Register transitions in an in vivo canine model as a function of intrinsic laryngeal muscle stimulation, fundamental frequency, and sound pressure level.

Phonatory instabilities and involuntary register transitions can occur during singing. However, little is known regarding the mechanisms which govern such transitions. To investigate this phenomenon, we systematically varied laryngeal muscle activation and airflow in an in vivo canine larynx model during phonation. We calculated voice range profiles showing average nerve activations for all combinations of fundamental frequency (F0) and sound pressure level (SPL). Further, we determined closed-quotient (CQ) and minimum-posterior-area (MPA) based on high-speed video recordings. While different combinations of muscle activation favored different combinations of F0 and SPL, in the investigated larynx there was a consistent region of instability at about 400 Hz which essentially precluded phonation. An explanation for this region may be a larynx specific coupling between sound source and subglottal tract or an effect based purely on larynx morphology. Register transitions crossed this region, with different combinations of cricothyroid and thyroarytenoid muscle (TA) activation stabilizing higher or lower neighboring frequencies. Observed patterns in CQ and MPA dependent on TA activation reproduced patterns found in singers in previous work. Lack of control of TA stimulation may result in phonation instabilities, and enhanced control of TA stimulation may help to avoid involuntary register transitions, especially in the singing voice.

LaDIVA: A neurocomputational model providing laryngeal motor control for speech acquisition and production.

Many voice disorders are the result of intricate neural and/or biomechanical impairments that are poorly understood. The limited knowledge of their etiological and pathophysiological mechanisms hampers effective clinical management. Behavioral studies have been used concurrently with computational models to better understand typical and pathological laryngeal motor control. Thus far, however, a unified computational framework that quantitatively integrates physiologically relevant models of phonation with the neural control of speech has not been developed. Here, we introduce LaDIVA, a novel neurocomputational model with physiologically based laryngeal motor control. We combined the DIVA model (an established neural network model of speech motor control) with the extended body-cover model (a physics-based vocal fold model). The resulting integrated model, LaDIVA, was validated by comparing its model simulations with behavioral responses to perturbations of auditory vocal fundamental frequency (fo) feedback in adults with typical speech. LaDIVA demonstrated capability to simulate different modes of laryngeal motor control, ranging from short-term (i.e., reflexive) and long-term (i.e., adaptive) auditory feedback paradigms, to generating prosodic contours in speech. Simulations showed that LaDIVA's laryngeal motor control displays properties of motor equivalence, i.e., LaDIVA could robustly generate compensatory responses to reflexive vocal fo perturbations with varying initial laryngeal muscle activation levels leading to the same output. The model can also generate prosodic contours for studying laryngeal motor control in running speech. LaDIVA can expand the understanding of the physiology of human phonation to enable, for the first time, the investigation of causal effects of neural motor control in the fine structure of the vocal signal.

Amount and spatial arrangement of muscle fibers in the human laryngeal Musculus ventricularis.

Textbooks and atlases of human macroscopic and microscopic anatomy of the larynx generally provide, if at all, only sparse information on the laryngeal Musculus ventricularis. However, several studies indicate that this muscle takes over the function of vestibular (ventricular) fold phonation after denervation of the Musculus vocalis. In the present study, 29 laryngeal specimens were coronally dissected at different levels, i.e. the anterior (L1), middle (L2), and posterior third of the vestibular fold (L3), and they underwent histological analysis. In all specimens the vestibular folds of both hemi-larynxes contained striated muscle bundles in variable amounts, representing a ventricularis muscle. These muscle bundles obviously originated from the lateral (external) and thyroepiglottic part of the thyroarytenoid muscle and the aryepiglottic part of the oblique arytenoid muscle, as has been described by other authors. The areas of vestibular folds and their amounts of ventricularis muscle bundles were measured using image analysis software (imageJ) by manual tracing. The mean area of the vestibular folds of both hemi-larynxes was 27.9 mm2 (SD [standard deviation] ± 9.17), and the area occupied by fibers of the ventricularis muscle was 1.5 mm2 (SD ± 1.78). Statistical analysis comparing the areas of both hemi-larynxes and levels resulted in no significant differences, except for the levels 2 and 3. In level 2, significantly more muscle fibers (2.0 mm2 ; SD ± 2.21) were detectable within the vestibular fold than in level 3 (0.9 mm2 ; SD ± 1.43). Level 1 also contained more muscle fibers (1.1 mm2 ; SD ± 1.06) than level 3, however, without significance. In conclusion, the laryngeal ventricularis muscle is present in the majority of reported cases. Since the muscle is of clinical relevance, it should be included in anatomical textbooks by default.

