CHAPTER 18

The Glossopharyngeal and Vagus Nerves

The glossopharyngeal (CN IX) and vagus (CN X) nerves are intimately related and similar in function. Both have motor and autonomic branches with nuclei of origin in the medulla. Both conduct general somatic afferent (GSA) as well as general visceral afferent (GVA) fibers to related or identical fiber tracts and nuclei in the brainstem, and both have a parasympathetic, or general visceral efferent, and a branchiomotor, or special visceral efferent (SVE), component. The two nerves leave the skull together, remain close in their course through the neck, and supply some of the same structures. They are often involved in the same disease processes, and involvement of one may be difficult to differentiate from involvement of the other. For these reasons, the two nerves are discussed together.

The Glossopharyngeal Nerve

Anatomy and Physiology

The glossopharyngeal, as its name implies, is distributed principally to the tongue and pharynx. It conveys general sensory as well as special sensory (taste) fibers from the posterior third of the tongue. It also provides general sensory innervation to the pharynx, the area of the tonsil, the internal surface of the tympanic membrane, and the skin of the external ear. It conveys GVAs from the carotid body and the carotid sinus. Its skeletomotor neurons innervate the stylopharyngeus muscle, and its parasympathetic component innervates the parotid gland.

Upper motor neuron influences on CN IX arise from the primary motor cortex and descend in the corticobulbar tracts to synapse in the rostral portion of the nucleus ambiguus in the dorsolateral medulla (Figure 18.1). The cortical innervation is bilateral. The cells in the nucleus ambiguus are branchiomotor and innervate muscles derived from the third, fourth, and fifth branchial arches. In keeping with the tendency of SVE axons to create internal loops, the fibers of CN IX first head posteromedially toward the floor of the fourth ventricle and then turn and sweep laterally and forward. The nerve emerges from the medulla as three to six rootlets in the groove between the inferior olive and the inferior cerebellar peduncle, between and in line with the emerging fibers of CN VII above and CN X below (Figure 11.3). These rootlets unite to form a single nerve, which leaves the skull through the jugular foramen.

FIGURE 18.1 Section through the medulla at the level of the inferior olivary nucleus.

CN IX exits the skull through the jugular foramen, lateral and anterior to CNs X and XI within a separate dural sheath. After leaving the skull, CN IX enters the carotid sheath, descends between the internal jugular vein and internal carotid artery, dips beneath the styloid process, and then passes between the internal and external carotid arteries. It curves forward, forming an arch on the side of the neck to reach the lateral pharyngeal wall, and then disappears under the hyoglossus muscle to divide into its terminal branches. Two ganglia lie on the nerve just caudal to the jugular foramen: the superior (jugular) and inferior (petrosal) glossopharyngeal ganglia (Figure 18.2). The superior glossopharyngeal ganglion is small, is inconstant, has no branches, and is often fused with the inferior ganglion. CN IX has six terminal branches: (a) the tympanic nerve (Jacobson’s nerve), (b) carotid, (c) pharyngeal, (d) muscular, (e) tonsillar, and (f) lingual branches. CN IX has important connections with CNs V, VII, and X and the cervical sympathetics.

FIGURE 18.2 Peripheral distribution of the parasympathetic branches of the glossopharyngeal nerve.

The branchiomotor fibers of CN IX go to the pharynx. The muscular branch follows along the posterior border of the stylopharyngeus muscle and then terminates in the belly of the muscle. Most of the pharyngeal muscles are supplied by both CNs IX and X. If CN IX supplies any muscle alone, it is the stylopharyngeus. The actions of the stylopharyngeus are described in Table 18.1.

