Objective: To study the role of spinal repMND in the paired-pulsed (PP) paradigms of SICI and ICF by combining PP with TMS, TST, QuadS and QuintS techniques and to explore their variability.
Background: Transcranial magnetic stimulation (TMS) leads to repetitive spinal motor neuron discharges (repMNDs). The paired-pulse TMS (PP-TMS) paradigm allows the exploration of the motor cortex physiology. The triple stimulation technique (TST) and an extended TST-technique including a 4th and 5th stimulation, Quadruple (QuadS) and Quintuple (QuintS) stimulation, respectively, allow a more precise exploration of the central motor conduction and of repMND.
Design/Methods: We explored the PP TMS paradigms of short intracortical inhibition (SICI) with an inter-stimulus interval (ISI) of 2ms and intracortical facilitation (ICF) with an ISI of 10ms in the conventional way (TMS), combined with the TST, and with the QuadS and QuintS in a randomized design in 20 healthy volunteers
Results: About half of the subjects have repMND following a single pulse TMS in the QuadS and QuintS condition (60% and 40%, respectively) and generally more in the QuadS than in the QuintS condition. In both the QuadS and the QuintS, there appear more repMND in the PP-TMS paradigm of ICF than with a single pulse TMS and less than latter in SICI. The variability differs considerably between subjects, but combining the PP paradigms with the TST reduced variability of SICI by -27% but not for ICF or SP. There were subjects showing inhibition and facilitation when the opposite was expected.
Conclusions: These results suggest that there is a contribution of repMND in the conditioned responses of PP-TMS. The large variability precludes their utility in clinical practice. This needs to be further explored.