Balance impairment in cerebellar disease: Can it be explained by disrupted sensory processing?

Purpose: Balance impairment is a common feature and often the presenting symptom for those with pure types of cerebellar disease. This study seeks an improved understanding of sensory mechanisms responsible for balance impairment in cerebellar disease. Relevance: If sensory processing for balance control is in some way impaired by cerebellar disease, knowledge of these impairments will facilitate future development of targeted balance therapies. Participants: Seventeen subjects with cerebellar disease (SCA6) and seventeen age, sex, height and weight matched healthy controls were recruited. Persons with genetically diagnosed SCA6 in the UK population were invited to participate via support group advertisements and invitations sent out to those known to the London specialist Ataxia centre. Subjects were required to be independently mobile (+/- walking sticks) and able to stand with their eyes closed for ten seconds. Subjects were only included if they had no known co-pathologies or past history of conditions that could affect balance. Methods: Whole body kinematics and ground reaction forces were collected, using integrated Codamotion and Kistler force plate technologies. Unperturbed balance was initially quantified over a range of stance widths and then isolated sensory system stimuli were used to perturb balance in standing subjects. Galvanic vestibular stimulation, muscle vibration and moving visual scenery modalities were used to drive isolated balance perturbations via vestibular, proprioceptive and visual channels, respectively. Two conditions per modality were designed to generate perturbations in two opposing directions. Ten randomised trial repeats were used per sensory condition during which whole body responses were collected. Same session baseline measures of disease severity and balance impairment were collected. Analysis: Measures of standing balance and responses to sensory perturbations were analysed using parametric techniques. T-tests and ANOVAs were employed to statistically analyse effects of group and parameters including stance width and perturbation direction. SCA6 response measures were compared with baseline measures using Pearson's correlation coefficients in order to assess the association between response characteristics and disease progression. Results: SCA6 unperturbed standing sway speeds were significantly increased across all stance widths investigated (p<0.05). Responses to vestibular and proprioceptive perturbations were largely normal. No significant group differences were reported for early response measures based on forces and whole body sway. Responses to moving visual scenery were delayed by 100ms and widespread significant increases in SCA6 response magnitudes were reported. Whole body sway response magnitudes and measures of centre-of-pressure magnitude in response to moving visual scenery significantly correlated with measures of disease severity (p<0.05). Conclusions: Processing of visual signals for balance control appears to be significant disrupted in persons with SCA6. Further work is now required to establish a better understanding of the patho-physiological mechanisms responsible for visual processing impairments. Implications: Therapies that aim at increasing the role of intact proprioceptive and vestibular systems in balance control may be effective means of improving balance in persons with SCA6. These findings could be relevant to other populations of cerebellar disease with similar regions of cerebellar damage to that of SCA6.