The influence of posture on the pattern of whole-body instability in freely-standing subjects with SCA6

Spino-cerebellar ataxia type 6 (SCA6) is a genetically identifiable cerebellar ataxia with relatively ‘pure’ pathology i.e. localised to the cerebellum. It is clinically characterised by impaired balance and ataxia of movement, frequently causing falls [1]. The balance control processes affected by the disease are unknown. As an important first step in understanding these processes, we have studied the pattern of whole-body instability in freely standing subjects and measured how the pattern is influenced by posture and whether it correlates with clinical scales of disease severity. In particular, we have studied whether the pattern of instability has a directional preponderance and whether it is localised to specific joints of the body. Seventeen subjects with SCA6 were recruited together with a group of seventeen matched healthy controls (HCs). Whole-body measures of posture and balance were collected using Coda 3D-motion capture (Charnwood Dynamics) over forty-second trial durations, whilst subjects stood quietly on a force plate (Kistler) with their eyes open and with a variable stance-width posture (from feet together to 32cm apart). Clusters of infrared-emitting diodes were fixed securely to each of the body segments (head, trunk, pelvis, thighs and feet). Anatomical landmarks were referred to these clusters for off-line analysis of joint angles using Visual3D software (C-Motion). Clinical assessment of disease severity was measured using the Scale for Assessment and Rating of Ataxia (SARA) [2]. Global measures of instability, from motion at the level of C7 and centre of pressure of ground reaction forces, revealed that SCA6 subjects were more unstable than control subjects in both anteroposterior (AP) and mediolateral (ML) directions at all stance widths. For both groups, standing with the feet closer together caused greater instability in both cardinal directions. However, this stance-width effect was disproportionally large in the SCA6 group in the ML direction (group x stance width interaction; p<0.001). Analysis of joint angle motion showed that the excessive instability of the SCA6 subjects when standing with their feet together occurred at all joints but with most angular motion taking place at the ankle joint. A number of measures of whole-body instability were found to correlate with disease severity. We conclude that the stance-width posture of SCA6 subjects is an important determinant of their pattern of standing instability and helps to explain why many subjects spontaneously adopt a wide-based posture. Correlations between quantitative stability measures and the SARA indicate that this score could be used not only to monitor overall disease severity but also as a balance impairment predictor. These findings will facilitate further investigation concerning the pathophysiology of balance control in SCA6 as well as contributing towards the assessment of future targeted therapies. [1] van de Warrenburg,B.P. et al., Falls in degenerative cerebellar ataxias. Movement Disorders. 2005; 20(4):497-500 [2] Schmidtz-Hubsch T. et al., Scale for the assessment and rating of ataxia: Development of a new clinical scale. Neurology. 2006; 66: 1717-1720