Laryngeal and tracheal afferent nerve stimulation evokes swallowing in anaesthetized guinea pigs

Cervical percutaneous interferential current stimulation improves citric acid cough tests in patients with Parkinson's disease on medication

Aspiration pneumonia is the leading cause of death in patients with Parkinson's disease. The incidence of silent aspiration is high in such patients owing to decreased pharyngeal and laryngeal sensation; thus, interventions for this condition may help prevent pneumonia. In this single-arm, open-label study, we used a cervical percutaneous interferential current stimulation device to activate pharyngeal and laryngeal sensory nerves. We evaluated its effectiveness in patients with Hoehn-Yahr stages 2-4 Parkinson's disease. The primary endpoint was the proportion of patients with a normal cough reflex after consuming 1% citric acid at the end of the intervention compared with baseline measurements. In total, 25 patients received neck percutaneous interferential current stimulation for 20 min twice weekly for 8 weeks. Afterward, the proportion of patients with a normal cough reflex after 1% citric acid consumption increased significantly (p = 0.001), whereas other indicators, such as tongue pressure, peak expiratory flow, and penetration or aspiration during videofluoroscopic examination, remained unchanged. A longer duration of illness, higher Unified Parkinson's Disease Rating Scale total scores, and higher levodopa equivalent daily doses were significantly associated with improved cough test outcomes. Hence, cervical percutaneous interferential current stimulation significantly improved cough reflexes and may improve silent aspiration. Trial Registration: Japan Registry of Clinical Trials, jRCTs062220013, first registered 09/05/2022.

Treatment of Post-stroke Dysphagia With Interferential Current: Three Case Reports and a Review of the Literature

Stroke is damage to the central nervous system due to vascular pathology. Stroke causes many complications. One of the most important of these complications is dysphagia. Dysphagia is a major cause of morbidity and mortality. In recent years, the benefits of using interferential current (IFC) stimulation in the treatment of dysphagia due to various etiologies have been demonstrated. However, there are significant gaps in the literature regarding patient populations, treatment procedures, and evaluation of treatment response. Here, we report the treatment of three cases of dysphagia after ischemic stroke with IFC stimulation and review the current literature. The patients had no previous treatment for dysphagia and were using only compensatory methods. This case report highlights the benefit of IFC stimulation in the treatment of post-stroke dysphagia both clinically and videofluoroscopically. It should be kept in mind that IFC stimulation may be an important alternative in the treatment of post-stroke dysphagia.

Sustained Effects of Capsaicin Infusion into the Oropharynx on Swallowing in Perfused Rats

OBJECTIVES

To examine the sustained effects of oropharyngeal capsaicin stimulation on the regulation of swallowing, we recorded the swallowing-related nerve activities during continuous infusion of capsaicin solution into the oropharynx.

METHODS

In 33 in situ perfused brainstem preparation of rats, we recorded the activities of the vagus, hypoglossal, and phrenic nerves during fictive swallowing. The interburst intervals (IBIs) of the swallowing-related nerves during sequential pharyngeal swallowing (sPSW) elicited by electrical stimulation of the superior laryngeal nerve (SLN) during concurrent capsaicin stimulation of 10, 1, and 0.1 μM (n = 28) were compared with those during oropharyngeal infusion of saline (control) (n = 5).

RESULTS

The IBIs during SLN-induced sPSW were reduced at 5 min after initiation of continuous infusion of 10 and 1 μM capsaicin solution. The IBIs showed significant decreases to -25.8 ± 6.9%, -25.9 ± 5.3, -18.3 ± 3.7, and -12.0 ± 1.6 at 30 min following 1 μM capsaicin stimulation at SLN stimulus conditions at 5 Hz of 1.2 times threshold, 10 Hz of 40 μA, 5 Hz of 60 μA, and 10 Hz of 60 μA, respectively. Continuous capsaicin stimulation of 0.1 μM solution did not show significant sustained effects.

CONCLUSION

Pharmacological stimulation of capsaicin could provide time-dependent effects on the likelihood of swallowing, particularly subserving sustained facilitation of swallowing reflex with appropriate concentration of capsaicin.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:305-314, 2024.

Transient receptor potential channels as an emerging therapeutic target for oropharyngeal dysphagia

Oropharyngeal dysphagia is a serious health concern in older adults and patients with neurological disorders. Current oropharyngeal dysphagia management largely relies on compensatory strategies with limited efficacy. A long-term goal in swallowing/dysphagia-related research is the identification of pharmacological treatment strategies for oropharyngeal dysphagia. In recent decades, several pre-clinical and clinical studies have investigated the use of transient receptor potential (TRP) channels as a therapeutic target to facilitate swallowing. Various TRP channels are present in regions involved in the swallowing process. Animal studies have shown that local activation of these channels by their pharmacological agonists initiates swallowing reflexes; the number of reflexes increases when the dose of the agonist reaches a particular level. Clinical studies, including randomized clinical trials involving patients with oropharyngeal dysphagia, have demonstrated improved swallowing efficacy, safety, and physiology when TRP agonists are mixed with the food bolus. Additionally, there is evidence of plasticity development in swallowing-related neuronal networks in the brain upon TRP channel activation in peripheral swallowing-related regions. Thus, TRP channels have emerged as a promising target for the development of pharmacological treatments for oropharyngeal dysphagia.

Detailed findings of videofluoroscopic examination among patients with Parkinson's disease on the effect of cervical percutaneous interferential current stimulation

Introduction

Parkinson's disease (PD) leads to various types of swallowing disorders. We investigated the effect of cervical percutaneous interferential current stimulation on dysphagia. By conducting detailed qualitative and quantitative analysis of videofluoroscopic examination, we aimed to understand dysphagia in patients with PD and investigate its effects on swallowing function.

