PURPOSE. In healthy subjects, the postural stability in orthostatic position is better when fixating at near than at far. Increase in the convergence angle contributes to this effect. Children with strabismus present a deficit in vergence. We evaluated postural control in children with respect to the vergence angle as they fixated at different depths, thereby engaging in active vergence movements. METHODS.A TechnoConcept platform was used to record the postural stability of 11 subjects (mean age 11.18 6 4.02 years) with convergent strabismus and 13 (mean age 11.31 6 3.54 years) with divergent strabismus in 3 conditions: fixation at 40 cm, at 2 m, and active vergence movements between 20 and 50 cm. RESULTS.The mediolateral body sway decreased significantly with proximity for convergent strabismus (from 3.78-2.70 mm) but increased significantly for divergent strabismus (from 3.27-3.97). Relative to fixation, vergence eye movements resulted in a statistically significant increase in mediolateral body sway for convergent strabismus (3.55 vs. 2.70) and a decrease for divergent strabismus (3.11 vs. 3.97, P ¼ 0.047). Vergence eye movements were associated with the least variance of speed (99 mm 2 /s 2 for convergent and 117 mm 2 / s 2 for divergent strabismus), so less energy was required to control body sway. CONCLUSIONS.The fixation depth at which postural stability is best is proximal for convergent strabismus and distal for divergent strabismus. Optimal postural stability might be mediated by preponderant eye movement signals related to the angle of strabismus. T o maintain the body in a stable position, the central nervous system uses multiple sensorimotor inputs that are at once visual, proprioceptive, vestibular, and somesthetic.Paulus et al. demonstrated that postural control improves while fixating at near rather than at far distances.1 They attributed what we henceforth will refer to as the stabilizing effect of proximity to the angular size of the retinal drift, such that at near distances the angular size of the retinal slip input resulting from body sway is higher than at far distances. The ensuing afferent signal feeds back into mechanisms of postural control potentially leading to an improvement in body stabilization.However, Kapoula and Lê showed that the stabilizing effect that results from the relative proximity of a fixation target also is due to oculomotor signals generated by the vergence angle itself. 2 Indeed, the efferent and afferent signals related to the vergence angle are stronger at near distances as a consequence of the relative increase in ocular convergence required to fixate a proximal target as opposed to a distal one. In their study, convergent prisms were used to make the eyes converge while the subjects were fixating at far distances. Despite the conflict with the physical distance, convergence of the eyes resulted in an improvement in postural stability. This stabilizing effect of proximity is present in children 3 and in the elderly. Bucci et al. showed that children with a vergen...

Vision is important for postural control as is shown by the Romberg quotient (RQ): with eyes closed, postural instability increases relative to eyes open (RQ = 2). Yet while fixating at far distance, postural stability is similar with eyes open and eyes closed (RQ = 1). Postural stability can be better with both eyes viewing than one eye, but such effect is not consistent among healthy subjects. The first goal of the study is to test the RQ as a function of distance for children with convergent versus divergent strabismus. The second goal is to test whether vision from two eyes relative to vision from one eye provides better postural stability. Thirteen children with divergent strabismus and eleven with convergent strabismus participated in this study. Posturtography was done with the Techno concept device. Experiment 1, four conditions: fixation at 40 cm and at 200 cm both with eyes open and eyes covered (evaluation of RQ). Experiment 2, six conditions: fixation at 40 cm and at 200 cm, with both eyes viewing or under monocular vision (dominant and non-dominant eye). For convergent strabismus, the groups mean value of RQ was 1.3 at near and 0.94 at far distance; for divergent, it was 1.06 at near and 1.68 at far. For all children, the surface of body sway was significantly smaller under both eyes viewing than monocular viewing (either eye). Increased RQ value at near for convergent and at far for divergent strabismus is attributed to the influence of the default strabismus angle and to better use of ocular motor signals. Vision with the two eyes improves postural control for both viewing distances and for both types of strabismus. Such benefit can be due to complementary mechanisms: larger visual field, better quality of fixation and vergence angle due to the use of visual inputs from both eyes.

Purpose In healthy subjects, the postural stability in orthostatic position is better when fixating at near than at far distance; increased convergence angle contributes to such effect. Children with strabismus present a deficit in vergence movements. The goal of this study was to evaluate the postural control in children in relation with the vergence angle while fixating at different depths, and also while making active vergence movements. Methods A TechnoConcept platform (Céreste, France) was used to record postural stability of 11 subjects (M= 11.18 years) with convergent strabismus and 13 (M=11.31 years) with divergent strabismus in 3 conditions: fixation at 40cm, at 2m and active vergence movement between two target at 20cm and 50cm. Results As for the controls, the standard deviation of medio‐lateral body sway decreased with proximity for convergent strabismus but increased for divergent strabismus. Relative to fixation, vergence eye movements caused an increase of medio‐lateral body sway for convergent strabismus but decrease for divergent strabismus. Yet, vergence eye movements were associated with the least variance of speed, indicating that less energy is needed to control body sway. Conclusion We conclude that the fixation depth at which postural stability is best in strabismus is that for which the appropriate vergence angle correspond to the strabismus angle. It might be mediated by preponderant eye movement signals related to the angle of strabismus. Reduction of variance of speed in the active vergence eye movement condition corroborates further such interpretation.

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