Electrophysiologic investigation of lower cranial nerve diseases by means of magnetically stimulated neuromyography of the larynx

Electrophysiological Changes on Laryngeal Motor Neuropathways Cause Voice Disorders for Postradiotherapy Patients with Nasopharyngeal Carcinoma

OBJECTIVE

This study explored electrophysiological changes in the laryngeal motor neuropathway and determined whether lesions in the laryngeal motor cortex (LMC) and its descending tract contribute to voice deterioration and peripheral nerve palsy in patients with nasopharyngeal carcinoma (NPC) postradiotherapy (RT).

STUDY DESIGNS

Prospective cohort study.

METHODS

Twenty-two patients with NPC at 2 to 4years post-RT (8 female and 14 male), 22 patients with NPC at 8 to 10years post-RT (8 female and 14 male), and 22 healthy individuals (9 female and 13 male) were selected to test their magnetic evoked potentials (MEP), motor nerve conduction, and voice quality using transcranial magnetic stimulation, laryngeal electromyography, and the XION DiVAS acoustic analysis software. Three groups were matched according to approximate age. Multiple comparisons were performed among the three groups.

RESULTS

The voice quality of post-RT patients with NPC deteriorated compared to that of healthy individuals. Bilateral LMC and their corticonuclear tracts to the bilateral ambiguous nuclei of post-RT patients with NPC were impaired according to multigroup comparisons of MEP amplitudes, latencies, and resting motor thresholds. The vagus and recurrent laryngeal nerves (RLN) of post-RT patients with NPC were impaired according to multigroup comparisons of the amplitude and latencies of the compound muscle action potential and latencies of f-waves.

CONCLUSIONS

The voice quality of patients with NPC deteriorated after RT. The pathogenesis of post-RT voice deterioration may involve radiation-induced injuries to the vagus, RLN, and bilateral LMC. Furthermore, radiation-induced injuries to the bilateral LMC may contribute to vagus and RLN palsies. These findings support the use of transcranial approaches to treating voice disorders and peripheral nerve palsies in post-RT patients with NPC.

TRIAL REGISTRATION

ChiCTR2100054425; Electrophysiological Study of Vocal-Fold Mobility Disorders After Radiotherapy for NPC Patients via Magnetic Evoked Potential and Their Correlation with Voice Quality Assessment; https://www.chictr.org.cn/bin/project/edit?pid=144429.

Transcranial Magnetic Stimulation and Transcranial Electrical Stimulation in Horses

Depending on the localization of the lesion, spinal cord ataxia is the most common type of ataxia in horses. Most prevalent diagnoses include cervical vertebral stenotic myelopathy (CVSM), equine protozoal myeloencephalitis (EPM), trauma and equine degenerative myeloencephalopathy (EDM). Other causes of ataxia and weakness are associated with infectious causes, trauma and neoplasia. A neurologic examination is indispensable to identify the type of ataxia. In addition, clinical neurophysiology offers tools to locate functional abnormalities in the central and peripheral nervous system. Clinical EMG assessment looks at the lower motoneuron function (LMN) and is used to differentiate between neuropathy in peripheral nerves, which belong to LMNs and myopathy. As LMNs reside in the spinal cord, it is possible to grossly localize lesions in the myelum by muscle examination. Transcranial (tc) stimulation techniques are gaining importance in all areas of medicine to assess the motor function of the spinal cord along the motor tracts to the LMNs. Applications in diagnostics, intraoperative neurophysiological monitoring (IONM), and evaluation of effects of treatment are still evolving in human medicine and offer new challenges in equine medicine. Tc stimulation techniques comprise transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES). TMS was first applied in horses in 1996 by Mayhew and colleagues and followed by TES. The methods are exchangeable for clinical diagnostic assessment but show a few differences. An outline is given on the principles, current clinical diagnostic applications and challenging possibilities of muscle evoked potentials (MEP) from transcranial stimulation in horses.

