Vestibular inputs tonically activate the anti-gravitative leg muscles during normal standing in humans, and visual information and proprioceptive inputs from the legs are very sensitive sensory loops for body sway control. This study investigated the postural control in a homogeneous population of 50 unilateral vestibular-deficient patients (Ménière's disease patients). It analyzed the postural deficits of the patients before and after surgical treatment (unilateral vestibular neurotomy) of their diseases and it focused on the visual contribution to the fine regulation of body sway. Static posturographic recordings on a stable force-plate were done with patients with eyes open (EO) and eyes closed (EC). Body sway and visual stabilization of posture were evaluated by computing sway area with and without vision and by calculating the percentage difference of sway between EC and EO conditions. Ménière's patients were examined when asymptomatic, 1 day before unilateral vestibular neurotomy, and during the time-course of recovery (1 week, 2 weeks, 1 month, 3 months, and 1 year). Data from the patients were compared with those recorded in 26 healthy, age- and sex-matched participants. Patients before neurotomy exhibited significantly greater sway area than controls with both EO (+52%) and EC (+93%). Healthy participants and Ménière's patients, however, displayed two different behaviors with EC. In both populations, 54% of the subjects significantly increased their body sway upon eye closure, whereas 46% exhibited no change or significantly swayed less without vision. This was statistically confirmed by the cluster analysis, which clearly split the controls and the patients into two well-identified subgroups, relying heavily on vision (visual strategy, V) or not (non-visual strategy, NV). The percentage difference of sway averaged +36.7%+/-10.9% and -6.2%+/-16.5% for the V and NV controls, respectively; +45.9%+/-16.8% and -4.2%+/-14.9% for the V and NV patients, respectively. These two distinct V and NV strategies seemed consistent over time in individual subjects. Body sway area was strongly increased in all patients with EO early after neurotomy (1 and 2 weeks) and regained preoperative values later on. In contrast, sway area as well as the percentage difference of sway were differently modified in the two subgroups of patients with EC during the early stage of recovery. The NV patients swayed more, whereas the V patients swayed less without vision. This surprising finding, indicating that patients switched strategies with respect to their preoperative behavior, was consistently observed in 45 out of the 50 Ménière's patients during the whole postoperative period, up to 1 year. We concluded that there is a differential weighting of visual inputs for the fine regulation of posture in both healthy participants and Ménière's patients before surgical treatment. This differential weighting was correlated neither with age or sex factors, nor with the clinical variables at our disposal in the patients. It can be accounted fo...

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The aim of the study was to analyse changes in the orientation and stabilization of the head and trunk and their recovery after complete unilateral loss of vestibular information in humans. The ability of nine Ménière's patients to orient and stabilize their heads and trunks in space was investigated during a simple dynamic task of knee-bends and compared with the performance of 10 healthy subjects. Patients' performance was recorded before unilateral vestibular neurotomy (UVN) and during the time-course of recovery (1 week, 1 month, 3 months). Experiments were performed both in eyes open (EO) and eyes closed (EC) conditions to evaluate the role of visual cues in the recovery process. Head and trunk mean angular position (orientation) and mean maximal angular rotation (stabilization) in the roll plane and the yaw plane were recorded using a video motion analysis system. The results indicate that, in the acute stage after UVN (1 week), patients exhibit marked impairments in head and trunk orientation in both visual conditions. In the EC condition, head and trunk were deviated towards the operated side in the roll plane and the yaw plane. Head and trunk stabilization in space was impaired in the roll plane and associated with increased stabilization of the head on the shoulders. Interestingly, vision caused a complete inversion of the orientation pattern, with head and trunk rotations towards the intact side in the roll plane and the yaw plane. Relative to darkness, vision also reduced head and trunk oscillations. Recovery from abnormal head orientation in the light and impaired head stability in both visual conditions was achieved within 1 month and 3 months after UVN, respectively. However, head and trunk orientation in the dark and trunk stabilization in the roll plane remained uncompensated 3 months post-lesion. These results suggest that unilateral vestibular loss leads to a postural syndrome similar to that described previously for various animal species. They confirm the necessity of vestibular inputs for properly stabilizing head and trunk during self-generated displacements in healthy subjects. They also support the notion that vestibular compensation relies on visual cues whose substitution role gradually decreases after UVN.