Cranial Nerves IX and X: Glossopharyngeal and Vagus

Swallowing Physiology

Swallowing is a complex process involving motor control and sensory feedback from anatomic structures within the oral cavity, pharynx, larynx, and esophagus ( Fig. 10.1 ). The trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX; Fig. 10.2 ), vagus (CN X; Fig. 10.3 ), and hypoglossal (CN XII) CNs are involved. A “normal swallow” comprises two major components, bolus transport and airway protection. The swallowing process is typically classified into four phases: oral preparatory, oral swallowing, pharyngeal, and esophageal ( Fig. 10.4 ). The pharyngeal phase of swallow is the most complex, and problems in this phase are most likely to cause aspiration.

• 1

  The oral preparatory phase involves voluntary motor function during which food or liquid is taken into the mouth, masticated, and mixed with saliva to form a cohesive bolus (see Fig. 10.4A ). This phase requires tension in the labial and buccal musculature (CN VII) while rotary mandible motion produces chewing (CN V ₃ ). Tongue mobility is the most important neuromuscular function involved in this first phase.

• 2

  The oral swallowing phase is initiated when the tongue (CN XII) sequentially squeezes the bolus posteriorly against the hard palate and initiates propulsion into the oropharynx (see Fig. 10.4B ). The soft palate (CN IX), critical to containing the bolus within the oral cavity during the oral preparatory phase, now moves posteriorly to allow the bolus to pass through the faucial arches and simultaneously prevents the bolus from entering the nasopharynx. The swallowing reflex is triggered as the bolus passes the anterior tonsillar pillars, which initiates the pharyngeal phase.

• 3

  The pharyngeal phase begins with the bolus passing into the throat, triggering the swallowing reflex and causing several pharyngeal physiologic actions to occur simultaneously, allowing food to pass into the esophagus (see Fig. 10.4C ). Intrinsic laryngeal muscles close the larynx, creating a seal that separates the airway from the digestive tract. The tongue is the major force pushing the bolus through the pharynx. Synergistic actions with CN X produce pharyngeal peristalsis as it innervates the pharyngeal constrictors. The food bolus moves around the closed airway and into the piriform sinuses, then is squeezed through the open upper esophageal sphincter and into the esophagus. The motion of the bolus is aided by forward and upward motion of the larynx, widening the esophageal inlet. Once the bolus passes into the esophagus, the larynx returns to its former position and the vocal folds open and respiration resumes. CN IX mediates the sensory portion of the pharyngeal gag but innervates just one muscle, the stylopharyngeus. The gag reflex is the best direct test of glossopharyngeal function, although it requires both CNs IX and X to be functioning. When swallowing is inefficient and aspiration occurs, a reflexive cough needs to occur as a respiratory defense against foreign matter. The cough reflex is induced by irritation of afferent CN IX and CN X sensory fibers in the larynx, trachea, and larger bronchi (see Figs. 10.2 and 10.3 ).

• 4

  The esophageal phase starts with the passage of the bolus through the upper esophageal sphincter (also called the cricopharyngeus) (see Fig. 10.4D ). CN X mediates the action of the cricopharyngeus, which relaxes to allow food to pass from the hypopharynx into the esophagus. Once the bolus enters the esophagus, serial contractions of the esophageal muscle push the bolus down to the lower esophageal sphincter (LES) and into the stomach.

Clinical Presentation

There are myriad presentations of dysphagia; although some complaints are more concerning than others, it is frequently difficult to determine the severity of a patient's dysphagia based solely on symptoms. Less concerning symptoms are a sensation of something being stuck in the throat (globus) or difficulty swallowing one's own saliva, and the most concerning presentation is recurrent aspiration pneumonia. In swallowing evaluations, two terms are commonly used to identify events that increase the risk of aspiration pneumonia: penetration and aspiration. Penetration is defined as the entry of material into the laryngeal introitus, where it is more likely to be aspirated. Aspiration is the entry of an inappropriate substance below the level of the vocal folds. If the aspirated material is not quickly and completely cleared, aspiration pneumonia can occur.

Odynophagia, or pain on swallowing, occurs most frequently due to infectious or neoplastic causes. The mechanism of swallowing is generally preserved, and aspiration is rare. If the symptoms are acute, bacterial and viral pathogens are more likely, and prompt evaluation is warranted because serious conditions such as epiglottis or neck abscess could be the cause. More chronic, indolent presentations tend to occur with fungal infections; these are more likely in patients with diabetes, patients using inhaled steroids, or in immunocompromised patients. Cancers of the base of tongue, supraglottis, or hypopharynx generally present with progressive odynophagia and, in the later stages, may cause swallowing dysfunction and malnutrition. Esophageal cancers are asymptomatic in the early stages but can cause symptoms of food sticking in the throat or chest in later stages.

