Postural Control in Nonamblyopic Children with Early-Onset Strabismus

Abstract

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...

Vergence and Standing Balance in Subjects with Idiopathic Bilateral Loss of Vestibular Function

“…The results suggest that sustained convergence and active vergence eye movements are both beneficial for postural regulation in patients; controls studied here showed similar benefit. The results are in line with those previously reported by Kapoula and Bucci [42] for dyslexic children, and Gaertner et al [43] in strabismus children. The hypothetical mechanism of action of vergence on posture has been discussed elsewhere [42].…”

Postural Patterns of the Subjects with Vergence Disorders: Impact of Orthoptic Re-education, a Pilot Study

“…Shortening vergence latency could help better integration of visual vestibular and somesthetic subtending postural control. This interpretation is in line with prior studies showing that vergence eye movements are important for postural control (Gaertner et al 2013;Matheron et al 2016).…”

Controlling Posture and Vergence Eye Movements in Quiet Stance: Effects of Thin Plantar Inserts

“…Our second hypothesis (hyp.2) concerns oculomotor control: there is strong evidence that vergence improves postural control [ 21 – 24 ], while saccades entail small changes in postural control (only a mild decrease of the standard deviation of the antero-posterior sway–see [ 25 ]), hence, we hypothesize that the plantar stimulations will act upon vergence neural networks only. Moreover, saccade is a significantly faster movement than vergence (50 milliseconds v.s 300–400 –see [ 26 ]) and podal inputs need between 100 and 200ms to be treated [ 16 , 27 ], which does not seem compatible with an influence upon saccades.…”