Oropharyngeal stimulation with air-pulse trains increases swallowing frequency in healthy adults

Efficacy of Transcranial Direct Current Stimulation Combined with Conventional Swallowing Rehabilitation Training on Post-stroke Dysphagia

To observe the clinical effects of transcranial direct current stimulation (tDCS) combined with conventional swallowing rehabilitation training on post-stroke dysphagia and explore its long-term efficacy. A total of 40 patients with dysphagia after the first stroke were randomly divided into a treatment group (n = 20) and a conventional group (n = 20). The treatment group received tDCS combined with conventional swallowing rehabilitation training, while the conventional group only received conventional swallowing rehabilitation training. The Standardized Swallowing Assessment (SSA) Scale and the Penetration-Aspiration Scale (PAS) were used to assess dysphagia before and after treatment, at the end of 10 treatments, and at the 3-month follow-up. The changes in infection indicators [the white blood cell (WBC), C-reactive protein (CRP) and procalcitonin (PCT)], the oxygenation indicator [arterial partial pressure of oxygen (PaO2)] and nutrition-related indicators [hemoglobin (Hb) and serum prealbumin (PAB)] were compared before and after treatment. The SSA and PAS scores were lower in both groups after treatment than before treatment, and the difference was statistically significant (P < 0.01). The SSA and PAS scores of the treatment group were lower than those of the conventional group before and after treatment and during follow-up, and the difference was statistically significant (P < 0.05, P < 0.01). A within-group comparison showed that WBC, CRP and PCT after treatment were lower than those before treatment, and the difference was statistically significant (P < 0.05). The PaO2, Hb and serum PAB were higher after treatment than before treatment, with a statistically significant difference (P < 0.05). The WBC, CRP and PCT of the tDCS group were lower than those of the conventional group, and PaO2, Hb and serum PAB were higher in the treatment group than in the conventional group, with a statistically significant difference (P < 0.01). The tDCS combined with conventional swallowing rehabilitation training can improve dysphagia with a better effect than conventional swallowing rehabilitation training and has a certain long-term efficacy. In addition, tDCS combined with conventional swallowing rehabilitation training can improve nutrition and oxygenation and reduce infection levels.

Accumulated Secretions and Associated Aerodigestive Function in Patients With Dysphagia

PURPOSE

Accumulated pharyngo-laryngeal secretions are associated with dysphagia, aspiration, and poor health outcomes. Despite ongoing developments in the evaluation of pharyngo-laryngeal secretions, understanding of the underlying mechanisms is limited. Pathophysiology associated with accumulated secretions is needed to guide tailored, targeted treatment pathways. This study reports the prevalence of accumulated pharyngo-laryngeal secretions in a large acute care caseload and explores the relationship between secretions and aerodigestive function.

METHOD

Consecutive inpatients (N = 222) referred for flexible endoscopic evaluation of swallow (FEES) with suspected dysphagia following assessment by a speech-language pathologist were recruited (43% neurological, 22% neurosurgical, 20% critical care, 15% other), and 250 standardized FEES were completed (222 first FEES, 28 repeat FEES). The assessment protocol included secretion assessment using the New Zealand Secretion Scale (NZSS), pharyngeal squeeze maneuver, laryngeal motor and sensory assessment, Penetration-Aspiration Scale, and Yale Pharyngeal Residue Severity Rating Scale. Urge-to-clear ratings were collected during endoscopy. Cough peak expiratory flow and swallow frequency measures were also collected, as well as clinical outcomes at time of discharge.

RESULTS

There was a high incidence of accumulated secretions, with 77% of inpatients having elevated NZSS (Mdn = 3, range: 0-7) and 37% with pooled laryngeal secretions. Accumulated secretions were associated with reduced swallow frequency, reduced laryngeal adductor reflex, impaired pharyngeal squeeze maneuver, and peak expiratory flow. NZSS scores also correlated with swallow measures (Penetration-Aspiration Scale and Yale Pharyngeal Residue Severity Rating Scale) and patient outcomes including diet recommendations at discharge and pneumonia during admission.

