Muscle Nerve. 2017 May;55(5):706-714. doi: 10.1002/mus.25384. 

An implantable system for In Vivo chronic electromyographic study in the larynx.

Li Y1,2, Huang S1, Zealear D1.

1 Department of Otolaryngology, Vanderbilt University Medical Center, Nashville, Tennessee, USA.
2 Department of Hearing and Speech Sciences, Vanderbilt University Medical Center, 602 Oxford House, 1313 21st Avenue South, Nashville, Tennessee, 37232-4480, USA.

Abstract

INTRODUCTION:

Electromyography (EMG) plays an important role in exploring the mechanisms of selective reinnervation. An implantable system can help provide chronological information regarding reinnervation of laryngeal muscles. This study was designed to develop an implantable system for repeated recordings of spontaneous and evoked EMG from laryngeal muscles.

METHODS:

This implantable system has 4 bipolar stimulus cuffs for bilateral recurrent laryngeal nerves (RLNs) and superior laryngeal nerves (SLNs), and 4 EMG recording electrodes for bilateral vocal fold adductors (thyroarytenoid-lateral cricoarytenoid, TA-LCA) and abductor (posterior cricoarytenoid, PCA) muscles. The system was implanted in 8 canines for up to 41 weeks.

RESULTS:

The system showed good compatibility. Consistent EMG signals were recorded from both PCA and TA-LCA muscles.

CONCLUSION:

We developed a long-term implantable EMG system that is simple and capable of obtaining stable EMG recordings from canine laryngeal muscles with minimal risk of device breakage, trauma, or infection. © 2016 Wiley Periodicals, Inc.

KEYWORDS:

electromyography; implant; laryngeal muscle; paralysis; reinnervation; vocal fold

PMID: 27543847

 

Supplement:

Normal laryngeal physiology requires complex coordination of muscle actions during voicing, airway protection and respiration. The posterior cricoarytenoid (PCA) muscle situated on the posterior larynx is the major abductor of the vocal fold. Contraction of this muscle enlarges the glottic area for inhalation (Figure 1A). The thyroarytenoid–lateral cricoarytenoid (TA-LCA) muscle complex is the major adductor of the vocal fold to close the glottic airway during voicing and airway protection (Figure 1B). Both abductor and adductor muscles are supplied by motor fibers in the recurrent laryngeal nerve (RLN). The internal branch of the superior laryngeal nerve (SLN) is a purely sensory nerve containing the afferent fibers of these receptors [1] (Figure 1C).

 

 

Figure 1. Normal laryngeal physiology and muscle actions. A. During inhalation, activation of bilateral PCA muscles abducts both vocal folds; B. During voicing or airway protection, activation of TA-LCA muscle complex moves vocal folds towards midline and closes the airway; C. Both abductor and adductor muscles are innervated by the RLN, while the internal branch of the SLN is purely sensory.

 

Bilateral vocal fold paralysis (BVFP), a debilitating clinical problem, results from damage to the RLNs and inappropriate neural regeneration to the PCA as well as the adductor muscles [2, 3]. Such misdirected reinnervation leads to a synkinetic and functionally paralyzed larynx, where contraction of abductor and adductor antagonists produces no net, or even “paradoxical” vocal fold motion during vocalization, inspiration, or airway protection.

 

In a previous qualitative study of the effects of electrical stimulation on canine PCA muscle reinnervation, there was indication that stimulation preferentially repressed reconnection by foreign motoneurons, thereby promoting correct reinnervation to the laryngeal muscles [4]. Recent quantitative studies further demonstrated that functional electrical stimulation (FES) on denervated PCA muscles with a low frequency (10pps) has successfully reduced synkinetic vocal fold motion with inspiration and restored animals’ exercise tolerance to normal [5]. However, the mechanism is unknown.

 

Electromyography (EMG) plays an important role in exploring the underlying mechanisms of selective reinnervation. However, in-vivo evoked EMG recording from intrinsic laryngeal muscles has been limited by the techniques available for chronic stimulation of nerves and recording from muscles. This limitation prevented repeated EMG recording that could provide chronological information regarding changes of innervation status of laryngeal muscles in different stages of reinnervation.

 

To overcome this limitation, Dr. Zealear’s research team from Vanderbilt University Medical Center have developed a simple, inexpensive, implantable system. It can be used for repeated EMG recordings from the larynx in a canine model. Drs. Yike Li, Shan Huang, and David Zealear published their findings in Muscle & Nerve in 2017 [6].

 

This system consisted of four bipolar nerve stimulus cuffs and four EMG recording electrodes, as well as an interface plug. During a sterile surgery, nerve stimulus cuffs were placed on the RLNs and the internal branches of SLNs bilaterally. The hook-wire recording electrodes were inserted into the TA-LCA complex and the PCA muscles bilaterally. EMG recordings from the PCA muscles during RLN stimulation gave an index of the overall magnitude of its reinnervation. EMG activity during SLN stimulation measured the amount of incorrect reinnervation of the PCA muscle by foreign reflex glottic closure motoneurons. The spontaneous EMG activity during hypercapnic breathing provided a good estimate of the magnitude of appropriate PCA muscle reinnervation by inspiratory motoneurons. Small metal female pins were attached at the other end of coiled electrode leads and inserted into the holes of the skin plug.  The plug was sutured to the anterior neck and served as an interface for connection between all implanted electrodes and the external equipment. This system has been implanted and tested in animals for as long as 10 months. It showed good compatibility throughout the study. Consistent EMG signals were recorded from the PCA and TA-LCA muscles. The shape and amplitude of the potentials were comparable to recordings from previous studies. Stimulus artifacts were minimal and did not impact the biological signals.

 

This implantable system proved to be a reliable platform for EMG measurement in any research application where chronological recordings are required over long periods of time. In fact, this technology has been used in the study of tongue plasticity [7]. It may also be combined with remote recording technology (e.g. telemetry) to provide a means for EMG recording in awake animals. In the future, this system will be applied to study the effect of electrical stimulation on Bell’s palsy, which is a common paralysis with synkinesis.

 

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