The effects of chronic electrical stimulation on laryngeal muscle physiology and histochemistry

Bilateral Functional Electrical Stimulation for the Treatment of Presbyphonia in a Sheep Model

OBJECTIVES

The aim of the study was to increase muscle volume and improve phonation characteristics of the aged ovine larynx by functional electrical stimulation (FES) using a minimally invasive surgical procedure.

METHODS

Stimulation electrodes were placed bilaterally near the terminal adduction branch of the recurrent laryngeal nerves (RLN). The electrodes were connected to battery powered pulse generators implanted subcutaneously at the neck region. Training patterns were programmed by an external programmer using a bidirectional radio frequency link. Training sessions were repeated automatically by the implant every other day for 1 week followed by every day for 8 weeks in the awake animal. Another group of animals were used as sham, with electrodes positioned but not connected to an implant. Outcome parameters included gene expression analysis, histological assessment of muscle fiber size, functional analysis, and volumetric measurements based on three-dimensional reconstructions of the entire thyroarytenoid muscle (TAM).

RESULTS

Increase in minimal muscle fiber diameter and an improvement in vocal efficiency were observed following FES, compared with sham animals.

CONCLUSION

This is the first study to demonstrate beneficial effects in the TAM of FES at molecular, histological, and functional levels. FES of the terminal branches of the RLN reversed the effects of age-related changes and improved vocal efficiency.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:848-854, 2024.

Dual laryngeal reinnervation in bilateral vocal fold paralysis: anatomical pitfalls

BACKGROUND

Bilateral laryngeal reinnervation can be a promising procedure for reanimation of laryngeal muscles, but currently not yet standardized. Besides patient conditions some intraoperative anatomical pitfalls need to be solved.

METHODS

Twelve human head and neck specimens (24 sides) have been studied using microdissection and histological serial sections of the nerves. The surgical anatomy of the dual reinnervation procedure according to JP Marie was investigated notably the branching pattern of the phrenic nerve (PN), the Ansa cervicalis (AC) and the recurrent laryngeal nerve (RLN).

RESULTS

Despite variations of the AC, a prominent inferior common trunk for sterno-hyoid and sterno-thyroid muscles can be used in more than 90% of the specimens. If the AC is missing because of previous surgery, the tiny nerve of the thyro-hyoid muscle can be used preferred. The PN display a double roots pattern from C3 to C4 cervical plexus in 50% of the specimens. A single root pattern can be found and an end-to-lateral neurorraphy can be used. Intra-laryngeal nerves pattern of the RLN display tiny collaterals which cannot be selected for abduction-adduction activity. Direct implantation of the Y-shape great auricular nerve within the posterior crico-arytenoid muscles can be a reliable method leading to challenging mechanical and functional conditions.

CONCLUSION

Several anatomical pitfalls, including intra-operative choices and variants of the donor nerves, but also the challenging intra-laryngeal dissection of the inferior laryngeal nerve need to be solved. A successful laryngeal reinnervation still needs further studies for a simplified procedure.

Functional Evaluation of Larynx Nerve Stimulator With EMG Acquisition Capability and Wireless Connectivity

Recently, an electrical stimulation of the paralyzed muscle, as a potential therapy for restoring function of a denervated muscle system, has been debated as an innovative treatment in the management of patients with laryngeal paralysis. Numerous studies in acute and chronic animal models have demonstrated that electrical stimulation of the paralyzed posterior cricoarytenoideus muscle (PCA) offers an approach to induce vocal fold abduction and restore ventilation through the glottis. The study aims to test applicability of the controlled opening of the rima glottides via direct electrical stimulation of the posterior cricoarytenoideus muscle. We developed for this purpose a novel instrument system for the controlled larynx nerve stimulation. An acute experiment on the 4 years old pig showed effectiveness of the engineered stimulator. The controlled opening of rima glottidis of both posterior cricoarytenoid muscles and afterwards of both PCA muscle contraction were observed as a result of the electrical stimulation with the applied current in the range of 0.1-3 mA and pulse width of 1 ms and 10 ms. Performed research indicates a large potential of the novel nerve stimulator for the human larynx stimulation.