Dysphagia, or difficulty swallowing, is usefully divided by the consistency of the swallowed bolus. Dysphagia to saliva but not to other consistencies is the least concerning. This symptom frequently occurs with globus sensation. This seems to occur due to minor sensory changes in the hypopharynx or due to mild edema of the larynx and is most often ascribed to laryngopharyngeal reflux. Some of these patients have anxiety about swallowing, which leads to oral aversive behaviors; they may refuse to eat specific foods or consistencies. Dysphagia to solids is often localizable to a narrow, specific area in the swallowing system. In this case, dysphagia starts with the largest and firmest boluses, such as large pills or meats, then progresses to smaller and less solid boluses. Some patients can point to a specific spot where the food becomes stuck. Patients may also develop dysphagia to solids due to loss of pharyngeal muscle strength. In these cases, all solids will be equally difficult, and they have frequent coughing or regurgitation of the bolus. Speech is frequently affected and may be wet and gurgly sounding due to accumulation of secretions in the larynx. Patients with dry mouth or throat may have difficulty with bread and crackers, which are inadequately moisturized and remain very sticky.

Dysphagia to liquids is the best indicator of a neurologic cause of dysphagia. Liquids do not form a cohesive bolus, and dysphagia can occur during any phase of the swallow. If oral control and sensation are lacking, the liquids can leak into the pharynx before the swallow is initiated. Sometimes they will trigger the swallow when they reach the larynx (secondary trigger of swallow), and the patient will protect the airway and successfully manage the swallow. In more concerning cases the liquids will enter the laryngeal introitus (penetration) and even make it to the trachea or beyond before a cough is triggered. Other patients can hold the liquids in the oral cavity without difficulty but have problems once the swallow is initiated. If the larynx does not close fully due to vocal fold paralysis or structural abnormality, the sphincteric effect is impaired and liquids may penetrate below the vocal folds during the swallow. In these cases the cough is also generally impaired, and it may be very difficult to clear any aspirated material. Finally, in some patients the coordination of swallowing is abnormal. One of the most common problems in coordination is early closure of the upper esophageal sphincter, leading to a portion of the bolus being trapped in the hypopharynx. The larynx then opens, and secondary aspiration of the residual material can occur.

Silent aspiration is the combination of swallowing dysfunction, leading to aspiration, and sensory loss, causing a lack of reflexive actions to clear the aspirated material. In patients with aspiration events, but normal sensation, reflexive coughing can efficiently clear the airway and prevent aspirated material from reaching the lung. These patients are at higher risk of aspiration pneumonia but can go years between episodes. Patients with abnormal sensation but normal swallow cannot sense aspiration but rarely or never have aspiration events that would normally trigger the cough reflex. Many of these patients have only rare aspiration pneumonias. The patients with silent aspiration are the ones at highest risk of pneumonia and are the least able to modify the swallow to increase safety. Many of these patients are maintained on no-oral feeding due to the frequency and severity of their aspiration pneumonias.

Diagnostic Approach

Occult swallowing problems are common in hospitalized patients, and failure to recognize the presence of dysphagia increases the risk of aspiration pneumonia, prolonged hospitalization, and death. Techniques for rapid universal dysphagia screening have been developed and are increasingly being implemented in the hospital setting. For patients who fail the dysphagia screening, formal assessment of swallowing function serves to confirm the severity of dysphagia and to identify therapeutic strategies to minimize the risk of aspiration.

Dysphagia screening begins with a simple evaluation of the patient's mental status and ability to follow commands. Hospitalized patients who are not oriented to person, place, and time or who cannot follow simple commands (“stick your tongue out”) should be kept NPO (nothing by mouth) unless evaluated and cleared for oral intake by a speech-language pathologist. Other patients can be screened with the 3-ounce swallow test: the patient is asked to drink a 3-oz cup of water continuously until empty. If they stop between swallows, cough or choke, or have a wet, gurgly vocal quality after drinking the water, the test is considered a failure. In a large study of 3000 patients, the 3-ounce swallow test was found to be highly sensitive but had a 51% false-positive rate. Some institutions have developed different screening protocols with better specificity; the 3-ounce swallow test has the advantage of being simple and quick to administer.

Flexible endoscopic evaluation of swallowing (FEES) allows direct evaluation of motor and sensory aspects of the pharyngeal swallow. It requires transnasal passage of a fiberoptic laryngoscope to view the larynx and surrounding structures. Laryngeal airway protection and the integrity of the oropharyngeal swallow are assessed by giving various foods tinted with coloring to enhance visualization. Velopharyngeal closure, abduction and adduction of the vocal folds, pharyngeal contraction, and the patient's ability to manage secretions are assessed. If abnormalities are detected, compensatory strategies and postures are also assessed.

Modified barium swallow (MBS), also called videofluoroscopy, is a functional radiologic evaluation intended to determine appropriate therapeutic intervention strategies to facilitate safe and efficient swallowing function.

FEES and MBS are complementary modalities; MBS provides better views of the upper esophageal sphincter and esophagus, whereas FEES can be performed by the bedside and uses real food. Sometimes one of these modalities will identify a problem missed by the other due to these differences. Both FEES and MBS have been validated as objective swallowing tests able to accurately evaluate dysphagia and identify patients at risk of aspiration.