Electromyography of the posterior cricoarytenoid muscles: a consensus guideline.

PURPOSE: Since the introduction of transcutaneous-transcricoidal needle approaches, electromyography (EMG) of the posterior cricoarytenoid muscle (PCA) became easier to perform and teach. Among the Neurolaryngology working group of the European Laryngological Society, several centers have adopted PCA EMG as part of their routine EMG workup in vocal fold immobility collectively gathering long-term experience. The purpose is to give an update and an extension to already existing guidelines on laryngeal EMG with specific regard to PCA EMG. METHODS: Consensus of all co-authors is based on continuous exchange of ideas and on joint laryngeal EMG workshop experiences over at least 7 years. A Delphi method of consensus development was used, i.e., the manuscript was circulated among the co-authors until full agreement was achieved. RESULTS: Step-by-step instructions on how to perform and interpret PCA EMG are provided. CONCLUSIONS: Further research should include the establishment of normal values for PCA and thyroarytenoid muscle (TA) EMG as well as studies on the nature of some unusual activation pattern commonly seen in chronically lesioned PCA.

Triangular body-cover model of the vocal folds with coordinated activation of the five intrinsic laryngeal muscles.

Poor laryngeal muscle coordination that results in abnormal glottal posturing is believed to be a primary etiologic factor in common voice disorders such as non-phonotraumatic vocal hyperfunction. Abnormal activity of antagonistic laryngeal muscles is hypothesized to play a key role in the alteration of normal vocal fold biomechanics that results in the dysphonia associated with such disorders. Current low-order models of the vocal folds are unsatisfactory to test this hypothesis since they do not capture the co-contraction of antagonist laryngeal muscle pairs. To address this limitation, a self-sustained triangular body-cover model with full intrinsic muscle control is introduced. The proposed scheme shows good agreement with prior studies using finite element models, excised larynges, and clinical studies in sustained and time-varying vocal gestures. Simulations of vocal fold posturing obtained with distinct antagonistic muscle activation yield clear differences in kinematic, aerodynamic, and acoustic measures. The proposed tool is deemed sufficiently accurate and flexible for future comprehensive investigations of non-phonotraumatic vocal hyperfunction and other laryngeal motor control disorders.

Laryngeal Muscle-Evoked Potential Recording as an Indicator of Vagal Nerve Fiber Activation.

BACKGROUND: Vagus nerve stimulation (VNS) is an adjunctive therapy for drug-resistant epilepsy. Noninvasive evoked potential recordings in laryngeal muscles (LMEPs) innervated by vagal branches may provide a marker to assess effective vagal nerve fiber activation. We investigated VNS-induced LMEPs in patients with epilepsy in acute and chronic settings. MATERIALS AND METHODS: A total of 17 of 25 patients underwent LMEP recordings at initiation of therapy (acute group); 15 of 25 patients after one year of VNS (chronic group); and 7 of 25 patients were tested at both time points (acute + chronic group). VNS-induced LMEPs were recorded following different pulse widths and output currents using six surface laryngeal EMG electrodes to calculate input/output curves and estimate LMEP latency, threshold current for minimal (Ithreshold), half-maximal (I50), and 95% of maximal (I95) response induction and amplitude of maximal response (Vmax). These were compared with the acute + chronic group and between responders and nonresponders in the acute and chronic group. RESULTS: VNS-induced LMEPs were present in all patients. Ithreshold and I95 values ranged from 0.25 to 1.00 mA and from 0.42 to 1.77 mA, respectively. Estimated mean LMEP latencies were 10 ± 0.1 milliseconds. No significant differences between responders and nonresponders were observed. In the acute + chronic group, Ithreshold values remained stable over time. However, at the individual level in this group, Vmax was lower in all patients after one year compared with baseline. CONCLUSIONS: Noninvasive VNS-induced LMEP recording is feasible both at initiation of VNS therapy and after one year. Low output currents (0.25-1.00 mA) may be sufficient to activate vagal Aα-motor fibers. Maximal LMEP amplitudes seemed to decrease after chronic VNS therapy in patients.

