Pre-pharyngeal Swallow Effects of Recurrent Laryngeal Nerve Lesion on Bolus Shape and Airway Protection in an Infant Pig Model

Exploring the interaction of viscosity and nipple design on feeding performance in an infant pig model

Infant feeding behaviors are modulated via sensorimotor feedback, such that sensory perturbations can significantly impact performance. Properties of the nipple and milk (e.g., nipple hole size and viscosity) are critical sources of sensory information. However, the direct effects of varying milk and nipple properties on infant motor output and the subsequent changes in feeding performance are poorly understood. In this study, we use an infant pig model to explore the interaction between nipple hole size and milk viscosity. Using high-speed videofluoroscopy and electromyography, we measured key performance metrics including sucks per swallow and suck duration, then synchronized these data with the onset and offset of activity of jaw opening and closing muscles. The combination of a small nipple hole and thick milk resulted in negative effects on both suck and swallow performance, with reduced feeding efficiency compared to the other treatments. It also appears that this combination of viscosity and hole size disrupts the coordination between correlates of tongue and jaw movements. We did not see a difference in feeding efficiency between viscosities when infants fed on the large-hole nipple, which may be the result of non-Newtonian fluid mechanics. Our results emphasize the importance of considering both fluid and nipple properties when considering alterations to an infant's feeding system.

Impact of volume and rate of milk delivery on coordination of respiration and swallowing in infant pigs

The coordination of respiration and swallowing is a life-critical function in infants. Varying volume and rate of milk delivery changes swallowing frequency and bolus volume but any impact on swallow-respiration coordination is unknown. Five infant pigs were filmed with simultaneous high speed videofluoroscopy and plethysmography while feeding from an automatic system delivering milk across a range of volumes and frequencies. Swallow inspiration delay, respiratory cycle duration, and distribution of inspiratory and expiratory swallows were calculated. At constant volume, there were more inspiratory phase swallows when frequency increased. At high constant frequency, increasing volume changed swallow-respiration coordination patterns, with increased occurrence of inspiratory phase swallows. Respiratory cycle duration did not change in response to changes in oral milk delivery. These results suggest that the observed pattern of expiratory swallowing in infants is achieved primarily by regulation of milk intake, not modulation of respiratory patterns by oral sensation.

The effect of stiffness and hole size on nipple compression in infant suckling

During infant feeding, the nipple is an important source of sensory information that affects motor outputs, including ones dealing with compression of the nipple, suction, milk bolus movement, and swallowing. Despite known differences in behavior across commercially available nipples, little is known about the in vivo effects of nipple property variation. Here we quantify the effect of differences in nipple stiffness and hole size on an easily measured metric representing infant feeding behavior: nipple compression. We bottle-fed 7-day old infant pigs (n = 6) on four custom fabricated silicone nipples. We recorded live X-ray fluoroscopic imaging data of feeding on nipples of two levels of hardness/stiffness and two hole sizes. We tested for differences in nipple compression at the nipple's maximum compression across different nipple types using a mixed model analysis of variance. Stiffer nipples and those with smaller holes were compressed less than compliant nipples and nipples with larger holes (p < 0.001). We also estimated the force applied on the nipple during feeding and found that more force was applied to the compliant nipple with disproportionately larger strains. Our results suggest that infant pigs' nipple compression depends on material type and hole size, which is likely detected by the infant pigs' initial assessment of compressibility and flow. By isolating nipple properties, we demonstrated a relationship between properties and suckling behavior. Our results suggest that sensory information affects feeding behaviors and may also inform clinical treatment of poor feeding performance.

Eliciting and Characterizing Porcine Vocalizations: When Pigs Fly

BACKGROUND/OBJECTIVES

While voice-related therapeutic interventions are often researched preclinically in the porcine model, there are no well-established methods to induce porcine glottic phonation. Described approaches, such as training animals to phonate for positive reinforcement are time-consuming and plagued by inherent variability in the type of phonation produced and contamination of background noise. Thus, a reliable method of assessing glottic phonation in the porcine model is needed.