CONCLUSIONS

This large study contributes to evidence associating both sensory and motor impairments with secretion accumulation and aspiration risk. Further exploration of the key physiological mechanisms contributing to accumulated secretions will serve as markers to provide proof of principle for targets for secretion management protocols.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24101667.

Evaluation of Spontaneous Swallow Frequency in Healthy People and Those With, or at Risk of Developing, Dysphagia: A Review

Dysphagia is a common and frequently undetected complication of many neurological disorders and of sarcopoenia in ageing persons. Spontaneous swallowing frequency (SSF) has been mooted as a possible tool to classify dysphagia risk. We conducted a review of the literature to describe SSF in both the healthy population and in disease-specific populations, in order to consider its utility as a screening tool to identify dysphagia. We searched Medline, Embase, Cochrane Database of Systematic Reviews and Cochrane Central Register of Controlled Trials databases. Metadata were extracted, collated and analysed to give quantitative insight. Three hundred and twelve articles were retrieved, with 19 meeting inclusion and quality criteria. Heterogeneity between studies was high (I² = 99%). Mean SSF in healthy younger sub-groups was 0.98/min [CI: 0.67; 1.42]. In the Parkinson's sub-group, mean SSF was 0.59/min [0.40; 0.87]. Mean SSF in healthy older, higher risk and dysphagic populations were similar (0.21/min [0.09; 0.52], 0.26/min [0.10; 0.72] and 0.30/min [0.16; 0.54], respectively). SSF is a novel, non-invasive clinical variable which warrants further exploration as to its potential to identify persons at risk of dysphagia. Larger, well-conducted studies are needed to develop objective, standardised methods for detecting SSF, and develop normative values in healthy populations.

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.

Reflex swallowing elicited by electrical stimulation in obstructive sleep apnea patients: A preliminary study

This study tested whether electrical stimulation of the pharyngeal mucosa is able to induce reliably the swallowing reflex in awake and asleep obstructive sleep apnea (OSA) patients, and whether the induced reflexes affect the sleep variables. In addition, the latency, occurrence, and morphology of swallows were evaluated. Eight patients received an esophageal catheter that was used on three consecutive nights for electrical stimulation and manometric recordings. The electrical stimulation proved itself safe, but its efficiency in inducing swallows sank from 80.0 % in awake to 37.4 % in sleeping subjects and was lowest in the sleep stage N3. The swallowing reflex was triggered with a mean latency of 3.69 ± 0.70 s, was predominantly induced in the hyperventilation phase, and had no significant effect on the subject's sleep variables. These findings indicate that electrical stimulation can more effectively trigger the swallowing reflex while the subjects are awake than during sleep without showing remarkable clinical benefits in terms of apnea-hypopnea index (AHI) improvement.

Laryngeal Vibration Increases Spontaneous Swallowing Rates in Chronic Oropharyngeal Dysphagia: A Proof-of-Principle Pilot Study

Previously, vibratory stimulation increased spontaneous swallowing rates in healthy volunteers indicating that sensory stimulation excited the neural control of swallowing. Here, we studied patients with severe chronic dysphagia following brain injury or radiation for head and neck cancer to determine if sensory stimulation could excite an impaired swallowing system. We examined (1) if laryngeal vibratory stimulation increased spontaneous swallowing rates over sham (no stimulation); (2) the optimal rate of vibration, device contact pressure, and vibratory mode for increasing swallowing rates; and (3) if vibration altered participants' urge to swallow, neck comfort, and swallow initiation latency. Vibration was applied to the skin overlying the thyroid lamina bilaterally in thirteen participants to compare vibratory rates 30, 70, 110, 150, or 70 + 110 Hz, different devices to neck pressures (2, 4, or 6 kilopascals), and pulsed versus continuous vibration. Swallows were confirmed from recordings of laryngeal accelerometry and respiratory apneas and viewing neck movement. Participants' swallowing rates, urge to swallow, discomfort levels, and swallow initiation latencies were measured. Vibration at 70 Hz and at 110 Hz significantly increased swallowing rates over sham. All vibratory frequencies except 70 + 100 Hz increased participants' urge to swallow, while no pressures or modes were optimal for increasing urge to swallow. No conditions increased discomfort. Vibration did not reduce measures of swallow initiation latency using accelerometry. In conclusion, as non-invasive neck vibration overlying the larynx increased swallowing rates and the urge to swallow without discomfort in patients with chronic dysphagia, the potential for vibratory stimulation facilitating swallowing during dysphagia rehabilitation should be investigated.

