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Katrina L Schmid, Stephanie D. Beavis, Joy Chen, Yi-Tang Chien, Terry Nguyen, Amy N Tran, Sophia I Wallace, Saulius R. Varnas, David A. Atchison; The effect of base-up and base-down vertically yoked prisms on binocular vision and accommodation.. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2960. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Vertically yoked prisms are sometimes prescribed for the treatment of binocular vision dysfunction despite minimal supporting evidence. The prisms cause objects to appear magnified and the visual field to appear displaced towards the apex; base-down (BD) yoked prisms give the sensation of farness and base-up (BU) prisms nearness. However, a study on the impact on near phoria reported no immediate effect of up to 5 Δ BD and BU yoked prisms. We investigated the effect of vertical yoked prisms on a range of binocular vision tasks and on accommodative responses, for greater prism powers and after an adaptation period. The prediction was that BD yoked prisms would shift the horopter away from the observer, decrease accommodation and cause an exophoric shift, with BU prisms doing the opposite.
Participants were 45 young adults aged 18 to 24 years. Twenty-three were myopic and wore soft contact lenses during procedures. In a random sequence each person wore 5 different pairs of spectacles, containing plano powered lenses with 8 Δ BD, 4 Δ BD, 0 Δ, 4 Δ BU, and 8 Δ BU prisms. Before spectacle wear, baseline measurements were taken of near heterophoria (Modified Thorington technique), negative and positive relative accommodation (NRA/PRA), accommodation stimulus response (right eye, Shin Nippon autorefractor SRW-5000, with binocular viewing at 6m, 1m, 50cm, 40cm and 33cm) and spatial perception with the Nonius horopter method. These measurements were repeated after 40 min of wear, then spectacles were removed, and the binocular functions remeasured 20 min later.
The majority of the measures were not affected by the yoked prisms. Where effects occurred, these were after 40 min of wear and had dissipated by 20 min after lens removal. There were statistically significant effects on accommodation, with BU prisms increasing accommodation, and on NRA (p < 0.01) with zero prism giving higher NRA than 8 Δ BU, 4 Δ BU, and 8 Δ BD prisms. Effects were small, often not as predicted and likely not of clinical importance. Tendencies were noted for prisms of both directions to reduce PRA and to move the horopter limits towards the observer.
Vertically yoked prisms have small effects on binocular vision and accommodation in young adults, at least during short term wear, but these are too small and unpredictable to be clinically useful in binocular vision training.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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