Generation, Coordination, and Evolution of Neural Circuits for Vocal Communication.

In many species, vocal communication is essential for coordinating social behaviors including courtship, mating, parenting, rivalry, and alarm signaling. Effective communication requires accurate production, detection, and classification of signals, as well as selection of socially appropriate responses. Understanding how signals are generated and how acoustic signals are perceived is key to understanding the neurobiology of social behaviors. Here we review our long-standing research program focused on Xenopus, a frog genus which has provided valuable insights into the mechanisms and evolution of vertebrate social behaviors. In Xenopus laevis, vocal signals differ between the sexes, through development, and across the genus, reflecting evolutionary divergence in sensory and motor circuits that can be interrogated mechanistically. Using two ex vivo preparations, the isolated brain and vocal organ, we have identified essential components of the vocal production system: the sexually differentiated larynx at the periphery, and the hindbrain vocal central pattern generator (CPG) centrally, that produce sex- and species-characteristic sound pulse frequencies and temporal patterns, respectively. Within the hindbrain, we have described how intrinsic membrane properties of neurons in the vocal CPG generate species-specific vocal patterns, how vocal nuclei are connected to generate vocal patterns, as well as the roles of neurotransmitters and neuromodulators in activating the circuit. For sensorimotor integration, we identified a key forebrain node that links auditory and vocal production circuits to match socially appropriate vocal responses to acoustic features of male and female calls. The availability of a well supported phylogeny as well as reference genomes from several species now support analysis of the genetic architecture and the evolutionary divergence of neural circuits for vocal communication. Xenopus thus provides a vertebrate model in which to study vocal communication at many levels, from physiology, to behavior, and from development to evolution. As one of the most comprehensively studied phylogenetic groups within vertebrate vocal communication systems, Xenopus provides insights that can inform social communication across phyla.

Savannah roars: The vocal anatomy and the impressive rutting calls of male impala (Aepyceros melampus) - highlighting the acoustic correlates of a mobile larynx.

A retractable larynx and adaptations of the vocal folds in the males of several polygynous ruminants serve for the production of rutting calls that acoustically announce larger than actual body size to both rival males and potential female mates. Here, such features of the vocal tract and of the sound source are documented in another species. We investigated the vocal anatomy and laryngeal mobility including its acoustical effects during the rutting vocal display of free-ranging male impala (Aepyceros melampus melampus) in Namibia. Male impala produced bouts of rutting calls (consisting of oral roars and interspersed explosive nasal snorts) in a low-stretch posture while guarding a rutting territory or harem. For the duration of the roars, male impala retracted the larynx from its high resting position to a low mid-neck position involving an extensible pharynx and a resilient connection between the hyoid apparatus and the larynx. Maximal larynx retraction was 108 mm based on estimates in video single frames. This was in good concordance with 91-mm vocal tract elongation calculated on the basis of differences in formant dispersion between roar portions produced with the larynx still ascended and those produced with maximally retracted larynx. Judged by their morphological traits, the larynx-retracting muscles of male impala are homologous to those of other larynx-retracting ruminants. In contrast, the large and massive vocal keels are evolutionary novelties arising by fusion and linear arrangement of the arytenoid cartilage and the canonical vocal fold. These bulky and histologically complex vocal keels produced a low fundamental frequency of 50 Hz. Impala is another ruminant species in which the males are capable of larynx retraction. In addition, male impala vocal folds are spectacularly specialized compared with domestic bovids, allowing the production of impressive, low-frequency roaring vocalizations as a significant part of their rutting behaviour. Our study expands knowledge on the evolutionary variation of vocal fold morphology in mammals, suggesting that the structure of the mammalian sound source is not always human-like and should be considered in acoustic analysis and modelling.