Guide to Enhancing Swallowing Initiation: Insights from Findings in Healthy Subjects and Dysphagic Patients

Purpose of Review

Difficulty in initiating swallowing is one of the main symptoms of oropharyngeal dysphagia. Therefore, enhancing swallowing initiation is an important approach for the treatment of oropharyngeal dysphagia. This review aims to introduce recent approaches to enhancing swallowing and to discuss their therapeutic potential.

Recent Findings

Both central interventions such as non-invasive brain stimulation and peripheral interventions such as electrical stimulation to peripheral tissues are conducted to enhance swallowing. Recent studies have paid more attention to generating neuroplasticity to produce long-lasting facilitative effect on swallowing.

Summary

Transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), pharyngeal electrical stimulation (PES), transcutaneous electrical stimulation, and somatic and chemical stimulation were introduced. Considerable evidence supports the therapeutic potential of TMS and PES. Other approaches need further studies to verify their efficacy (e.g., duration of the effect and a limit of effectiveness) and/or possible risk of adverse effects.

Vibration over the larynx increases swallowing and cortical activation for swallowing

Sensory input can alter swallowing control in both the cortex and brainstem. Electrical stimulation of superior laryngeal nerve afferents increases reflexive swallowing in animals, with different frequencies optimally effective across species. Here we determined 1) if neck vibration overlying the larynx affected the fundamental frequency of the voice demonstrating penetration of vibration into the laryngeal tissues, and 2) if vibration, in comparison with sham, increased spontaneous swallowing and enhanced cortical hemodynamic responses to swallows in the swallowing network. A device with two motors, one over each thyroid lamina, delivered intermittent 10-s epochs of vibration. We recorded swallows and event-related changes in blood oxygenation level to swallows over the motor and sensory swallowing cortexes bilaterally using functional near infrared spectroscopy. Ten healthy participants completed eight 20-min conditions in counterbalanced order with either epochs of continuous vibration at 30, 70, 110, 150, and 70 + 110 Hz combined, 4-Hz pulsed vibration at 70 + 110 Hz, or two sham conditions without stimulation. Stimulation epochs were separated by interstimulus intervals varying between 30 and 45 s in duration. Vibration significantly reduced the fundamental frequency of the voice compared with no stimulation demonstrating that vibration penetrated laryngeal tissues. Vibration at 70 and at 150 Hz increased spontaneous swallowing compared with sham. Hemodynamic responses to swallows in the motor cortex were enhanced during conditions containing stimulation compared with sham. As vibratory stimulation on the neck increased spontaneous swallowing and enhanced cortical activation for swallows in healthy participants, it may be useful for enhancing swallowing in patients with dysphagia.NEW & NOTEWORTHY Vibratory stimulation at 70 and 150 Hz on the neck overlying the larynx increased the frequency of spontaneous swallowing. Simultaneously vibration also enhanced hemodynamic responses in the motor cortex to swallows when recorded with functional near-infrared spectroscopy (fNIRS). As vibrotactile stimulation on the neck enhanced cortical activation for swallowing in healthy participants, it may be useful for enhancing swallowing in patients with dysphagia.

Using devices to upregulate nonnutritive swallowing in typically developing infants