METHODS

In this study, we have created a novel pulley-based apparatus with harness for "pig-lifting" with surrounding acoustic insulation and high-directional microphone with digital recorder for recording phonation. Praat and Matlab were used to analyze all porcine vocalizations for fundamental frequency (F0), intensity, duration of phonation and cepstral peak prominence (CPP). Glottic phonation was detected using F0 (≥2000 hz), duration (≥3 seconds) and researcher perceptual judgment. Partial-glottic phonations were also analyzed. Reliability between researcher judgment and acoustic measures for glottic phonation detection was high.

RESULTS

Acoustic analysis demonstrated that glottic and partial-glottic phonation was consistently elicited, with no formal training of the minipigs required. Glottic vocalizations increased with multiple lifts. Glottic phonation continued to be elicited after multiple days but became less frequent. Glottic and partial-glottic phonations had similar CPP values over the 6 experimental days.

CONCLUSION

Our cost-effective, reliable method of inducing and recording glottic phonation in the porcine model may provide a cost effective, preclinical tool in voice research.

Advances in Swallowing Neurophysiology across Pediatric Development: Current Evidence and Insights

Purpose of Review

This review article analyzes current evidence on the neurophysiology of swallowing during development and offers expert opinion on clinical implications and future research directions.

Recent Findings

In the past five years, basic and clinical research has offered advances in our understanding of pediatric swallowing neurophysiology. Animal models have elucidated the role of brainstem circuits and the peripheral and central nervous system in neonatal swallowing. Recent human studies have further showcased that fetal and infant swallowing require cerebral inputs in order to develop functionally. Finally, neurophysiological and neuroimaging studies are starting to better define these cerebral inputs, as well as neuroplastic adaptations that may be needed for optimal feeding development.

Summary

The neural development of swallowing is a complex and dynamic process. Continued research is needed to better understand influences on swallowing neural development, which can be essential for improving prevention, diagnosis, and interventions for pediatric dysphagia.

Increased viscosity of milk during infant feeding improves swallow safety through modifying sucking in an animal model

Infants experiencing frequent aspiration, the entry of milk into the airway, are often prescribed thickened fluids to improve swallow safety. However, research on the outcomes of thickened milk on infant feeding have been limited to documenting rates of aspiration and the rheologic properties of milk following thickening. As a result, we have little insight into the physiologic and behavioral mechanisms driving differences in performance during feeding on high viscosity milk. Understanding the physiologic and behavioral mechanisms driving variation in performance at different viscosities is especially critical, because the structures involved in feeding respond differently to sensory stimulation. We used infant pigs, a validated animal model for infant feeding, to test how the tongue, soft palate, and hyoid respond to changes in viscosity during sucking and swallowing, in addition to measuring swallow safety and bolus size. We found that the tongue exhibited substantive changes in its movements associated with thickened fluids during sucking and swallowing, but that pharyngeal transit time as well as hyoid and soft palate movements during swallowing were unaffected. This work demonstrates the integrated nature of infant feeding and that behaviors associated with sucking are more sensitive to sensorimotor feedback associated with changes in milk viscosity than those associated with the pharyngeal swallow, likely due to its reflexive nature.

Effects of Superior Laryngeal Nerve Lesion on Kinematics of Swallowing and Airway Protection in an Infant Pig Model

The superior laryngeal nerve provides detailed sensory information from the mucosal surfaces of laryngeal structures superior to the vocal folds, including the valleculae. Injury to this nerve results in airway penetration and aspiration. Furthermore, such injuries might have an impact on the function of multiple structures involved in intraoral transport and swallowing due to connections within the brainstem. We sought to determine the effects of a surgical lesion of the superior laryngeal nerve on kinematics of the tongue, hyoid, and epiglottis during swallowing. We implanted radio-opaque markers into five infant pigs under anesthesia. Then we fed milk mixed with contrast agent to the pigs while they were recorded via video fluoroscopy, before and after a surgery to transect the superior laryngeal nerve. We digitized and rated airway protection in 177 swallows. We found that in most animals, swallow duration was shorter after nerve lesion. The hyoid also traveled a shorter distance after lesion. Frequently, individuals reacted differently to the same nerve lesion. We suggest that these differences are due to individual differences in neurological connections. When comparing hyoid kinematics between swallows with successful or failed airway protection, we found more consistency among individuals. This indicates that protecting the airway requires specific sets of kinematic events to occur, regardless of the neurological differences among individuals.