Evaluation of Laryngeal Motor Neuropathy Using Transcranial Magnetic Stimulation-Mediated Evoked Potentials

OBJECTIVES/HYPOTHESIS

Contemporary evaluation of vocal fold motion impairment largely relies on clinical laryngoscopy, with the diagnoses of vocal fold paresis (VFPa) and paralysis (VFP) being based on identification of partial and complete restriction of gross vocal fold motion, respectively. No consensus exists on the diagnostic criteria of VFPa. Laryngeal electromyography does not offer any insight into nerve conduction velocity without the adjunction of nerve conduction studies, which are impractical to perform on laryngeal nerves due to their anatomic location. The present study aims to assess the feasibility of laryngeal nerve conduction studies using transcranial magnetic stimulation (TMS)-mediated myogenic evoked potentials in the evaluation of laryngeal motor nerve function.

STUDY DESIGN

Prospective controlled cohort study.

METHODS

Enrollment of three groups of subjects defined as healthy volunteers, subjects with clinically diagnosed unilateral VFP, and subjects with clinically diagnosed unilateral VFPa of peripheral etiology. Electrodiagnostic studies consisting of bilateral stimulation of the laryngeal motor cortex, proximal cisternal, and peripheral portions of the vagus nerves were performed using figure-of-eight magnetic stimulation coils, and myogenic evoked potentials recorded from bilateral thyroarytenoid, cricothyroid, and posterior cricoarytenoid muscles using indwelling hook wire electrodes. Conduction latencies were plotted against demographic and anthropometric variables. Values obtained in healthy volunteers were used as normative references and compared to aggregated latencies of VFP and VFPa groups.

RESULTS

Enrolled subjects included 19 healthy volunteers, 5 subjects with VFP, and 4 subjects with VFPa. Normative laryngeal nerve conduction latency ranges measured in healthy subjects were comparable to prior published values, and recorded latencies increased in positive correlation with age. VFPa subjects exhibited increased latencies in affected nerve sites, while VFP subjects presented more variability in electrophysiologic manifestations, mostly dependent on their degree of compensatory reinnervation. Aberrant and synkinetic reinnervation patterns were more predominant in the VFP group than the VFPa group.

CONCLUSIONS

Laryngeal nerve conduction studies using TMS-mediated myogenic evoked potentials are safely feasible. They may serve as a useful complement to laryngeal electromyography in the evaluation of motor laryngeal neuropathy and represent a promising diagnostic modality in the evaluation of VFPa. Based on the present study's findings, the commonly accepted notion of VFPa as a manifestation of a less severe form of neuropathy than VFP may be unsubstantiated. Aging may contribute to progressive motor nerve dysfunction. Future investigations are needed to ascertain the role of nerve conduction studies in clinical laryngology practice.

LEVEL OF EVIDENCE

3 Laryngoscope, 2022.

Transcranial magnetic stimulation and functional magnet resonance imaging evaluation of adductor spasmodic dysphonia during phonation

BACKGROUND

Reduced intracortical inhibition is a neurophysiologic finding in focal dystonia that suggests a broader problem of impaired cortical excitability within the brain. A robust understanding of the neurophysiology in dystonia is essential to elucidate the pathophysiology of the disorder and develop new treatments. The cortical silent period (cSP) is a reliable, non-invasive method to measure intracortical inhibition in the primary motor cortex associated with a muscle of interest. In adductor spasmodic dysphonia (AdSD), cSP of the laryngeal motor cortex (LMC) which directly corresponds to the affected musculature, the thyroarytenoid (TA), has not been examined.

OBJECTIVE

This work evaluated the cSP of the LMC and the relationship between cSP and functional magnetic resonance imaging (fMRI) blood-oxygen-level dependent (BOLD) activation in people with AdSD (n = 12) compared to healthy controls (CTL, n = 14).

RESULTS

Shortened LMC cSP were observed bilaterally in people with AdSD vs CTL (F(1, 99) = 19.5226, p < 0.0001), with a large effect size (η2 = 0.1834). Between-group fMRI analysis revealed greater activation in bilateral LMC in the AdSD > CTL contrast as compared to CTL > AdSD contrast. Correlation analysis showed that people with AdSD have positive correlation of left LMC BOLD activation and the cSP. Further, the right LMC cSP lacks either positive or negative associations with BOLD activation. CTL individuals displayed both positive and negative correlations between cSP and BOLD activation in the left LMC. In CTL, the LMC cSP and BOLD activation showed exclusively negative correlations in both hemispheres.

CONCLUSION

In AdSD, the cortical activation during phonation may not be efficiently or effectively associated with inhibitory processes, leading to muscular dysfunction. These findings may give insight into the maladaptive cortical control during phonation in people with AdSD.