TABLE 18.1 Branches of the Glossopharyngeal and Vagus Nerves, the Muscles Innervated, and Their Actions

CN IX supplies parasympathetic innervation to the parotid gland and to the mucous membranes of the posterior-inferior mouth and pharynx (Figure 18.2). The parasympathetic nuclei in the lower brainstem are the superior and inferior salivatory and the dorsal motor nucleus of CN X (DMNX), also known as the dorsal motor or dorsal efferent nucleus of the vagus. The autonomic fibers of CN IX arise primarily from the inferior salivatory nucleus, with some from the DMNX. The parasympathetics pass through the superior and inferior glossopharyngeal ganglia without synapsing. Just below the inferior ganglion, they exit to form the tympanic nerve, which ascends to the tympanic cavity through a small canal on the undersurface of the temporal bone between the carotid canal and the jugular fossa (tympanic canaliculus). The tympanic nerve ramifies in the tympanic cavity to form part of the tympanic plexus. The lesser petrosal nerve is a continuation of the tympanic nerve that leaves the tympanic plexus, enters the middle cranial fossa briefly, and then exits through the foramen ovale to synapse in the otic ganglion. Postganglionic fibers join the auriculotemporal branch of the mandibular division of CN V for distribution to the parotid gland; this is the nerve involved in gustatory sweating (Chapter 15).

Sensory neurons of CN IX are located in the superior and inferior glossopharyngeal ganglia. There are GSA fibers that convey ordinary exteroceptive sensation; GVA fibers that convey information from the carotid body and carotid sinus, as well as visceral sensation from the pharynx; and special visceral afferents that convey taste sensation. The GSA fibers convey exteroceptive sensation from the mucous membranes of the tympanic cavity, mastoid air cells, and auditory canal via the tympanic plexus and tympanic branch. Sensation from the pharynx, tonsil, and posterior third of the tongue travels via the pharyngeal, tonsillar, and lingual branches. Central processes of these cells terminate in the trigeminal nuclei, and their central connections are the same as for other GSA fibers. One of the most important functions of CN IX is to carry visceral afferent fibers from the carotid body and sinus involved in the reflex control of heart rate, blood pressure, and respiration. The carotid branch of CN IX (carotid sinus nerve) arises just below the jugular foramen and descends on the internal carotid artery to the carotid sinus and carotid body. It conveys impulses from carotid body chemoreceptors and carotid sinus baroreceptors and terminates centrally on cells in the middle third of the nucleus of the solitary tract. Other fibers carrying visceral afferent fibers from the mucous membranes of the pharynx, soft palate, and posterior third of the tongue pass through the petrous ganglion to terminate in the solitary tract and nucleus. The lingual branches of CN IX carry taste fibers (primarily sour and bitter) from the circumvallate papillae, mucous membranes of the base and taste buds on the posterior third of the tongue, glossoepiglottic and pharyngoepiglottic folds, and lingual surface of the epiglottis. These fibers terminate in the rostral part of the nucleus of the solitary tract (gustatory nucleus). Their central connections are the same as for the taste fibers of CN VII.

Clinical Examination

CN IX is difficult to examine because most or all of its functions are shared by other nerves and because many of the structures it supplies are inaccessible. It is possible to examine pain and touch sensation of the pharynx, tonsillar region and soft palate, and the gag reflex. Bedside testing of taste on the posterior third of the tongue is difficult and seldom attempted. It is not possible to isolate the motor functions from those of the vagus. The small area of cutaneous exteroceptive sensory supply is shared by other nerves. Patients with CN IX lesions might theoretically have detectable sensory loss, but it is not possible to find in patients who have undergone ninth nerve section for glossopharyngeal neuralgia.

The only muscle to receive its motor innervation purely from CN IX is the stylopharyngeus. The only deficit that might be detectable is a slight lowering of the palatal arch at rest on the involved side. Other palatal motor functions are subserved by either CN X or the two nerves working together. The salivary reflex is flow of saliva from the parotid duct after gustatory stimuli. The afferent limb is through taste fibers and the efferent through the parasympathetic outflow of the superior and interior salivatory nuclei.

