Altered Laryngeal Sensation: A Potential Cause of Apnea of Infancy

Chronic intermittent hypoxia reveals role of the Postinspiratory Complex in the mediation of normal swallow production

Obstructive sleep apnea (OSA) is a prevalent sleep-related breathing disorder that results in multiple bouts of intermittent hypoxia. OSA has many neurological and systemic comorbidities, including dysphagia, or disordered swallow, and discoordination with breathing. However, the mechanism in which chronic intermittent hypoxia (CIH) causes dysphagia is unknown. Recently, we showed the postinspiratory complex (PiCo) acts as an interface between the swallow pattern generator (SPG) and the inspiratory rhythm generator, the preBötzinger complex, to regulate proper swallow-breathing coordination (Huff et al., 2023). PiCo is characterized by interneurons co-expressing transporters for glutamate (Vglut2) and acetylcholine (ChAT). Here we show that optogenetic stimulation of ChATcre:Ai32, Vglut2cre:Ai32, and ChATcre:Vglut2FlpO:ChR2 mice exposed to CIH does not alter swallow-breathing coordination, but unexpectedly disrupts swallow behavior via triggering variable swallow motor patterns. This suggests that glutamatergic-cholinergic neurons in PiCo are not only critical for the regulation of swallow-breathing coordination, but also play an important role in the modulation of swallow motor patterning. Our study also suggests that swallow disruption, as seen in OSA, involves central nervous mechanisms interfering with swallow motor patterning and laryngeal activation. These findings are crucial for understanding the mechanisms underlying dysphagia, both in OSA and other breathing and neurological disorders.

Chronic Intermittent Hypoxia reveals role of the Postinspiratory Complex in the mediation of normal swallow production

Obstructive sleep apnea (OSA) is a prevalent sleep-related breathing disorder that results in multiple bouts of intermittent hypoxia. OSA has many neurologic and systemic comorbidities including dysphagia, or disordered swallow, and discoordination with breathing. However, the mechanism in which chronic intermittent hypoxia (CIH) causes dysphagia is unknown. Recently we showed the Postinspiratory complex (PiCo) acts as an interface between the swallow pattern generator (SPG) and the inspiratory rhythm generator, the preBötzinger Complex, to regulate proper swallow-breathing coordination (Huff et al., 2023). PiCo is characterized by interneurons co-expressing transporters for glutamate (Vglut2) and acetylcholine (ChAT). Here we show that optogenetic stimulation of ChATcre:Ai32, Vglut2cre:Ai32, and ChATcre:Vglut2FlpO:ChR2 mice exposed to CIH does not alter swallow-breathing coordination, but unexpectedly disrupts swallow behavior via triggering variable swallow motor patterns. This suggests, glutamatergic-cholinergic neurons in PiCo are not only critical for the regulation of swallow-breathing coordination, but also play an important role in the modulation of swallow motor patterning. Our study also suggests that swallow disruption, as seen in OSA, involves central nervous mechanisms interfering with swallow motor patterning and laryngeal activation. These findings are crucial for understanding the mechanisms underlying dysphagia, both in OSA and other breathing and neurological disorders.

Laryngeal Adductor Reflex Movement Latency Following Tactile Stimulation

OBJECTIVE

To measure the latency of laryngeal adductor reflex (LAR) motion onset at 2 laryngopharyngeal subsites using calibrated aesthesiometers.

STUDY DESIGN

Cross-sectional.

SETTING

Academic institution.

METHODS

Twenty-one asymptomatic, healthy subjects (11 male, 10 female) underwent laryngopharyngeal sensory testing with tactile stimuli delivered to the aryepiglottic fold and medial pyriform sinus using 30-mm Cheung-Bearelly monofilaments (4-0 and 5-0 nylon sutures) via channeled flexible laryngoscope. The LAR onset latency, defined as the first visual detection of ipsilateral vocal fold adduction following tactile stimulation, was measured with frame-by-frame analysis of video recordings.

