Enhanced Efficacy of M1 Intermittent Theta Burst Stimulation with Greater Stimulation Intensity

Research Objectives: Intermittent theta burst stimulation (iTBS) is a type of repetitive transcranial magnetic stimulation that can be applied on primary motor cortex and other brain areas. iTBS applied on primary motor cortex can produce a persisting increase in the corticospinal excitability. Although iTBS has been increasingly used in recent years, the inter-individual variability of responses to iTBS protocol is high and the reasons of which remain poorly understood. The objective of this study was to examine how increase in stimulation intensity impacts the efficacy of iTBS and whether it can increase the number of responders – individuals who show increased corticospinal excitability after iTBS.

Design: We designed sham-controlled, single-blinded experiment to test the iTBS aftereffects.

Setting: University research laboratory

Participants: 17 healthy individuals (45±19 years old; 11 males)

Interventions: Participants underwent 3 iTBS sessions in a randomized order; 1) iTBS applied at 80% active motor threshold (AMT), 2) iTBS applied at 80% resting motor threshold (RMT), and 3) sham iTBS.

Main Outcome Measures: Motor evoked potentials (MEPs) amplitudes was assessed as a measure of corticospinal excitability. MEP was measured in two different coil orientations, posterior-anterior (PA) and anterior-posterior (AP) coil rotations, to test the differential effects of iTBS on cortical neuronal networks. Blocks of 15 PA and AP MEPs were obtained before and after each iTBS session.

Results: The amplitude of PA MEPs increased 15.4±9.9% after 80% AMT iTBS and increased further, 50.8±18.8%, after 80% RMT iTBS. The 4 out of 5 non-responders in 80% RMT iTBS showed increase in PA MEP amplitudes after an additional iTBS session when the iTBS was applied at 100% RMT intensity. Similarly, the amplitude of AP MEPs increased 3.9±12.9% after 80% AMT iTBS and 65.7±28.6% after 80% RMT iTBS. After sham iTBS, PA MEPs decreased 0.8±5.5% and AP MEPs decreased 9.1±6.7%.

Conclusions: The results showed that a higher intensity of iTBS more effectively increases corticospinal excitability. Regardless of inter-individual variability related to cortical plasticity, participants who were non-responders with conventional 80% AMT iTBS responded to iTBS applied at higher intensities of 80% or 100% RMT. Our results suggest that iTBS protocols could be optimized to more effectively induce neuroplasticity in the primary motor cortex.

Author(s) Disclosures: None.