The role of various sensory stimuli for stimulating swallowing in infants may be of importance for assisting infants to develop oral feeding. We evaluated the swallowing mechanism response to two devices for increasing the rate of nonnutritive swallowing in two typically developing infant age groups, ages 2-4 mo and 7-9 mo. One device was a pacifier familiar to the infant; the other was a small vibrator placed on the skin overlying the thyroid cartilage. The rate of nonnutritive swallowing while infants were awake was compared in three 10-min conditions: at rest without stimulation (spontaneous); during nonnutritive sucking with a pacifier; and over 10 min containing 18 epochs of vibratory stimulation for 10 s each. To assess whether vibration on the throat over the laryngeal area altered respiration, the mean cycle length was compared between 10-min intervals either containing vibratory stimulation or without stimulation at rest. Both the pacifier and laryngeal vibration stimulation doubled the rate of swallowing in the infants with a mean age of 3 mo 16 days and infants with a mean age of 8 mo 8 days. No differences occurred in the mean respiratory cycle length between intervals with and without vibration in either age group. Results suggest that nonnutritive sucking, vibration, or both might be beneficial in enhancing swallowing in young infants. Because vibration on the neck would not interfere with oral transfer of liquid, it might provide additional stimulation for swallowing during oral feeding. Both stimulation types should be evaluated for enhancing swallowing in infants with immature swallowing skills.

Central Nervous System Control of Voice and Swallowing

This review of the central nervous control systems for voice and swallowing has suggested that the traditional concepts of a separation between cortical and limbic and brain stem control should be refined and be more integrative. For voice production, a separation of the nonhuman vocalization system from the human learned voice production system has been posited based primarily on studies of nonhuman primates. However, recent humans studies of emotionally based vocalizations and human volitional voice production have shown more integration between these two systems than previously proposed. Recent human studies have shown that reflexive vocalization as well as learned voice production not involving speech involve a common integrative system. However, recent studies of nonhuman primates have provided evidence that some cortical activity vocalization and cortical changes occur with training during vocal behavior. For swallowing, evidence from the macaque and functional brain imaging in humans indicates that the control for the pharyngeal phase of swallowing is not primarily under brain stem mechanisms as previously proposed. Studies suggest that the initiation and patterning of swallowing for the pharyngeal phase is also under active cortical control for both spontaneous as well as volitional swallowing in awake humans and nonhuman primates.

Laryngeal Reflexes: Physiology, Technique, and Clinical Use

This review examines the current level of knowledge and techniques available for the study of laryngeal reflexes. Overall, the larynx is under constant control of several systems (including respiration, swallowing and cough) as well as sensory motor reflex responses involving glossopharyngeal, pharyngeal, laryngeal, and tracheobronchial sensory receptors. Techniques for the clinical assessment of these reflexes are emerging and need to be examined for sensitivity and specificity in identifying laryngeal sensory disorders. Quantitative assessment methods for the diagnosis of sensory reductions and sensory hypersensitivity may account for laryngeal disorders, such as chronic cough, paradoxical vocal fold disorder, and muscular tension dysphonia. The development of accurate assessment techniques could improve our understanding of the mechanisms involved in these disorders.

A framework for understanding shared substrates of airway protection

Deficits of airway protection can have deleterious effects to health and quality of life. Effective airway protection requires a continuum of behaviors including swallowing and cough. Swallowing prevents material from entering the airway and coughing ejects endogenous material from the airway. There is significant overlap between the control mechanisms for swallowing and cough. In this review we will present the existing literature to support a novel framework for understanding shared substrates of airway protection. This framework was originally adapted from Eccles' model of cough²⁸ (2009) by Hegland, et al.⁴² (2012). It will serve to provide a basis from which to develop future studies and test specific hypotheses that advance our field and ultimately improve outcomes for people with airway protective deficits.

Electrophysiological evaluation of oropharyngeal Dysphagia in Parkinson's disease

Parkinson’s disease (PD) is a chronic, neurodegenerative movement disorder that typically affects elderly patients. Swallowing disorders are highly prevalent in PD and can have grave consequences, including pneumonia, malnutrition, dehydration and mortality. Neurogenic dysphagia in PD can manifest with both overt clinical symptoms or silent dysphagia. Regardless, early diagnosis and objective follow-up of dysphagia in PD is crucial for timely and appropriate care for these patients. In this review, we provide a comprehensive summary of the electrophysiological methods that can be used to objectively evaluate dysphagia in PD. We discuss the electrophysiological abnormalities that can be observed in PD, their clinical correlates and the pathophysiology underlying these findings.

