Focal hand dystonia: lack of evidence for abnormality of motor representation at rest.

OBJECTIVE

To examine whether the corticospinal system emanating from the primary motor cortex may be organized to facilitate generation of dystonic movements.

METHODS

In this cross-sectional observational study, finger movement (FM) representations were assessed in 10 patients with focal hand dystonia (FHD) and 10 matched healthy controls by transcranial magnetic stimulation during rest. Evoked finger movements of the right hand were recorded using an instrumented data glove. Patterns of finger joint movements were analyzed using cluster analysis. Principal component analysis and centers of gravity for finger movement representations and motor evoked potentials recorded from the abductor pollicis brevis and abductor digiti minimi muscles were computed. For comparison, high-resolution somatosensory evoked potentials (SSEP) were recorded after electrical stimulation of the thumb (D1) or little finger (D5) in the same patients. Source reconstruction for the N20 SSEP component was performed using a dual-dipole model.

RESULTS

Stimulation of the resting motor cortex did not reveal overt abnormalities in FHD, neither with respect to finger joint movement patterns nor with respect to the topologic organization of finger movements or intrinsic hand muscle representations. However, in line with previous reports, the distance between the dipole sources of D1 and D5 in the somatosensory cortex (S1) was smaller in patients with FHD, suggesting disruption of homuncular finger representations in S1.

CONCLUSIONS

Our findings may imply that abnormality of motor organization in focal hand dystonia arises principally only during activation, when abnormal somatosensory representations are functionally integrated.