Cranial Nerve IX: Glossopharyngeal Nerve and Swallowing

Clinical Vignette

A 70-year-old man with a history of hypertension and paroxysmal atrial fibrillation presented to the emergency department with acute onset of slurred speech, left-sided paresis, left neglect, and a left inferior quadrantanopsia. MRI revealed a right middle cerebral artery stroke. An initial oral peripheral and cranial nerve examination revealed bilateral depressed gag reflex and diminished soft palate elevation, left central facial weakness, reduced labial retraction, and mild tongue deviation to the right side with protrusion. The patient was to receive nothing per mouth (NPO) and was referred to a speech pathologist.

Clinical swallowing evaluation demonstrated hoarse and moderately dysarthric speech that retained fair intelligibility. Graduated sized boluses of thin liquids, nectar thick liquids, and purees were administered. The patient had significant difficulty with oral containment, bolus formation, and posterior transport through the oral cavity, on the left more than on the right. Posterior placement on the right side facilitated oral swallowing. Reduced orolabial seal benefited from the use of a straw. Attempts to administer a soft solid bolus were unsuccessful because of delayed triggering of oropharyngeal swallow. Mild voice changes after liquids, characterized by a wet vocal quality, indicated laryngeal penetration and possible silent aspiration. The patient was able to clear with cues and to use a throat clear/reswallow strategy. Use of a chin tuck swallowing posture with nectar thick liquids eliminated clinical signs of aspiration.

He was placed on a modified pureed diet with nectar thick liquids, and medications were crushed in applesauce. Aspiration precautions included remaining upright 90° during and 45 minutes after oral intake and single, small boluses. Swallowing strategies included a chin tuck posture and right posterior placement of food in the oral cavity to decrease anterior leakage and to assist in oral transport. A flexible endoscopic evaluation demonstrated bilateral vocal cord movement, but sensory testing revealed severe left laryngopharyngeal sensory deficit and silent aspiration.

Swallowing malfunctioning, or dysphagia, prevents adequate nutrition intake and may predispose to significant aspiration with a risk for potentially fatal pneumonia. Dysphagia occurs in a variety of central and peripheral neurologic disorders and can rarely be the presenting sign of neurologic disease. It is seen with central disorders including acute cerebral infarction, brainstem stroke, Parkinson disease, and demyelinating disease such as multiple sclerosis. Dysphagia may be a prominent and progressive problem in motor neuron disease, syringobulbia, or primary pontomedullary meningeal-based tumors. Peripheral disorders of the nerve (e.g., Guillain-Barré syndrome), neuromuscular junction (e.g., myasthenia gravis), and hereditary or acquired muscle disease (e.g., oculopharyngeal dystrophy, myotonic dystrophy and dermatomyositis) can also compromise swallowing function.

Physiology

Swallowing is a complex process involving motor control with sensory feedback from many anatomic structures within the oral cavity, pharynx, larynx, and esophagus (Fig. 9-1). The trigeminal (CN-V), facial (CN-VII), glossopharyngeal (CN-IX; Fig. 9-2), vagus (CN-X; Fig. 9-3), and hypoglossal (CN-XII) cranial nerves are involved. A “normal swallow” comprises two major components, bolus transport and airway protection. The swallowing process is typically classified into four phases (Fig. 9-4).

Figure 9-1 Neuroregulation of Deglutition.

Figure 9-2 Glossopharyngeal Nerve (IX): Schema.

Figure 9-3 Vagus Nerve (X): Schema.

Figure 9-4 Deglutition.

1. The oral preparatory phase involves voluntary motor function during which food or liquid is taken into the mouth, masticated (CN-V), and mixed with saliva to form a cohesive bolus (Fig. 9-4, nos. 1 and 2). This phase requires coordination of several cranial nerves and corresponding structures. Tension in the labial and buccal musculature closes the anterior and lateral sulci (CN-VII) while rotary mandible motion produces chewing (CN-V₃). Lateral rolling tongue motion (CN-XII) and bulging of the soft palate forward (widening the nasal airway, and sealing the posterior oral cavity) properly positions the bolus for the swallowing (CN-IX). Tongue mobility is the most important neuromuscular function involved in this first phase. The mid and lower divisions of CN-V provide sensory feedback for positioning the bolus. Saliva derived from the parotid, sublingual, and submandibular glands (innervated by secretomotor fibers of CN-IX and -VII) contain digestive enzymes that act as an emollient to soften and shape the bolus.

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 (Fig. 9-4, nos. 3 and 4). Lips and buccal muscles contract (CN-V and CN-VII) with elevation of the velum (CN-V and CN-X) providing the valving process that generates pressure to seal the nasopharynx, preventing reflux and nasal regurgitation. CN-V is responsible for the afferent (sensory) feedback for the entire oral cavity and tongue. 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 prevent the bolus from entering the nasopharynx. The swallowing reflex is triggered as the bolus passes the anterior tonsillar pillars, which initiates the pharyngeal phase.

Taste for the anterior two thirds of the tongue is carried by CN-VII, whereas the afferent CN-IX controls taste for the posterior one third of the tongue and the posterior pharyngeal wall. CN-X supplies primary innervation to the palatal muscles, pharyngeal constrictors, laryngeal musculature, and cricopharyngeus. Afferent fibers also provide critical sensory feedback from the larynx and esophageal inlet.

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 (Fig. 9-4, nos. 5–7). Once pharyngeal swallowing is elicited, essential functions of airway protection occur. Intrinsic laryngeal muscles innervated by CN-X close the larynx at the aryepiglottic, false vocal, and at the true vocal folds, creating a seal that separates the airway from the digestive tract protecting the laryngeal vestibule from foreign material aspiration. The tongue (CN XII) is the major force pushing the bolus through the pharynx. Synergistic actions with CN-X produce pharyngeal peristalsis as it innervates the pharyngeal constrictors and carries afferents from the lower pharynx.

CN-IX mediates the sensory portion of the pharyngeal gag but innervates just 1 muscle, the stylopharyngeus. The absence of the gag reflex is not the sole indicator of a patient’s swallowing abilities. A study of the risk of aspiration in patients with dysphagia and absent gag reflexes demonstrated that the majority could tolerate a modified diet. Additionally, the gag reflex was absent in 10–13% of nondysphagic individuals in the control group.

Poor airway protection and delayed triggering of pharyngeal swallow may cause aspiration. 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 (Figs. 9-2 and 9-3). If a reflexive cough is not elicited in response to foreign material within the airway, silent aspiration results; it is radiographically documented in 50% of aspiration cases.

CN-IX is the primary afferent of the swallowing response, whereas CN-X is the secondary afferent; both nerves terminate in the swallowing center located in the medulla within the nucleus solitarius. Sensory events initiating swallowing occur with stimulation to jaw, posterior tongue, faucial pillars, and upper pharynx and are mediated through CN-V, CN-IX, and CN-X. These afferent fibers converge on the nucleus solitarius in the medulla and communicate with the nucleus ambiguous via interneurons stimulating the motor response.

4. The esophageal phase

Only gold members can continue reading. Log In or Register to continue