Effects of perceptible and imperceptible galvanic vestibular stimulation on the postural control of patients with bilateral vestibulopathy

Galvanic vestibular stimulation (GVS) has increasingly been used to stimulate the vestibular system in health and disease. While perceptible supra-threshold GVS destabilizes postural control in healthy control (HC) subjects, imperceptible 'noisy' GVS (nGVS) is reported to improve postural control in patients with bilateral vestibulopathy (BV) and therapeutic devices using nGVS are currently under development. We questioned (1) whether perceptible GVS destabilizes postural control of BV patients, expecting any effect to be smaller than in healthy subjects due to the patients' vestibulopathy, and (2) whether imperceptible nGVS improves postural control in comparison to an active sham stimulus in context-dependent conditions, hypothesizing that it fades off once postural control becomes more challenging with respect to its sensory (standing on foam) or cognitive (dual task) complexity. We tested postural responses of 30 BV patients to bimastoidal perceptible (lowGVS, highGVS) or imperceptible (nGVS, sham, noGVS) GVS in comparison to 24 age-matched HC. Perceptible GVS intensities were applied according to the participants' individual motion perception thresholds. Postural sway speed (PSS) was analyzed in a 4-factorial experimental design with the factors group (BV, HC), vision (eyes open/closed), condition (baseline, proprioception, cognition) and stimulation (noGVS, sham, nGVS, lowGVS, highGVS). With eyes open (EO), there were no group-related PSS differences in the baseline and cognition condition in response to either stimulations. With EO on foam and with eye closed (EC) in all conditions, patients showed larger PSS than HC, irrespective of the stimulation type. PSS differed with GVS intensities within each group but not between the groups. PSS under nGVS on EC was only smaller in patients when compared to perceptible GVS, but it was not different from noGVS or sham stimulation. Moreover, this nGVS effect was only found in the baseline but not in the more challenging dual task and foam condition. Almost half of the patients showed higher individual thresholds of motion perception of GVS compared to HC. Interestingly, this high-threshold subgroup showed significantly larger PSS with EC as compared to HC and the low-threshold patient subgroup, although both patient subgroups did not differ in vestibular parameters. We conclude, first, that perceptible GVS is able to destabilize BV patients similarly to HC subjects, suggesting sufficient vestibular afferent processing of GVS during vestibulo-spinal postural control. Second, the effect of the hitherto observed improved postural control by nGVS appears to be small during more demanding postural control conditions (foam, cognitive distraction) that are closer to the patients' everyday life, when active sham stimuli are used as control stimuli. These findings underline the meaning of active control conditions when the efficacy of nGVS is tested, e.g. in portable GVS devices in the attempt to improve postural control in BV patients. However, differential GVS effects on vestibulo-perceptional and vestibulo-spinal thresholds should be taken into account. Finally, our data suggest that individual motion perception thresholds for GVS could potentially serve as a predictor of postural control safety and falling risk in BV.