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Lisa Asper, Richard Lin, Edmund Tsui, Kathleen Watt; The effect of vertical yoked prism on horizontal fixation disparity. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1511.
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© 2017 Association for Research in Vision and Ophthalmology.
Vertically yoked prisms (VYP) are prisms of identical magnitude and direction, and they are sometimes, and somewhat controversially, prescribed in the management of binocular dysfunction. A prior study found no immediate effect of 2 prism dioptres (Δ) or 5Δ VYP on horizontal dissociated heterophoria. The purpose of this study is to investigate what changes occur in horizontal fixation disparity, an associated measure of binocular function, with VYP wear in young, non-strabismic, non-amblyopic adults.
Thirty-two participants (21 M) mean age 20.8 years (range 18-31) were recruited. All were naïve to any potential effects of VYP, and wore their habitual distance refractive correction. Posture was kept constant by use of chin and forehead rests. Using the Sheedy disparometer at 40 cm, horizontal fixation disparity was measured at baseline, with VYP of 2Δ base up (BU), 2Δ base down (BD), 8Δ BU, 8Δ BD, and control lenses of +0.125D OU. Following baseline, all lens conditions were pseudo-randomly introduced. Each measurement was taken after 1 minute of prism or control lens wear. A washout period of 1 minute between lens conditions was used. The investigator who measured the fixation disparity and the participant were masked as to lens condition.
Habitual fixation disparity at 40 cm ranged from 2 arcmin (‘) eso to 25’ exo (mean 4.7’ exo +/- 6.3’). The mean difference in baseline and control lens fixation disparity was 0.8’ and was not statistically significant (paired t-test, p>0.05). The mean difference between baseline and any prismatic lens condition was never greater than 0.1’, and there was no statistically significant difference in horizontal fixation disparity across lens conditions (repeated measures ANOVA, p>0.05). Post-hoc comparisons showed no significant difference between fixation disparity in any individual lens condition, even between 8Δ BU and 8Δ BD. Participants with high exo fixation disparity results showed greater variability with VYPs (BD>BU), but did not show a consistent eso or exo shift.
No change in horizontal fixation disparity was found when viewing through 2 or 8Δ VYP (either BU or BD) in young adults, when posture was controlled. Further studies into participants with binocular dysfunction and into the effect of VYP without controlling posture may be warranted.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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