Reproducibility of the effects of theta burst stimulation on motor cortical plasticity in healthy participants

Abstract

Objective: Theta-burst stimulation (TBS) is a repetitive transcranial magnetic stimulation (TMS) protocol, capable of enhancing or suppressing the amplitude of contralateral motor-evoked potentials (MEP) for several minutes after stimulation over the primary motor cortex. Continuous TBS (cTBS) produces a long-term depression (LTD)-like reduction of cortical excitability. The purpose of this study was to assess the test-retest reproducibility of the effects of cTBS and to investigate which neurophysiologic markers of cTBS-induced plasticity are most reproducible.

Methods: In ten healthy participants we evaluated in two different sessions the effects of cTBS (using AP-PA current direction, opposite to most commercial rTMS stimulators) on MEPs induced by single-pulse suprathreshold TMS (using AP-PA or PA current direction) over left motor cortex in the first dorsal interosseus (FDI) muscle.

Results: Results demonstrate that the marker of cTBS induced-plasticity with highest within-subject reproducibility is the modulation of corticospinal excitability measured 5min after cTBS.

Conclusion: Overall the effects of cTBS modulation show limited test-retest reproducibility and some measures of the cTBS effects are more reproducible than others.

Significance: Studies comparing cTBS effects in healthy subjects and patients need to proceed with care. Further characterization of the effects of TBS and identification of the best metrics warrant future studies.

Keywords: Neurophysiological markers; Primary motor cortex; Transcranial magnetic stimulation; Variability.

Copyright © 2013 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Figures

Fig. 1

(A) Group-averaged evolution of excitability at visits A and B measured 5, 10, 20, 30, 40, 50, 60, 75 and 90 min after cTBS. For visualization are drawn the spline interpolations and areas of inhibition (visit A: dark and light grey areas; visit B: dark grey area). (B–D) Group-averaged for visit A and visit B and group-averaged difference between visits (visit B – visit A) of: B. MEP at baseline; C. time to baseline; D. area of suppression; E. modulation at T5, T10, T20, T30 and maximum suppression. (Mean ± SE).

Fig. 2

Box plots of the individual difference between the two visits for: A. MEP at baseline; B. time to baseline; C. area of suppression; D. modulation at T5, T10, T20 and T30 and maximum suppression. The box horizontal line is the median individual differences between visits; the tops and bottoms of each box are the 25th and 75th percentiles; the whiskers extend to the further observations beside outliers. Open dots are outliers, more than 1.5 times the interquartile range (i.e. distance top–bottom) away from the top or bottom of the box. Dashed horizontal lines are zero, plus and minus the inter-individual variability (averaged between the two visits).

Fig. 3

Correlation between the variability of RMT and the variability of modulation at T5.