The Cortical Silent Period in the Cricothyroid Muscle as a Neurophysiologic Feature for Dystonia Observation: E-Field-Navigated Transcranial Magnetic (TMS) Study

Abstract

The cortical silent period (cSP) is a period of electrical silence following a motor-evoked potential (MEP) in the electromyographic signal recorded from a muscle. The MEP can be elicited by transcranial magnetic stimulation (TMS) over the primary motor cortex site corresponding with the muscle. The cSP reflects the intracortical inhibitory process mediated by GABA_A and GABA_B receptors. The study aimed to investigate the cSP in the cricothyroid (CT) muscle after applying e-field-navigated TMS over the laryngeal motor cortex (LMC) in healthy subjects. Then, a cSP as a neurophysiologic feature for laryngeal dystonia was observed. We applied a single-pulse e-field-navigated TMS to the LMC over both hemispheres with hook-wire electrodes positioned in the CT muscle in nineteen healthy participants, which triggered the elicitation of contralateral and ipsilateral corticobulbar MEPs. The subjects were engaged in a vocalization task, and then we assessed the following metrics: LMC intensity, peak-to-peak MEP amplitude in the CT muscle, and cSP duration. The results showed that the cSP duration from the contralateral CT muscle was distributed from 40 ms to 60.83 ms, and from the ipsilateral CT muscle, from 40 ms to 65.58 ms. Also, no significant difference was found between the contralateral and ipsilateral cSP duration (t(30) = 0.85, p = 0.40), MEP amplitude in the CT muscle (t(30) = 0.91, p = 0.36), and LMC intensity (t(30) = 1.20, p = 0.23). To conclude, the applied research protocol showed the feasibility of recording LMC corticobulbar MEPs and observing the cSP during vocalization in healthy participants. Furthermore, an understanding of neurophysiologic cSP features can be used to study the pathophysiology of neurological disorders that affect laryngeal muscles, such as laryngeal dystonia.

Keywords: MEP; TMS; cortical silent period; dystonia; laryngeal dystonia; motor-evoked potentials; navigated TMS; spasmodic dysphonia; transcranial magnetic stimulation.

Figure 1

MEP responses and detection of cSP from laryngeal CT muscle. Six individual trials are presented as mean (red dashed line) and median (blue dashed line) for a male subject (No. 9). The pink vertical dashed line represents the offset of the cSP response, whilst the magnetic stimulation onset is represented as the zero time. The time on the x-axis is expressed in milliseconds (ms), and on the y-axis is the MEP amplitude in microvolts (µV).

Figure 2

Graphical representation of cSP duration for contralateral and ipsilateral responses. The central red line specifies the median and mean value, and the bottom and top edges of the box show the 25th and 75th percentiles. The whiskers represent mean ± 1.96*SE of data. p > 0.05. SE: standard error of the mean.

Figure 3

The 3D graphical representation of the repeatability of cSP in CT muscles in a single subject (No. 13). On the x-axis, the time in milliseconds is shown and on the y-axis, “Vpp” (peak-to-peak amplitude) denotes the MEP amplitude value expressed in microvolts. A total of twelve single trials are presented with MEPs in CT muscles followed by cSP. Pre-EMG activity is recorded preceding MEPs and following cSP.