Cross-innervation of the thyroarytenoid muscle by a branch from the external division of the superior laryngeal nerve

Role of the postinspiratory complex in regulating swallow-breathing coordination and other laryngeal behaviors

Breathing needs to be tightly coordinated with upper airway behaviors, such as swallowing. Discoordination leads to aspiration pneumonia, the leading cause of death in neurodegenerative disease. Here, we study the role of the postinspiratory complex (PiCo) in coordinating breathing and swallowing. Using optogenetic approaches in freely breathing anesthetized ChATcre:Ai32, Vglut2cre:Ai32 and intersectional recombination of ChATcre:Vglut2FlpO:ChR2 mice reveals PiCo mediates airway protective behaviors. Activation of PiCo during inspiration or the beginning of postinspiration triggers swallow behavior in an all-or-nothing manner, while there is a higher probability for stimulating only laryngeal activation when activated further into expiration. Laryngeal activation is dependent on stimulation duration. Sufficient bilateral PiCo activation is necessary for preserving the physiological swallow motor sequence since activation of only a few PiCo neurons or unilateral activation leads to blurred upper airway behavioral responses. We believe PiCo acts as an interface between the swallow pattern generator and the preBötzinger complex to coordinate swallow and breathing. Investigating PiCo's role in swallow and laryngeal coordination will aid in understanding discoordination with breathing in neurological diseases.

Distinct modulation of tracheal and laryngopharyngeal cough via superior laryngeal nerve in cat

Unilateral and bilateral cooling and bilateral transsection of the superior laryngeal nerve (SLN) were employed to modulate mechanically induced tracheobronchial (TB) and laryngopharyngeal (LPh) cough in 12 anesthetized cats. There was little effect of SLN block or cut on TB. Bilateral SLN cooling reduced the number of LPh (<50 %, p < 0.05), amplitudes of diaphragm EMG activity (<55 %, p < 0.05), and cough expiratory efforts (<40 %, p < 0.01) during LPh. Effects after unilateral SLN cooling were less pronounced. Temporal analysis of LPh showed only shortening of diaphragm and abdominal muscles burst overlap in the inspiratory-expiratory transition after unilateral SLN cooling. Bilateral cooling reduced both expiratory phase and total cough cycle duration. There was no significant difference in the average effects of cooling left or right SLN on LPh or TB as well as no differences in contralateral and ipsilateral diaphragm and abdominal EMG amplitudes. Our results show that reduced afferent drive in the SLN markedly attenuates LPh with virtually no effect on TB.

Extubation and the Risks of Coughing and Laryngospasm in the Era of Coronavirus Disease-19 (COVID-19)

The coronavirus disease-19 (COVID-19) pandemic has prompted new interest among anesthesiologists and intensivists in controlling coughing and expectoration of potentially infectious aerosolized secretions during intubation and extubation. However, the fear of provoking laryngospasm may cause avoidance of deep or sedated extubation techniques which could reduce coughing and infection risk. This fear may be alleviated with clear understanding of the mechanisms and effective management of post-extubation airway obstruction including laryngospasm. We review the dynamic function of the larynx from the vantage point of head-and-neck surgery, highlighting two key concepts:

1. The larynx is a complex organ that may occlude reflexively at levels other than the true vocal folds;

2. The widely held belief that positive-pressure ventilation by mask can “break” laryngospasm is not supported by the otorhinolaryngology literature.

We review the differential diagnosis of acute airway obstruction after extubation, discuss techniques for achieving smooth extubation with avoidance of coughing and expectoration of secretions, and recommend, on the basis of this review, a clinical pathway for optimal management of upper airway obstruction including laryngospasm to avoid adverse outcomes.

Investigation on EMG Profiles of the Superior Laryngeal Nerve in a In Vivo Porcine Model

BACKGROUND

The electromyographic (EMG) profiles of external branch of the superior laryngeal nerve (EBSLN) have been defined and the optimal intensity of the stimulation of EBSLN in an in vivo porcine model has been explored.

MATERIALS

EMG was simultaneously registered by the surface of endotracheal tube and needle electrodes by applying a monopolar stimulation probe in 12 piglets (22 EBSLNs). Vagal nerve (VN), RLN and EBSLN were excited to record the EMG tracings and cricothyroid muscle twitch (CTM). VN, RLN and EBSLN were stimulated from 0.1 to 1.0 mA. Cmin and Cmax have been defined as the minimum and maximal stimulation to evoke an EMG response.