Muscle activity and kinematics show different responses to recurrent laryngeal nerve lesion in mammal swallowing

Understanding the interactions between neural and musculoskeletal systems is key to identifying mechanisms of functional failure. Mammalian swallowing is a complex, poorly understood motor process. Lesion of the recurrent laryngeal nerve, a sensory and motor nerve of the upper airway, results in airway protection failure (liquid entry into the airway) during swallowing through an unknown mechanism. We examined how muscle and kinematic changes after recurrent laryngeal nerve lesion relate to airway protection in eight infant pigs. We tested two hypotheses: 1) kinematics and muscle function will both change in response to lesion in swallows with and without airway protection failure, and 2) differences in both kinematics and muscle function will predict whether airway protection failure occurs in lesion and intact pigs. We recorded swallowing with high-speed videofluoroscopy and simultaneous electromyography of oropharyngeal muscles pre- and postrecurrent laryngeal nerve lesion. Lesion changed the relationship between airway protection and timing of tongue and hyoid movements. Changes in onset and duration of hyolaryngeal muscles postlesion were less associated with airway protection outcomes. The tongue and hyoid kinematics all predicted airway protection outcomes differently pre- and postlesion. Onset and duration of activity in only one infrahyoid and one suprahyoid muscle showed a change in predictive relationship pre- and postlesion. Kinematics of the tongue and hyoid more directly reflect changes in airway protections pre- and postlesion than muscle activation patterns. Identifying mechanisms of airway protection failure requires specific functional hypotheses that link neural motor outputs to muscle activation to specific movements.NEW & NOTEWORTHY Kinematic and muscle activity patterns of oropharyngeal structures used in swallowing show different patterns of response to lesion of the recurrent laryngeal nerve. Understanding how muscles act on structures to produce behavior is necessary to understand neural control.

Swallow Safety is Determined by Bolus Volume During Infant Feeding in an Animal Model

Feeding difficulties are especially prevalent in preterm infants, although the mechanisms driving these difficulties are poorly understood due to a lack of data on healthy infants. One potential mechanism of dysphagia in adults is correlated with bolus volume. Yet, whether and how bolus volume impacts swallow safety in infant feeding is unknown. A further complication for safe infant swallowing is recurrent laryngeal nerve (RLN) injury due to patent ductus arteriosus surgery, which exacerbates the issues that preterm infants face and can increase the risk of dysphagia. Here, we used a validated animal model feeding freely to test the effect of preterm birth, postnatal maturation and RLN lesion and their interactions on swallow safety. We also tested whether bolus size differed with lesion or birth status, and the relationship between bolus size and swallow safety. We found very little effect of lesion on swallow safety, and preterm infants did not experience more penetration or aspiration than term infants. However, term infants swallowed larger boluses than preterm infants, even after correcting for body size. Bolus size was the primary predictor of penetration or aspiration, with larger boluses being more likely to result in greater degrees of dysphagia irrespective of age or lesion status. These results highlight that penetration and aspiration are likely normal occurrences in infant feeding. Further, when comorbidities, such as RLN lesion or preterm birth are present, limiting bolus size may be an effective means to reduce incidences of penetration and aspiration.

Premature birth impacts bolus size and shape through nursing in infant pigs

BACKGROUND

The formation of a bolus of food is critical for proper feeding function, and there is substantial variation in the size and shape of a bolus prior to a swallow. Preterm infants exhibit decreased abilities to acquire and process food, but how that relates to their bolus size and shape is unknown. Here, we test two hypotheses: (1) that bolus size and shape will differ between term and preterm infants, and (2) bolus size and shape will change longitudinally through development in both term and preterm infants.

METHODS

To test these hypotheses, we measured bolus size and shape in preterm and term infant pigs longitudinally through nursing using high-speed videofluoroscopy.

RESULTS

Preterm infant pigs swallowed smaller volumes of milk. Although term infants increased the amount of milk per swallow as they aged, preterm infants did not. These changes in bolus volume were also correlated with changes in bolus shape; larger boluses became more elongate as they better filled the available anatomical space of the valleculae.