The Internal Superior Laryngeal Nerve in Humans: Evidence for Pure Sensory Function

OBJECTIVES

To determine if the internal branch of the superior laryngeal nerve (iSLN) provides direct motor innervation to the interarytenoid muscle, a laryngeal adductor critical for airway protection. We studied the iSLN-evoked motor response in the interarytenoid and other laryngeal muscles. If the iSLN is purely sensory, there will be no detectable short latency motor response upon supramaximal stimulation, indicating the absence of a direct efferent conduction path.

STUDY DESIGN

Intraoperative case series.

METHODS

In seven anesthetized patients undergoing laryngectomy for unilateral laryngeal carcinoma, the iSLN of the unaffected side was electrically stimulated intraoperatively with 0.1-ms pulses of progressive intensities until supramaximal stimulation was reached. Electromyographic responses were measured in the ipsilateral interarytenoid, thyroarytenoid, and cricothyroid muscles.

RESULTS

None of the subjects exhibited short-latency interarytenoid motor responses to iSLN stimulation. Supramaximal electrical stimulation of the intact iSLN evoked ipsilateral motor responses with long latencies: 18.7-38.5 ms in the interarytenoid (n = 6) and 17.8-24.9 ms in the thyroarytenoid (n = 5). Supramaximal stimulation of the recurrent laryngeal nerve evoked ipsilateral motor responses with short latencies: 1.6-3.9 ms in the interarytenoid (n = 6) and 1.6-2.7 ms in the thyroarytenoid (n = 6).

CONCLUSION

The iSLN provides no functional efferent motor innervation to the interarytenoid muscles. The iSLN exclusively evokes an interarytenoid motor response via afferent activation of central neural circuits that mediate the laryngeal reflex arc. These findings suggest that the role of the iSLN in vital laryngopharyngeal functions, such as normal swallowing and protection of the airway from aspiration, is purely sensory.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:E207-E211, 2021.

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.

Evoked Electromyographic Technique for Quantitative Assessment of the Innervation Status of Laryngeal Muscles

Objectives:

The purpose of this study was to develop a minimally invasive, noninjurious evoked electromyographic technique that could accurately quantitate the level of innervation of laryngeal muscles with recurrent laryngeal nerve stimulation.

Methods:

A four-phase study was conducted in 24 canines, including 1) identification of the best stimulation-recording configuration, 2) statistical analysis of sensitivity and accuracy, 3) evaluation of safety, and 4) identification of the laryngeal muscle(s) that contribute to the evoked response.

Results:

The results demonstrated that an entirely noninvasive technique is not feasible. The stimulating cathode must be invasive to ensure discrete activation of the recurrent laryngeal nerve, whereas both recording electrodes should remain on the surface with one overlying the thyroid ala. This configuration proved to be highly accurate, with an error rate of only 6% to 7%, and with sensitivity sufficient to detect a signal in a nerve with fewer than 1% of the axons intact. There was no evidence of nerve injury in any animal over the course of 350 stimulus needle penetrations. By use of neuromuscular blockade to identify those muscles generating the surface response, the thyroarytenoid muscle was found to be the primary contributor, whereas the posterior cricoarytenoid muscle was uninvolved.

Conclusions:

This evoked electromyographic technique could provide quantitative information regarding the extent of muscle innervation during denervation and regeneration in case of laryngeal paralysis.

Peripheral component of laryngeal and pharyngeal motor evoked potentials

In this study, the responses of thyroarytenoid (TA) and cricopharyngeus (CP) muscles were simultaneously recorded to peripheral magnetic stimulation of the vagus nerve. Recordings were performed in 13 subjects by means of concentric needle EMG electrodes inserted in the TA and CP. Magnetic shocks were delivered to the vagus nerve with a round coil placed occipitally, while EMG was silent in the TA. In all subjects, clear-cut responses were obtained simultaneously in both muscles. In TA compared to CP, the maximum amplitude of the responses were higher, whereas the onset latency was shorter. Our results revealed that simultaneous recordings of TA and CP motor responses to occipital magnetic stimulation enabled a reliable evaluation of their peripheral innervation by the vagus nerve.

Human cortical motor representation of the larynx as assessed by transcranial magnetic stimulation (TMS)

OBJECTIVES

To analyze characteristic features and details on motor-evoked potentials (MEPs) of the cricothyroid and vocalis muscles from single-pulse cortical transcranial magnetic stimulation (TMS) in normal subjects to characterize cortical motor representation of laryngeal muscles.