Methods

Patients received cervical percutaneous interferential current stimulation for 20 min twice a week for 8 weeks. In this exploratory study, we evaluated aspiration/laryngeal penetration, oral cavity residue, vallecular residue, and pharyngeal residue. In addition, we performed temporal analysis.

Results

Twenty-five patients were completely evaluated. At baseline, the proportions of laryngeal penetration/aspiration, oral cavity residue, epiglottic vallecula residue, and pharyngeal residue were 40.0, 88.0, 72.0, 60.0, and 16.0%, respectively. Conversely, pharyngeal transit time, laryngeal elevation delay time, pharyngeal delay time, and swallowing reflex delay were nearly within the normal ranges. Cervical percutaneous interferential current sensory stimulation improved only oral cavity residue at the end of the intervention, from 88.0 to 56.0%.

Discussion

Patients with PD demonstrated remarkably high frequencies of residues in the oral and pharyngeal regions. The usefulness of cervical interferential current stimulation was partially demonstrated for oral cavity residue. Considering that PD exhibits diverse symptoms, further accumulation of cases and knowledge is warranted.

Trial registration

jRCTs062220013.

Atropine facilitates water-evoked swallows via central muscarinic receptors in anesthetized rats

Anticholinergic medication causes impaired swallowing with hyposalivation. However, the underlying mechanisms by which these drugs modulate the swallowing reflex remain unclear. This study investigated the effects of the muscarinic acetylcholine receptor (mAChR) nonspecific antagonist atropine on the initiation of swallowing. Experiments were performed on 124 urethane-anesthetized rats. A swallow was evoked by either topical laryngeal application of a small amount of distilled water (DW), saline, citric acid, or capsaicin; upper airway distention with a continuous airflow; electrical stimulation of the superior laryngeal nerve (SLN); or focal microinjection of N-methyl-d-aspartate (NMDA) into the lateral region of the nucleus of the solitary tract (L-nTS). Swallows were identified by electromyographic bursts of the digastric and thyrohyoid muscles. Either atropine, the peripheral mAChR antagonist methylatropine, or antagonists of mAChR subtypes M1-M5 were intravenously delivered. Atropine at a dose of 1 mg/kg increased the number of DW-evoked swallows compared with baseline and did not affect the number of swallows evoked by saline, citric acid, capsaicin, or upper airway distention. Methylatropine and M1-M5 antagonists did not significantly change the number of DW-evoked swallows. Bilateral SLN transection completely abolished DW-evoked swallows, and atropine decreased the swallowing threshold of SLN electrical stimulation. Finally, microinjection of NMDA receptor antagonist AP-5 into the L-nTS inhibited DW-evoked swallows, and atropine facilitated the initiation of swallowing evoked by NMDA microinjection into this region. These results suggest that atropine facilitates DW-evoked swallows via central mAChR actions.NEW & NOTEWORTHY Atropine facilitated the distilled water (DW)-evoked swallows in anesthetized rats. Atropine decreased the swallowing threshold evoked by electrical stimulation of the superior laryngeal nerve, which is a primary sensory nerve for the initiation of DW-evoked swallows. Atropine facilitated the swallows evoked by N-methyl-d-aspartate microinjection into the lateral region of the nucleus of the solitary tract, which is involved in the DW-evoked swallows. We speculate that atropine facilitates the DW-evoked swallows via central muscarinic receptor actions.

Impact of neck percutaneous interferential current sensory stimulation on swallowing function in patients with Parkinson's disease: A single-arm, open-label study protocol

Background

Parkinson's disease (PD) can lead to swallowing dysfunction, resulting in aspiration pneumonia. Among the types of swallowing disorders, a characteristic and serious problem associated with PD is silent aspiration due to pharyngeal and laryngeal hypoesthesia.

Methods

This single-arm, open-label study aims to evaluate the effectiveness of percutaneous neck interferential current sensory stimulation in enhancing swallowing function in patients with PD. The efficacy and safety of percutaneous neck interferential current sensory stimulation will be investigated for patients diagnosed with PD, based on the Movement Disorder Society criteria, of Hoehn-Yahr stages 2-4. The patients will receive neck percutaneous interferential current sensory stimulation for 20 min twice a week for 8 weeks using a Gentle Stim® (FoodCare Co., Ltd., Kanagawa, Japan) device. Once the intervention is initiated, evaluations will be performed every 4 weeks for a 16-week period. The primary endpoint to be assessed is the proportion of patients with normal cough with 1% citric acid at the end of the intervention (8 weeks after intervention initiation) compared with that at the beginning. This clinical trial will examine the usefulness of percutaneous neck interferential current sensory stimulation in patients with PD. In addition, this study will use novel instruments, such as multichannel surface electromyography and electronic stethoscope, to evaluate swallowing function.

Discussion

This novel evaluation can provide insights into dysphagia in patients with PD and the usefulness of percutaneous neck interferential current stimulation. This exploratory study is limited by its single-arm, open-label design and small size.

Trial registration number

jRCTs062220013; pre-results.

Efficacy of interferential current transcutaneous electrical sensory stimulation through the neck skin for treating dysphagia in children with disabilities: A case series

Finding a suitable treatment for dysphagia has been challenging and the efficacy of neuromuscular electrical stimulation has been recognized. Moreover, the beneficial effect of interferential current transcutaneous electrical sensory stimulation has recently been described. However, the efficacy of interferential current transcutaneous electrical sensory stimulation in children with disabilities is unknown. Therefore, the aim of this study was to confirm the efficacy of interferential current transcutaneous electrical sensory stimulation in children with disabilities. Four children with disabilities of various types underwent interferential current transcutaneous electrical sensory stimulation once a week. All patients showed improved symptoms after interferential current transcutaneous electrical sensory stimulation treatment. Videoendoscopic examination showed reduced accumulation of secretion in all patients and decreased residual bolus in two. We also felt an increased forcefulness when swallowing in two. In addition, the questionnaire results regarding dysphagia indicated improvements. No significant side effects were observed. The interferential current transcutaneous electrical sensory stimulation treatment may be effective and safe in children with disabilities. The effect of this treatment on swallowing ability needs to be further investigated by studying more cases.