CONCLUSIONS

These results suggest that preterm birth reduces the ability of preterm pigs to increase bolus size as they grow, affecting development in this fragile population. These results highlight that studies on term infant feeding may not translate to preterm infants.

Swallow Safety in Infant Pigs With and Without Recurrent Laryngeal Nerve Lesion

Aerodigestive coordination is critical for safe feeding in mammals, and failure to do so can result in aspiration. Using an infant pig model, we analyzed the impact of recurrent laryngeal nerve (RLN) lesion on aerodigestive coordination and swallow safety at two time points prior to weaning. We used high-speed videofluoroscopy to record 23 infant pigs longitudinally at two ages (7 days, 17 days) feeding on barium milk. We measured respiration with a plethysmograph and used the Infant Mammalian Penetration-Aspiration Scale (IMPAS) to identify unsafe swallows. We tested for changes in swallow safety longitudinally in control and lesion pigs, and whether there was any interaction between the four different groups. On postnatal day 7, lesioned pigs exhibited differences in the frequency distribution of IMPAS scores relative to control pigs on day 7, and 17 day old lesion and control pigs. There were longitudinal changes in performance following RLN lesion through time, suggesting that the impact of RLN lesion decreases with time, as older lesioned pigs performed similarly to older control pigs. We found minimal differences in the impact of aerodigestive coordination on swallow safety, with shorter delays of inspiration onset reflecting higher rates of penetration in young lesioned pigs. Healthy pigs aspirated at a similar rate to those with an RLN lesion indicating that the occasional occurrence of dysphagia in infants may be a normal behavior.

Suckling, Feeding, and Swallowing: Behaviors, Circuits, and Targets for Neurodevelopmental Pathology

All mammals must suckle and swallow at birth, and subsequently chew and swallow solid foods, for optimal growth and health. These initially innate behaviors depend critically upon coordinated development of the mouth, tongue, pharynx, and larynx as well as the cranial nerves that control these structures. Disrupted suckling, feeding, and swallowing from birth onward-perinatal dysphagia-is often associated with several neurodevelopmental disorders that subsequently alter complex behaviors. Apparently, a broad range of neurodevelopmental pathologic mechanisms also target oropharyngeal and cranial nerve differentiation. These aberrant mechanisms, including altered patterning, progenitor specification, and neurite growth, prefigure dysphagia and may then compromise circuits for additional behavioral capacities. Thus, perinatal dysphagia may be an early indicator of disrupted genetic and developmental programs that compromise neural circuits and yield a broad range of behavioral deficits in neurodevelopmental disorders.

Adaptations to Oral and Pharyngeal Swallowing Function Induced by Injury to the Mylohyoid Muscle

Muscle injury is a frequent side effect of radiation treatment for head and neck cancer. To understand the pathophysiology of injury-related dysfunction, we investigated the effects of a single muscle injury to the mylohyoid on oropharyngeal swallowing function in the rat. The mylohyoid protects the airway from food/liquid via hyolaryngeal elevation and plays an active role during both oral and pharyngeal swallowing. We hypothesized (1) that fibrosis to the mylohyoid alters swallowing bolus flow and licking patterns and (2) that injury to the mylohyoid changes normal activity of submental, laryngeal, and pharyngeal muscles during swallowing. A chilled cryoprobe was applied to the rat mylohyoid muscle to create a localized injury. One and two weeks after injury, swallowing bolus transit was assessed via videofluoroscopy and licking behavior via an electrical lick sensor. The motor activity of five swallow-related muscles was analyzed immediately after injury using electromyography (EMG). Comparisons were made pre- and post-injury. Fibrosis was confirmed in the mylohyoid at 2 weeks after injury by measuring collagen content. One week after injury, bolus size decreased, swallowing rate reduced, and licking patterns were altered. Immediately post-injury, there was a significant depression in mylohyoid and thyropharyngeus EMG amplitudes during swallowing. Our results demonstrated that injury to the mylohyoid is sufficient to cause changes in deglutition. These disruptions in oral and pharyngeal swallowing were detected prior to long-term fibrotic changes, including delays in tongue movement, alterations in bolus flow, and changes in sensorimotor function. Therefore, injuring a single important swallowing muscle can have dramatic clinical effects.

