Processing vestibular, proprioceptive and visual information for balance control in pure cerebellar disease (SCA6)

BACKGROUND AND AIM: Balance impairment is a feature of cerebellar disease [1-2], but causal mechanisms are largely unknown. SCA6 is a prevalent type of inherited cerebellar disease, rarely accompanied by any extra-cerebellar signs or symptoms [3]. This study evaluates the processing of individual sensory channels for the control of standing balance in subjects with SCA6 using single-channel balance perturbations. METHODS: Whole body balance responses to sensory perturbations while standing were evoked in 16 subjects with SCA6 and compared with those from 16 age-, sex- and height-matched healthy controls. Galvanic vestibular stimulation (GVS), muscle vibration (VIB) and moving visual scenery (MVS) were used to perturb vestibular, proprioceptive and visual sensory channels respectively. Twenty trials of each stimulus were randomised and intermixed with additional no-stimulation trials to produce unpredictable perturbations. 3D whole-body motion (Coda) and ground reaction forces (Kistler) were recorded throughout each eight-second trial period. Ten trial repeats per direction, per modality were used to assess response habituation and to generate mean traces of response activity over time in two cardinal directions (pitch and roll). From the mean traces, trunk sway responses (0.2-1.0s) and shear-force responses (0.2-0.4s) were measured and groups compared using t-tests. ANOVAs evaluated habituation of responses (group x trial repeat) measured from single trials. RESULTS: All subjects generated clearly identifiable responses to all stimuli. Response directions were normal in the SCA6 group for all perturbation modalities. Response magnitude to GVS did not differ significantly between groups (force, p=0.964; trunk sway, p=0.142). Response magnitude to VIB was larger in SCA6 for trunk sway but not force (force, p=0.169; trunk sway, p=0.020). Response magnitude to MVS was highly significantly greater in SCA6 for both measures (force, p=0.002; trunk sway, p<0.001). Only the visual perturbation revealed a significant positive correlation between SCA6 response magnitude and disease severity, rated using the SARA score (MVS: r=0.536, p=0.018; GVS: r=0.021, p=0.940; VIB: r=0.378, p=0.149). Habituation to the stimuli was not found for any sensory modality in either group. CONCLUSION: In SCA6 the balance responses to visual perturbations were abnormally large, scaled with disease severity, and were not caused by habituation problems. The increased response magnitude could indicate an increased reliance on vision as compensation for a deficit elsewhere. Alternatively, it could point to a problem in visual processing for balance control. One possibility is that it reflects impairment in distinguishing self-motion from environmental motion which could lead to instability, particularly in busy environments.