The gag reflex is elicited by touching the pharynx or palate. Some sources make a distinction between the pharyngeal reflex and the palatal reflex, referring only to the former as the gag reflex. In common clinical usage, no distinction is made between these two and either is referred to as the gag reflex. The reflex is elicited by touching the lateral oropharynx in the region of the anterior faucial pillar with a tongue blade, applicator stick, or similar object (pharyngeal reflex), or by touching one side of the soft palate or uvula (palatal reflex). The pharyngeal reflex is the more active of the two. The reflex also occurs with touching the base of the tongue or posterior pharyngeal wall. The afferent limb of the reflex is mediated by CN IX and the efferent limb through CNs IX and X. The reflex center is in the medulla. The motor response is constriction and elevation of the oropharynx. This causes the midline raphe of the palate and the uvula to elevate and the pharyngeal constrictors to contract. The activity on the two sides is compared. The gag reflex is protective; it is designed to prevent noxious substances or foreign objects from going beyond the oral cavity. There are three motor components: elevation of the soft palate to seal off the nasopharynx, closure of the glottis to protect the airway, and constriction of the pharynx to prevent entry of the substance.

When unilateral pharyngeal weakness is present, the raphe will deviate away from the weak side and toward the normal side. This movement is usually dramatic (see Video Link 18.1). Minor movements of the uvula and trivial deviations of the midline raphe are not of clinical significance. In normal adults, both palatal and pharyngeal reflexes are usually present, but there may be inter- and intraindividual variation in the intensity of the stimulus required. The gag reflex may be bilaterally absent in some normal individuals. Unilateral absence signifies a lower motor neuron lesion. Like most bulbar muscles, the pharynx receives bilateral supranuclear innervation, and a unilateral cerebral lesion does not cause detectable weakness.

The gag reflex is often used to predict whether or not a patient will be able to swallow. A poor gag reflex in an awake patient with an acute deficit may be a predictor of swallowing difficulties. In fact, the gag reflex has little to do with normal swallowing. Normal deglutition is a smooth coordinated sequence of muscle contractions that propel a bolus of food from the mouth into the esophagus. A normal swallow bears little resemblance to the chaos of a gag reflex. Higher cortical centers have to inhibit the gag response during normal swallowing. The gag reflex is useful but limited in assessing airway protection. A decreased gag reflex in a patient with depressed consciousness may portend inadequate guarding of the airway and increased aspiration risk, but the status of the gag reflex is not a completely reliable indicator. Patients with an apparently intact gag reflex may still aspirate, and a patient with a depressed gag reflex may not.

Davies et al. found that the gag reflex is absent in 37% of normals, which may explain its low predictive value in the assessment of aspiration risk. Leder and Espinosa concluded that the clinical examination, a major component of which is the status of the gag reflex, underestimated the probability of aspiration in patients who were at risk and overestimated it in patients who were not. The trigeminal nerve contributes to palatal sensation and may allow for paradoxical preservation of the gag reflex in the face of a CN IX lesion. The gag reflex may be hyperactive in some normal individuals, even to the point of causing retching and vomiting. A hyperactive gag reflex may occur with bilateral cerebral lesions, as in pseudobulbar palsy and amyotrophic lateral sclerosis (ALS).

Disorders of Function

Unilateral supranuclear lesions cause no deficit because of the bilateral corticobulbar innervation. Bilateral supranuclear lesions may cause pseudobulbar palsy (Chapter 21).

Isolated lesions of CN IX are extremely rare if they ever occur. In all instances, the nerve is involved along with other CNs, especially CN X. Nuclear and infranuclear processes that may affect CN IX include intramedullary and extramedullary neoplasms and other mass lesions (e.g., glomus jugulare tumor), trauma (e.g., basilar skull fracture or surgical dissection), motor neuron disease, syringobulbia, retropharyngeal abscess, demyelinating disease, birth injury, and brainstem ischemia. Surgical section or other trauma to the carotid branch may cause transient or sustained hypertension. Involvement of CN IX may be related to the cardiovascular dysautonomia that sometimes accompanies Guillain-Barré syndrome. CN IX may be involved along with other CNs in lesions of the skull base, for example, the jugular foramen syndrome (Chapter 21).