Occurrences of yawn and swallow are temporally related

Yawning is a stereotyped motor behavior characterized by deep inhalation and associated dilation of the respiratory tract, pronounced jaw opening, and facial grimacing. The frequency of spontaneous yawning varies over the diurnal cycle, peaking after waking and before sleep. Yawning can also be elicited by seeing or hearing another yawn, or by thinking about yawning, a phenomenon known as "contagious yawning". Yawning is mediated by a distributed network of brainstem and supratentorial brain regions, the components of which are shared with other airway behaviors including respiration, swallowing, and mastication. Nevertheless, the possibility of behavioral coordination between yawning and other brainstem-mediated functions has not been examined. Here we show, with a double-blind methodology, a greater-than-fivefold increase in rest (saliva) swallowing rate during the 10-s period immediately following contagious yawning elicited in 14 adult humans through the viewing of videotaped yawn stimuli. Sixty-five percent of yawns were followed by a swallow within 10 s and swallows accounted for 26 % of all behaviors produced during this post-yawn period. This novel finding of a tight temporal coupling between yawning and swallowing provides preliminary evidence that yawning and swallowing are physiologically related, thus extending current models of upper airway physiology and neurophysiology. Moreover, our finding suggests the possibility that yawning plays a role in eliciting rest swallowing, a view not considered in previous theories of yawning. As such, the present demonstration of a temporal association between yawning and swallowing motivates a re-examination of the longstanding question, "Why do we yawn?".

Neuromagnetic detection of the laryngeal area: Sensory-evoked fields to air-puff stimulation

The sensory projections from the oral cavity, pharynx, and larynx are crucial in assuring safe deglutition, coughing, breathing, and voice production/speaking. Although several studies using neuroimaging techniques have demonstrated cortical activation related to pharyngeal and laryngeal functions, little is known regarding sensory projections from the laryngeal area to the somatosensory cortex. The purpose of this study was to establish the cortical activity evoked by somatic air-puff stimulation at the laryngeal mucosa using magnetoencephalography. Twelve healthy volunteers were trained to inhibit swallowing in response to air stimuli delivered to the larynx. Minimum norm estimates was performed on the laryngeal somatosensory evoked fields (LSEFs) to best differentiate the target activations from non-task-related activations. Evoked magnetic fields were recorded with acceptable reproducibility in the left hemisphere, with a peak latency of approximately 100ms in 10 subjects. Peak activation was estimated at the caudolateral region of the primary somatosensory area (S1). These results establish the ability to detect LSEFs with an acceptable reproducibility within a single subject and among subjects. These results also suggest the existence of laryngeal somatic afferent input to the caudolateral region of S1 in human. Our findings indicate that further investigation in this area is needed, and should focus on laryngeal lateralization, swallowing, and speech processing.

Validation and demonstration of an isolated acoustic recording technique to estimate spontaneous swallow frequency

Spontaneous swallowing is considered a reflexive, pharyngeal clearance mechanism. Reductions in spontaneous swallow frequency may be a sensitive index for dysphagia and related morbidities. This study evaluated an acoustic recording technique as a measure to estimate spontaneous swallow frequency. Initially, a multichannel physiologic (surface electromyography, swallow apnea, cervical auscultation) recording technique was validated and subsequently compared to an isolated acoustic (microphone) recording technique on a sample of younger (25 ± 2.8 years) and older (68 ± 5.3 years) healthy adult participants. Sensitivity (94 %), specificity (99 %), and classification accuracy (98 %) were high for swallow identification from the multichannel physiologic recording technique. Interjudge reliability was high (k = 0.94, 95 % CI = 0.92-0.96). No significant differences in spontaneous swallow frequency were observed between the multichannel physiologic recordings and the acoustic recordings (0.85 vs. 0.81 swallows per minute). Furthermore, these two techniques were highly correlated (r = 0.95). Interjudge reliability for swallow identification via acoustic recordings was high (k = 0.96, 95 % CI = 0.94-0.99). Preliminary evaluation of the temporal stability of spontaneous swallow frequency measured from acoustic recordings indicated that time samples as short as 5 min produce viable results. Age differences were identified in spontaneous swallow frequency rates, with older participants swallowing less frequently than younger participants (0.47 vs. 1.02 swallows per minute). Collectively, these results indicate that an isolated acoustic recording technique is a valid approach to estimate spontaneous swallow frequency.