Specific Vagus Nerve Lesion Have Distinctive Physiologic Mechanisms of Dysphagia

Swallowing is complex at anatomical, functional, and neurological levels. The connections among these levels are poorly understood, yet they underpin mechanisms of swallowing pathology. The complexity of swallowing physiology means that multiple failure points may exist that lead to the same clinical diagnosis (e.g., aspiration). The superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN) are branches of the vagus that innervate different structures involved in swallowing. Although they have distinct sensory fields, lesion of either nerve is associated clinically with increased aspiration. We tested the hypothesis that despite increased aspiration in both case, oropharyngeal kinematic changes and their relationship to aspiration would be different in RLN and SLN lesioned infant pigs. We compared movements of the tongue and epiglottis in swallows before and after either RLN or SLN lesion. We rated swallows for airway protection. Posterior tongue ratio of safe swallows changed in RLN (p = 0.01) but not SLN lesioned animals. Unsafe swallows post lesion had different posterior tongue ratios in RLN and SLN lesioned animals. Duration of epiglottal inversion shortened after lesion in SLN animals (p = 0.02) but remained unchanged in RLN animals. Thus, although SLN and RLN lesion lead to the same clinical outcome (increased aspiration), the mechanisms of failure of airway protection are different, which suggests that effective therapies may be different with each injury. Understanding the specific pathophysiology of swallowing associated with specific neural insults will help develop targeted, disease appropriate treatments.

Recurrent laryngeal nerve transection in mice results in translational upper airway dysfunction

The recurrent laryngeal nerve (RLN) is responsible for normal vocal-fold (VF) movement, and is at risk for iatrogenic injury during anterior neck surgical procedures in human patients. Injury, resulting in VF paralysis, may contribute to subsequent swallowing, voice, and respiratory dysfunction. Unfortunately, treatment for RLN injury does little to restore physiologic function of the VFs. Thus, we sought to create a mouse model with translational functional outcomes to further investigate RLN regeneration and potential therapeutic interventions. To do so, we performed ventral neck surgery in 21 C57BL/6J male mice, divided into two groups: Unilateral RLN Transection (n = 11) and Sham Injury (n = 10). Mice underwent behavioral assays to determine upper airway function at multiple time points prior to and following surgery. Transoral endoscopy, videofluoroscopy, ultrasonic vocalizations, and whole-body plethysmography were used to assess VF motion, swallow function, vocal function, and respiratory function, respectively. Affected outcome metrics, such as VF motion correlation, intervocalization interval, and peak inspiratory flow were identified to increase the translational potential of this model. Additionally, immunohistochemistry was used to investigate neuronal cell death in the nucleus ambiguus. Results revealed that RLN transection created ipsilateral VF paralysis that did not recover by 13 weeks postsurgery. Furthermore, there was evidence of significant vocal and respiratory dysfunction in the RLN transection group, but not the sham injury group. No significant differences in swallow function or neuronal cell death were found between the two groups. In conclusion, our mouse model of RLN injury provides several novel functional outcome measures to increase the translational potential of findings in preclinical animal studies. We will use this model and behavioral assays to assess various treatment options in future studies.