Genetic identification of a hindbrain nucleus essential for innate vocalization.

Vocalization in young mice is an innate response to isolation or mechanical stimulation. Neuronal circuits that control vocalization and breathing overlap and rely on motor neurons that innervate laryngeal and expiratory muscles, but the brain center that coordinates these motor neurons has not been identified. Here, we show that the hindbrain nucleus tractus solitarius (NTS) is essential for vocalization in mice. By generating genetically modified newborn mice that specifically lack excitatory NTS neurons, we show that they are both mute and unable to produce the expiratory drive required for vocalization. Furthermore, the muteness of these newborns results in maternal neglect. We also show that neurons of the NTS directly connect to and entrain the activity of spinal (L1) and nucleus ambiguus motor pools located at positions where expiratory and laryngeal motor neurons reside. These motor neurons control expiratory pressure and laryngeal tension, respectively, thereby establishing the essential biomechanical parameters used for vocalization. In summary, our work demonstrates that the NTS is an obligatory component of the neuronal circuitry that transforms breaths into calls.

Voice production in a MRI-based subject-specific vocal fold model with parametrically controlled medial surface shape.

The goal of this study was to investigate how realistic changes in medial surface shape, as occur in human phonation, affect voice production. In a parametric magnetic resonance imaging-based three-dimensional vocal fold model, the superior and inferior portions of the medial surface were systematically manipulated to produce different medial surface contours similar to those observed in previous excised larynx and in vivo canine larynx experiments. Voice simulations were performed to investigate the differences in the resulting voice production. The results showed that both superior-medial bulging and inferior-medial bulging of the medial surface, which led to an increased vertical thickness and a more rectangular glottal configuration, increased the closed quotient of vocal fold vibration. Changes in medial surface shape also had significant effects on the phonation threshold pressure. The degree of these effects of changes in medial surface shape was larynx specific, and varied significantly depending on the vocal fold cross-sectional geometry and its variation along the anterior-posterior direction. The results suggest that, in addition to vocal fold approximation, surgical interventions of voice disorders should also aim at restoring a rectangular and sufficiently thick medial surface.

Comparison between Intramuscular Multichannel Electrodes and Supramysial Multichannel Electrodes via EMG Measurements for Potential Use as Larynx Stimulation Electrodes: In Vivo Animal Analysis.

One of the most common causes for larynx paralysis is the injury of the recurrent laryngeal nerve which, among others, causes the paralysis of the posterior cricoarytenoideus muscle (PCA). Electrical stimulation of PCA offers an approach to retaining the function of the paralyzed larynx muscle. The study aim was to test the applicability of an intramuscular multichannel array electrode as a measuring electrode for myoelectrical potentials and as a possible electrode for stimulation, e.g., posterior cricoarytenoideus muscle stimulation. For this purpose, two different kinds of electrodes were compared. 42 intramuscular multichannel array electrodes and 11 supramysial multichannel electrodes were implanted into the triceps brachii muscle of rats. The triceps brachii muscle of rats is suitable to serve as a substitute muscle for the human PCA muscle in an in vivo animal model. It has the same striated muscle cells, is of comparable size, and fundamentally serves a similar function to the human PCA muscle during normal respiration. Walking and breathing are circular functions that cause minimal muscle fatigue when carried out steadily. In total, the myoelectrical activity of 6703 steps could be recorded, allowing a comparison and statistical analysis of the EMG amplitudes and EMG activation patterns. Small differences can be detected between the EMG signals of both electrode types which, however, can be explained physiologically. Both electrode types reveal the basic characteristics of the triceps brachii muscle activity, namely the muscle contraction strength and the coordination pattern. This indicates that the intramuscular electrode may be applied for a detailed analysis of the human larynx.