Effects of the GABA(B) agonist baclofen on volitional swallowing in normal subjects

The GABA(B) receptor agonist baclofen is known to suppress the rate of spontaneous swallowing but not pharyngeal muscle contraction. The extent to which baclofen may alter volitional swallowing is not currently known. We investigated the effects of baclofen in healthy subjects, hypothesizing that baclofen exposure would alter volume-regulation and/or piecemeal deglutition behaviors during volitional swallowing attempts. Pharyngeal high-resolution manometry impedance (P-HRM-I) protocol was used to assess swallowing function of 22 healthy adult volunteers (median 29 years) who were investigated on two occasions, receiving 40 mg baclofen (oral) 1 h before study, or placebo (randomized). Standard swallow function variables recommended by the pharyngeal HRM Working Group were derived for 5 ml, 10 ml, and 20 ml volumes of thin and extremely thick liquid challenges. Multiple swallow behaviors, comprising two swallows <5 s apart, were characterized. The spontaneous swallow rate was also determined. Baclofen exposure had no overall significant effect on swallow variables. Upper esophageal sphincter pressure was weaker during exposure to baclofen, during both the pre-deglutitive and post-deglutitive phases of the swallow (p < 0.05 during thick liquid swallows). Piecemeal swallows, where the bolus is separated in two potions, were significantly more common during 20 ml boluses (p = 0.002). Baclofen decreased the frequency of piecemeal deglutition overall. Baclofen has limited to no effect on volitional swallowing measures, however, does reduce the likelihood of initiation of piecemeal deglutition to large volume challenges.

Oropharyngeal Capsaicin Exposure Improves Infant Feeding Performance in an Animal Model of Superior Laryngeal Nerve Damage

Sensorimotor feedback is critical to safe and effective swallowing. Because of this, sensory interventions have the potential to treat dysphagia. One such treatment may be found in capsaicin, which activates the internal branch of the superior laryngeal nerve (iSLN). The iSLN initiates the pharyngeal swallow, and a more sensitive iSLN should more readily elicit swallowing and improve swallow safety. We explored the neurophysiological mechanism by which capsaicin improves swallow performance using an infant pig model with a unilateral iSLN lesion. Using high-speed videofluoroscopy, we collected oropharyngeal kinematic data while pigs suckled on bottles, before and after applying capsaicin to the posterior tongue and valleculae. We found that capsaicin application decreased maximal bolus sizes, which improved swallow safety. Furthermore, capsaicin improved performance when infant pigs swallowed more moderately sized boluses. However, capsaicin did not change swallow frequency, the number of sucks prior to each swallow, nor total pharyngeal transit time (TPT). Similarly, excursions of the hyoid, thyroid, and posterior tongue were unchanged. TPT and hyoid and thyroid excursions maintained relationships with bolus size post-capsaicin, suggesting that these variables are less sensitive to sensory intervention. The timing and extent of posterior tongue movement were only correlated with bolus size pre-capsaicin, which could imply that capsaicin fundamentally changes in relationships between tongue movements and bolus size. Our results provide insight into the neural control of swallowing and capsaicin's mechanism of action, and suggest that capsaicin may be beneficial in treating acute infant dysphagia.

Antitussive effects of NaV 1.7 blockade in Guinea pigs

Our previous studies implicated the voltage-gated sodium channel subtype NaV 1.7 in the transmission of action potentials by the vagal afferent nerves regulating cough and thus identified this channel as a rational therapeutic target for antitussive therapy. But it is presently unclear whether a systemically administered small molecule inhibitor of NaV 1.7 conductance can achieve therapeutic benefit in the absence of side effects on cardiovascular function, gastrointestinal motility or respiration. To this end, we have evaluated the antitussive effects of the NaV 1.7 selective blocker Compound 801 administered systemically in awake guinea pigs or administered topically in anesthetized guinea pigs. We also evaluated the antitussive effects of ambroxol, a low affinity NaV blocker modestly selective for tetrodotoxin resistant NaV subtypes. Both Compound 801 and ambroxol dose-dependently inhibited action potential conduction in guinea pig vagus nerves (assessed by compound potential), with ambroxol nearly 100-fold less potent than the NaV 1.7 selective Compound 801 in this and other NaV 1.7-dependent guinea pig and human tissue-based assays. Both drugs also inhibited citric acid evoked coughing in awake or anesthetized guinea pigs, with potencies supportive of an NaV 1.7-dependent mechanism. Notably, however, the antitussive effects of systemically administered Compound 801 were accompanied by hypotension and respiratory depression. Given the antitussive effects of topically administered Compound 801, we speculate that the likely insurmountable side effects on blood pressure and respiratory drive associated with systemic dosing make topical formulations a viable and perhaps unavoidable therapeutic strategy for targeting NaV 1.7 in cough.

Characterisation of cough evoked by inhaled treprostinil and treprostinil palmitil

Cough is induced by inhaled prostacyclin analogues including treprostinil (TRE), and, at higher doses, treprostinil palmitil (TP), a prodrug of TRE. In this report, we have investigated mechanisms involved in TRE- and TP-induced cough, using a dry powder formulation of TP (TPIP) to supplement previous data obtained with an aqueous suspension formulation of TP (TPIS).

