Impact of repetitive theta burst stimulation on motor cortex excitability

Theta burst stimulation (TBS) alters cortical excitability in inhibitory or facilitatory directions depending on the pattern of stimulation used. Although continuous TBS (cTBS) decreases motor cortex excitability, intermittent TBS (iTBS) increases excitability by introducing an 8-second stimulation interval after 2 seconds of TBS. The after-effects induced by TBS last from 30 minutes up to 1 hour. Optimization of TBS techniques might be possible through manipulation of a variety of parameters such as number of pulses, stimulus intensity, duration of stimulation, and repetitive stimulation. The aim of this study was to assess the after-effects induced by introducing an interval between two TBS interventions to identify more efficient protocols. The study was divided in two groups, iTBS protocols and cTBS protocols, each of them with four sessions: classical TBS, TBS - 2 minutes - TBS, TBS - 5 minutes - TBS, TBS - 20 minutes - TBS. Our results show that cTBS - 20 minutes - cTBS and iTBS - 2 minutes - iTBS resulted in similar after-effects as those accomplished by a single TBS session, whereas a suppression of after-effects was observed in the other break durations. Repeated TBS with short break durations does not seem to be suitable to prolong the duration of excitability changes accomplished by single TBS. These results might be relevant for clinical applications of TBS, when long-lasting excitability alterations are needed.