Coupling relations underlying the production of speech articulator movements and their invariance to speech rate.

Since the seminal works of Bernstein (The coordination and regulation of movements. Pergamon Press, Oxford, 1967) several authors have supported the idea that, to produce a goal-oriented movement in general, and a movement of the organs responsible for the production of speech sounds in particular, individuals activate a set of coupling relations that coordinate the behavior of the elements of the motor system involved in the production of the target movement or sound. In order to characterize the configurations of the coupling relations underlying speech production articulator movements, we introduce an original method based on recurrence analysis. The method is validated through the analysis of simulated dynamical systems adapted to reproduce the features of speech gesture kinematics and it is applied to the analysis of speech articulator movements recorded in five German speakers during the production of labial and coronal plosive and fricative consonants at variable speech rates. We were able to show that the underlying coupling relations change systematically between labial and coronal consonants, but are not affected by speech rate, despite the presence of qualitative changes observed in the trajectory of the jaw at fast speech rate.

Coupling between a fiber-reinforced model and a Hill-based contractile model for passive and active tissue properties of laryngeal muscles: A finite element study.

In this work, a three-dimensional fiber-reinforced model was used to simulate passive stress response of vocal fold muscle tissue undergoing a series of isometric force measurement and a dynamic stretching. It was found that, with proper material constants, the fiber-reinforced model is able to reproduce literature data with acceptable deviation. A Hill-based contractile model was then coupled with the fiber-reinforced model to enable simulations of stretching-induced and activation-induced stress at the same time. For dynamic, concurrent tissue stimulation and stretching, the coupled model demonstrated a good agreement with past experimental data.

Contextually dependent cue realization and cue weighting for a laryngeal contrast in Shanghai Wu.

Phonological categories are often differentiated by multiple phonetic cues. This paper reports a production and perception study of a laryngeal contrast in Shanghai Wu that is not only cued in multiple dimensions, but also cued differently on different manners (stops, fricatives, sonorants) and in different positions (non-sandhi, sandhi). Acoustic results showed that, although this contrast has been described as phonatory in earlier literature, its primary cue is in tone in the non-sandhi context, with vowel phonation and consonant properties appearing selectively for specific manners of articulation. In the sandhi context where the tonal distinction is neutralized, these other cues may remain depending on the manner of articulation. Sonorants, in both contexts, embody the weakest cues. The perception results were largely consistent with the aggregate acoustic results, indicating that speakers adjust the perceptual weights of individual cues for a contrast according to manner and context. These findings support the position that phonological contrasts are formed by the integration of multiple cues in a language-specific, context-specific fashion and should be represented as such.

Bilateral patterns and motor function of the extralaryngeal branching of the recurrent laryngeal nerve.