Role of sensory stimulation in amelioration of obstructive sleep apnea

Obstructive sleep apnea (OSA), characterized by recurrent upper airway (UA) collapse during sleep, is associated with significant morbidity and disorders. Polysomnogram is employed in the evaluation of OSA and apnea-hypopnea number per hour reflects severity. For normal breathing, it is essential that the collapsible UA is patent. However, obstruction of the UA is quite common in adults and infants. Normally, important reflex mechanisms defend against the UA collapse. The muscle activity of UA dilators, including the genioglossus, tensor palatini (TP), and pharyngeal constrictors, is due to the integrated mechanism of afferent sensory input → to motor function. Snoring is harsh breathing to prevent UA obstruction. Unfortunately, snoring vibrations, pharyngeal suction collapse, negative pressure, and hypoxia cause pathological perturbations including dysfunctional UA afferent sensory activity. The current paper posits that peripheral sensory stimulation paradigm, which has been shown to be efficacious in improving several neurological conditions, could be an important therapeutic strategy in OSA also.

Voluntary versus spontaneous swallowing in man

This review examines the evidence regarding the clinical and neurophysiological differences between voluntary and spontaneous swallows. From the clinical point of view, voluntary swallow (VS) occurs when a human has a desire to eat or drink during the awake and aware state. Spontaneous swallow (SS) is the result of accumulated saliva and/or food remnants in the mouth. It occurs without awareness while awake and also during sleep. VS is a part of eating behavior, while SS is a type of protective reflex action. In VS, there is harmonized and orderly activation of perioral, lingual, and submental striated muscles in the oral phase. In SS, the oral phase is bypassed in most cases, although there may be partial excitation. Following the oral phase, both VS and SS have a pharyngeal phase, which is a reflex phenomenon that protects the upper airway from any escape of food and direct the swallowed material into the esophagus. This reflexive phase of swallowing should not be confused with SS. VS and SS are similar regarding their dependence on the swallowing Central Pattern Generator (CPG) at the brainstem, which receives sensory feedback from the oropharynx. There are differences in the role of the corticobulbar input between VS and SS.

Sensory input pathways and mechanisms in swallowing: a review

Over the past 20 years, research on the physiology of swallowing has confirmed that the oropharyngeal swallowing process can be modulated, both volitionally and in response to different sensory stimuli. In this review we identify what is known regarding the sensory pathways and mechanisms that are now thought to influence swallowing motor control and evoke its response. By synthesizing the current state of research evidence and knowledge, we identify continuing gaps in our knowledge of these mechanisms and pose questions for future research.

Effects of oropharyngeal air-pulse stimulation on swallowing in healthy older adults

While previous research has shown that air-pulse stimulation of the oropharynx facilitates saliva swallowing in young adults, the effects of air pulses in older adults have not been examined. Responses to air-pulse stimulation may differ in young and older adults given age-related changes in sensation, swallowing physiology, and swallow-related brain activation. Therefore, this study sought to determine the effects of oropharyngeal air-pulse stimulation on saliva swallowing rates in 18 healthy older adults. Saliva swallowing rates were monitored across six conditions: baseline without mouthpiece, baseline with mouthpiece in situ, unilateral right oropharyngeal stimulation, unilateral left oropharyngeal stimulation, bilateral oropharyngeal stimulation, and sham stimulation. Results indicated that bilateral oropharyngeal air-pulse stimulation was associated with a statistically significant increase in mean saliva swallowing rate compared to baseline without mouthpiece, baseline with mouthpiece in situ, and sham stimulation. In previous studies, young adults reported an irrepressible urge to swallow in response to oropharyngeal air-pulse delivery, but the older adults in the current study did not perceive the air-pulse stimulation as being associated with swallowing or other behaviors. These findings indicate that oropharyngeal air-pulse stimulation facilitates the elicitation of saliva swallowing in older adults.