Assessment of Changes in Laryngeal Configuration and Voice Parameters Among Different Frequencies of Neuromuscular Electrical Stimulation (NMES) and Cumulative Effects of NMES in a Normophonic Subject: A Pilot Study

INTRODUCTION

Neuromuscular electrical stimulation (NMES) is a complementary resource to voice therapy that can be used for the treatment of hypofunctional voice disorders. Although positive clinical studies have been reported, neutral and even potentially harmful effects of NMES are also described in the literature. Furthermore, in the studies examined by the authors, the use of different methods of NMES have been identified, which further contributes to the inconsistent results found among studies. Moreover, limited rationale is provided for the chosen NMES parameters such as electrode placement, frequency of NMES and length of treatment. The aims of this pilot study were to investigate the a) impact of different frequencies of NMES on glottal configuration and vocal fold vibration patterns and b) changes in laryngeal configuration and vocal output across 12 minutes of NMES.

METHOD

Three experiments were carried out looking at changes in laryngeal configuration and voice output using different imaging techniques (fibreoptic nasolaryngoscopy and high-speed video), acoustical analysis (F0, formant analysis, SPL, CPPS and LHSR values), electroglottography (EGG) and Relative Fundamental Frequency (RFF) analyses. Glottal parameters and acoustical measures were recorded before, during, and after stimulation. Data was collected at rest and during phonation.

RESULTS

Overall the results showed global changes in laryngeal configuration from normal to hyperfunctional (ie, increased RFF, SPL, CQ, and stiffness). Changes were more pronounced for lower frequencies of NMES and were significant within less than three minutes of application.

CONCLUSION

NMES is an effective resource for the activation of intrinsic laryngeal muscles producing significant levels of adduction within few minutes of application. Lower NMES frequencies produced greater muscle activation when compared to higher frequencies.

Unilateral and Bilateral Laryngeal Pacing for Bilateral Vocal Fold Paralysis

Purpose of Review

Present the state-of-the-art overview of laryngeal pacing for treatment of bilateral vocal fold paralysis. A minimally invasive unilateral pacing system and a fully implantable bilateral pacing system are currently in clinical trials. The relative advantages and disadvantages of each are discussed.

Recent Findings

Research in functional electrical stimulation for the reanimation of the posterior cricoarytenoid muscle has successfully translated from animal models to human clinical trials for unilateral pacing and bilateral pacing. Current findings suggest unilateral pacing in humans significantly improves ventilation but only marginally better than cordotomy. Bilateral pacing in canines increases glottal opening greater than 2-fold over unilateral pacing and restores exercise tolerance to normal.

Summary

Unilateral pacing can be considered a breathing assist device and may not be appropriate for active individuals. Bilateral pacing may be preferable for patients who wish to engage in strenuous exercise. Minimally invasive systems may be ideal for patients who prefer less invasive implantation and are not concerned with cosmesis. Fully implantable pacing systems offer greater electrode redundancy and stability, resulting in a system that is robust against electrode migration or damage.

Current Treatment Options for Bilateral Vocal Fold Paralysis: A State-of-the-Art Review

Vocal fold paralysis (VFP) refers to neurological causes of reduced or absent movement of one or both vocal folds. Bilateral VFP (BVFP) is characterized by inspiratory dyspnea due to narrowing of the airway at the glottic level with both vocal folds assuming a paramedian position. The primary objective of intervention for BVFP is to relieve patients’ dyspnea. Common clinical options for management include tracheostomy, arytenoidectomy and cordotomy. Other options that have been used with varying success include reinnervation techniques and botulinum toxin (Botox) injections into the vocal fold adductors. More recently, research has focused on neuromodulation, laryngeal pacing, gene therapy, and stem cell therapy. These newer approaches have the potential advantage of avoiding damage to the voicing mechanism of the larynx with an added goal of restoring some physiologic movement of the affected vocal folds. However, clinical data are scarce for these new treatment options (i.e., reinnervation and pacing), so more investigative work is needed. These areas of research are expected to provide dramatic improvements in the treatment of BVFP.

Basic Science: The Foundation of Evidence-Based Voice Therapy

Basic science research is part of the circle of translational research that provides the scientific underpinning for evidence-based practice. The translation from bench to bedside, however, is sometimes not obvious. This short review seeks to demonstrate ways in which basic science can inform our clinical practice as voice therapists. From in vitro molecular and cellular studies to in vivo animal models, basic science can investigate biological mechanisms of vocal health, such as vocal fold hydration, and voice use, such as voice rest and vocal exercise, in ways that are impossible in human clinical studies. Knowledge of these mechanisms inform and guide our clinical investigations and help provide evidence for behavioral voice therapy.

Toward an implantable functional electrical stimulation device to correct strabismus

PURPOSE

To investigate the feasibility of electrically stimulating the lateral rectus muscle to recover its physiologic abduction ability in cases of complete sixth cranial (abducens) nerve palsy.

