms is higher than other configurations. However, the base surface obscuration hitting the base surface of the prism (red dotted surface) further limits the effective range of visual eccentricities in the flat outward prism serration.

Figure 5.

(A) Calculated and (B) measured binocular field diagrams of a patient with left homonymous hemianopia with two configurations of bilateral yoked full-field prisms. Left and right columns show the effects of full-field prisms in the meniscus and flat outward prism serration (OPS), respectively. Results for 7Δ and 12Δ are in the top and bottom row, respectively. The dashed line shows the binocular visual field without prisms. Due to the fixation shift, there is no field-of-view shift into the blind side (left). The compensatory head rotation toward the seeing (right) side for the fixation shift is expected to result in far right temporal field expansion on the seeing side, which is verified by the measurements. The calculated apical scotomas shown are from the right prism, and the upper parts of these may be compensated for by the nasal field of the left eye as seen in the measurements. The apical scotomas in the mid-periphery are wider than the magnitude of the temporal field shift. Note that the patient had incomplete hemianopia, which was consistent with his prior perimetry records.

Figure 6.

(A) Calculated and (B) measured field diagrams of a patient with right acquired monocular vision with different configurations of full-field prisms. Left and right columns show the effects of full-field prisms in the meniscus and the flat outward prism serration (OPS), respectively. Results for 7Δ and 12Δ are in the top and bottom row, respectively. The dashed line shows the visual field without prisms. Due to the head and eye rotation required for the fixation shift, the prisms show slightly farther temporal field-of-view (non-interesting side). With the flat outward prism serration full-field prisms (right columns), the eye rotation resulted in a slight field-of-view shift into the blind nasal side as the visual field was no longer blocked by the nose tip and wing at primary position of gaze. Taking into consideration the apical scotoma, this resulted in field substitution at best.