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.
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.
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.
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.
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.
1Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, Massachusetts *email@example.com
Supplemental Digital Content: Appendix 1: the flat eyeward prism serration full-field prism is less practical than other configurations because of the mechanical issues (i.e., touching eyelashes or nose). Because we could not measure the effect perimetrically, we calculated field diagram using the result of ray tracining simulation. As shown in Fig. A1 (available at http://links.lww.com/OPX/A349), no field expansion into the blind side is expected with eye and/or head rotations to the right to fixate on the perimeter fixation target through full-field prisms.
The flat eyeward prism serration full-field prisms for acquired monocular vision (Fig. A1B) extend the FoV slightly farther into the nasal blindside because of higher effective prism power at the base than at the primary position of gaze. However, this configuration also results in the widest apical scotoma (15°). Therefore, this also provides field substitution at best.
Submitted: January 30, 2018
Accepted: June 19, 2018
Funding/Support: National Institute of Health (R01EY023385; to EP) and National Institute of Health (P30EY003790).
Conflict of Interest Disclosure: EP has patents rights (assigned to Schepens Eye Research Institute) for the peripheral oblique prisms and the multiplexing prisms (both licensed to Chadwick Optical).
Author Contributions and Acknowledgments: Conceptualization: J-HJ, EP; Data Curation: J-HJ; Formal Analysis: J-HJ; Funding Acquisition: EP; Investigation: J-HJ, EP; Methodology: J-HJ; Project Administration: J-HJ, EP; Resources: J-HJ, EP; Software: J-HJ, EP; Supervision: J-HJ, EP; Validation: J-HJ; Visualization: J-HJ; Writing – Original Draft: J-HJ; Writing – Review & Editing: J-HJ, EP.
The authors thank Merve Tuccar, Sailaja Manda, and Rachel Castle for their help with subject tests and preparation of figures.
Supplemental Digital Content: Direct URL links are provided within the text.