A Surgical Mouse Model for Advancing Laryngeal Nerve Regeneration Strategies

Iatrogenic recurrent laryngeal nerve (RLN) injury is a morbid complication of anterior neck surgical procedures. Existing treatments are predominantly symptomatic, ranging from behavioral therapy to a variety of surgical approaches. Though laryngeal reinnervation strategies often provide muscle tone to the paralyzed vocal fold (VF), which may improve outcomes, there is no clinical intervention that reliably restores true physiologic VF movement. Moreover, existing interventions neglect the full cascade of molecular events that affect the entire neuromuscular pathway after RLN injury, including the intrinsic laryngeal muscles, synaptic connections within the central nervous system, and laryngeal nerve anastomoses. Systematic investigations of this pathway are essential to develop better RLN regenerative strategies. Our aim was to develop a translational mouse model for this purpose, which will permit longitudinal investigations of the pathophysiology of iatrogenic RLN injury and potential therapeutic interventions. C57BL/6J mice were divided into four surgical transection groups (unilateral RLN, n = 10; bilateral RLN, n = 2; unilateral SLN, n = 10; bilateral SLN, n = 10) and a sham surgical group (n = 10). Miniaturized transoral laryngoscopy was used to assess VF mobility over time, and swallowing was assessed using serial videofluoroscopy. Histological assays were conducted 3 months post-surgery for anatomical investigation of the larynx and laryngeal nerves. Eight additional mice underwent unilateral RLN crush injury, half of which received intraoperative vagal nerve stimulation (iVNS). These 8 mice underwent weekly transoral laryngoscopy to investigate VF recovery patterns. Unilateral RLN injury resulted in chronic VF immobility but only acute dysphagia. Bilateral RLN injury caused intraoperative asphyxiation and death. VF mobility was unaffected by SLN transection (unilateral or bilateral), and dysphagia (transient) was evident only after bilateral SLN transection. The sham surgery group retained normal VF mobility and swallow function. Mice that underwent RLN crush injury and iVNS treatment demonstrated accelerated and improved VF recovery. We successfully developed a mouse model of iatrogenic RLN injury with impaired VF mobility and swallowing function that can serve as a clinically relevant platform to develop translational neuroregenerative strategies for RLN injury.

Reduced Coordination of Hyolaryngeal Elevation and Bolus Movement in a Pig Model of Preterm Infant Swallowing

Preterm infants often have dysphagia. Because reducing lifetime cumulative exposure to radiation in the context of diagnosis and treatment is a continuing goal of all medical fields which use X-ray imaging, efforts exist to reduce reliance on the gold standard diagnostic tool for dysphagia, VFSS. Alternatives, such as video of external hyolaryngeal movement using video recordings of the anterior surface of the neck, must be evaluated and validated against videofluoroscopy, a task for which non-human animal models are appropriate. In this study, we tested the hypotheses that (1) swallows could be identified equally well from video of external hyolaryngeal movement and bolus movement in videofluoroscopy, and that (2) the two measures would be tightly temporally linked in both term and preterm infant pigs. We recorded 222 swallows in simultaneous and precisely synchronized high-speed videofluoroscopy and high-speed camera films of 4 preterm and 3 term infant pigs drinking milk from a bottle. In term pigs, the two measures consistently identified the same swallows in each image stream. However, in preterm pigs there was a high rate of false positives (~ 10% per feeding sequence) and false negatives (~ 27% per feeding sequence). The timing of hyolaryngeal elevation (external video) and bolus movement (videofluoroscopy) was correlated and consistent in terms pigs, but not in preterm pigs. Magnitude of hyolaryngeal elevation was less in preterm pig swallows than term pig swallows. Absence of epiglottal inversion in preterm pigs was not linked to variation in the timing of the two swallow events. Video of external hyolaryngeal movement, though a reliable swallow indicator in term infant pigs, was unreliable in preterm infant pigs. The coordination of swallowing events differs in preterm and term infant pigs. More research is needed into the distinctive biomechanics of preterm infant pigs.

Preterm birth disrupts the development of feeding and breathing coordination

All mammals must breathe and breathe continuously from birth. Similarly, all mammals, including infants, have high functional demands for feeding. However, the pathway that food takes through the pharynx interrupts respiration. The coordination between swallowing and breathing is therefore critical for all infant mammals. Clinically, this coordination differs between term and preterm infants. However, the neurological mechanisms underlying this coordination and how it matures as infants grow are poorly understood. Here, we integrate high-resolution data from multiple physiologic processes across a longitudinal time frame to study suck-swallow-breathe dynamics in a preterm animal model, the infant pig. In doing so, we test the hypothesis that preterm birth will have an impact on some, but not all, behaviors associated with suck-swallow-breath performance. We hypothesize that coordination will be disrupted, reflecting incomplete connections in the brainstem. We found that preterm pigs became rhythmic and mature in sucking and swallowing behaviors, suggesting substantial postnatal maturation in the coordination of these behaviors. However, their ability to coordinate swallowing and breathing never developed. These results have implications for the nature of clinical care of human infants, as well as for how feeding processes develop in mammals. Clinically, they provide a foundation for developing interventions for preterm infants. Additionally, these results suggest that the lack of coordination between swallowing and breathing may be a significant factor in determining the minimum gestation time across mammals. NEW & NOTEWORTHY Preterm infants face a variety of challenges associated with safe feeding, but obtaining high-resolution longitudinal data to understand these challenges in humans is challenging. We used a pig model to acquire high-speed videofluoroscopic and respiratory inductance plethysmograph data throughout the nursing period to show that preterm birth does not have substantial impacts on the ability of infants to perform isolated behaviors. However, it does decrease the ability of preterm infants to coordinate among behaviors during feeding.

