Abnormal acute changes in upper limb muscle cortical representation areas in the patients with writer's cramp during co-activation of distal and proximal muscles

Aim We analysed cortical muscle representation areas during single muscle activation and during the co-activation of several upper arm muscles in the patients with writer's cramp to determine the possible occurrence of abnormal dynamic somatotopic changes in M1, in addition to the static map abnormalities already described in this form of dystonia. Methods Using transcranial magnetic stimulation, we assessed cortical representations of medial deltoid, extensor carpi radialis and the first dorsal interosseus muscles in eight patients with writer's cramp and in eight healthy control subjects. Cortical maps were obtained during distal muscles' activation either in isolation or in conjunction with voluntary medial deltoid co-activation. Results This study showed a difference in the organization of cortical representations of these muscles between the patients with dystonia and control subjects. The first dorsal interosseus and the extensor carpi radialis cortical representation areas were larger in the dystonic group. The cortical representations became larger when the medial deltoid was simultaneously co-activated, and this effect was not observed in the control group. In the dystonic group, the three cortical muscle representations largely overlapped and their centres of gravity were closer. Conclusion Patients with dystonia showed not only a different spatial organization of muscle cortical representation areas, but also abnormal acute somatotopic changes during proximal muscle co-activation. Task-specific motor impairment in writer's cramp may result not only from lack of cortical inhibition and the well-known anomalous cortical organization observed in these patients, but also from abnormal patterns of proximo-distal functional muscle coupling.