PURPOSE: To evaluate the bilateral patterns and motor function of the extralaryngeal branches (ELB) of the recurrent laryngeal nerve(RLN). METHODS: This study included 500 consecutive patients who underwent total thyroidectomy. Intraoperative nerve monitoring (IONM) was used in 230 patients. Demographic data, indications for surgery, the bilateral patterns of ELB of the RLN, electromyographic activity of the ELB, distance between the branching point to the entrance into the larynx, and the rate of postoperative morbidity were analyzed. RESULTS: The overall rate of ELB was 27.6% (276/1000). A single trunk of the RLN on both sides was found in 269 (54%) patients, whereas ELB on both sides was observed in 45 (9%) patients. The rates of ELB on the left and right sides were 26.6 and 28.6%, respectively. Of the 89 branched nerves which were dissected using IONM, an evoked motor response was present in 100% of the anterior branches and 5.6% of the posterior branches. The mean branching distance of the RLN was significantly greater in female patients than in male patients on the left side (p = 0.031). The patterns of ELB showed no significant difference in male and female patients. The rates of postoperative transient and permanent hypoparathyroidism and unilateral RLN palsy were 21.6 and 2.8%, and 3.2 and 0.8%, respectively. The rate of RLN palsy was higher in branched nerves compared to those with a single trunk (0.75 vs 0.3%; p = 0.2). CONCLUSION: Unilateral ELB of the RLN might be observed in approximately 1/4 of the patients, while bilateral branching is rare. A few number of posterior branches of the RLN can have motor function. The RLN's with ELB might have a higher risk of injury compared to those with a single trunk.

Structural Organization of the Laryngeal Motor Cortical Network and Its Implication for Evolution of Speech Production.

The laryngeal motor cortex (LMC) is essential for the production of learned vocal behaviors because bilateral damage to this area renders humans unable to speak but has no apparent effect on innate vocalizations such as human laughing and crying or monkey calls. Several hypotheses have been put forward attempting to explain the evolutionary changes from monkeys to humans that potentially led to enhanced LMC functionality for finer motor control of speech production. These views, however, remain limited to the position of the larynx area within the motor cortex, as well as its connections with the phonatory brainstem regions responsible for the direct control of laryngeal muscles. Using probabilistic diffusion tractography in healthy humans and rhesus monkeys, we show that, whereas the LMC structural network is largely comparable in both species, the LMC establishes nearly 7-fold stronger connectivity with the somatosensory and inferior parietal cortices in humans than in macaques. These findings suggest that important "hard-wired" components of the human LMC network controlling the laryngeal component of speech motor output evolved from an already existing, similar network in nonhuman primates. However, the evolution of enhanced LMC-parietal connections likely allowed for more complex synchrony of higher-order sensorimotor coordination, proprioceptive and tactile feedback, and modulation of learned voice for speech production. SIGNIFICANCE STATEMENT: The role of the primary motor cortex in the formation of a comprehensive network controlling speech and language has been long underestimated and poorly studied. Here, we provide comparative and quantitative evidence for the significance of this region in the control of a highly learned and uniquely human behavior: speech production. From the viewpoint of structural network organization, we discuss potential evolutionary advances of enhanced temporoparietal cortical connections with the laryngeal motor cortex in humans compared with nonhuman primates that may have contributed to the development of finer vocal motor control necessary for speech production.

Contributions of rapid neuromuscular transmission to the fine control of acoustic parameters of birdsong.

Neural control of complex vocal behaviors, such as birdsong and speech, requires integration of biomechanical nonlinearities through muscular output. Although control of airflow and tension of vibrating tissues are known functions of vocal muscles, it remains unclear how specific muscle characteristics contribute to specific acoustic parameters. To address this gap, we removed heparan sulfate chains using heparitinases to perturb neuromuscular transmission subtly in the syrinx of adult male zebra finches (Taeniopygia guttata). Infusion of heparitinases into ventral syringeal muscles altered their excitation threshold and reduced neuromuscular transmission changing their ability to modulate airflow. The changes in muscle activation dynamics caused a reduction in frequency modulation rates and elimination of many high-frequency syllables but did not alter the fundamental frequency of syllables. Sound amplitude was reduced and sound onset pressure was increased, suggesting a role of muscles in the induction of self-sustained oscillations under low-airflow conditions, thus enhancing vocal efficiency. These changes were reversed to preinfusion levels by 7 days after infusion. These results illustrate complex interactions between the control of airflow and tension and further define the importance of syringeal muscle in the control of a variety of acoustic song characteristics. In summary, the findings reported here show that altering neuromuscular transmission can lead to reversible changes to the acoustic structure of song. Understanding the full extent of muscle involvement in song production is critical in decoding the motor program for the production of complex vocal behavior, including our search for parallels between birdsong and human speech motor control. NEW & NOTEWORTHY: It is largely unknown how fine motor control of acoustic parameters is achieved in vocal organs. Subtle manipulation of syringeal muscle function was used to test how active motor control influences acoustic parameters. Slowed activation kinetics of muscles reduced frequency modulation and, unexpectedly, caused a distinct decrease in sound amplitude and increase in phonation onset pressure. These results show that active control enhances the efficiency of energy conversion in the syrinx.

