Disrupt of anticipatory postural adjustments in cerebellar ataxia

Each voluntary movement is known to induce postural perturbations which are counteracted by anticipatory postural adjustments (APAs): i.e. unconscious feed-forward muscular activities, aimed at building up a muscular chain toward an available fixation points (Massion, 1992). The importance of a proper whole-body stabilisation is apparent when considering standing subjects performing intentional movements involving large masses; in these motor acts, the equilibrium-perturbing forces would cause a whole-body equilibrium disturbance, by displacing both the trunk and the whole-body CoM (Bouisset and Do, 2008). On the other hand, the importance of an accurate segmental stabilisation may look less obvious in those tasks in which the whole-body equilibrium is not threatened, such an index finger flexion. It has been reported that an APA chain also develops in several upper-limb muscles when the index finger is flexed. In this case the prime mover Flexor Digitorum Superficialis (FDS) is preceded by an excitatory burst in Triceps Brachii (TB) and the tonically active Biceps Brachii (BB) and Anterior Deltoid (AD) are carved by inhibitory APAs (Caronni and Cavallari, 2009a). In the present study, APAs and prime mover activation were recorded in seven patients with cerebellar degeneration and on an equal number of healthy subjects. All patients suffered from a slowly progressive cerebellar syndrome, without involvement of the sensory and motor systems. This was witnessed by a standard 1.5-T brain MRIs showing cerebellar atrophy without cerebral cortex, pons, medulla or white matter lesions. Two cases were sporadic, five had a positive family history for autosomal dominant cerebellar ataxia type III. Meann onset age was 31±14 years while age at examination was 52 ±13. All patients presented pure gait ataxia, four-limb dysmetria, mild dysarthria, and occasionally mild increase of deep tendon reflexes, without spasticity. Cognition was normal. Scale for the Assessment and Rating of Ataxia was applied to all patients. The pattern of EMG activities was similar between the two groups, while modifications in the timing of EMG onsets were observed in cerebellar patients, in which no anticipatory activity could be observed in postural muscles. Both inhibition in BB and AD and excitation in TB lagged the onset of the prime mover. Although the role of the cerebellum in the genesis or in the transmission of the APAs still remains open, these data supports the idea that both the prime mover onset and the temporal distribution of the related APAs are set by the “cerebellar timing machine”.