Writer's cramp: cortical excitability in tasks involving proximo-distal coordination

The aim of this work was to analyse how writer's cramp patients coordinate each element of the proximal to distal upper arm muscle chain during voluntary movement.

Using transcranial magnetic stimulation, we have assessed motor cortex excitability properties in patients by recording motor-evoked potentials and silent periods in both the extensor carpi radialis (ECR) and the first dorsal interosseus muscles (FDI), activated either in isolation, or in conjunction with voluntary medial deltoid (MD) co-activation during performance of precise tasks. Ten dystonic patients and ten healthy controls were tested.

In both test groups, the ECR muscle displayed a similar active motor threshold, but the excitability curves reached higher plateau values, when the proximal MD muscle was co-activated. In the dystonic group, the FDI muscle excitability curves reached higher plateau values when the MD was co-activated, whereas co-activation had no effect on the control group. In the control group, silent periods, in both the ECR and the FDI were longer when the MD was co-activated. This effect was not observed in the dystonic group.

In the dystonic group, facilitation of the FDI was observed during a task involving proximo-distal coordination. No differences in silent periods were observed when the muscle was activated alone. Our results suggest that such abnormal facilitation is not only an impairment of the central inhibitory mechanisms reported for dystonic patients, but, in addition, represents true abnormality in cortical muscle activation strategies.