METHODS

In the feline lateral rectus muscle model, the effects of a charge-balanced, biphasic, current-controlled stimulus on the movement of the eye were investigated while stimulation frequency, amplitude, and pulse duration was varied. Eye deflection was measured with a force transducer. Denervated conditions were simulated by injection of botulinum toxin A.

RESULTS

Three chemically denervated and 4 control lateral rectus muscles were analyzed. In control lateral rectus muscles, the minimum fusion frequency was approximately 170 Hz, and the maximum evoked abduction was 27 degrees. The minimum fusion frequency was unchanged after 4 weeks of chemical denervation. Stimulation of chemically denervated lateral rectus muscle resulted in 17 degrees of abduction. For both innervated and chemically denervated lateral rectus muscle, frequencies greater than 175 Hz yielded very little increase in abduction. Modulating amplitude produced noticeable movement throughout the tested range (0.2 to 9 mA).

CONCLUSIONS

Results from the feline lateral rectus muscle showed that electrical stimulation is a feasible approach to evoke a contraction from a denervated lateral rectus muscle. The degree of denervation of the feline lateral rectus muscle was indeterminate. Varying the stimulation amplitude allowed greater eye movement. It is very likely that both frequency and amplitude must be modulated for finer control of static eye position.

Evaluation of a deep brain stimulation electrode for laryngeal pacing

OBJECTIVES

The purpose of the present study was to evaluate the suitability of a deep brain stimulation electrode for laryngeal pacing. Of interest was whether the smaller and more closely spaced channels could provide sufficient channel redundancy, controlled current distribution, and discrete activation of the posterior cricoarytenoid (PCA) muscle.

METHODS

A study was conducted in dogs under differing states of PCA muscle innervation representing complete denervation to complete synkinetic reinnervation. In 3 animals, stimulated glottal opening was assessed in the innervated state and after chemical denervation by pancuronium bromide. In 3 additional dogs, the left side of the larynx was surgically denervated and compared to the innervated, right side to study an anatomic model of clinical paralysis.

RESULTS

The thresholds were lower and the maximum level of abduction was greater for the innervated state. The stimulated glottal opening equaled that of a spontaneously breathing animal. Abductory responses were obtained across all channels in the array, demonstrating its anatomic and physiological compatibility for this application. In the denervated state, responses were only 20% of that of the innervated state with a pulse duration of 0.5 ms. The response could be enhanced to 40% and 60% by increasing the pulse duration to 1 and 2 ms.

CONCLUSIONS

A deep brain stimulation electrode could effectively reanimate the PCA muscle to a normal level in a case of synkinetic reinnervation and to as much as 60% of the normal level in a case of complete denervation.

Neuroanatomy of the equine dorsal cricoarytenoid muscle: surgical implications

REASON FOR PERFORMING STUDY

Studies are required to define more accurately and completely the neuroanatomy of the equine dorsal cricoarytenoid muscle as a prerequisite for developing a neuroprosthesis for recurrent laryngeal neuropathy.

OBJECTIVES

To describe the anatomy, innervation, fibre types and function of the equine dorsal cricoarytenoid muscle.

METHODS

Thirty-one larynges were collected at necropsy from horses with no history of upper airway disease and 25 subjected to gross dissection. Thereafter, the following preparations were made on a subset of larynges: histochemical staining (n = 5), Sihler's and acetylcholinesterase staining for motor endplates (n = 2). An additional 6 larynges were collected and used for a muscle stimulation study.

RESULTS

Two neuromuscular compartments (NMC), each innervated by a primary nerve branch of the recurrent laryngeal nerve, were identified in all larynges. Stimulation of the lateral NMC produced more lateral displacement of the arytenoid cartilage than the medial NMC (P<0.05). The medial NMC tended to rotate the arytenoid cartilage dorsally. Motor endplates were identified at the junction of the middle and caudal thirds of each NMC. If fibre type grouping was present it was always present in both NMCs.

CONCLUSIONS

The equine dorsal cricoarytenoid muscle has 2 distinct muscle NMCs with discrete innervation and lines of action. The lateral NMC appears to have a larger role in increasing cross-sectional area of the rima glottidis.

POTENTIAL RELEVANCE

This information should assist in planning surgical reinnervation procedures and development of a neuroprosthesis for recurrent laryngeal neuropathy.