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

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. Muscle Nerve 55: 706-714, 2017.

The functional role of the pharyngeal plexus in vocal cord innervation in humans.

Classical understanding of the function of the pharyngeal plexus in humans is that it relies on both motor branches for innervation of the majority of pharyngeal muscles and sensory branches for the pharyngeal wall sensation. To date there has been no reported data on the role of the pharyngeal plexus in vocal cord innervation. The aim of this study is to evaluate whether or not the plexus pharyngeus contributes to the innervation of the vocal cords. One hundred twenty-five sides from 79 patients (59 female, 20 male) undergoing thyroid surgery with intraoperative neuromonitoring were prospectively evaluated. While vocal cord function was evaluated with endotracheal tube surface electrodes, cricothyroid and cricopharyngeal muscle electromyographic recordings were obtained with a pair of needle electrodes. The ipsilateral pharyngeal plexus, external branch of the superior laryngeal nerve, and recurrent laryngeal nerve were stimulated with a monopolar probe at 1 mA. With stimulation of the plexus pharyngeus on 125 operated sides, positive electromyographic waveforms were detected from five ipsilateral vocal cords (accounting for 3.2% of all vocal cords monitored and 6.3% of patients). The mean EMG amplitude of the vocal cords with stimulation of the plexus pharyngeus was 147 ± 35.5 µV (range 110-203). In one case, the long latency time of 19.8 ms correlated with innervation by the glottic closure reflex pathway. The short latencies seen in the other four cases [3.9 ± 1.1 ms (range 3.2-5.5)] correlated with direct innervation. In some cases, the plexus pharyngeus may contribute to vocal cord innervation by reflex or direct innervation patterns in humans.

Intraoperative monitoring of the recurrent laryngeal nerve by vagal nerve stimulation in thyroid surgery.

The aim of the present study was to evaluate the thyroarytenoid muscle response during bilateral thyroid surgery using vagal nerve stimulation. 195 patients (390 nerves at risk) underwent a total thyroidectomy. The recurrent laryngeal nerve's function was checked by analyzing the amplitude and the latency of the thyroarytenoid muscle's responses after a vagal nerve's stimulation (0.5 and 1 mA) using the NIM3 Medtronic system. All patients were submitted to preoperative and postoperative laryngoscopy. 20 patients get no thyroarytenoid muscle response to the vagal nerve stimulation, and 14 postoperative recurrent laryngeal nerve palsies were confirmed (3.8 %). Two palsies were present after 6 months (0.51 %). All the patients with muscle's response have normal mobility vocal fold. The test sensitivity was 100 % and the test specificity was 98 %. Physiologically, the mean latencies of the muscular potentials for the right RLN were, respectively, 3.89 and 3.83 ms (p > 0.05) for the stimulation at 0.5 and 1 mA. The mean latencies for the left RLN were, respectively, 6.25 and 6.22 ms for the stimulation at 0.5 and 1 mA (p > 0.05). The difference of the latencies between the right and the left nerve was 2.30 ms (1.75-3.25 ms) with a stimulation of 0.5 or 1 mA (p < 0.05). Thyroarytenoid muscle's response via a vagal nerve stimulation showed a functional asymmetry of the laryngeal adduction with a faster right response. Surgically, this method can predict accurately an immediate postoperative vocal folds function in patients undergoing a bilateral thyroid surgery.