Abstract
Posture control is based on central integration of multisensory inputs, and on internal representation of body orientation in space. This multisensory feedback regulates posture control and continuously updates the internal model of body's position which in turn forwards motor commands adapted to the environmental context and constraints. The peripheral localization of the vestibular system, close to the cochlea, makes vestibular damage possible following cochlear implant (CI) surgery. Impaired vestibular function in CI patients, if any, may have a strong impact on posture stability. The simple postural task of quiet standing is generally paired with cognitive activity in most day life conditions, leading therefore to competition for attentional resources in dual-tasking, and increased risk of fall particularly in patients with impaired vestibular function. This study was aimed at evaluating the effects of postlingual cochlear implantation on posture control in adult deaf patients. Possible impairment of vestibular function was assessed by comparing the postural performance of patients to that of age-matched healthy subjects during a simple postural task performed in static (stable platform) and dynamic (platform in translation) conditions, and during dual-tasking with a visual or auditory memory task. Postural tests were done in eyes open (EO) and eyes closed (EC) conditions, with the CI activated (ON) or not (OFF). Results showed that the postural performance of the CI patients strongly differed from the controls, mainly in the EC condition. The CI patients showed significantly reduced limits of stability and increased postural instability in static conditions. In dynamic conditions, they spent considerably more energy to maintain equilibrium, and their head was stabilized neither in space nor on trunk: they behaved dynamically without vision like an inverted pendulum while the controls showed a whole body rigidification strategy. Hearing (prosthesis on) as well as dual-tasking did not really improve the dynamic postural performance of the CI patients. We conclude that CI patients become strongly visual dependent mainly in challenging postural conditions, a result they have to be awarded of particularly when getting older.
Highlights
The peripheral vestibular system is made of angular and linear sensors providing the brain with sensory signals about three-dimensional head rotations and translations
Postural Instability Index (PII) calculated from the 3D posturographic map of the center of pressure recordings obtained with the wavelet analysis
Recordings were made in subjects standing quietly on the platform with (EO) or without (EC) vision, with (ON) or without (OFF) hearing, and during dual-tasking (DT) with a concomitant cognitive task consisting of a visual or an auditory memory task
Summary
The peripheral vestibular system is made of angular (semicircular canals) and linear (otoliths) sensors providing the brain with sensory signals about three-dimensional head rotations and translations. The vestibular signals project to the vestibular nuclei, which in turn project to the spinal cord, the cerebellum, the thalamus, the parieto-insular vestibular cortex and related cortical areas processing the vestibular input These descending and ascending pathways are known for their functional role in posture control and equilibrium function, gaze stabilization, self-motion perception, and spatial navigation (see Lopez and Blanke, 2011, for review). Very uncommon loss of vestibular function (one patient among eleven) was shown with the head impulse test, a physiologically relevant stimulation able to detect subtle changes in the functioning of individual semicircular canals (Migliaccio et al, 2005) Significant improvement of both static (Kluenter et al, 2010) and dynamic (Buchman et al, 2004) balance were even observed in CI patients, and significant increase in vestibular responsiveness was noted in CI patients during air-caloric stimulation (Ribári et al, 1999)
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