RESULTS

The overall mean LAR latency across both subsites and stimulation forces was 176.6 (95% CI, 170.3-183.0) ms, without significant difference between subsites or forces. The critical value for LAR response latency prolongation at the .01 significance level was 244 ms. At 30 frames/s video capture resolution, LAR response latency ≥8 frame intervals would indicate abnormal prolongation.

CONCLUSION

Aesthesiometer-triggered LAR latency appears to be invariant over an 8.7-dB force range and between the aryepiglottic fold and medial pyriform sinus subsites in controls. Laryngeal adductor reflex latency incongruences between stimulation forces or laryngopharyngeal subsites may serve as pathophysiological features to dissect mechanisms of upper aerodigestive tract disorders.

LEVEL OF EVIDENCE

Level 3B.

Novel Adaptation of a Validated Tactile Aesthesiometer to Evaluate Laryngopharyngeal Sensation

OBJECTIVES

To evaluate laryngopharyngeal sensation at specific subsites using a novel adaptation of a buckling force aesthesiometer for delivery of calibrated tactile stimuli.

STUDY DESIGN

Cross-sectional.

METHODS

Twenty-two healthy adults (12 men, 10 women) were tested for responses to tactile forces, using 30-mm 6-0, 5-0, and 4-0 nylon monofilaments to map sensation of the aryepiglottic (AE) fold, lateral pyriform sinus (PS), and medial PS bilaterally. The outcome measures were the laryngeal adductor reflex (LAR) and patient reported rating of perceptual strength.

RESULTS

Rates of triggered LAR response grew monotonically with increasing tactile force at a mean (SD) stimulus duration of 663 (164) msec across all three subsites. The AE fold and medial PS had similar profiles and were the most responsive, while the lateral PS was the least responsive. Low force (6-0) response rate was ≤14% for all subsites. High force (4-0) response rate was 91% for AE fold and medial PS, and 23% for lateral PS. The perceptual strength gradient was in the lateral to medial trajectory.

CONCLUSION

Normative data for LAR response rates to low, medium, and high stimulation forces will be useful to assess sensory dysfunction in a variety of laryngopharyngeal disorders, including aspiration, dysphagia, chronic cough, and spasmodic dysphonia. In turn, that information will guide the creation of innovative treatments. LAR response profiles to low and high force stimuli will inform the development of screening tools to diagnose laryngopharyngeal hypersensitivity and hyposensitivity conditions.

LEVEL OF EVIDENCE

3b Laryngoscope, 131:1324-1331, 2021.

Relationships between vocal pitch perception and production: a developmental perspective

The purpose of this study was to examine the relationships between vocal pitch discrimination abilities and vocal responses to auditory pitch-shifts. Twenty children (6.6-11.7 years) and twenty adults (18-28 years) completed a listening task to determine auditory discrimination abilities to vocal fundamental frequency (fo) as well as two vocalization tasks in which their perceived fo was modulated in real-time. These pitch-shifts were either unexpected, providing information on auditory feedback control, or sustained, providing information on sensorimotor adaptation. Children were subdivided into two groups based on their auditory pitch discrimination abilities; children within two standard deviations of the adult group were classified as having adult-like discrimination abilities (N = 11), whereas children outside of this range were classified as having less sensitive discrimination abilities than adults (N = 9). Children with less sensitive auditory pitch discrimination abilities had significantly larger vocal response magnitudes to unexpected pitch-shifts and significantly smaller vocal response magnitudes to sustained pitch-shifts. Children with less sensitive auditory pitch discrimination abilities may rely more on auditory feedback and thus may be less adept at updating their stored motor programs.

