A team of researchers from Boston suggests that full-field prisms are not effective field expansion devices due to the inevitable fixation shift they cause, which provides a mere field substitution at best, the team notes.

The researchers fit patients with meniscus and flat full-field 7∆ and 12∆ yoked prisms. They then conducted Goldmann perimetry in homonymous hemianopia and acquired monocular vision (AMV) and compared the results.

They found the rated prism power was in effect at the primary position of gaze for all prisms and the meniscus prism power was nearly constant at all eccentricities. To fixate on the perimetry target, they note, participants needed to turn their heads and eyes away from the blind side, which negated the field shift into the blind side. They discovered that, in homonymous hemianopia, no difference in the perimetry results existed on the blind side with any of the prisms. In acquired monocular vision, they noticed that the lower nasal field of view was slightly shifted into the blind side with the flat prisms but not with the meniscus prisms.

The study concludes, “there is potential for a small field shift with the flat full-field prism in AMV, but such lenses cannot incorporate refractive correction.”