RESULTS

The stimulation resulted in a dose-response curve. Cmin were 0.19 mA (0.04-0.4), 0.19 mA (0.08-0.3) and 0.21 mA (0.1-0.4) for EBSLN, RLN and VN (p > 0.05) respectively. Cmax were 0.6 mA along with an amplitude value of 396 ± 330 μV, 0.5 mA including 1058 ± 382 μV, 0.8 mA coupled with 870 ± 382 μV, equally for EBSLN, RLN and VN (p > 0.05) respectively. No asymmetry of amplitude responses each side for EBSLN, RLN and VN (p = 0.317, p = 0.203 and p = 0.468, respectively) was noted. The amplitudes of EBSLN were significantly lower than RLNs and VN (42% of RLN and 50% of VN amplitude rates). Also, CTM twitch was always detectable with the stimulation of EBSLN.

CONCLUSIONS

Cmin and Cmax of EBSLN were comparable to RLN and VN standards. The amplitude stimulus-response curves of RLN, VN and EBSLN were highly variable. It has been suggested to apply a stimulation of 1.0 mA and a visual appreciation of CTM twitch for the identification of EBSLN.

Recovery of Voice After Reconstruction of the Recurrent Laryngeal Nerve and Adjuvant Nimodipine

Background

Transection injury to the recurrent laryngeal nerve (RLN) has been associated with permanent vocal fold palsy, and treatment has been limited to voice therapy or local treatment of vocal folds. Microsurgical repair has been reported to induce a better function. The calcium channel antagonist nimodipine improves functional recovery after experimental nerve injury and also after cranial nerve injury in patients. This study aims to present voice outcome in patients who underwent repair of the RLN and received nimodipine during regeneration.

Methods

From 2002–2016, 19 patients were admitted to our center with complete unilateral injury to the RLN and underwent microsurgical repair of the RLN. After nerve repair, patients received nimodipine for 2–3 months. Laryngoscopy was performed repeatedly up to 14 months postoperatively. The Voice Handicap Index (VHI) was administered, and patients’ maximum phonation time (MPT) was recorded during the follow-up.

Results

All patients recovered well after surgery, and nimodipine was well tolerated with no dropouts. None of the patients suffered from atrophy of the vocal fold, and some patients even showed a small ab/adduction of the vocal fold on the repaired side with laryngoscopy. During long-term follow-up (>3 years), VHI and MPT normalized, indicating a nearly complete recovery from unilateral RLN injury.

Conclusions

In this cohort study, we report the results of the first 19 consecutive cases at our center subjected to reconstruction of the RLN and adjuvant nimodipine treatment. The outcome of the current strategy is encouraging and should be considered after iatrogenic RLN transection injuries.

Anatomy of the external branch of the superior laryngeal nerve in Asian population

Injury to the external branch of the superior laryngeal nerve (eSLN) can cause a hoarse or weak voice with dysergia of the cricothyroid. The present study provided the topographic information of the eSLN in the Asian and verified anatomical validity of the landmarks previously recruited to localize the eSLN. Thirty specimens were dissected from 16 human embalmed cadavers (12 men and four women; mean age: 80.5 years). The vertical distance between the eSLN and the apical pole of the thyroid gland (AP) was 8.2 ± 4.2 mm. It descended over the AP with <1 cm distance in 51.7%, >1 cm distance in 27.6% and under the AP in 20.7%. The piercing point (PP) of the eSLN to the muscles located 26.0 ± 5.5 mm posterior and 14.7 ± 5.0 mm inferior to the laryngeal prominence. Generally, the PP located superoposterior to the midpoint of the joint between the joint of inferior constrictor and cricothyroid (ICJ). The distance between the PP and the midpoint was 8.7 ± 5.1 mm. We found that 1) the Asian had the eSLN located over the AP with <1 cm distance about half cases, 2) the PP can be a consistent reference for the eSLN identification, 3) the ICJ can be a useful landmark to preserve the eSLN at the PP.