Liquiritin apioside attenuates laryngeal chemoreflex but not mechanoreflex in rat pups

Liquiritin apioside (LA), a main flavonoid component of licorice, reportedly suppresses cough responses to inhalation of aerosolized capsaicin [CAP; a stimulant to transient receptor potential vanilloid 1 (TRPV1)] in conscious guinea pigs via acting on peripheral nerves. However, the evidence of LA having a direct effect on airway sensory fibers is lacking. Considering the important role laryngeal chemoreceptors and mechanoreceptors play in triggering apnea and cough, we studied whether LA suppressed the apneic responses to stimulation of these receptors via directly acting on the superior laryngeal nerve (SLN). Intralaryngeal delivery of chemical [CAP, HCl, and distilled water (DW)] and mechanical [an air-pulse (AP)] stimulations was applied in anesthetized rat pups to evoke the apnea. These stimuli were repeated after intralaryngeal LA treatment or peri-SLN LA treatment to determine the direct effect of LA on the SLN. Our results showed that all stimuli triggered an immediate apnea. Intralaryngeal LA treatment significantly attenuated the apneic response to chemical but not mechanical stimulations. The same attenuation was observed after peri-SLN LA treatment. Owing that TRPV1 receptors of laryngeal C fibers are responsible for the CAP-triggered apneas, the LA impact on the activity of laryngeal C neurons retrogradely traced by DiI was subsequently studied using a patch-clamp approach. LA pretreatment significantly altered the electrophysiological kinetics of CAP-induced currents in laryngeal C neurons by reducing their amplitudes, increasing the rise times, and prolonging the decay times. In conclusion, our results, for the first time, reveal that LA suppresses the laryngeal chemoreceptor-mediated apnea by directly acting on the SLN (TRPV1 receptors of laryngeal C fibers).

Mapping regional laryngopharyngeal mechanoreceptor response

Objectives

To map mechanoreceptor response in various regions of the laryngopharynx.

Methods

Five patients with suspected laryngopharyngeal reflux and six healthy control subjects underwent stimulation of mechanoreceptors in the hypopharynx, interarytenoid area, arytenoids, aryepiglottic folds, and pyriform sinuses. The threshold stimuli evoking sensation and eliciting laryngeal adductor reflex were recorded.

Results

In controls, an air pulse with 2 mmHg pressure evoked mechanoreceptor response in all regions, except bilateral aryepiglottic folds of one control. In patients, stimulus intensity to elicit mechanoreceptor response ranged between 2 mmHg and 10 mmHg and varied among the regions. Air pulse intensity differed between right and left sides of laryngopharyngeal regions in the majority of patients.

Conclusion

Laryngopharyngeal mechanoreceptor response was uniform among regions and subjects in the healthy group. Patients with suspected laryngopharyngeal reflux showed inter- and intra-regional variations in mechanoreceptor response. Laryngopharyngeal sensory deficit in patients with suspected laryngopharyngeal reflux is not limited to aryepiglottic folds.

Symptom index P-value and symptom sensitivity index P-value to determine symptom association between apnea and reflux in premature infants at term

The current method to determine temporal association (TA) between reflux and symptoms is the symptom association probability (SAP), but this method has limitations due to the constraints of binning and the violation of statistical principles of the Fisher's exact test that lead to an invalid estimation of TA. The aim of this study is to develop improved methods of computing the TA between apneic and reflux events using simulation and permutation methods and to compare these to the SAP. TA was analyzed between polysomnographic obstructive apneas and multichannel intraluminal impedance (MII) reflux events. Three new numerical methods were compared to the SAP in four former premature infants with persistent apneas at term. The experimentally found association was compared to the association observed in simulated or permuted data consistent with the lack of association beyond what is expected by chance alone. Temporal association was computed based on symptom and symptom sensitivity indices, SI and SSI, with varying window of association (WA) times from 15 to 300 s. The three new methods estimated P-values at varying WA that generally followed the same pattern of the SAP which had a more erratic pattern. The WA that gave the lowest P-value was approximately 120 s. Each of the novel methods produced P-value results consistent with each other and the SAP yet not subject to its limitations. The variation of WA gave a temporal profile of TA providing clues to its etiology. These new metrics are called Symptom Index (SIP) and Symptom Sensitivity Index (SSIP) P-values.

