Somatosensory loss increases vestibulospinal sensitivity

To determine whether subjects with somatosensory loss show a compensatory i ncrease in sensitivity to vestibular stimulation, we compared the amplitude of postural lean in response to four different intensities of bipolar galv anic stimulation in subjects with diabetic peripheral neuropathy (PNP) and age-matched control subjects. To determine whether healthy and neuropathic subjects show similar increases in sensitivity to galvanic vestibular stimu lation when standing on unstable surfaces, both groups were exposed to galv anic stimulation while standing on a compliant foam surface. In these exper iments, a 3-s pulse of galvanic current was administered to subjects standi ng with eyes closed and their heads turned toward one shoulder (anodal curr ent on the forward mastoid). Anterior body tilt, as measured by center of f oot pressure (CoP), increased proportionately with increasing galvanic vest ibular stimulation intensity for all subjects. Subjects with peripheral neu ropathy showed larger forward CoP displacement in response to galvanic stim ulation than control subjects. The largest differences between neuropathy a nd control subjects were at the highest galvanic intensities, indicating an increased sensitivity to vestibular stimulation. Neuropathy subjects showe d a larger increase in sensitivity to vestibular stimulation when standing on compliant foam than control subjects. The effect of galvanic stimulation was larger on the movement of the trunk segment in space than on the body' s center of mass (CoM) angle, suggesting that the vestibular system acts to control trunk orientation rather than to control whole body posture. This study provides evidence for an increase in the sensitivity of the postural control system to vestibular stimulation when somatosensory information fro m the surface is disrupted either by peripheral neuropathy or by standing o n an unstable surface. Simulations from a simple model of postural orientat ion incorporating feedback from the vestibular and somatosensory systems su ggest that the increase in body lean in response to galvanic current in sub jects with neuropathy could be reproduced only if central vestibular gain w as increased when peripheral somatosensory gain was decreased. The larger e ffects of galvanic vestibular stimulation on the trunk than on the body's C oM suggest that the vestibular system may act to control postural orientati on via control of the trunk in space.