Injury of the external branch of the superior laryngeal nerve in thyroid surgery

The external branch of the superior laryngeal nerve (EBSLN) innervates the cricothyroid muscle (CTM) to promote lengthening and thinning of the vocal fold, thus increasing voice pitch. The close relation with the superior thyroid vessels (STV) puts the EBSLN in risk every time the superior pole of the thyroid is dissected. It travels downward to innervate the CTM, lateral to the thyroid cartilage and to the inferior pharyngeal constrictor muscle (IPCM), being eventually covered by this muscle fibers as it approaches its entry point. During its descending course, the EBSLN curves and crosses the STV posteriorly. The lower this crossing occurs in the neck, the higher the risk of surgical damage to the nerve by transection, traction, entrapment, thermal damage or disrupted blood supply. The chances of surgical trauma are also increased by size and weight of the specimen, shorter neck length and non-white ethnicity. Voice changes following thyroid surgery are common and multifactorial. The actual rate of vocal impairment due to EBSLN injury is unclear, since changes to the everyday speaking voice can be minimal and laryngeal findings are usually subtle and controversial. CTM electroneuromyography (EMG) is the most accurate tool to diagnose abnormal EBSLN conductivity, but it is technically difficult and barely applicable in routine practice. Recommended approaches to prevent injury include: (I) individual distal ligature of the STV by the thyroid capsule; (II) visual identification of the nerve and its trajectory and (III) electrostimulation with either observation of CTM twitch or intraoperative nerve monitoring (IONM) via dedicated endotracheal tube electrodes. There is accumulating evidence that a combination of visual and standardized electrophysiological EBSLN identification with meticulous division of the STV improves preservation rates. IONM bears the additional benefits of prognostication, quantification and documentation of neural function once it allows intraoperative laryngeal EMG.

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 electrophysiology of thyroid surgery: electrophysiologic and muscular responses with stimulation of the vagus nerve, recurrent laryngeal nerve, and external branch of the superior laryngeal nerve

OBJECTIVES/HYPOTHESIS

Correlation of physiologically important electromyographic (EMG) waveforms with demonstrable muscle activation is important for the reliable interpretation of evoked waveforms during intraoperative neural monitoring (IONM) of the vagus nerve, recurrent laryngeal nerve (RLN), and external branch of the superior laryngeal nerve (EBSLN) in thyroid surgery.

STUDY DESIGN

Retrospective chart review.

METHODS

Data were reviewed retrospectively for thyroid surgery patients with laryngeal nerve IONM from January to December, 2015. EMG responses to monopolar stimulation of the vagus/RLN and EBSLN were recorded in bilateral vocalis, cricothyroid (CTM), and strap muscles using endotracheal tube-based surface and intramuscular hook electrodes, respectively. Target muscles for vagal/RLN and EBSLN stimulation were the ipsilateral vocalis and CTM, respectively. All other recording channels were nontarget muscles.

RESULTS

Fifty surgical sides were identified in 37 subjects. All target muscle mean amplitudes were significantly higher than in nontarget muscles. With vagal/RLN stimulation, target ipsilateral vocalis mean amplitude was 1,095.7 μV (mean difference range = -814.1 to -1,078 μV, P < .0001). For EBSLN stimulation, target ipsilateral CTM mean amplitude was 6,379.3 μV (mean difference range = -6,222.6 to -6,362.3 μV, P < .0001). Target muscle large-amplitude EMG responses correlated with meaningful visual or palpable muscular responses, whereas nontarget EMG responses showed no meaningful muscle activation.

CONCLUSIONS

Target and nontarget laryngeal muscles are differentiated based on divergence of EMG response directly correlating with presence or absence of visual and palpable muscle activation. Low-amplitude EMG waveforms in nontarget muscles with neural stimulation can be explained by the concept of far-field artifactual waveforms and do not correspond to a true muscular response. The surgeon should be aware of these nonphysiologic waveforms when interpreting and applying IONM during thyroid surgery.

LEVEL OF EVIDENCE

4 Laryngoscope, 127:764-771, 2017.

Office-Based Laryngoscopic Observations of Recurrent Laryngeal Nerve Paresis and Paralysis

Objectives:

To evaluate the endoscopic criteria of recurrent laryngeal nerve disorders, we performed a retrospective evaluation of videolaryngoscopic recordings from 50 patients with recurrent laryngeal nerve disorders.