Perhaps, the most important lesion of the ninth nerve is glossopharyngeal (or vagoglossopharyngeal) neuralgia or “tic douloureux of the ninth nerve.” In this condition, the patient experiences attacks of severe lancinating pain originating in one side of the throat or tonsillar region and radiating along the course of the eustachian tube to the tympanic membrane, external auditory canal, behind the angle of the jaw, and adjacent portion of the ear. As in trigeminal neuralgia, there may be trigger zones; they are usually in the pharyngeal wall, fauces, tonsillar regions, or base of the tongue. The pain may be brought on by talking, eating, swallowing, or coughing. It can lead to syncope, convulsions, and rarely cardiac arrest because of stimulation of the carotid sinus reflex. Glossopharyngeal neuralgia must be differentiated from other craniofacial neuralgias and from pain because of a structural lesion of the nerve. Some authorities differentiate between glossopharyngeal neuralgia, in which the pain radiates from the throat to the ear, and Jacobson’s neuralgia, in which the pain is limited to the ear and eustachian tube. Glossopharyngeal neuralgia is most often idiopathic but has been reported with lesions involving the peripheral distribution of the nerves. Multiple sclerosis only rarely causes glossopharyngeal neuralgia, although it is commonly associated with trigeminal neuralgia.

Carotid sinus hypersensitivity is due to inadvertent activation of the baroreceptors in the carotid sinus causing bradycardia and hypotension. Identifiable etiologies may include constriction around the neck (e.g., tight collar) or a mass in the neck impinging on the sinus, but many cases are idiopathic.

The Vagus Nerve

Anatomy and Physiology

The vagus (L. “wandering,” because of its wide distribution) is the longest and most widely distributed CN (Figure 18.3). Some of the nuclei of origin are the same as for CN IX, and it shares many functions with CN IX. It connects with four brainstem nuclei: the nucleus ambiguus, the DMNX, the nucleus of the spinal tract of CN V, and the nucleus of the solitary tract. It conveys exteroceptive GSA sensation from the pharynx, larynx, ear, and meninges and GVA fibers from the larynx, viscera of the thorax and abdomen, and receptors in the aorta. CN X carries skeletomotor axons from the nucleus ambiguus to the pharynx and larynx and parasympathetic axons from the DMNX to the smooth muscles and glands of the pharynx and larynx and to the thoracic and abdominal viscera. Its terminal ramifications reach the splenic flexure of the colon.

FIGURE 18.3 Peripheral distribution of the branches of the vagus nerve.

The vagus emerges from the medulla as a series of rootlets just below those of the glossopharyngeal. CN X leaves the skull through the jugular foramen in the same neural sheath as the cranial root of CN XI and behind CN IX. In the jugular foramen, the nerve lies close to the jugular bulb, a dilatation of the internal jugular vein that houses the glomus jugulare (tympanic body). The glomus jugulare has functions similar to the carotid body. CN X descends the neck in the carotid sheath, lying between the carotid artery and internal jugular vein to the upper border of the thyroid cartilage and then between the vein and common carotid to the base of the neck. Branches leave in the jugular foramen to supply the meninges and ear; other branches leave distal to the foramen to supply the pharynx and larynx. The major portion of the nerve enters the thorax. The vagus has two sensory ganglia. The superior (jugular) vagal ganglion is located in the jugular fossa of the temporal bone; the inferior (nodose) ganglion is located just distal to the jugular foramen. There are 10 major terminal branches that arise at different levels: (a) meningeal, (b) auricular, (c) pharyngeal, (d) carotid, (e) superior laryngeal, (f) recurrent laryngeal, (g) cardiac, (h) esophageal, (i) pulmonary, and (j) gastrointestinal. The terminal branches are summarized in Table 18.2.

TABLE 18.2 The Terminal Branches of the Vagus Nerve

The Motor Portion

The cortical center regulating vagus function lies in the lower portion of the precentral gyrus; the supranuclear innervation is bilateral but primarily crossed. Fibers descend in the corticobulbar tracts to synapse in the nucleus ambiguus. The vagal branchiomotor fibers follow the same looping intramedullary course as the fibers of CN IX. There are three major branchiomotor branches: pharyngeal, superior laryngeal, and recurrent laryngeal. The actions of the muscles innervated by the vagus are summarized in Table 18.1.