A novel fluoroscopic method for multidimensional evaluation of swallowing function

OBJECTIVE

Dynamic videofluoroscopic swallow study (VFSS) is used to investigate swallowing movements. However, it requires prolonged radiation exposure and mainly provides qualitative information. Herein, we present a multi-dimensional method for analyzing swallowing based on a pulsed, low-dose fluoroscopy technique that uses serial-shot images and evaluates the size, position, and temporal profile of the bolus to obtain a more comprehensive and realistic analysis of swallowing movements.

METHODS

Fifteen healthy adults drank two liquids: 20mL of pure water followed by 20mL of contrast medium mixture in a fluoroscopic study. Data were recorded in serial-shot images (7.5 frames/second, 1024×1024-pixel resolution, DICOM format). The images from the water and contrast swallows were inverted, synchronized, and subtracted to visualize the bolus in each frame. The pathway of the bolus was divided into 15 parts traversing the oropharynx, hypopharynx, and upper esophagus, and the total gray value was measured in each section. The results were presented as contour graphs.

RESULTS

The contour graphs allowed for information on the size, anatomical location, and temporal location of the bolus during swallowing to be displayed simultaneously. Two distinct swallowing patterns were observed in the subjects. The bolus showed two peaks-one in the hypopharynx and one in the upper esophagus-in all subjects. However, in nine of the 15 subjects, the two peaks were in different frames, whereas in six of the subjects, the two peaks were in the same frame.

CONCLUSION

We developed a new method for quantitatively evaluating swallowing. The technique allows for multidimensional assessment of the size, position, and temporal profile of the movement of the bolus across the pharynx. This method evaluates the swallowing movements using sharp, high-resolution images obtained by serial-shot, pulsed fluoroscopy with low radiation exposure. Additional studies are required to further clarify the variability of swallowing patterns and their clinical relevance in the evaluation of swallowing movements in healthy subjects and in patients with swallowing disorders.

Impact of recurrent laryngeal nerve lesion on oropharyngeal muscle activity and sensorimotor integration in an infant pig model

The successful performance of a swallow requires dynamic integration between a wide range of sensory inputs and muscle activities to produce the coordinated kinematics of oropharyngeal structures. Damage to the recurrent laryngeal nerve (RLN) produces dysphagia in infants, with food or liquid entering the airway despite this nerve having minimal direct sensory or motor connections to the act of swallowing, apart from vocal fold closure. Previous results have demonstrated that a complete RLN lesion disrupts both performance and kinematics before initiation of the pharyngeal swallow in infants. We tested the hypothesis that a RLN lesion produces changes in the normal activity of oral floor, tongue, and infrahyoid muscles during a swallow. We recorded swallowing in our validated infant pig model, with synchronous high-speed imaging and fine-wire, chronic electromyography. We found changes in the timing, duration, and amplitude of the motor pattern in an array of muscles that are supplied by several different cranial and cervical nerves. Some of these changes in muscle activity are associated with the preparatory aspects of bolus aggregation or movement and so occur before the pharyngeal swallow. Taken with previous biomechanical results, these patterns suggest an intricate brain stem sensorimotor integration that occurs as part of a swallow. In particular, the execution of oral motor function is changed as a result of this simple lesion. NEW & NOTEWORTHY Damage to the recurrent laryngeal nerve compromises swallowing despite an absent or minimal contribution to either the motor or sensory aspects of this function. This study documents EMG changes, following RLN lesion, to non-RLN innervated muscles that are active during swallowing in an infant model. Some of these muscles fire before the pharyngeal swallow and are associated with the preparatory aspects of bolus aggregation and movement, suggesting important sensorimotor integration at a brain stem level.

