Cortical silent period in the tongue induced by transcranial magnetic stimulation

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

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.

Evaluation of the Cortical Silent Period of the Laryngeal Motor Cortex in Healthy Individuals

Objective: This work aimed to evaluate the cortical silent period (cSP) of the laryngeal motor cortex (LMC) using the bilateral thyroarytenoid (TA) muscles with transcranial magnetic stimulation (TMS).

Methods: In 11 healthy participants, fine-wire electromyography (EMG) was used to record bilateral TA muscle responses to single pulse TMS delivered to the LMC in both hemispheres. Peripheral responses to stimulation over the mastoid, where the vagus nerve exits the skull, were collected to verify the central origin of the cortical stimulation responses by comparing the latencies.

Results: The cSP duration ranged from 41.7 to 66.4 ms. The peripherally evoked motor-evoked potential (MEP) peak occurred 5–9 ms earlier than the cortical responses (for both sides of TAs: p < 0.0001) with no silent period. The right TA MEP latencies were earlier than the left TA responses for both peripheral and cortical measures (p ≤ 0.0001).

Conclusion: These findings demonstrate the feasibility of measuring cSP of LMC based on intrinsic laryngeal muscles responses during vocalization in healthy volunteers.

Significance: The technique could be used to study the pathophysiology of neurological disorders that affect TA muscles, such as spasmodic dysphonia. Further, the methodology has application to other muscles of the head and neck not accessible using surface electrodes.

Bilateral sensory deprivation of trigeminal afferent fibres on corticomotor control of human tongue musculature: a preliminary study

Transcranial magnetic stimulation (TMS) has demonstrated changes in motor evoked potentials (MEPs) in human limb muscles following modulation of sensory afferent inputs. The aim of this study was to determine whether bilateral local anaesthesia (LA) of the lingual nerve affects the excitability of the tongue motor cortex (MI) as measured by TMS. The effect on MEPs after bilateral LA of the lingual nerve was studied, while the first dorsal interosseous (FDI) muscle served as a control in ten healthy participants. MEPs were measured on the right side of the tongue dorsum in four different conditions: (i) immediately prior to anaesthesia (baseline), (ii) during bilateral LA block of the lingual nerve, (iii) after anaesthesia had subjectively subsided (recovery) and (iv) 3 h after bilateral lingual block injection. MEPs were assessed using stimulus-response curves in steps of 10% of motor threshold (T). Eight stimuli were given at each stimulus level. The amplitudes of the tongue MEPs were significantly influenced by the stimulus intensity (P < 0·001) but not by condition (P = 0·186). However, post hoc tests showed that MEPS were statistically significantly higher during bilateral LA block condition compared with baseline at T + 40%, T + 50% and T + 60% (P < 0·028) and also compared with recovery at T + 60% (P = 0·010) as well as at 3 h after injection at T + 50% and T + 60% (P < 0·029). Bilateral LA block of the lingual nerve seems to be associated with a facilitation of the corticomotor pathways related to the tongue musculature.

Assessing the temporal reproducibility of human esophageal motor-evoked potentials to transcranial magnetic stimulation

BACKGROUND

Although the electrophysiological properties and reproducibility of somatic limb motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are well characterized, little is known about the reproducibility of MEPs for viscerosomatic structures such as the esophagus.

AIM

To determine the temporal reproducibility of esophageal MEPs to TMS.

METHODS

MEPs to TMS were recorded from the proximal esophagus, using a swallowed catheter housing a pair of electrodes, in eight healthy subjects at five stimulus intensities (SI) (motor threshold [MT] to 20% above MT). For each SI, 20 consecutive TMS stimuli at 5-second intervals were delivered over a single scalp site (dominant hemisphere at site exhibiting MT at lowest SI) and repeated 40 and 80 minutes thereafter. MEP amplitudes and latencies were measured, and means were sequentially calculated for each SI and then log-transformed. The repeatability coefficients (RC) for the three time points were calculated across each set of 20 stimuli and presented as an exponential ratio.

RESULTS

Best RC (amplitude/latency) were achieved at 120% SI relative to MT, being 1.8/1.2 (optimal = 1.0). For lower intensities of 115%, 110%, 105%, and 100% SI, the RC were 2.1/1.2, 2.1/1.1, 2.4/1.2, and 2.6/1.4, respectively. For all SI, the greatest reductions in RC occurred over the first 10 stimuli, with little additional gain beyond this number.

