Altered dorsal premotor-motor interhemispheric pathway activity in focal arm dystonia

Given the possible role of dorsal premotor cortex(PMd) in the pathophysiology of dystonia, we used transcranialmagnetic stimulation (TMS) methods to study PMd and PMd–primary motor cortex (M1) interactions in patients with focalarm dystonia. Here, we tested the connectivity between leftPMd and right M1 as well as the intracortical excitability ofPMd in 11 right-handed patients with focal arm/hand dystoniaand nine age-matched healthy controls. The results showed thatexcitability of the inhibitory connection between PMd and M1was reduced in patients, but there was no significant differenceto healthy subjects in the excitability of the facilitatory con-nection. A triple stimulation technique in which pairs of TMSpulses are given over PMd and their interaction measured interms of the effect on the baseline PMd-M1 connection failedto reveal the usual pattern of interaction between the pairs ofPMd stimuli. Indeed, the results in patients were similar tothose seen in a group of young healthy subjects after theexcitability of PMd had been changed by pretreatment withhigh-frequency rTMS. We suggest that reduced transcallosalinhibition from the PMd may be involved in the altered patternof abnormal muscle contractions of agonists and antagonists(overflow)