Experiments in guinea pigs and rats investigated the prostanoid receptor subtype producing cough and whether it involved activation of sensory nerves in the airways and vasculature. Experiments involved treatment with prostanoid, tachykinin and bradykinin receptor antagonists, a cyclooxygenase inhibitor and TRE administration to the isolated larynx or intravenously.

In guinea pigs, cough with inhaled TRE (1.23 µg·kg⁻¹) was not observed with an equivalent dose of TPIP and required higher inhaled doses (12.8 and 35.8 µg·kg⁻¹) to induce cough. TRE cough was blocked with IP and tachykinin NK₁ receptor antagonists but not with EP₁, EP₂, EP₃, DP₁ or bradykinin B₂ antagonists or a cyclooxygenase inhibitor. TRE administered to the isolated larynx or intravenously in rats produced no apnoea or swallowing, whereas citric acid, capsaicin and hypertonic saline had significant effects.

The mechanisms inducing cough with inhaled TRE likely involves the activation of prostanoid IP receptors on jugular C-fibres in the tracheobronchial airways. Cough induced by inhaled dry powder and nebulised formulations of TP occurs at higher inhaled doses than TRE, presumably due to the slow, sustained release of TRE from the prodrug resulting in lower concentrations of TRE at the airway sensory nerves.

Cough induced by inhaled treprostinil and treprostinil palmitil involves the activation of prostacyclin (IP) receptors located on airway tachykinin nerveshttps://bit.ly/37sXz1I

Comparison of physical properties of voluntary coughing, huffing and swallowing in healthy subjects

Coughing, huffing and swallowing protect the airway from aspiration. This study was conducted to compare the physical properties of voluntary coughing, huffing and swallowing in healthy subjects. Ten healthy men were asked to huff, cough and swallow repeatedly. Electromyograms (EMGs) were recorded from the left side of the external oblique (EO), sternocleidomastoid, suprahyoid (SH) and thyrohyoid muscles. Airflow was recorded using a face mask with two-way non-rebreathing valves. The expiratory velocity of huffing and coughing and the SH EMG of all actions presented high intraclass correlation coefficients (> 0.8). The inspiratory and expiratory velocities did not differ significantly between coughing and huffing. The expiratory acceleration of coughing was significantly higher than that of huffing, whereas the expiratory volume of coughing was significantly smaller than that of huffing. The EO EMG of coughing and huffing were significantly larger than that of swallowing. The EO EMG activity during the expiratory phase was significantly higher than that of the other phases of both coughing and huffing. The SH EMG of coughing and huffing were significantly smaller than that of swallowing. Correlation analysis revealed that the expiratory velocity of coughing was strongly positively correlated with that of huffing. The expiratory volume of huffing was significantly positively correlated with hand grip strength. These results suggest that EO and SH muscle activities during huffing or coughing differ those during swallowing, and huffing and coughing may work similarly in expiratory function.

Involvement of capsaicin-sensitive nerves in the initiation of swallowing evoked by carbonated water in anesthetized rats

Capsaicin powerfully evokes the swallowing reflex and is a known therapeutic agent for improving dysphagia and preventing aspiration pneumonia. However, the role of capsaicin-sensitive nerves in the initiation of swallowing evoked by various natural stimuli remains unclear. To explore this question, we blocked laryngeal capsaicin-sensitive nerves following the coapplication of QX-314 and capsaicin (QX/Cap), and investigated the effects on swallowing evoked by mechanical and chemical stimulation in anesthetized rats. Swallows were evoked by capsaicin, carbonated water (CW), distilled water (DW), and punctate mechanical stimulation using von Frey filaments applied topically to the larynx. Swallows were documented by recording electromyographic activation of the suprahyoid and thyrohyoid muscles. The initiation of swallowing by capsaicin was strongly suppressed at 5 min following QX/Cap treatment and returned in a time-dependent manner. CW-evoked swallows at 5 min following QX/Cap treatment were significantly diminished compared with before and 30 min after treatment. In contrast, DW-evoked and mechanically evoked swallows were unchanged by QX/Cap treatment. Furthermore, CW-evoked swallows were virtually abolished by transection of the superior laryngeal nerves and significantly decreased by the topical application of acid-sensing ion channel-3 (ASIC3) inhibitor APETx2, but they were not affected by the nonselective transient receptor potential channel inhibitor ruthenium red or the ASIC1 inhibitor mambalgin-1. Taken together, we speculate that capsaicin-sensitive nerves play an important role in the initiation of CW-evoked swallows.NEW & NOTEWORTHY The initiation of swallowing evoked by laryngeal capsaicin and carbonated water application was diminished by the coapplication of QX-314 and capsaicin. Carbonated water-evoked swallows were also abolished by transection of the superior laryngeal nerves and were inhibited by the acid-sensing ion channel-3 inhibitor. Capsaicin-sensitive nerves are involved in the initiation of carbonated water-evoked swallows.

Diagnosis and treatments for oropharyngeal dysphagia: effects of capsaicin evaluated by newly developed ultrasonographic method

Oropharyngeal dysphagia (OD) is a common symptom in the older people, and may cause fatal complications such as aspiration pneumonia. However, there is no established treatment for OD. The relationship between the transient receptor potential vanilloid 1 (TRPV1) and substance P released by activated TRPV1 was recently demonstrated. Further, there are several reports showing that capsaicin, a specific agonist of TRPV1, can improve OD. Currently, the evaluation of swallowing is mainly performed by videofluoroscopic examination. However, there are no reports on the clinical application of ultrasonography using tissue Doppler imaging. In this review, we describe the pathophysiology and treatments for OD, introduce our novel US method to evaluate cervical esophageal motility, and then outline our clinical study examining the effects of capsaicin, a specific TRPV1 agonist, in older patients with OD.

