Effects on Posture of a Two-Diopter Horizontal Prism Base Out on the Non-Dominant Eye

Abstract

Background/Objectives: Ocular proprioception is implicated in balance control and heterophoria is associated with abnormal posture, though previous research focused mainly on the role of vertical phoria and the use of vertical prisms. This study aims to evaluate whether ocular misalignment and prismatic correction of horizontal phoria affect posture. Methods: Sixty-nine (N = 69) young healthy subjects were included and equally divided by horizontal distance phoria: orthophoria (n = 23), esophoria (n = 23) and exophoria (n = 23). A prism of low power (two-diopter) was placed base out on the non-dominant eye, reducing misalignment in esophorics and increasing it in exophorics more than in orthophorics. Dynamic computerized posturography was performed with the sensory organization test protocol (SOT) of the EquiTest^® NeuroCom^® version 8 platform both without and with prism, always maintaining subjects unaware of prism use. A mixed model for repeated measures analysis of variance was run to evaluate the main effect of prism and the interaction effect of prism with baseline phoria. Results: Composite movement strategy score without prism was 88.1 ± 2.8% (ankle-dominant strategy) and slightly increased to 89.0 ± 3.1% with prism insertion (p = 0.004), further shifting toward ankle strategy. Composite equilibrium score without prism was 80.3 ± 6.5% and remained stable with prism insertion (81.3 ± 8.2%, p = 0.117), medio-lateral and antero-posterior projection of center of gravity did not displace significantly under prism insertion (p = 0.652 and p = 0.270, respectively). At baseline, posturographic parameters were statistically independent of individual phoria, and no significant interaction between prism insertion and individual phoria was documented for any parameters (p > 0.05 for all). Secondary analysis and pairwise comparisons confirmed that the effect of prism was strongly selective on condition SOT 5 (eyes-closed, platform sway-referenced) with improvement of equilibrium (70.4 ± 9.7% with prism vs. 65.7 ± 11.6% without) and more use of ankle strategy (81.6 ± 5.3% with prism vs. 78.2 ± 6.0% without), without any interaction of phoria and ocular dominance, while the other conditions were comparable with and without prism. Conclusions: A two-diopter prism base out on the non-dominant eye induces the body to use the ankle joint more independently of individual phoria, suggesting a small improvement in postural control, while maintaining oscillations of the center of gravity unaltered. Prism seems to enhance the function of vestibular system selectively. Phoria adjustments with prismatic correction enable intervention in postural behavior. Extraocular muscles could act as proprioceptors influencing postural stability.

Keywords: EquiTest NeuroCom; balance control; heterophoria; horizontal phoria; ocular proprioception; posture; posturography; prism; sensory organization test; vergence.

Figure 1

Distributions of horizontal distance phoria (Δ, prismatic diopter) and stereoacuity (log arcsec) without and with prism. Column and bar respectively represent median and interquartile range of phoria, and mean and standard error of stereoacuity. Wilcoxon signed-rank test statistical significance (P) and effect size (r) showed a significant exophoric shift in all groups after prism insertion. Two-way ANOVA statistical significance (P) and partial effect size (η²) of main effect (Prism) and interaction (Prism × Phoria) showed a significant small increase in logarithm of stereoacuity seconds of arc for all groups.

Figure 2

Composite equilibrium (CES) and composite movement strategy scores (CMS) without (at baseline) and with prism, sorted by baseline phoria. A higher CSS denotes a better performance (less sway); a higher CMS indicates a more predominant ankle strategy (versus hip). Dot and bar represent mean and standard error, respectively. Two-way ANOVA statistical significance (P) and partial effect size (η²) of main effect (Prism) and interaction (Prism × Phoria) showed only a significant shift of CMS toward ankle strategy for all groups.

Figure 3

Medio-lateral (ML/Px) and antero-posterior (AP/Py) projection of center of gravity (COG) without (at baseline) and with prism, sorted by baseline phoria. Right and forward displacements are positive, left and backward negative. Dot and bar represent mean and standard error, respectively. Two-way ANOVA statistical significance (P) and partial effect size (η²) of main effect (Prism) and interaction (Prism × Phoria) showed no significant displacement of COG, though COG-Px displacement of exophoric subjects was divergent (rightward) from esophoric and orthophoric subjects (leftward).