Methods:

The videolaryngoscopic examination was performed with rigid and flexible endoscopes. The range of motion of three laryngeal structures was assessed: the vocal ligament, the vocal process, and the arytenoid “hump” (mainly the corniculate region).

Results:

Comparison of movement of these three structures revealed discrepancies. In 16 of 45 patients (36%) rigid endoscopy showed movements of the arytenoid hump associated with absence of any mobility of the vocal process and vocal ligament. In 5 patients the extent of movement of the vocal process and vocal ligament was less than that of the arytenoid hump. Only in 24 of 45 cases were the ratings for the vocal process, vocal ligament, and arytenoid hump identical. The findings of fiberscopy were comparable.

Conclusions:

In assessing recurrent laryngeal nerve disorders via laryngoscopy, sole judgment of the arytenoid hump movement can mislead. Our interpretation suggests that visible movement of the mucosa covering the arytenoid and accessory cartilages is not always paralleled by movement of the arytenoid cartilage itself. It was shown that the best criterion to rely on in endoscopy is movement of the vocal process or the vocal ligament.

External Branch of the Superior Laryngeal Nerve Mediated Glottic Closing Force in the Porcine Model

Objectives:

Based on our laboratory’s newly confirmed motor pathway for glottic closure, we measured the glottic closing force (GCF) during isolated stimulation of the external branch of the superior laryngeal nerve (eSLN) in the porcine model. Glottic closure is 1 of the primary mechanisms for prevention of aspiration during deglutition.

Methods:

The recurrent laryngeal nerve (RLN) and eSLN were identified bilaterally in 4 porcine necks. Subsequently, GCF was measured with a pressure transducer as the distal ends of individual nerves were stimulated in 4 animals. The RLN mediated GCF was measured first, followed by isolated eSLN mediated GCF, followed by transection of the RLN and repeat measurement of the eSLN GCF. Ultimately, the cricothyroid (CT) muscle attachment was released and the GCF measured.

Results:

The GCF during isolated eSLN stimulation before and after RLN transection is approximately 89% of the RLN mediated GCF in each animal. The GCF after CT release is approximately 84% of the RLN perceived GCF. Transection of the RLN did not alter the eSLN observed GCF.

Conclusions:

The GCF obtained during isolated eSLN stimulation is adequate for delivery of an appropriate laryngeal protective response and may be considered a target motor nerve for augmenting GCF in selected rehab settings.

Intraoperative neuromonitoring of the external branch of the superior laryngeal nerve during thyroidectomy: the need for evidence-based data and perioperative technical/technological standardization

The external branch of the superior laryngeal nerve (EBSLN) is surgically relevant since its close anatomical proximity to the superior thyroid vessels. There is heterogeneity in the EBSLN anatomy and EBSLN damage produces changes in voice that are very heterogenous and difficult to diagnose. The reported prevalence of EBSLN injury widely ranges. EBSLN iatrogenic injury is considered the most commonly underestimated complication in endocrine surgery because vocal assessment underestimates such event and laryngoscopic postsurgical evaluation does not show standardized findings. In order to decrease the risk for EBSLN injury, multiple surgical approaches have been described so far. IONM provides multiple advantages in the EBSLN surgical approach. In this review, we discuss the current state of the art of the monitored approach to the EBSLN. In particular, we summarize, providing our additional remarks, the most relevant aspects of the standardized technique brilliantly described by the INMSG (International Neuromonitoring Study Group). In conclusion, in our opinion, there is currently the need for more prospective randomized trials investigating the electrophysiological and pathological aspects of the EBSLN for a better understanding of the role of IONM in the EBSLN surgery.

Contribution of the pharyngeal plexus to vocal cord adduction

OBJECTIVES/HYPOTHESIS

The aim of this study was to elucidate whether the pharyngeal plexus (PP) contributes to vocal cord adduction in the porcine model.

STUDY DESIGN

Experimental study.

METHODS

Thyroarytenoid (TA) muscle contraction was recorded electromyographically with electrical stimulation of the internal branch of the superior laryngeal nerve (iSLN) and PP in seven pigs.