Sustained laryngeal transient receptor potential vanilloid 1 activation inhibits mechanically induced swallowing in anesthetized rats

A major component of gastric acid is hydrochloric acid (HCl), which can activate transient receptor potential vanilloid 1 (TRPV1). In the present study, we investigated how sustained laryngeal TRPV1 activation affects the frequency of the swallowing reflex. Experiments were carried out on 85 male Sprague-Dawley rats. The effects of short and sustained application of chemicals (3 µl of 0.1 N HCl or capsaicin) on the frequency of swallowing and on time-dependent changes in the occurrence of swallowing evoked by supralaryngeal nerve stimulation were determined. To evaluate vascular permeability of the larynx, Evans blue dye was intravenously injected after 5 or 60 min of sustained TRPV1 activation. SB366791 (a TRPV1 inhibitor) and Cap/QX-314 (a TRPV1-expressed neuronal inhibitor) significantly inhibited HCl/capsaicin-induced swallowing, but air flow-induced swallowing was not affected. Although the number of air flow-induced swallows followed by capsaicin stimulation was not affected within 5 min, it was significantly reduced by 60-min capsaicin or HCl application. The swallowing threshold associated with supralaryngeal nerve stimulation did not significantly change throughout the recording period. Evans blue dye concentrations in the larynx were significantly higher at 60 min in the 10^-5 M capsaicin group than in the control group. Our results suggest that sustained TPRV1 activation not only desensitizes TRPV1 but also inactivates mechanoreceptors, which may be attributed to increases in vascular permeability and edema, as part of an inflammatory process.NEW & NOTEWORTHY Although a transient receptor potential vanilloid 1 (TRPV1) inhibitor or TRPV1-expressed neuronal inhibitor significantly inhibited HCl/capsaicin-evoked swallowing, air flow-induced swallowing was not affected. The number of air flow-induced swallows was significantly reduced within 60 min of TRPV1 activation. Evans blue dye concentration in the larynx increased 60 min after capsaicin application. TPRV1 activation not only desensitizes TRPV1 but also inactivates mechanoreceptors caused by increases in vascular permeability and edema.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

Effects of Carbonation and Temperature on Voluntary Swallowing in Healthy Humans

We investigated how swallowing behaviors are affected by the temperature and carbonation of water in healthy humans. Twenty-nine healthy volunteers were instructed to drink as much natural water, carbonated water, or cider as they wanted, and we recorded the volume of solution swallowed and electromyographic (EMG) activity of the masseter and suprahyoid muscles. Sensory tests regarding the ease of holding the solution in the mouth and ease of swallowing were also performed. The volume of carbonated water swallowed was significantly lower than that of natural water and cider. The ease of holding and swallowing the solution significantly differed between solution types such that natural water was the easiest solution to hold and swallow, followed by cider and then carbonated water in both tests. EMG activity was also affected by the solution type. Masseter EMG activity was significantly lower when swallowing natural water compared with carbonated water. Suprahyoid EMG activity was significantly lower when swallowing natural water compared with carbonated water and cider. The volume of solution swallowed was significantly correlated with the ease of holding and swallowing the solution, but not with masseter or suprahyoid EMG activities. The ease of holding and swallowing the solution significantly affected masseter and suprahyoid EMG activities. The results suggested that when participants experienced difficulty holding and swallowing the solution, masseter and suprahyoid EMG activity increased. Considering our findings that mechanical stimulation with bubbles decreased the volume of solution swallowed and increased EMG activities, carbonated water swallowing may be useful in treating deglutition disorders.

Elicitation of the Swallowing Reflex by Esophageal Stimulation in Healthy Subjects: An Evaluation Using High-Resolution Manometry

The purposes of this human study using high-resolution manometry were to verify whether the swallowing reflex can be evoked by intra-esophageal fluid injection and whether the reflex latency and manometric variables differ depending on the injected location, amount, or speed. Ten healthy individuals participated in this study. The tip of the intranasal catheter for injection was placed at 5 cm (upper), 10 cm (upper-middle), 15 cm (lower-middle), or 20 cm (lower) from the distal end of the upper esophageal sphincter (UES). An intra-esophageal injection of 3 mL or 10 mL of thickened water was administered and controlled at 3 mL/s or 10 mL/s. Latencies from the start of the injection to the onset of UES relaxation were compared regarding injection locations, amounts, and rates. Manometric variables of intra-esophageal injection and voluntary swallowing were compared. The latency became shorter when the upper region was injected. Latency after the 10-mL injection was shorter than that after the 3-mL injection (p < 0.01) when faster injection (10 mL/s) was used. Faster injection induced shorter latency (p < 0.01) when a larger volume (10 mL) was injected. Pre-maximum and post-maximum UES pressures during voluntary swallowing or during spontaneous swallowing when injecting the upper esophageal region were significantly higher than spontaneous swallowing at other regions (p < 0.01). Intra-esophageal fluid injection induces the swallowing reflex in humans. The most effective condition for inducing the swallowing reflex involved a larger fluid amount with a faster injection rate in the upper esophagus.

Involvement of the epithelial sodium channel in initiation of mechanically evoked swallows in anaesthetized rats

KEY POINTS

Afferents carried by the superior laryngeal nerve play a primary role in the initiation of laryngeal mechanically evoked swallows in anaesthetized rats. Amiloride and its analogues inhibit swallowing evoked by mechanical stimulation, but not swallowing evoked by chemical and electrical stimulation. The epithelial sodium channel is probably involved in the initiation of laryngeal mechanically evoked swallows.

