Stimulus-response properties of motor system in patients with cerebellar ataxia

The aim of the study was to examine the stimulus-response properties of the excitatory and inhibitory components of corticospinal projections at rest and during voluntary contraction in cerebellar patients.

We investigated motor evoked potential (MEP) and cortical silent period recruitment curves in response to increasing intensities of transcranial magnetic stimulation in 8 patients with 'pure' cerebellar syndromes and in 14 age-matched controls. The transcranial magnetic stimulation intensity was increased from 90 to 180% of the resting motor threshold. MEP recruitment curves were recorded at rest and during voluntary contraction in the right abductor pollicis brevis muscle.

No statistical differences were found between patients and controls in MEP recruitment curves in either the resting or active condition. A significant difference was found between patients and controls in the cortical silent period threshold (patients: 33.2+/-3.4% of maximal stimulator output; controls 39.4+/-3.2%; P=0.01) and recruitment curve, the duration of the cortical silent period being longer in patients at transcranial magnetic stimulation intensities ranging from 90 to 130% of the resting motor threshold (patients: 135-191 ms; controls: 53-158 ms). No changes were found in the silent period evoked by peripheral nerve stimulation.

Inhibitory components of corticospinal projections were recruited with a lower threshold in patients. No abnormalities were found in the recruitment of the excitatory networks. Our data show a prevalence of inhibitory phenomena in the motor cortex of cerebellar patients. These findings would appear to be specific to cerebellar diseases and are the opposite of those previously documented in movement disorders such as dystonia and Parkinson's disease. Our results suggest that the cerebellum and the basal ganglia may counteract each other in modulating the level of motor system excitability.