RESULTS

Glottic closure reflex (GCR) was detected with stimulation of the iSLN in all subjects. Electrical stimulation of the PP elicited a response from the TA in six of the seven pigs. In one subject, TA electromyography (EMG) mean latency was noted to be 18.8 msec, approximating that of GCR elicited by iSLN stimulation. Antidromic nerve pulses generated the responses from the TA with a medium latency (mean, 6.32 msec) in three of the seven pigs. TA EMG with a short latency (mean, 1.8 msec) was identified in two of the seven pigs.

CONCLUSIONS

Electrical stimulation of both central and distal ends of the PP elicited vocal cord adduction. These results support the promise of future rehabilitative uses of the PP to treat dysfunctions of the iSLN or as a means of addressing many age-related or metabolic causes of protective laryngeal paresis.

LEVEL OF EVIDENCE

NA.

Spontaneous laryngeal reinnervation following chronic recurrent laryngeal nerve injury

OBJECTIVES/HYPOTHESIS

To enhance understanding of spontaneous laryngeal muscle reinnervation following severe recurrent laryngeal nerve injury by testing the hypotheses that 1) nerve fibers responsible for thyroarytenoid muscle reinnervation can originate from multiple sources and 2) superior laryngeal nerve is a source of reinnervation.

STUDY DESIGN

Prospective, controlled, animal model.

METHODS

A combination of retrograde neuronal labeling techniques, immunohistochemistry, electromyography, and sequential observations of vocal fold mobility were employed in rat model of chronic recurrent laryngeal nerve injury. The current study details an initial set of experiments in sham surgical and denervated group animals and a subsequent set of experiments in a denervated group.

RESULTS

At 3 months after recurrent laryngeal nerve resection, retrograde brainstem neuronal labeling identified cells in the characteristic superior laryngeal nerve cell body location as well as cells in a novel caudal location. Regrowth of neuron fibers across the site of previous recurrent laryngeal nerve resection was seen in 87% of examined animals in the denervated group. Electromyographic data support innervation by both the superior and recurrent laryngeal nerves following chronic recurrent laryngeal nerve injury.

CONCLUSIONS

Following chronic recurrent laryngeal nerve injury in the rat, laryngeal innervation is demonstrated through the superior laryngeal nerve from cells both within and outside of the normal cluster of cells that supply the superior laryngeal nerve. The recurrent laryngeal nerve regenerates across a surgically created gap, but functional significance of regenerated nerve fibers is unclear.

New animal model for assessment of functional laryngeal motor innervation

OBJECTIVES

The functional motor innervation of the larynx is not fully understood because of the complexity of the peripheral neuroanatomy. Since the late 19th century, there has been controversy regarding the role of the superior laryngeal nerve, which may have wider motor projections than are currently acknowledged. The aim of this study was to develop a large animal model to characterize and quantify the functional motor input to the intrinsic laryngeal muscles.

METHODS

We performed invasive electrophysiology (evoked electromyography) in normal pigs.

RESULTS

The thyroarytenoid, lateral cricoarytenoid, and posterior cricoarytenoid muscles receive dual innervation from both the superior and recurrent laryngeal nerves, whereas the cricothyroid muscle is innervated only by the superior laryngeal nerve.

CONCLUSIONS

The dual innervation pattern from both laryngeal nerves supports the concept of a laryngeal nerve plexus. The motor input through the external branch of the superior laryngeal nerve was surprisingly high. The animal model presented here may be used in future investigations of laryngeal reinnervation following nerve injury.

Superior laryngeal nerve loop: patterns and surgical implications

This study clarifies the patterns of the superior laryngeal nerve loop (SLN loop), connecting the cervical sympathetic chain (CSC) and the SLN and its branches, so as to provide an anatomic basis for decreasing the risk of injury to the external laryngeal nerve (ELN) during neck surgery. Fifty Chinese adult human cadavers fixed with 4 % formalin were dissected, and their SLN loop patterns were analyzed and summarized. In 98 of 100 sides the CSC anastomosed with the SLN and its branches, forming a looped nerve structure which we called the SLN loop. The SLN loops could be divided into five types: e ( n ), t ( n ), i ( n ), t ( n ) e ( n ), and i ( n ) e ( n ) based on morphological variations. The results demonstrated that e ( n ) was most frequently found in the samples (82/100) followed by t ( n ) (9/100), i ( n ) (3/100), t ( n ) e ( n ) (2/100), and i ( n ) e ( n ) (2/100). Comparing with the previous work, we identified additional 18 subtypes of the SLN loop. The relations of the SLN loop to the surrounding structures were complicated, which brought more challenges to thyroidectomy. Thus, we do not advocate routine identification of ELN/ELN loop during the process of thyroidectomy, especially systematic identification of ELN during operation. However, this study introduces the possibility that nerve injury can be avoided by exposure of the nerve via careful dissection in the region of the superior pole of the thyroid gland to the extent that we can initiate individual ligation of the superior polar vessels, along with the help of neuromonitors, video monitors, and magnifying loupes.