ABSTRACT

The swallowing reflex plays a critical role in airway protection. Because impaired laryngeal mechanosensation is associated with food bolus aspiration, it is important to know how the laryngeal sensory system regulates swallowing initiation. This study was performed to clarify the neuronal mechanism of mechanically evoked swallows. Urethane-anaesthetized Sprague-Dawley male rats were used. A swallow was identified by activation of the suprahyoid and thyrohyoid muscles on electromyography. The swallowing threshold was measured by von Frey filament and electrical stimulation of the larynx. The number of swallows induced by upper airway distension and capsaicin application (0.03 nmol, 3 μl) to the vocal folds was counted. The effects of topical application (0.3-30 nmol, 3 μl) of the epithelial sodium channel (ENaC) blocker amiloride and its analogues (benzamil and dimethylamiloride), acid-sensing ion channel (ASIC) inhibitors (mambalgine-1 and diminazene) and gadolinium to the laryngeal mucosa on swallowing initiation were evaluated. A nerve transection study indicated that afferents carried by the superior laryngeal nerve play a primary role in the initiation of laryngeal mechanically evoked swallows. The mechanical threshold of swallowing was increased in a dose-dependent manner by amiloride and its analogues and gadolinium, but not by ASIC inhibitors. The number of swallows by upper airway distension was significantly decreased by benzamil application. However, the initiation of swallows evoked by capsaicin and electrical stimulation was not affected by benzamil application. We speculate that the ENaC is involved in the initiation of laryngeal mechanically evoked swallows.

Reflex regulation of breathing by the paratrigeminal nucleus via multiple bulbar circuits

Sensory neurons of the jugular vagal ganglia innervate the respiratory tract and project to the poorly studied medullary paratrigeminal nucleus. In the present study, we used neuroanatomical tracing, pharmacology and physiology in guinea pig to investigate the paratrigeminal neural circuits mediating jugular ganglia-evoked respiratory reflexes. Retrogradely traced laryngeal jugular ganglia neurons were largely (> 60%) unmyelinated and expressed the neuropeptide substance P and calcitonin gene-related peptide, although a population (~ 30%) of larger diameter myelinated jugular neurons was defined by the expression of vGlut1. Within the brainstem, vagal afferent terminals were confined to the caudal two-thirds of the paratrigeminal nucleus. Electrical stimulation of the laryngeal mucosa evoked a vagally mediated respiratory slowing that was mimicked by laryngeal capsaicin application. These laryngeal reflexes were modestly reduced by neuropeptide receptor antagonist microinjections into the paratrigeminal nucleus, but abolished by ionotropic glutamate receptor antagonists. D,L-Homocysteic acid microinjections into the paratrigeminal nucleus mimicked the laryngeal-evoked respiratory slowing, whereas capsaicin microinjections evoked a persistent tachypnoea that was insensitive to glutamatergic inhibition but abolished by neuropeptide receptor antagonists. Extensive projections from paratrigeminal neurons were anterogradely traced throughout the pontomedullary respiratory column. Dual retrograde tracing from pontine and ventrolateral medullary termination sites, as well as immunohistochemical staining for calbindin and neurokinin 1 receptors, supported the existence of different subpopulations of paratrigeminal neurons. Collectively, these data provide anatomical and functional evidence for at least two types of post-synaptic paratrigeminal neurons involved in respiratory reflexes, highlighting an unrecognised complexity in sensory processing in this region of the brainstem.

New insights into the pathophysiology of postoperative hemorrhage in thyroid surgery: An experimental study in a porcine model

BACKGROUND

Postoperative cervical hemorrhage is a rare but life-threatening complication that can cause severe morbidity. Different mechanisms leading to asphyxia have been described based only on clinical observation.

METHODS

We performed a series of in vivo animal studies simulating post-thyroidectomy hemorrhage and its effect on respiratory drive. Three series of tests were carried out in 12 German domestic pigs under general anesthesia. The pigs were breathing spontaneously with secured airways. An additional series using functional magnetic resonance imaging of the pigs' brainstem was also conducted.

RESULTS

The first experimental series carried out on 2 animals revealed an obvious difference between the effects of cervical hemorrhage and external bleeding with development of hemorrhagic shock. An experimental setting for the repeated simulation of cervical hemorrhage was established. A pressure-dependent mechanism was discovered that led to apnea in every animal despite the secured airway. In 8 of 10 animals, relief of cervical pressure led to complete respiratory recovery. The test was repeated up to 6 times per pig. Apnea was induced in 25 of 25 test procedures (100%) and was followed by respiratory recovery in 22 of 25 tests (88%). The threshold pressure at which the respiratory rate started to decrease was 47 ± 14 Torr when blood was used to increase the cervical compartment pressure. When silicone oil was used in a further experimental series, the threshold pressure was similar at 44 ± 21 Torr. The cervical compartment pressure needed to induce apnea was 74 ± 18 Torr using blood and 74 ± 39 Torr using silicone oil, both of which exceeded the mean arterial pressure by 28 Torr during apnea. Functional magnetic resonance imaging revealed a decrease in brainstem activity during phases of increased cervical compartment pressure, which suggests a possible role for cerebral vascular perfusion.

CONCLUSION

Respiratory drive can be suppressed by increased pressure in the cervical compartment, possibly because of a pressure-dependent impairment in cerebral perfusion through a form of cervical compartment phenomenon or, less likely, a pressure-dependent reflex (nervous) mechanism.

