Variability of the motor evoked potential

Wright, Christopher W. (2016). Variability of the motor evoked potential. University of Birmingham. M.Sc.

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Abstract

\(Background\) - The use of transcranial magnetic stimulation (TMS) to assess corticospinal excitability is troubled by trial-to-trial variability of the motor evoked potential (MEP). Many factors have been suggested to contribute to this variability including a range of methodological factors. Coil movement in the yaw axis has been studied extensively however it is unclear how the MEP is affected by coil movement in the roll and pitch axes. Moreover, setting the stimulation intensity to evoke MEPs with a 1 mV amplitude is becoming increasingly common however the reliability of measures taken using this method has yet to be investigated.
\(Objective\) – (1) To assess the short term (≈ 1 h) reliability of MEPs. (2) To investigate the effect of coil rotation in the roll and pitch axes on MEP amplitude, providing and defining the tolerance of coil movement in these two axes.
\(Method\) – Two experiments were performed. (1) Reliability of 1 mV MEPs was assessed by acquiring 8 sets of 30 stimuli, one set every 4 min, with an interstimulus interval of 4 s. Bland-Altman plots were used to assess reliability. (2) The effect of coil tilt in the roll and pitch axes was assessed by acquiring 25 MEPs in 3° increments over a range of 60° around each axis. 70% of peak MEP amplitude defined the range of angles in which MEP amplitude is not affected by coil tilt.
\(Results\) – (1) Bland-Altman plots revealed a consistent lack of agreement between MEP averages compared to the first set with a tendency for MEP amplitude to increase on average 0.37 ± 0.06 mV. (2) The range at which MEP amplitude is not affected by coil tilt is 29.59° ± 10.02° for the roll axis and 17.4° ± 4.36° for the pitch axis.
\(Conclusions\) – The method of setting stimulation intensity to generate 1 mV MEPs is not reliable and is therefore not recommended to be used in future TMS research. Additionally a range of angles within which coil tilt does not affect MEP amplitude has been defined. This study has also highlighted the need for experimenters to be cautious of natural MEP variation and attempt to control as many influential factors as possible.

Type of Work:

Thesis (Masters by Research > M.Sc.)

Award Type:

Masters by Research > M.Sc.

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