Chiropractic care for a patient with spasmodic dysphonia associated with cervical spine trauma

OBJECTIVE

To discuss the diagnosis and response to treatment of spasmodic dysphonia in a 25-year-old female vocalist following an auto accident.

CLINICAL FEATURES

The voice disorder and neck pain appeared after the traumatic incident. Examination of the cervical spine revealed moderate pain, muscle spasm and restricted joint motion at C-1 and C-5 on the left side. Cervical range of motion was reduced on left rotation. Bilateral manual muscle testing of the trapezius and sternocleidomastoid muscles, which share innervation with the laryngeal muscles by way of the spinal accessory nerve, were weak on the left side. Pre and post accident voice range profiles (phonetograms) that measure singing voice quality were examined. The pre- and post-accident phonetograms revealed significant reduction in voice intensity and fundamental frequency as measured in decibels and hertz.

INTERVENTION AND OUTCOME

Low-force chiropractic spinal manipulative therapy to C-1 and C-5 was employed. Following a course of care, the patient's singing voice returned to normal, as well as a resolution of her musculo- skeletal complaints.

CONCLUSION

It appears that in certain cases, the singing voice can be adversely affected if neck or head trauma is severe enough. This case proposes that trauma with irritation to the cervical spine nerve roots as they communicate with the spinal accessory, and in turn the laryngeal nerves, may be contributory in some functional voice disorders or muscle tension dysphonia.

The central projections of the laryngeal nerves in the rat

The larynx serves respiratory, protective, and phonatory functions. The motor and sensory innervation to the larynx controlling these functions is provided by the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN). Classical studies state that the SLN innervates the cricothyroid muscle and provides sensory innervation to the supraglottic cavity, whereas the RLN supplies motor innervation to the remaining intrinsic laryngeal muscles and sensory innervation to the infraglottic cavity, but recent data suggest a more complex anatomical and functional organisation. The current neuroanatomical tracing study was undertaken to provide a comprehensive description of the central brainstem connections of the axons within the SLN and the RLN, including those neurons that innervate the larynx. The study has been carried out in 41 adult male Sprague-Dawley rats. The central projections of the laryngeal nerves were labelled following application of biotinylated dextran amines onto the SLN, the RLN or both. The most remarkable result of the study is that in the rat the RLN does not contain any afferent axons from the larynx, in contrast to the pattern observed in many other species including man. The RLN supplied only special visceromotor innervation to the intrinsic muscles of the larynx from motoneurons in the nucleus ambiguus (Amb). All the afferent axons innervating the larynx are contained within the SLN, and reach the nucleus of the solitary tract. The SLN also contained secretomotor efferents originating from motoneurons in the dorsal motor nucleus of the vagus, and special visceral efferent fibres from the Amb. In conclusion, the present study shows that in the rat the innervation of the larynx differs in significant ways from that described in other species.