The pharyngeal branch runs between the internal and external carotid arteries and enters the pharynx, where it ramifies to form the pharyngeal plexus. The plexus also receives fibers from the external laryngeal branch, CN IX, and the sympathetic trunk. The vagus, with a contribution from the bulbar portion of CN XI, supplies all the striated muscles of the soft palate, pharynx, and larynx except for the stylopharyngeus (CN IX) and tensor veli palatini (CN V).

The superior laryngeal nerve arises distal to the pharyngeal branch and divides into an internal and external branch. The internal branch is primarily sensory. The external branch supplies the cricothyroid. All of the other intrinsic laryngeal muscles are supplied by the recurrent nerves, except for the arytenoid, which may receive some fibers from the internal branch of the superior laryngeal. The recurrent laryngeal nerves both descend deep into the thorax and then loop back to the larynx. On the right, the recurrent laryngeal arches around the subclavian artery and on the left around the aortic arch. Each nerve gives off cardiac, tracheal, and esophageal branches, ending on each side as the inferior laryngeal nerve to supply the intrinsic muscles of the larynx.

The Parasympathetic Portion

The parasympathetic component of CN X arises from the DMNX, a long cell column just dorsolateral to the hypoglossal nucleus extending from the upper pole of the inferior olive to the lower portion of the medulla. Some parasympathetic neurons lie immediately adjacent in the medial part of the nucleus ambiguus. The neurons in the nucleus ambiguus innervate the heart, and those in DMNX supply the other vagally innervated viscera. The fibers stream ventromedially and merge with the branchiomotor fibers coming from the nucleus ambiguus. The autonomic fibers leave the medulla as preganglionic fibers of the craniosacral division of the autonomic nervous system. They terminate in ganglia close to the viscera they supply and send short postganglionic fibers directly to the muscular and glandular structures they innervate. The vagus is the longest parasympathetic nerve in the body and mediates many important functions, which are discussed in Chapter 45. In brief, a vagal discharge causes bradycardia, hypotension, bronchoconstriction, bronchorrhea, increased peristalsis, increased gastric secretion, and inhibition of adrenal function. The vagal centers in the medulla that control these functions are themselves under the control of higher centers in the cortex and hypothalamus. Inhibition of vagal function produces the opposite effects.

In its course through the thorax, the right vagus nerve gives off pulmonary and esophageal branches, passes through the esophageal opening in the diaphragm posterior to the esophagus, and then divides into gastric and celiac branches. The left vagus also gives off pulmonary and esophageal branches and then enters the abdomen anterior to the esophagus and divides into several gastric branches.

The Sensory Portion

The superior vagal ganglion is located in the upper part of the jugular foramen. It communicates through several delicate branches with the cranial portion of CN XI and with the petrous ganglion of CN IX, with CN VII, and with the superior cervical ganglion. The inferior vagal ganglion lies just beneath the jugular foramen. The cranial root of the CN XI passes through it to join CN X. The inferior ganglion also communicates with CN XII, the superior cervical ganglion, and the loop between C1 and C2. Both vagal ganglia are sensory, containing unipolar neurons that mediate general somatic, special visceral, and general visceral afferents. The branchiomotor and parasympathetic axons pass through the ganglia without synapsing. The superior ganglion primarily conveys somatic sensation, and most of its communication is with the auricular nerve. The inferior ganglion relays general visceral sensation and taste.

The somatic sensory portion of the vagus conveys pain, temperature, and touch sensation from the pharynx, larynx, ear canal, external surface of the tympanic membrane, and meninges of the posterior fossa. In the larynx, GSA fibers from above the vocal folds travel in the internal laryngeal branch of the superior laryngeal nerve; fibers from below the vocal folds travel with the recurrent laryngeal nerve. Visceral afferents follow the same pathways. General sensory fibers from the region of the ear, ear canal, and tympanic membrane travel in the auricular branch (nerve of Arnold). Stimulation of the auricular branch, as by tickling the ear canal, can produce reflex activation of DMNX with coughing, vomiting, and even syncope (mitempfindung; see Chapter 31). The GSA fibers in CN X synapse in the nucleus of the spinal tract of CN V and are relayed to the thalamus and to the sensory cortex.