Maturation of the Coordination Between Respiration and Deglutition with and Without Recurrent Laryngeal Nerve Lesion in an Animal Model

The timing of the occurrence of a swallow in a respiratory cycle is critical for safe swallowing, and changes with infant development. Infants with damage to the recurrent laryngeal nerve, which receives sensory information from the larynx and supplies the intrinsic muscles of the larynx, experience a significant incidence of dysphagia. Using our validated infant pig model, we determined the interaction between this nerve damage and the coordination between respiration and swallowing during postnatal development. We recorded 23 infant pigs at two ages (neonatal and older, pre-weaning) feeding on milk with barium using simultaneous high-speed videofluoroscopy and measurements of thoracic movement. With a complete linear model, we tested for changes with maturation, and whether these changes are the same in control and lesioned individuals. We found (1) the timing of swallowing and respiration coordination changes with maturation; (2) no overall effect of RLN lesion on the timing of coordination, but (3) a greater magnitude of maturational change occurs with RLN injury. We also determined that animals with no surgical intervention did not differ from animals that had surgery for marker placement and a sham procedure for nerve lesion. The coordination between respiration and swallowing changes in normal, intact individuals to provide increased airway protection prior to weaning. Further, in animals with an RLN lesion, the maturation process has a larger effect. Finally, these results suggest a high level of brainstem sensorimotor interactions with respect to these two functions.

A Method to Administer Agents to the Larynx in an Awake Large Animal

PURPOSE

This research note describes an adapted experimental methodology to administer an exogenous agent to the larynx and upper airway of awake animals. The exogenous agent could be a perturbation. In the current study, the agent was isotonic saline. Isotonic saline was selected because it is safe, of similar composition to extracellular fluid, and used in voice studies. The described approach allowed large animals such as pigs to be comfortably restrained without chemical sedation or anesthesia for extended periods while receiving the agent.

METHOD

Six Sinclair pigs were successfully trained with positive reinforcement to voluntarily enter and then be restrained in a Panepinto Sling. Once restrained, the pigs accepted a nose cone that delivered nebulized isotonic saline. This procedure was repeated 3 times per day for 20 days. At the end of the study, the larynx and airway tissues were excised and examined using histology and transmission electron microscopy.

RESULTS

Pathology related to the procedure (i.e., nebulized inhaled isotonic saline or stress) was not identified in any examined tissues.

CONCLUSIONS

This methodology allowed for repeated application of exogenous agents to awake, unstressed animals. This method can be used repeatedly in the laboratory to test various therapeutics for safety, toxicity, and dosage. Future studies will specifically manipulate the type of agent to further our understanding of laryngeal pathobiology.

LVC Timing in Infant Pig Swallowing and the Effect of Safe Swallowing

Recurrent laryngeal nerve (RLN) injury in neonates, a complication of head and neck surgeries, leads to increased aspiration risk and swallowing dysfunction. The severity of resulting sequelae range from morbidity, such as aspiration pneumonia, to mortality from infection and failure to thrive. The timing of airway protective events including laryngeal vestibule closure (LVC) is implicated in aspiration. We unilaterally transected the RLN in an infant pig model to observe changes in the timing of swallowing kinematics with lesion and aspiration. We recorded swallows using high-speed video-fluoroscopic swallow studies (VFSS) and scored them using the Infant Mammalian Penetration and Aspiration Scale (IMPAS). We hypothesized that changes would occur in swallowing kinematics (1) between RLN lesion and control animals, and (2) among safe swallows (IMPAS 1), penetration swallows (IMPAS 3), and aspiration swallows (IMPAS 7). We observed numerous changes in timing following RLN lesion in safe and unsafe swallows, suggesting pervasive changes in the coordination of oropharyngeal function. The timing of LVC, posterior tongue, and hyoid movements differed between pre- and post-lesion in safe swallows. Posterior tongue kinematics differed for post-lesion swallows with penetration. The timing and duration of LVC and posterior tongue movement differed between aspiration swallows pre- and post-lesion. After lesion, safe swallows and swallows with aspiration differed in timing of LVC, laryngeal vestibule opening, and posterior tongue and hyoid movements. The timing of thyrohyoid muscle activity varied with IMPAS, but not lesion. Further study into the pathophysiology of RLN lesion-induced swallowing dysfunction is important to developing novel therapies.