Abstract
Dystonia is a movement disorder associated with the involuntary co-contraction of agonist and antagonist muscles. In focal dystonia (FD), these co-contractions are localized to a specific limb. One mechanism that may underlie FD is an abnormality in surround inhibition – a neural mechanism hypothesized to hone selected movement patterns by inhibiting the activation of muscles that are not directly involved in the selected movement. The purpose of the present study was to investigate whether abnormal inhibitory mechanisms extend beyond the affected limb in FD. To this end, seven participants with cervical dystonia (CD) and seven healthy, age-matched, individuals were asked to perform simple index finger abduction movements. Single-pulse transcranial magnetic stimulation was applied at time intervals from 3-1000ms into the movements and EMG was recorded from the first dorsal interosseous (FDI, the agonist) and from abductor digiti minimi (ADM, an uninvolved muscle). If abnormal surround inhibition occurs outside of the affected limb in CD, then the MEPs in the FDI and ADM will be of similar amplitude in the CD group only. In contrast to this prediction, the results showed that MEP amplitudes of the FDI were significantly greater than those of the ADM early in the movement (<500ms post EMG onset) in both CD and control groups. These results suggest that abnormalities in surround inhibition may not extend beyond the affected limb in people with CD.Acknowledgments: This research was supported by funding from the Natural Sciences and Engineering Research Council of Canada, the Ontario Ministry of Research and Innovation, and the Naitonal Science Foundation.