The effect of pulse shape in theta-burst stimulation: Monophasic vs biphasic TMS.

Background: Intermittent theta-burst stimulation (i) (TBS) is a transcranial magnetic stimulation (TMS) plasticity protocol. Conventionally, TBS is applied using biphasic pulses due to hardware limitations. However, monophasic pulses are hypothesised to recruit cortical neurons more selectively than biphasic pulses, predicting stronger plasticity effects. Monophasic and biphasic TBS can be generated using a custom-made pulse-width modulation-based TMS device (pTMS).
Objective: Using pTMS, we tested the hypothesis that monophasic iTBS would induce a stronger plasticity effect than biphasic, measured as induced increases in motor corticospinal excitability.
Methods: In a repeated-measures design, thirty healthy volunteers participated in three separate sessions, where monophasic and biphasic iTBS was applied to the primary motor cortex (M1 condition) or the vertex (control condition). Plasticity was quantified as increases in motor corticospinal excitability after versus before iTBS, by comparing peak-to-peak amplitudes of motor evoked potentials (MEP) measured at baseline and over 60 min after iTBS.
Results: Both monophasic and biphasic M1 iTBS led to significant increases in MEP amplitude. As predicted, linear mixed effects (LME) models showed that the iTBS condition had a significant effect on the MEP amplitude (χ ² (1) = 27.615, p < 0.001) with monophasic iTBS leading to significantly stronger plasticity than biphasic iTBS (t (693) = 2.311, p = 0.021). Control vertex iTBS had no effect.
Conclusions: In this study, monophasic iTBS induced a stronger motor corticospinal excitability increase than biphasic within participants. This greater physiological effect suggests that monophasic iTBS may also have potential for greater functional impact, of interest for future fundamental and clinical applications of TBS.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Timothy Denison, Majid Memarian Sorkhabi and Karen Wendt have pending patent applications for TMS device circuits and control algorithms. Timothy Denison and Karen Wendt have received grant funding (including an MRC iCASE studentship) and materials through collaboration agreements from Magstim Ltd. Majid Memarian Sorkhabi is currently employed by Magstim Ltd. Jacinta O'Shea has acted as a consultant for Welcony Inc. And serves on the Scientific Advisory Board of Plato Science. Charlotte J. Stagg and Melanie K. Fleming have no conflicts of interest.
(Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)