Pseudo-bilateral hand motor responses evoked by transcranial magnetic stimulation in patients with deep brain stimulators

Objectives: In 3 of 5 patients with dystonia and bilaterally implanted deep brain stimulating electrodes, focal transcranial magnetic stimulation (TMS) of one motor cortex elicited bilateral hand motor responses. The aim of this study was to clarify the origin of these ipsilateral responses. Methods: TMS and electrical stimulation of corticospinal fibres by the implanted electrodes were performed and the evoked hand motor potentials were analysed. Results: In comparison with responses elicited by contralateral motor cortex stimulation, ipsilateral responses were smaller in amplitude (3.0±1.4 versus 5.8±1.5 mV), had shorter peak latencies (first negative peak: 20.9±0.8 versus 25.1±0.4 ms) and were followed by a shorter-lasting silent period (46±4 versus 195±35 ms). Ipsilateral responses following TMS had similar peak latencies to responses elicited subcortically by deep brain stimulation (DBS) (20.4±0.9 ms). Conclusions: Hand motor responses ipsilateral to TMS result from a subcortical activation of corticospinal fibres, via the implanted electrode in the other hemisphere, secondary to currents induced by TMS in subcutaneous wire loops that underlie the magnetic coil. Studies of TMS in patients with DBS have to take this potential source of confounding into account.