Neurophysiology and Clinical Implications of the Laryngeal Adductor Reflex

The laryngeal adductor reflex (LAR) is an involuntary protective response to stimuli in the larynx. The superior laryngeal nerve (SLN) acts as the afferent limb and the recurrent laryngeal nerve (RLN) as the efferent limb of this reflex, which is modulated by the central nervous system. Perhaps the most clinically significant application of the LAR is its use in laryngopharyngeal (LP) sensory discrimination testing. Importantly, aberrations in the LAR may predict dysphagia or portend clinical phenotypes of chronic cough, vocal cord dysfunction or pediatric apneas. LP sensation is a potential target for interventions addressing the aforementioned conditions though currently remains an area of active investigation.

Assessment of laryngopharyngeal sensation in children with dysphagia

OBJECTIVES/HYPOTHESIS

To assess laryngopharyngeal sensation, prevalence of laryngopharyngeal sensory deficit and abnormal swallowing function parameters in children with dysphagia.

STUDY DESIGN

Retrospective chart review.

METHODS

The medical records of children who underwent flexible endoscopic evaluation of swallowing with sensory testing (FEESST) were reviewed. Laryngopharyngeal sensory threshold (LPST) was assessed based on the threshold intensity of air pulse stimulation eliciting laryngeal adductor reflex. Swallowing function parameters including pharyngeal residue, hypopharyngeal pooling of secretions, premature spillage, laryngeal penetration, and aspiration were evaluated. Prevalence of abnormal swallowing function parameters in children with normal and impaired LPST was compared.

RESULTS

Forty children with dysphagia (28 male, 12 female; age range, 3 months to 17 years) underwent FEESST. LPST was normal in six patients, moderately impaired in 20 patients, and severely impaired in 10 patients. LPST could not be measured in four patients. Children showed one or more abnormal swallowing function parameters. The prevalence of abnormal swallowing parameters in patients with normal LPST was lower than that of patients with moderately or severely impaired LPST (P < .05). The prevalence of pharyngeal residue, hypopharyngeal pooling of secretions, and spillage in patients with severely impaired LPST was higher than that of patients with moderately impaired LPST (P < .05).

CONCLUSIONS

The majority of children with dysphagia have impaired LPST. The prevalence of abnormal swallowing function parameters in children with normal LPST is lower than that in children with moderately or severely impaired LPST. Prevalence of aspiration tends to increase when the abnormal swallowing function parameters are associated with severely impaired LPST.

Neuronal mechanisms underlying the laryngeal adductor reflex

OBJECTIVES

Electromyographic studies of the laryngeal adductor reflex, glottal closure occurring in response to laryngeal stimulation, have demonstrated an early ipsilateral response (R1) and a late bilateral response (R2). To better define the physiologic properties of these responses, we recorded responses from expiratory laryngeal motoneurons (ELMs) in rats during stimulation of the superior laryngeal nerve (SLN).

METHODS

Single unit extracellular recordings were obtained from 5 ELMs, identified by their antidromic responses to recurrent laryngeal nerve stimulation and postinspiratory firing pattern, in 4 Sprague-Dawley rats.

RESULTS

Unilateral stimulation of the SLN (at 20 Hz) stopped both phrenic nerve inspiratory activity and ELM postinspiratory activity. However, the ELMs displayed robust tonic firing, consisting of non-respiratory burst activity and single action potentials. The single action potentials were identified as short-latency ones (5 to 10 ms) activated by ipsilateral SLN stimulation, with an occurrence rate of 90%, and long-latency ones (20 to 50 ms) activated by bilateral SLN stimulation, with occurrence rates of 47% on the ipsilateral side and 58% on the contralateral side.

CONCLUSIONS

The R1 response appears to be the result of the short-latency action potentials, orthodromically activated by ipsilateral stimulation of the SLN. The R2 response is likely to be a result of the long-latency action potentials that can be recorded from ELMs on both sides.