EMG analysis of the thenar muscles as a model for EMG-triggered larynx stimulation

Paralysis of one or both sides of the larynx musculature compromises breathing and speech function. Currently there is no surgical remedy to restore adequate function of the larynx. A plausible alternative solution is triggered electrical stimulation of the paralysed larynx site using a laryngeal pacemaker. Triggering of the pacemaker succeeds via constant EMG measurement of the muscle activity of the healthy larynx side. The EMG data analysis described in this work is one possible approach for regulating pacemaker triggering. In this study we used EMG data from the thenar muscles as a model to calculate a trigger point.

Recent advances in laryngeal sensorimotor control for voice, speech and swallowing

PURPOSE OF REVIEW

This article reviews advances in knowledge on laryngeal sensorimotor control affecting the assessment, understanding, and treatment of laryngeal motor control disorders in voice, speech, and swallowing. Three topics are covered: new knowledge on laryngeal innervation and central nervous system control from basic research studies, the role of laryngeal sensation in normal swallowing and dysphagia in patients, and new approaches to the restoration of laryngeal motor control after recurrent laryngeal nerve disorders.

RECENT FINDINGS

A significant advance this year was tracing the efferent pathways from the cortex to the brainstem in monkeys. This provided new information on subcortical and brainstem connections in the laryngeal efferent pathways. Laryngeal sensory feedback continued to receive attention, and the role of sensory feedback in the control of the pharyngeal phase of swallowing is now well established. Further developments in neuromotor monitoring of the recurrent laryngeal nerve during thyroidectomy were seen, and a large case series recommended that these techniques become standard practice for surgery for thyroid benign recurrence or malignancy. Finally, the first tissue engineering papers in the field of vocal fold tissue and nerve restoration were published this year, beginning an exciting new approach to restoration of laryngeal motor control.

SUMMARY

Considerable attention has been given to laryngeal muscle physiology, denervation, and sensation in neurolaryngology. Relatively limited understanding is available regarding the central nervous system integrative control of laryngeal function for speech, respiration, and swallowing.

Neurophysiology of vocal fold paralysis

Although a tremendous volume of energy and literature has been devoted to laryngeal paralysis in the past decade, there are still substantial gaps in our understanding of fundamental issues. Oddly enough, controversy remains regarding the actual innervation pathways of the larynx and whether the paralyzed larynx is truly denervated or dysfunctionally reinnervated. An appreciation of these basic issues is prerequisite to making prudent decisions regarding the most appropriate type of intervention. The purpose of this article is to provide a brief overview of basic laryngeal anatomy and neurophysiology to prepare the reader for a subsequent discussion of futuristic research for treatment of laryngeal paralysis.A novel approach is described, which can induce selective reinnervation of individual laryngeal muscles by their original motor fibers within the recurrent laryngeal nerve.

Effect of chronic electrical stimulation of laryngeal muscle on voice

Conventional surgical therapies for bilateral laryngeal paralysis sacrifice voice to enlarge the airway. Electrical pacing of the posterior cricoarytenoid (PCA) muscle to restore glottal opening and allow ventilation offers a new treatment approach. The purpose of this investigation was to determine whether long-term stimulation of the PCA muscle altered perceptual, acoustic, and aerodynamic parameters of voice. Two patients underwent implantation of a Medtronic Itrel II laryngeal pacemaker. Voice evaluation was performed before surgery and at monthly postoperative sessions with the pacemaker off. Months of PCA stimulation did not change perceptual descriptors of voice quality. Measures of fundamental frequency and intensity, upper and lower limits of the dynamic frequency and intensity range, and phonatory flow rates were largely unaltered. The results indicated that there was no effect of laryngeal pacing on voice.

Determination of the optimal conditions for laryngeal pacing with the Itrel II implantable stimulator

OBJECTIVE

To determine the optimal stimulus paradigm, electrode orientation, and configuration of an implantable stimulator used to reanimate the posterior-cricoarytenoid (PCA) muscle in case of bilateral vocal fold paralysis (BVFP).

STUDY DESIGN

Acute studies were conducted on 13 canines implanted with Itrel II systems with or without PCA innervation. PCA stimulus-response characteristics were obtained by measuring stimulated vocal fold displacement endoscopically.

RESULTS

The denervated PCA was only 10% to 25% as responsive to stimulation as the innervated PCA. However, the response could be increased to 38% and 61% if the Itrel was modified to deliver 1 and 2 msec pulses, respectively. Stimuli delivered centrally to the muscle 5 mm from the median raphe improved performance.

CONCLUSION AND SIGNIFICANCE

The optimal stimulus paradigm identified in this study (1 msec pulses delivered at 30 to 40 Hz and 2 to 8.5 mA) has been applied to implanted BVFP patients and improved outcome. Information regarding optimal electrode orientation could also be important to future clinical trials.