Effects of Galvanic Vestibular Stimulation on Visual Verticality and Standing Posture Differ Based on the Polarity of the Stimulation and Hemispheric Lesion Side in Patients With Stroke

Abstract

Introduction: There is growing evidence supporting the relationship of vertical misperception and poor balance control with asymmetrical standing posture in patients with stroke. Although the vestibular system has been shown to be responsible for vertical misperception and balance disorders, the effect of galvanic vestibular stimulation (GVS) on both vertical misperception and postural asymmetry after stroke remains elusive. The aim of this study was to investigate the effects of GVS on visual verticality and postural asymmetry after stroke and to clarify whether the effects differ depending on the polarity of the stimulation and hemispheric lesion side. Methods: We measured the subjective visual vertical (SVV) and body weight distribution on each foot in an upright stance in 24 patients with a hemispheric stroke (10 with a left hemisphere lesion and 14 with a right hemisphere lesion) and nine age-matched healthy controls. During the measurements, bipolar GVS (1.5 mA) was applied over the bilateral mastoid processes in three stimulation conditions: contralesional-anodal and ipsilesional-cathodal vestibular stimulation, ipsilesional-anodal and contralesional-cathodal vestibular stimulation, and no stimulation. To examine whether GVS modulates visual verticality and standing posture, SVV and weight-bearing in the three conditions were analyzed. Results: During no stimulation, the SVV deviated to the contralesional side in patients with a right hemisphere lesion, while more weight-bearing was observed on the ipsilesional limb than on the contralesional limb in both patient groups than in the controls. The SVV was modulated by reversing the polarity of GVS in all the groups when the cathodal stimulus side was either ipsilateral or contralateral to the lesion while the ipsilesional-cathodal vestibular stimulation reduced weight-bearing asymmetry in only the patients with a right hemisphere lesion. Conclusions: These findings demonstrate that the effects of GVS on the SVV and standing posture differ depending on the polarity of GVS and the hemispheric lesion side. Patients with a right hemisphere lesion have difficulty maintaining their preferred standing posture under visual verticality modulation evoked by GVS. The application of GVS may clarify whether the vestibular system has neural redundancy after stroke to suppress any effects of the stimulation, including modulation of the visual verticality, on balance.

Keywords: cerebrovascular disorder; hemiparesis; postural balance; subjective visual vertical; vestibular control.

Figure 1

Effects of bipolar galvanic vestibular stimulation on the visual verticality in the controls, patients with a left hemisphere lesion, and patients with a right hemisphere lesion. Each color bar indicates the mean of the subjective visual vertical (SVV) during each type of bipolar galvanic vestibular stimulation (GVS). Positive values indicate rotations to the right side (for the controls) and the ipsilesional side (for the patients with stroke). Each error bar indicates a standard deviation. Each gray line indicates changes in the SVV during each type of bipolar GVS in each participant. NS, no stimulation; rtVS, left-anodal and right-cathodal vestibular stimulation; ltVS, right-anodal and left-cathodal vestibular stimulation; ipsiVS, contralesional-anodal and ipsilesional-cathodal vestibular stimulation; contraVS, ipsilesional-anodal and contralesional-cathodal vestibular stimulation. *p < 0.05; ^†p < 0.01.

Figure 2

Effects of bipolar galvanic vestibular stimulation on weight-bearing in the controls, patients with a left hemisphere lesion, and patients with a right hemisphere lesion. Each color bar indicates the mean weight-bearing ratio (WBR) during each type of bipolar galvanic vestibular stimulation (GVS). More than 50% of WBR indicates more weight-bearing on the right side (for the controls) and the ipsilesional side (for the patients with stroke). Each error bar indicates a standard deviation. Each gray line indicates changes in the WBR during each type of bipolar GVS in each participant. NS, no stimulation; rtVS, left-anodal and right-cathodal vestibular stimulation; ltVS, right-anodal and left-cathodal vestibular stimulation; ipsiVS, contralesional-anodal and ipsilesional-cathodal vestibular stimulation; contraVS, ipsilesional-anodal and contralesional-cathodal vestibular stimulation. *p < 0.05; ^†p < 0.01.