Abstract
Purpose:
Vertically yoked prisms (VYP) are prisms of same magnitude and direction, and they have been controversially prescribed in the management of lateral binocular dysfunction. The purpose of this study is to investigate what changes occur in lateral heterophoria (phoria) with the introduction of VYP in young, non-strabismic, non-amblyopic adults.
Methods:
Forty participants (26F, 14M) mean age 19 years (range 18-28) were recruited. All were naïve to any potential effects of VYP, and wore their full habitual distance refractive correction. Posture was kept constant by use of chin and forehead rests. Using Modified Thorington Cards at distance (3m) and near (40cm), horizontal phoria was measured at baseline, with VYP of 2 prism dioptre (Δ) base up (BU), 2Δ base down (BD), 5Δ VYP BU, 5Δ VYP BD, and control lenses of +0.125D OU. Following baseline, all lens conditions were pseudo-randomly introduced. Distance phoria was determined before near phoria for each lens condition. A washout period of 1 minute between lens conditions was used. The investigator who measured the phoria and the participant were masked as to lens condition.
Results:
Habitual phorias ranged from 3 eso to 9 exo (mean 1.2 exo±2.6) at distance and 5 eso to 18 exo (mean 2.7 exo± 4.6) at near. The mean difference in baseline and control lens phorias was 0.29Δ at distance and 0.10Δ at near, and was not statistically significant (Wilcoxon test, p>0.05). The mean difference between baseline and any prismatic lens condition was never greater than 0.27 Δ, and there was no statistically significant difference in horizontal phoria across lens conditions for either distance or near (Friedman, p>0.05). Post-hoc comparisons showed no significant difference between phorias in any individual lens condition, even between 5Δ BU and 5Δ BD. Two participants with high baseline near exophoria showed relatively large magnitude changes in heterophoria. However, the direction of change in phoria was inconsistent.
Conclusions:
No change in horizontal phoria was found when viewing through 2 or 5Δ VYP (either BU or BD) in young adults, when posture was controlled. This investigation supplies no evidence to support the use of VYP to affect changes in lateral binocular dysfunction. Further studies with participants with a wider range of baseline phoria or with binocular dysfunction may be warranted.
Keywords: 434 binocular vision/stereopsis •
751 vergence