The clinical interest of the ary-thyro-cricoid fascicle

The aim of this work was to study the prevalence and form of the ary-thyro-cricoid (ATC) muscular fascicle, a variable muscular slip connecting the oblique and/or transverse arytenoid muscles with the thyroarytenoid (TA) and/or lateral cricoarytenoid (LCA) muscles resembling a sphincter encircling the glottis. Thirty larynges obtained from necropsies of individuals with no known laryngeal pathology were dissected. The ATC fascicle was observed in 96.7% of the larynges. It appeared bilaterally in 60% of subjects and unilaterally in 36.7%. The posterior attachment of the muscular slip was observed to be in common with either the transverse arytenoid (34%), or the oblique arytenoid (28%) or both muscles (38%). Its fibers terminated by intermingling with either those of the LCA muscle (10.6%), or the TA muscle (38.3%) or both (51.1%). These variable attachments mean that there are nine possible variants of this muscular fascicle. The ATC fascicle was supplied by branches originating bilaterally from the recurrent laryngeal (RLN) and internal laryngeal nerves. The existence of this ATC fascicle could explain the variable position (intermediate, paramedian or lateral) adopted by the vocal folds after lesion of the RLN. The bilateral disposition and innervation of the fascicle could also complicate the interpretation of electromyographic techniques used for testing laryngeal nerve function.

Transverse cordotomy and cuneiform cartilage amputation for the management of laryngeal paralysis in a dog

This paper describes the novel approach of transverse cordotomy and cuneiform cartilage amputation for the surgical management of laryngeal paralysis in a dog.

[Function and dysfunction of the superior laryngeal nerve]

BACKGROUND

Diseases of the superior laryngeal nerve occur more often than thought. Paresis of the superior laryngeal nerve is one of the most frequent causes of dysfunction of this nerve, mainly occurring in the context of thyroid surgery.

RESULTS AND CONCLUSION

To avoid iatrogenic damage of the superior laryngeal nerve, knowledge of the exact topography of the course of this nerve is necessary. A lesion of the superior laryngeal nerve may be associated with changes in the voice as well as with swallowing disorders with aspiration. Diagnosis of this disease is clinically challenging because the clinical symptoms are frequently not pronounced. Besides laryngoscopy, stroboscopy and electromyography are required for exact diagnosis.

Collateral reinnervation by the superior laryngeal nerve after recurrent laryngeal nerve injury

This study investigates the role of the intact superior laryngeal nerve (SLN) in the reinnervation process of one of the laryngeal muscles, the posterior cricoarytenoid muscle (PCA), following recurrent laryngeal nerve (RLN) injury. Using a chronic RLN injury model in the adult rat, PCA reinnervation was assessed by retrograde double-tracing techniques in combination with electrophysiology and immunohistochemistry of muscle sections. The results demonstrate that the PCA receives dual innervation from both laryngeal nerves even in the uninjured system. Functionally significant collateral reinnervation originates from intact SLN fibers following RLN injury, mainly due to intramuscular sprouting rather than by recruitment of more motor neurons. This may be important when choosing surgical and/or medical treatment for patients with RLN injury.

The human cricothyroid muscle: three muscle bellies and their innervation patterns

We hypothesized that the phonatory and respiratory functions of the human cricothyroid (CT) muscle are subserved by separately controlled muscle bellies. In this work, 30 autopsied adult human hemilarynges were used to determine the neuromuscular organization of the CT muscle using microdissection, histology, and Sihler's stain. The results showed that the human CT was composed of three bellies: rectus, oblique, and horizontal. External superior laryngeal nerve (ESLN) was found to enter into the CT muscle as a single trunk (37.5%) or multiple (two to five) branches (62.5%). Within the CT muscle, the ESLN gave off three to seven branches to innervate the rectus belly and one or two branches to supply the oblique and horizontal bellies, respectively. Notably, ESLN also gave off branches to innervate the ipsilateral thyroarytenoid muscle (46%) and subglottic mucosa (67%) or connect with the recurrent laryngeal nerve (25%). These findings suggest that the CT bellies appear to be functionally designed for different motor tasks. The data are also useful for further clarifying the functions of the CT bellies and the ESLN branches and for developing belly-based reinnervation procedures to treat laryngeal paralysis.

Is the external laryngeal nerve an exclusively motor nerve? The cricothyroid connection branch

OBJECTIVES

To obtain an accurate morphological description of the nerve that provides communication between the external laryngeal nerve and the recurrent laryngeal nerve in a large sample of human larynges.

STUDY DESIGN

Morphological study of human larynges.

METHODS

Microdissection of 103 human larynges obtained from necropsies was performed. Both the external laryngeal nerve and the recurrent laryngeal nerve were identified and dissected. The existence of a communication between the two nerves was evaluated.