The neurobiology of swallowing and dysphagia

The neurobiological study of swallowing and its dysfunction, defined as dysphagia, has evolved over two centuries beginning with electrical stimulation applied directly to the central nervous system, and then followed by systematic investigations that have used lesioning, transmagnetic stimulation, magnetoencephalography, and functional magnetic resonance imaging. The field has evolved from mapping the central neural pathway and peripheral nerves, to defining the importance of specific regions of the lower brain stem in terms of interneurons that provide sequential control for multiple muscles in the most complex reflex elicited by the nervous system, the pharyngeal phase of swallowing. The field is now emerging into defining how the higher cortical regions interact with this brain stem control and is providing a broader perspective of how the intact nervous system functions to control the three phases of swallowing (i.e., oral, pharyngeal, and esophageal). Much of the present interest focuses on how to retrain a damaged nervous system using a variety of stimulus techniques, which follow fundamentals in rehabilitation of the nervous system.

Facilitation of voluntary swallowing by chemical stimulation of the posterior tongue and pharyngeal region in humans

In this study, we investigated the functional difference between chemical stimulations of the posterior tongue (PT) and pharyngeal region (PR) for facilitation of voluntary swallowing in humans. The PT or PR stimulation consisted of infusion of water (distilled water), 0.3 M NaCl solution or olive oil (non-chemical stimulant) into the PT or the PR through a fine tube at a very slow infusion rate (0.2 ml/min). Water was used as a stimulant of water receptors. A solution of 0.3 M NaCl was used as an inhibitor of the response of water receptors and as a stimulant of salt taste receptors. Excitation of the mucosal receptors would facilitate voluntary swallowing and diminution of sensory inputs from the oral mucosa would induce difficulty in swallowing. Swallowing intervals (SIs) during voluntary swallowing were measured by submental electromyographic activity. Infusion of water into the PR shortened SI (facilitation of swallowing) and infusion of 0.3 M NaCl or olive oil into the same region prolonged it (difficulty in swallowing). On the other hand, infusion of water into the PT prolonged SI and infusion of 0.3 M NaCl into the same region shortened it. The results suggest that water receptors are localized in the PR and that salt taste receptors are almost absent in the PR and present in the PT. With diminution of sensory inputs from the oral mucosa, central inputs would play a dominant role in initiating swallowing voluntarily, and SI would be prolonged. With weak stimulation (infusion of 0.3 M NaCl into the PR or infusion of water into the PT), SI was prolonged and inter-individual variation in SI was pronounced, suggesting that the ability of the central regulation of swallowing to perform repetitive voluntary swallowing varies among subjects. With stimulation of water receptors or salt taste receptors, SI was shortened and inter-individual variation in SI was moderate, suggesting that sensory inputs are important for performing voluntary swallowing smoothly and that the sensory inputs compensate for the difficulty in performing swallowing caused by the central mechanism.

Sensory stimulation activates both motor and sensory components of the swallowing system

Volitional swallowing in humans involves the coordination of both brainstem and cerebral swallowing control regions. Peripheral sensory inputs are necessary for safe and efficient swallowing, and their importance to the patterned components of swallowing has been demonstrated. However, the role of sensory inputs to the cerebral system during volitional swallowing is less clear. We used four conditions applied during functional magnetic resonance imaging to differentiate between sensory, motor planning, and motor execution components for cerebral control of swallowing. Oral air pulse stimulation was used to examine the effect of sensory input, covert swallowing was used to engage motor planning for swallowing, and overt swallowing was used to activate the volitional swallowing system. Breath-holding was also included to determine whether its effects could account for the activation seen during overt swallowing. Oral air pulse stimulation, covert swallowing and overt swallowing all produced activation in the primary motor cortex, cingulate cortex, putamen and insula. Additional regions of the swallowing cerebral system that were activated by the oral air pulse stimulation condition included the primary and secondary somatosensory cortex and thalamus. Although air pulse stimulation was on the right side only, bilateral cerebral activation occurred. On the other hand, covert swallowing minimally activated sensory regions, but did activate the supplementary motor area and other motor regions. Breath-holding did not account for the activation during overt swallowing. The effectiveness of oral-sensory stimulation for engaging both sensory and motor components of the cerebral swallowing system demonstrates the importance of sensory input in cerebral swallowing control.