STUDY DESIGN

Prospective, experimental investigation on healthy volunteers.

METHOD

MEPs of the cricothyroid and vocalis muscles elicited by cortical TMS with a figure-8-shaped coil were investigated in two groups of six healthy subjects each, with special regard to MEP amplitude as a function of the coil position on the head surface along the interaural line.

RESULTS

Bilateral reproducible responses of the cricothyroid and the vocalis muscles could be observed in all subjects. For the cricothyroid muscle, maximal responses were obtained at mean stimulus positions of 7.5 +/- 1.4 cm (contralateral) and of 7.3 +/- 1.3 cm (ipsilateral), respectively. For the vocalis muscle, we found maximal responses at mean stimulus positions of 10.3 +/- 1.9 cm (contralateral) and of 9.6 +/- 1.6 cm (ipsilateral), respectively. Despite a considerable overlap of these coil positions, from which reproducible MEPs could be elicited in both groups of the laryngeal muscles, statistically significant separation of the cricothyroid-and vocalis-associated cortical representation areas was possible.

CONCLUSIONS

Our observations point to two different cortical motor representation areas, with the cricothyroid muscle-related area being located more medially.

Electrophysiological study of vocal-fold mobility disorders using a magnetic stimulator

In the field of neurolaryngology, there has been a great interest in neurophysiological studies, such as neurography, for the assessment of the integrity of the laryngeal neural pathway. Such tools provide an indication about the site and the nature of the nerve lesion. We have tried to use a magnetically evoked potential to assess the corticolaryngeal pathway in order to provide normative data on laryngeal nerve conductivity and to evaluate the integrity of the laryngeal neural system in patients with vocal-fold mobility disorders. This study was conducted on 26 subjects (10 normal volunteers and 16 patients with vocal-fold immobility) who were primarily selected on the basis of a comprehensive laryngeal evaluation including laryngo-videostroboscopy assessment. Transcranial (cortical) and mastoid (peripheral) magnetic stimulations were performed to evoke muscle action potentials of the thyro-arytenoid (TA) and cricothyroid muscles (CT). In normal volunteers, cortical stimulation leads to contralateral responses (cortical latency) after 10.9 and 11.3 ms and ipsilateral responses after 8.3 and 9.4 ms for right CT and TA muscles, respectively. There was a significant prolongation of cortical latency of the left TA compared with the right TA muscle, whilst no such significant difference was observed in the CT muscles. Peripheral stimulation evoked response (peripheral latency) after 2.8 and 2.7 ms in the right CT and TA, respectively, with the same significant prolongation of the left TA response compared with the right side. Amongst the patient groups, variable patterns of laryngeal muscle response latencies occurred, including normal response latency, lack of response of CT and TA muscles, prolonged peripheral latency with secondary prolonged cortical latency and prolonged cortical latency with normal peripheral latency. The results indicate that the magnetically evoked potential of laryngeal muscles offers an easy, non-invasive technique and could have a role in the assessment of the integrity of corticolaryngeal pathways.

Cricopharyngeal sphincter muscle responses to transcranial magnetic stimulation in normal subjects and in patients with dysphagia

OBJECTIVE

Cricopharyngeal (CP) muscle of the upper oesophageal sphincter (UES) has a significant role in the pharyngo-esophageal phase of deglutition. The linkage between the CP muscle of UES and the motor cortex has not been previously studied electrophysiologically in healthy humans and in patients with neurogenic dysphagia.

METHODS

Needle recordings of EMG responses were carried out from the CP sphincter muscle following transcranial magnetic stimulation (TMS) over the vertex around the Cz electrode position (cortical MEP), and on the parieto-occipital skull and the occiput ipsilaterally (peripheral MEP) in 14 healthy control subjects and in 26 patients with and without neurogenic dysphagia. Needle recordings obtained from the cricothyroid muscle of the larynx were also evaluated in six healthy subjects.