CONCLUSIONS

Latencies of esophageal MEP to TMS across intensities are highly reproducible, whereas amplitudes are more stimulus intensity-dependent, being most reliable and reproducible at the highest stimulus strengths.

SIGNIFICANCE

Using careful parameters, TMS can be used reliably in future studies of viscerosomatic structures, although the size of the response variability needs to be taken into account when assessing changes in cortico-fugal activity.

Influence of sensory deprivation and perturbation of trigeminal afferent fibers on corticomotor control of human tongue musculature

Several recent studies with transcranial magnetic stimulation (TMS) have demonstrated changes in motor evoked potentials (MEPs) in human limb muscles following modulation of sensory afferent inputs, but little is known about the regulation of the human tongue motor control. To test the effect of local anesthesia (LA) of the lingual nerve and topical application of capsaicin stimulation on tongue MEPs. Fourteen volunteers participated (21-30 years) in two randomized sessions; before, during a nerve block of the lingual nerve or topical capsaicin application (30 microl 5%) on the tongue, and after anesthesia or pain had subsided. EMG electrodes were placed on the tongue and the first dorsal interosseous (FDI) muscle (control). EMG signals were amplified, filtered (20 Hz-1 kHz), and sampled at 4 kHz (Nicolet, USA). TMS were delivered with a figure-of-eight coil (Magstim 200, UK). Scalp sites at which EMG responses were evoked in the relaxed tongue or FDI at the lowest stimulus strength were determined, i.e., motor threshold (T). MEPs were assessed using stimulus-response curves in steps of 10% T. Eight stimuli were presented at each stimulus level. The proximal hypoglossal nerve was activated by TMS delivered over the parieto-occipital skull distal to the right ear. Eight stimuli were delivered at 50% of maximum stimulator output. ANOVAs were used to analyze latency and peak-to-peak amplitudes. Capsaicin evoked mild pain (2.8+/-0.5), and a strong burning sensation (6.2+/-0.4) on 0-10 visual analogue scales. MEP amplitudes in tongue and FDI were not influenced by capsaicin (P>0.44) but by stimulus strength (P<0.001). MEP latencies in tongue (8.9+/-0.2 ms) and FDI (22.4+/-0.4 ms) were not affected by capsaicin (P>0.19). Hypoglossal nerve stimulation evoked a short-latency (3.6+/-0.9 ms) response (mean amplitude 65+/-9 microV); but was unaffected by capsaicin (P>0.54). LA did not have any effect on FDI MEPs but was associated with a significant facilitation of tongue MEPs at T+50% and T+60% about 50 min after the nerve block in the recovery phase. Also in this condition, the direct motor responses evoked by hypoglossal nerve stimulation remained constant. No direct effect of a strong burning sensation could be shown on peripheral or central corticomotor pathways to the relaxed tongue musculature, however, LA of the lingual nerve (cranial nerve V) seems able to induce a delayed change in corticomotor control of tongue musculature (cranial nerve XII) possibly related to unmasking effects at the cortical level but not completely excluding excitability changes at the brain stem level.

Evaluation of periodic leg movements and associated transcranial magnetic stimulation parameters in restless legs syndrome

Restless legs syndrome (RLS), a sensorimotor disorder characterized by unpleasant sensations commonly localized in the legs, is frequently associated with periodic limb movements (PLMs) during sleep. We investigated the role of transcranial magnetic stimulation (TMS) and cortical silent period (CSP) duration as diagnostic and monitoring tools in 20 patients with primary RLS before and after 1 month of treatment and also studied 15 normal age- and gender-matched subjects. Polysomnographic assessment was undertaken and the PLM index determined in 17 of the 20 patients. We also studied the correlation between sleep efficiency index and CSP duration because of the increasing severity of the sleep disturbance and PLMs in patients with RLS. Our results demonstrate that the duration of the CSP was reduced in patients with RLS, and that dopaminergic treatment normalized this duration. There was no correlation between the PLM index and CSP duration. It may be speculated that PLMs and the CSP are due to different inhibitory mechanisms and they may be used separately as diagnostic and monitoring tools in patients with primary RLS.