Involvement of hypoglossal and recurrent laryngeal nerves on swallowing pressure

Swallowing pressure generation is important to ensure safe transport of an ingested bolus without aspiration or leaving residue in the pharynx. To clarify the mechanism, we measured swallowing pressure at the oropharynx (OP), upper esophageal sphincter (UES), and cervical esophagus (CE) using a specially designed manometric catheter in anesthetized rats. A swallow, evoked by punctate mechanical stimulation to the larynx, was identified by recording activation of the suprahyoid and thyrohyoid muscles using electromyography (EMG). Areas under the curve of the swallowing pressure at the OP, UES, and CE from two trials indicated high intrasubject reproducibility. Effects of transecting the hypoglossal nerve (12N) and recurrent laryngeal nerve (RLN) on swallowing were investigated. Following bilateral hypoglossal nerve transection (Bi-12Nx), OP pressure was significantly decreased, and time intervals between peaks of thyrohyoid EMG bursts and OP pressure were significantly shorter. Decreased OP pressure and shortened times between peaks of thyrohyoid EMG bursts and OP pressure following Bi-12Nx were significantly increased and longer, respectively, after covering the hard and soft palates with acrylic material. UES pressure was significantly decreased after bilateral RLN transection compared with that before transection. These results suggest that the 12N and RLN play crucial roles in OP and UES pressure during swallowing, respectively. We speculate that covering the palates with a palatal augmentation prosthesis may reverse the reduced swallowing pressure in patients with 12N or tongue damage by the changes of the sensory information and of the contact between the tongue and a palates. NEW & NOTEWORTHY Hypoglossal nerve transection reduced swallowing pressure at the oropharynx. Covering the hard and soft palates with acrylic material may reverse the reduced swallowing function caused by hypoglossal nerve damage. Recurrent laryngeal nerve transection reduced upper esophageal sphincter negative pressure during swallowing.

Effects of pharyngeal electrical stimulation on swallowing performance

Pharyngeal electrical stimulation (PEStim) has been found to facilitate voluntary swallowing. This study investigated how PEStim contributed to modulation of swallowing function in 15 healthy humans. In the involuntary swallowing test, water was injected onto the pharynx at 0.05 ml/s and the onset latency of the first swallow was measured. In the voluntary swallowing test, subjects swallowed their own saliva as quickly as possible for 30 s and the number of swallows was counted. Voluntary and involuntary swallowing was evaluated before (baseline), immediately after, and every 10 min after 10-min PEStim for 60 min. A voluntary swallowing test with simultaneous 30-s PEStim was also conducted before and 60 min after 10-min PEStim. The number of voluntary swallows with simultaneous PEStim significantly increased over 60 min after 10-min PEStim compared with the baseline. The onset latency of the first swallow in the involuntary swallowing test was not affected by 10-min PEStim. The results suggest that PEStim may have a long-term facilitatory effect on the initiation of voluntary swallowing in healthy humans, but not on peripherally-evoked swallowing. The physiological implications of this modulation are discussed.

Development of an oesophageal stimulation method to elicit swallowing reflex in humans

Swallowing reflex is known to be evoked by gastroesophageal regurgitation or oesophageal stimulation in animal studies. However, details regarding the stimulating material, bolus size and stimulation area remain unclear for the stimulation-induced type of swallowing reflex in humans. Here, we evaluated the effects of different kinds of stimulation via water and air injection of the oesophagus on the initiation of the swallowing reflex. Nine healthy individuals participated in this study. A fibre-optic endoscope was passed transnasally, and a thin catheter for injection was passed through the other side. The tip of the catheter was placed at the upper, upper middle, lower middle or lower region of the oesophagus, and the rate of injection was controlled at 0.2 mL/s. Swallowing reflex latency was calculated as the time from injection via air or thin/thick fluid until the onset of white-out in endoscopic images. Reflex latency was significantly shorter when injection occurred at the upper region of the oesophagus than at the lower region, for both thin and thick fluids (P < .01). At the upper region of the oesophagus, the latency was significantly shorter after injection of thin fluid than with thick fluid (P < .05). Injection of air did not induce the swallowing reflex at all sites. These findings suggest that while the swallowing reflex is evoked by stimulation via fluid injection of the oesophagus in humans, sensitivity is greatest in the upper region of the oesophagus compared with the lower region and can vary depending on the injecting material.

Interferential current sensory stimulation, through the neck skin, improves airway defense and oral nutrition intake in patients with dysphagia: a double-blind randomized controlled trial

Background

Neuromuscular electrical stimulation with muscle contraction, administered through the skin of the neck, improves a patient’s swallowing ability. However, the beneficial effects of transcutaneous electrical sensory stimulation (TESS), without muscle contraction, are controversial. We investigated the effect of TESS, using interferential current, in patients undergoing dysphagia rehabilitation.

Methods

This double-blind, randomized controlled trial involved 43 patients who were prescribed in-hospital dysphagia rehabilitation for ≥3 weeks. Patients were randomly assigned to the sensory stimulation (SS) or sham groups; all received usual rehabilitative care plus 2 weeks of SS or sham intervention. Outcome measures included cough latency times against a 1% citric acid mist, functional oral intake scale (FOIS) scores, and oral nutritional intake – each determined after the second and third week following treatment initiation.

Results

Mean patient age was 84.3±7.5 years; 58% were women. The SS and sham groups had similar baseline characteristics. Changes in cough latency time at 2 weeks (−14.1±14.0 vs −5.2±14.2 s, p=0.047) and oral nutrition intake at 3 weeks (437±575 vs 138±315 kcal/day, p=0.042) improved more in the SS group than in the sham group. Changes in cough frequency and FOIS scores indicated better outcomes in the SS group, based on substantial effect sizes.