RESULTS

The cortical motor latency of CP sphincter muscle was 10.7+/-0.5 ms with an amplitude of 0.8+/-0.2 mV in healthy subjects. Both the latency and amplitude of CP-MEP were facilitated during swallowing. The peripheral MEP of the CP muscle was very stable in all normal subjects (5.1+/-0.3 ms; 1.3+/-0.3 mV) and swallowing did not influence these parameters. The cortically elicited CP-MEP was significantly longer than the cortical MEPs obtained from the cricothyroid muscle of the larynx. In 10 dysphagic patients with corticobulbar tract involvement (6 ALS and 4 pseudobulbar palsy) and with pathologic and hyperreflexic EMG of the CP-sphincter muscle, the cortical MEP of CP muscle of the upper esophageal sphincter could not be elicited, although the peripheral CP-MEPs were obtained. TMS never produced a swallowing movement in neither healthy subjects nor patients.

CONCLUSION

The CP muscle of the upper esophageal sphincter can produce MEPs by cortical TMS and by stimulation at the root/nerve levels of vagus nerve. The MEP latency values and central motor delay suggest that there is an oligosynaptic corticobulbar pathway to the motoneurons of CP muscles. When the pathway is affected by a pathology (i.e. ALS or pseudobulbar palsy) the CP sphincter becomes hyperreflexic due to disinhibition and the cortical MEP of the CP muscle disappears due to degeneration of the corticobulbar pathway. These mechanisms appear to be responsible for the pathogenesis of dysphagia.

Effects of Stimulus Intensity on Laryngeal Long Latency Responses in Awake Humans

Percutaneous electrical stimulation applied to the internal branch of the superior laryngeal nerve (ISLN) results in two long latency laryngeal adductor responses in awake humans: an ipsilateral thyroarytenoid (TA) R1 muscle response at 16 ms, and later bilateral TA R2 muscle responses at 60 ms. The purpose of this study was to determine whether a functional relationship existed between the R1 and R2 responses by gradually increasing the level of electrical stimulation from threshold to supramaximal levels. R1 amplitude increased linearly with stimulation intensity in 9 of the 11 subjects, whereas R2 only had a positive linear relationship in 3 subjects and a negative relationship with stimulation intensity in 1 subject. Significant negative relationships were found between response latency and stimulation intensify in 3 subjects for the R1 responses and 3 other subjects for the R2 responses. Overall, R1 amplitudes increased systematically, whereas R2 responses varied in latency and amplitude with increasing stimulus Intensity. Neither the latencies nor the amplitudes of the two responses were related after adjusting for stimulation intensity within subjects by using partial correlation coefficients. The R1 and R2 responses were functionally unrelated and most likely have different neural components.

Assessing the Clinical Utility of the Magnetic Stimulator for Measuring Response Latencies in the Laryngeal Muscles

Our purpose was to assess the use of magnetic stimulation for measuring conduction time of the recurrent and superior laryngeal nerves in 10 normal volunteers (7 male, 3 female). Subjects underwent laryngeal electromyography and magnetic stimulation of the vagus nerve bilaterally at the mastoid tip with a figure 8 coil. Mean muscle response latencies were measured and examined for consistent differences. Thyroarytenoid muscle response latencies were consistently longer than those in the cricothyroid muscle. Left thyroarytenoid muscle latencies were consistently longer than those on the right in agreement with bilateral asymmetry of these nerves. No appreciable differences were observed in cricothyroid muscle latencies when the right side was compared with the left. Results were consistent and reproducible within a broad range, but appreciable intersubject variability was observed. The limited sample size was unable to support a correlation with anthropometric variables, although an association was indicated. Magnetic stimulation with this technique has great potential for use in neurolaryngologic studies.

Limitations of electromyography and magnetic stimulation for assessing laryngeal muscle control

The development of new phonosurgical techniques has increased the level of interest in the field of neurolaryngology. This field requires valid techniques for determining if muscle activation is normal. Laryngeal electromyography is being used more frequently to assess muscle innervation and synkinesis. Further, magnetic stimulation has been introduced as a noninvasive technique for nerve stimulation. Technical limitations that affect the clinical utility of both these techniques are reviewed: 1) difficulties obtaining selective and accurate electromyographic laryngeal muscle recordings, 2) normal variation in movement and muscle activation patterns within and between normal individuals when producing the same speech syllables, and 3) variation in laryngeal muscle response latencies between and within normal subjects during peripheral magnetic stimulation. Given the normal variation in laryngeal electromyography and magnetic stimulation response latencies, these techniques may not yet be reliable or accurate for assessing reinnervation or synkinesis following recurrent laryngeal nerve injury.