Temporal association of polysomnographic cardiorespiratory events with GER detected by MII-pH probe in the premature infant at term

OBJECTIVES

The aim of the study was to examine temporal association (TA) between polysomnographic cardiorespiratory (CR) events and gastroesophageal reflux (GER) in premature infants with persistent CR events at >39 weeks postmenstrual age and determine whether the use of multichannel intraluminal impedance (MII)-pH probe improves sensitivity of the TA compared with pH probe alone.

PATIENTS AND METHODS

Seven infants born between 24 and 29 weeks' gestational age with persistent CR events at 39 to 48 weeks' postmenstrual age underwent a polysomnography with MII-pH probe. Symptom index (SI) and symptom-associated probability were calculated for diverse types of reflux and CR events. SI and a Fisher exact test with variable association windows were calculated for obstructive apnea (OA). Odds ratios for an OA given a reflux event and for a reflux event given an OA were determined.

RESULTS

With a Fisher exact test, a subject-specific association between MII events and OA was found in the 3 patients who required a fundoplication or had the worse clinical GER. Some level of TA was found with SI and symptom-associated probability in 6 of 7 infants. Association was found for pH > 4 and pH ≤ 4 reflux events. pH-only events with no change of MII had only a limited role in generating CR events.

CONCLUSIONS

TA between CR events and GER was found in a single-subject-level analysis in some infants with persistent CR events at term. This TA suggests a causal relation between CR and reflux events that was further strengthened by the clinical outcomes of each infant.

Impact of Supraglottoplasty on Aspiration in Severe Laryngomalacia

Objectives:

We examined the incidence and significance of aspiration in infants with severe laryngomalacia (LM) who undergo supraglottoplasty.

Methods:

We performed a 5-year retrospective review of a prospective database from 2 tertiary care pediatric institutions. The studied patients were 50 consecutive infants with severe LM who underwent supraglottoplasty (median age, 4.5 months) and functional endoscopic evaluation of swallowing (FEES) with or without laryngopharyngeal stimulation testing. The intervention was cold-knife microlaryngeal supraglottoplasty and reflux management. The main outcome measure was aspiration resolution.

Results:

Preoperative FEES identified laryngeal penetration in 44 infants (88%) with severe LM. Laryngeal penetration with aspiration beyond the vocal folds was noted in 36 infants (72%). Postoperative FEES (median follow-up, 3.8 months) indicated resolution of laryngeal penetration and aspiration in 36 (81.8%) and 31 (86.1%) of these patients, respectively. The 14 patients without preoperative aspiration showed no evidence of aspiration after supraglottoplasty. In patients with aspiration, the mean preoperative laryngopharyngeal stimulation test threshold was 8.45 mm Hg. This improved on average by 4.0 mm Hg after supraglottoplasty (paired t-test, p < 0.0001). Multiple medical comorbidities were present in the 5 patients who had persistent aspiration after supraglottoplasty, including congenital heart disease (all 5 patients), congenital syndromes (4 patients), neurologic disorders (2 patients), and a need for tracheostomy (2 patients).

Conclusions:

Laryngeal penetration and aspiration improve after cold-knife supraglottoplasty. Supraglottoplasty does not cause aspiration in patients who do not have preoperative aspiration. Supraglottoplasty may not improve aspiration in patients with multiple medical comorbidities.

Abnormal Sensorimotor Integrative Function of the Larynx in Congenital Laryngomalacia: A New Theory of Etiology

OBJECTIVES/HYPOTHESIS

Laryngomalacia is an enigmatic disease in which laryngeal tone is weak, resulting in dynamic prolapse of tissue into the larynx. Sensorimotor integrative function of the brainstem and peripheral reflexes are responsible for laryngeal tone and airway patency. The goal of this study was to elucidate the etiology of decreased laryngeal tone through evaluating the sensorimotor integrative function of the larynx. The secondary goal was to evaluate factors and medical comorbidities that contribute to the wide spectrum of symptoms and outcomes.

STUDY DESIGN

Prospective and retrospective collection of evaluative data on infants with congenital laryngomalacia at two tertiary care pediatric referral centers.