Conclusion

TESS, using interferential current through the neck, improved airway defense and nutrition in patients suffering from dysphagia. Further large-scale studies are needed to confirm the technique’s effect on swallowing ability.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Central inhibition of initiation of swallowing by systemic administration of diazepam and baclofen in anaesthetized rats

Dysphagia is caused not only by neurological and/or structural damage but also by medication. We hypothesized memantine, dextromethorphan, diazepam, and baclofen, all commonly used drugs with central sites of action, may regulate swallowing function. Swallows were evoked by upper airway (UA)/pharyngeal distension, punctate mechanical stimulation using a von Frey filament, capsaicin or distilled water (DW) applied topically to the vocal folds, and electrical stimulation of a superior laryngeal nerve (SLN) in anesthetized rats and were documented by recording electromyographic activation of the suprahyoid and thyrohyoid muscles and by visualizing laryngeal elevation. The effects of intraperitoneal or topical administration of each drug on swallowing function were studied. Systemic administration of diazepam and baclofen, but not memantine or dextromethorphan, inhibited swallowing evoked by mechanical, chemical, and electrical stimulation. Both benzodiazepines and GABA_A receptor antagonists diminished the inhibitory effects of diazepam, whereas a GABA_B receptor antagonist diminished the effects of baclofen. Topically applied diazepam or baclofen had no effect on swallowing. These data indicate that diazepam and baclofen act centrally to inhibit swallowing in anesthetized rats.NEW & NOTEWORTHY Systemic administration of diazepam and baclofen, but not memantine or dextromethorphan, inhibited swallowing evoked by mechanical, chemical, and electrical stimulation. Both benzodiazepines and GABA_A receptor antagonists diminished the inhibitory effects of diazepam, whereas a GABA_B receptor antagonist diminished the effects of baclofen. Topical applied diazepam or baclofen was without effect on swallowing. Diazepam and baclofen act centrally to inhibit swallowing in anesthetized rats.

Protective Effects of Ferulic Acid against Chronic Cerebral Hypoperfusion-Induced Swallowing Dysfunction in Rats

Ferulic acid (FA), a phenolic phytochemical, has been reported to exert antioxidative and neuroprotective effects. In this study, we investigated the protective effects of FA against the dysfunction of the swallowing reflex induced by ligation of bilateral common carotid arteries (2VO) in rats. In 2VO rats, topical administration of water or citric acid to the pharyngolaryngeal region evoked a diminished number of swallowing events with prolonged latency compared to sham-operated control rats. 2VO rats had an increased level of superoxide anion radical, and decreased dopamine and tyrosine hydroxylase enzyme levels in the striatum, suggesting that 2VO augmented cerebral oxidative stress and impaired the striatal dopaminergic system. Furthermore, substance P (SP) expression in the laryngopharyngeal mucosa, which is believed to be positively regulated by dopaminergic signaling in the basal ganglia, was decreased in 2VO rats. Oral treatment with FA (30 mg/kg) for 3 weeks (from one week before 2VO to two weeks after) improved the swallowing reflex and maintained levels of striatal dopamine and laryngopharyngeal SP expression in 2VO rats. These results suggest that FA maintains the swallowing reflex by protecting the dopamine-SP system against ischemia-induced oxidative damage in 2VO rats.

Differential response properties of peripherally and cortically evoked swallows by electrical stimulation in anesthetized rats

We compared onset latency, motor-response patterns, and the effect of electrical stimulation of the cortical masticatory area between peripherally and cortically evoked swallows by electrical stimulation in anesthetized rats. The number of swallows and the motor patterns were determined using electromyographic recordings from the thyrohyoid, digastric, and masseter muscles. The onset latency of the first swallow evoked by electrical stimulation of the cortical swallowing area (Cx) was significantly longer than that evoked by stimulation of the superior laryngeal nerve (SLN). The duration of thyrohyoid burst activity associated with SLN-evoked swallows was significantly longer than that associated with either Cx-evoked or spontaneous swallows. Combining Cx with SLN stimulation increased the number of swallows at low levels of SLN stimulation. Finally, A-area (the orofacial motor cortex) stimulation inhibited Cx-evoked swallows significantly more than it inhibited SLN-evoked swallows. These findings suggest that peripherally and cortically evoked swallows have different response properties and are affected differently by the mastication network.

Changes in the frequency of swallowing during electrical stimulation of superior laryngeal nerve in rats

The aim of the present study was to investigate the adaptation of the swallowing reflex in terms of reduced swallowing reflex initiation following continuous superior laryngeal nerve stimulation. Forty-four male Sprague Dawley rats were anesthetized with urethane. To identify swallowing, electromyographic activity of the left mylohyoid and thyrohyoid muscles was recorded. To evoke the swallowing response, the superior laryngeal nerve (SLN), recurrent laryngeal nerve, or cortical swallowing area was electrically stimulated. Repetitive swallowing evoked by continuous SLN stimulation was gradually reduced, and this reduction was dependent on the resting time duration between stimulations. Prior SLN stimulation also suppressed subsequent swallowing initiation. The reduction in evoked swallows induced by recurrent laryngeal nerve or cortical swallowing area stimulation was less than that following superior laryngeal nerve stimulation. Decerebration had no effect on the reduction in evoked swallows. Prior subthreshold stimulation reduced subsequent initiation of swallowing, suggesting that there was no relationship between swallowing movement evoked by prior stimulation and the subsequent reduction in swallowing initiation. Overall, these data suggest that reduced sensory afferent nerve firing and/or trans-synaptic responses, as well as part of the brainstem central pattern generator, are involved in adaptation of the swallowing reflex following continuous stimulation of swallow-inducing peripheral nerves and cortical areas.

Airway protective mechanisms

Cough and swallow are highly coordinated reflex behaviors whose common purpose is to protect the airway. The pharynx is the common tube for air and food/liquid movement from the mouth into the thorax, has been largely overlooked, and is potentially seen as just a passive space. The thyropharyngeus muscle responds to cough inducing stimuli to prepare a transient holding area for material that has been removed from the subglottic airway. The cricopharyngeus muscle participates with the larynx to ensure regulation of pressure when a bolus/air is moving from the upper airway through to the thorax (i.e., inspiration or swallow) or the reverse (i.e., expiration reflex or vomiting). These vital mechanisms have not been evaluated in clinical conditions but could be impaired in many neurodegenerative diseases, leading to aspiration pneumonia. These newly described airway protective mechanisms need further study, especially in healthy and pathologic human populations.