In moderate to severely impaired stroke patients, single pulse TMS, with or without background facilitation, may not be able to evoke a motor response in muscles of the upper extremity, thereby hindering potential studies of stroke patients using TMS. Paired pulse TMS has been shown to facilitate responses in distal muscles of healthy subjects. In this study, our aim was to investigate thirteen muscles of the upper extremity in moderate to severely impaired stroke patients and determine the paired pulse interstimulus interval (ISI) that was optimal for facilitation of the TMS response.
Methods: We recruited 8 moderate to severely impaired stroke patients and 5 healthy controls. A hotspot was found that could activate the greatest number of the 13 target upper extremity muscles. 16 ISIs were tested.
Results: In healthy controls, an ISI range of 3-50 ms in the left hemisphere and 8-40 ms in the right hemisphere was optimal for activating the contralateral arm. In the stroke patients, stimulation of the non-lesioned hemisphere at an ISI of 8-50 ms was optimal for contralateral responses, similar to the control subjects, while stimulation of the lesioned hemisphere had an optimal ISI range of 12-50 ms. Ipsilateral responses in the paretic limb were frequent and the optimal ISI range was much later than the contralateral responses in stroke or controls occurring at 25-40 ms.
Conclusion: In stroke and control subjects, across muscles and contralateral or ipsilateral pathways, an interstimulus interval of 25-40 ms was optimal to evoke a TMS response and resulted in the greatest degree of facilitation.
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