METHODS

Two hundred one infants with laryngomalacia were divided into three groups on the basis of disease severity (mild, moderate, severe). Patients were followed prospectively every 8 to 12 weeks until symptom resolution or loss to follow-up. Sensorimotor integrative function of the larynx was evaluated in 134 infants by laryngopharyngeal sensory testing (LPST) of the laryngeal adductor reflex (LAR) by delivering a duration- (50 ms) and intensity- (2.5-10 mm Hg) controlled air pulse to the aryepiglottic fold to induce the LAR. Medical records were retrospectively reviewed for medical comorbidities.

RESULTS

The initial LPST was higher (P < .001) in infants with moderate (6.8 mm Hg) and severe disease (7.4 mm Hg) compared with those with mild disease (4.1 mm Hg). At 1, 3, and 6 months, infants with moderate and severe disease continued to have a higher LPST compared with those with mild disease. At 9 months, the LPST decreased in all subjects (3.1-3.5 mm Hg, P = .14), which also correlated with symptom resolution. Neurologic, genetic, and cardiac diseases were more common in infants with severe disease (P < .001). Gastroesophageal reflux disease (GERD) and feeding problems more common in those with moderate and severe disease (P < .001). Apgar scores were lower in those with severe disease (P < .001). Most symptoms resolved within 12 months of presentation. Those with GERD benefited from treatment. Supraglottoplasty resulted in few complications. Multiple comorbidities (>3) influenced the need for tracheotomy.

CONCLUSIONS

Laryngeal tone and sensorimotor integrative function of the larynx is altered. The degree of alteration correlated with disease severity, indicating that factors that alter the peripheral and central reflexes of the LAR have a role in the etiology of signs and symptoms of laryngomalacia. GERD, neurologic disease, and low Apgar scores influenced disease severity and clinical course, explaining the spectrum of disease symptoms and outcomes. Sensorimotor integrative function improved as symptoms resolved.

The role of sensory dysfunction in the development of voice disorders, chronic cough and paradoxical vocal fold movement

Sensory function may be important in the pathogenesis of Chronic Cough (CC) and Paradoxical Vocal Fold Movement (PVFM). This paper aims to explore sensory issues related to the pathogenesis, classification, assessment and management of these conditions. Sensory disruption of the vagus nerve can occur through neural plasticity whereby a change occurs in the way a central neuron reacts to an incoming stimulus. Such disruption can be demonstrated through assessment of cough reflex sensitivity and extrathoracic airway hyperresponsiveness both of which may be increased in CC and PVFM. In addition, sensory function may be determined by measuring the laryngeal adductor reflex, however this phenomenon is yet to be explored in CC and PVFM. The similarity in sensory dysfunction between CC and PVFM provides support for a link between the two conditions. There are also similarities in underlying medical conditions and symptom profiles between CC/PVFM and voice disorders such as muscle tension dysphonia. Although coughing and throat clearing may be contributing factors in the development and maintenance of voice disorders, they may occur in response to extrathoracic airway hyperresponsiveness. Dysphonia can occur in CC/PVFM and may improve following behavioural treatment of CC.

Infant apnea and gastroesophageal reflux: a critical review and framework for further investigation

Gastroesophageal reflux and apnea of prematurity are both common occurrences in premature infants. However, a causal relationship between the two remains controversial. Strong physiologic evidence indicates that a variety of protective reflex responses may elicit laryngeal adduction and apnea. Although a potential link between gastroesophageal reflux and apnea may exist through this pathway, clinical studies can be cited to either support or refute such a link in premature infants. The majority of gastroesophageal reflux episodes do not appear to be related to apnea. In a specific subset of events, a causal relationship may exist. Whether this is related to the character of the reflux episode or to a predisposition in a subpopulation of infants is unclear. This review presents the evidence for and against an association between gastroesophageal reflux and apnea, discusses techniques used in their evaluation, and identifies approaches for future investigation.