RESULTS

A neural communication between the external laryngeal nerve and the recurrent laryngeal nerve was found in 85% of the larynges studied (bilaterally in 44% and unilaterally in 41%). Two patterns of communication (with single or double branches) were found. The communication between the nerves can appear at different levels, giving off collateral branches to the cricothyroid muscle (23%), the cricothyroid joint capsule (49%), and the subglottic mucosa (50%).

CONCLUSIONS

The nerve supply of the human larynx is more complex than classically considered. Our study does not support the classic neuroanatomical belief that the external laryngeal nerve is purely a motor nerve to the cricothyroid muscle. Our results reveal that it carries sensory and motor fibers on to other muscles (thyroarytenoid) or regions (subglottis mucosa and cricothyroid joint), as has been demonstrated in experimental studies in cats and dogs.

Impact of vocal cord paralysis on cricoarytenoid joint

To demonstrate structural changes in the cricoarytenoid joint after recurrent laryngeal nerve paralysis, we performed a laboratory investigation of fixed arytenoid cartilages from adult humans obtained during laser surgical arytenoidectomy in cases of bilateral vocal fold paralysis, analyzing the articular cartilage, the joint capsule, and the attached laryngeal musculature. Ten arytenoid cartilages from adult humans were studied by means of histology, as well as scanning and transmission electron microscopy. After long-standing denervation (>6 months), all arytenoid cartilages showed degenerative changes in their joint surface structure at various levels of intensity. The articular surface revealed fibrillation in some places, demasking of collagen fibrils next to the joint surface, and formation of chondrocyte clusters near the joint surface. All specimens also showed muscle atrophy. We conclude that long-standing recurrent laryngeal nerve paralysis does not result in ankylosis of the cricoarytenoid joint, as assumed, but the articular cartilage undergoes structural changes comparable to those in osteoarthritis. Structural changes in the articular cartilage and in the surrounding musculature hamper efforts at joint function recovery, as do procedures aiming solely at either medialization or lateralization of the vocal fold.

Lack of evoked laryngeal electromyography response in patients with a clinical diagnosis of vocal cord paralysis

There has been recent debate about whether patients with vocal cord immobility have a neurologic paralysis or whether synkinesis, the misdirection of axons to competing laryngeal muscles, is responsible for the lack of voluntary vocal cord motion. This issue was studied in 15 patients with vocal cord paralysis who underwent laryngeal reinnervation. Evoked electromyography was performed with a surface electrode endotracheal tube. The recurrent laryngeal nerve (RLN) was identified and stimulated with constant current. Of the 15 patients, only 1 produced a compound muscle action potential upon nerve stimulation. The remaining 14 patients had no evoked response during RLN stimulation. A control group of 8 patients with normal vocal cord mobility was studied, and each had a normal evoked electromyography response after RLN stimulation. These results support the assertion that patients who require treatment for vocal cord paralysis do not have synkinesis produced by RLN reinnervation.

An anatomical study of anastomoses between the laryngeal nerves

OBJECTIVE

To systematize the anatomy of the connecting branches between laryngeal nerves.

METHODS

Microdissection of 90 larynges obtained from necropsies (57 men and 33 women; age range, 41-95 y).

RESULTS

Anastomoses between the internal and recurrent nerves appeared in four different patterns: 1) Galen's anastomosis, as the connection between the dorsal branches of both nerves (100%); 2) arytenoid plexus, as the connection between the arytenoid branches of both nerves, in relation with the arytenoid muscle, and divided in a deep part (100%) and a superficial part (86%); 3) cricoid anastomosis, previously only described in cows, located in the front of the cricoid lamina (6/10 cases); and 4) thyroarytenoid anastomosis, as the connection of a descending branch of the internal laryngeal nerve and an ascending branch of the recurrent nerve (14%). Anastomosis between the internal laryngeal and the external laryngeal nerves appeared as a connecting branch throughout the foramen thyroideum (21%). Anastomosis between the external laryngeal and recurrent nerves appeared as a connecting branch throughout the cricothyroid muscle (68%).

CONCLUSION

At least two anastomoses (Galen's anastomosis and arytenoid plexus) appeared in 21% of hemilarynges, and 79% of cases had three or more anastomoses between the laryngeal nerves. The different prevalence of this complex anastomotic pattern suggests functional differences in the sensory and motor innervation of individual subjects.