Interventional repetitive I-wave transcranial magnetic stimulation (TMS): the dimension of stimulation duration.

Background: A range of transcranial magnetic stimulation (TMS) techniques are now available to modulate human corticomotor excitability and plasticity. One presumably critical aspect of these interventions is their duration of application.

Objective: In the current study, we investigated whether doubling the duration of an intervention would offer any additional benefit, or invoke self-limiting mechanisms controlling corticomotor excitability or synaptic plasticity.

Methods: We compared (in a cross-over design) corticomotor excitability (to the first dorsal interosseous muscle) during and after a 15-minute (I15) and 30-minute (I30) TMS intervention targeting indirect (I-) wave interaction (iTMS). The interventions consisted of equi-intensity paired stimuli with an interpulse interval (IPI) of 1.5 milliseconds, corresponding to I-wave periodicity, delivered at a frequency of 0.2 Hz.

Results: During both the I15 and I30 interventions, paired-pulse (I-wave) motor evoked potential (iMEP) amplitude significantly increased (by 98.3% and 120.6%, respectively, last versus first minute, P = .001). The increase for I30 occurred in the first 15 minutes, and there was no further change during the remainder of the intervention. Both interventions were equally effective overall. Postintervention, single-pulse MEP amplitude increased by a mean of 91% and 106% (I15 and I30, respectively, P < .01) with no significant difference between interventions.

Conclusions: We conclude that repetitive iTMS can increase corticomotor excitability after a relatively short intervention period of stimulation, and that a longer stimulation period has no additional benefit or detriment, perhaps as a result of the action of regulatory mechanisms.