Vertically yoked prisms have minor effects on accommodation and binocular vision during short-term wear in young adults with normal binocular vision, an Australian study suggests. These prisms are often used to treat binocular vision dysfunction, although little evidence exists to support their use, the researchers note. Few studies explore the impact on accommodation, accommodation vergence interactions or the horopter. “We found that vertically yoked prisms have minor effects during short-term wear in young adults,” researchers wrote in their paper.

The investigation enrolled 45 participants between the ages of 18 and 24, including 23 myopes who wore distance-correcting soft contact lenses. In a random arrangement, the study had each patient wear spectacles containing plano lenses with either 8Δ base-up, 4Δ base-up, zero, 4Δ base-down and 8Δ base-down prisms. Before spectacle wear, researchers took baseline measurements of near heterophoria, accommodation response, negative and positive relative accommodations, fusional vergence and Nonius-horopter spatial perception. After 40 minutes, patients removed the spectacles, and investigators repeated the measurements and tested again 20 minutes later. The study also included a 22-participant subset, where on a separate occasion, investigators measured heterophoria, accommodation response and relative accommodation immediately after the spectacles were fitted.

Investigators reported most changes relative to baseline were not significant.

“Where effects occurred, these were nearly all associated with prism presence rather than adaptation. There were significant effects on accommodation response, but these seem to be refraction effects produced by pantoscopic tilt-induced power changes rather than perceptual effects altering accommodation,” researchers wrote.

The study noted statistically significant effects were observed on negative relative accommodation, with zero prism giving more negative relative accommodation than 8Δ base-down prisms. Investigators also observed tendencies for prisms to move horopter limits toward the observer, but the effects were small and likely not of clinical relevance.

Schmid KL, Beavis SD, Wallace SI, et al. The effect of vertically yoked prisms on binocular vision and accommodation. Optom Vis Sci. May 17, 2019. [Epub ahead of print].