Fibers carrying GVAs from the pharynx, larynx, vagally innervated viscera, and baroreceptors and chemoreceptors in the aorta travel over the peripheral processes of neurons in the inferior vagal ganglion. The central processes terminate in the caudal portion of the solitary tract. Collaterals to the reticular formation, DMNX, and other CN nuclei mediate important visceral reflexes and are involved in the regulation of cardiovascular, respiratory, and gastrointestinal function. There are some taste fibers from the region of the epiglottis and arytenoids, which travel with the taste fibers of CN IX to terminate in the rostral solitary tract.

Normal Functions

Normal functions mediated by CNs IX and X include swallowing, phonation, and airway protection and modulation. The complex process of swallowing is divided into two stages, controlled primarily by CNs IX, X, and XII. In the first stage, the food bolus is driven back into the fauces by tongue action. During the second stage, the epiglottis closes over the entrance to the larynx, and the bolus glides along its posterior surface. The muscles of the soft palate and nasopharynx close above the bolus to prevent passage into the nasopharynx. The bolus is directed downward and backward into the pharynx, and then the constrictors contract to propel it downward into the esophagus.

The larynx is composed of several cartilages. The thyroid and cricoid cartilages form part of the outer casing. The arytenoids are paired cartilages lying in the interior; they have a muscular process and a vocal process. The true vocal cords are mucous membranes that cover the vocal ligaments, which extend from the vocal processes of the arytenoids to the thyroid cartilage. The larynx is controlled by myriad small muscles. The arytenoids may either slide or pivot; either action changes the configuration of the vocal cords. The glottic rim is the passageway between the vocal cords. Contraction and relaxation of the intrinsic laryngeal muscles change the tension or shape of the vocal cords and alter the aperture of the glottic rim. The muscles of the larynx perform three basic functions: They abduct and open the glottic rim to allow air entry and exit, they adduct and close the glottic rim to protect the airway during swallowing, and they regulate the tension on the vocal cords to allow phonation. The cricothyroids, posterior and lateral cricoarytenoids, and thyroarytenoids are paired muscles. The arytenoid muscle is unpaired. The actions of the intrinsic laryngeal muscles are summarized in Table 18.1 and Figure 18.4.

FIGURE 18.4 The cricothyroid muscles (not shown) tilt the thyroid cartilage forward on the cricoid cartilage, tensing the vocal cords. The thyroarytenoid muscles run from the thyroid cartilage to the arytenoid cartilages; contraction tenses the vocal cords. The other muscles attach to the cricoid cartilage. The paired arytenoids may either slide or pivot. Contraction of the arytenoid muscle pulls the arytenoid cartilages together, adducting the cords and closing the glottic rim. The lateral cricoarytenoid muscle causes the vocal process of the arytenoid to pivot medially, adducting the cords. The posterior cricoarytenoid causes the vocal process to rotate laterally, abducting the cords.

Clinical Examination

Despite its size and importance, CN X is difficult to evaluate at the bedside. Formal autonomic function assessment can sometimes provide useful information.

Examination of the Motor Functions

The motor branches of CN X supply the soft palate, pharynx, and larynx in the same distribution as for CN IX and are examined in the same manner. The gag reflex is discussed in the section on CN IX.

The character of the voice and the ability to swallow provide information about the branchiomotor functions of the vagus. With acute unilateral lesions, the speech may have a nasal quality and dysphagia is often present; this is more marked for liquids than solids, with a tendency to nasal regurgitation because of velopharyngeal insufficiency. Examination of the soft palate includes observation of the position of the palate and uvula at rest and during quiet breathing and phonation. The median raphe of the palate rises in the midline on phonation. With a unilateral lesion of the vagus, there is weakness of the levator veli palatini and musculus uvulae, which causes a droop of the palate and flattening of the palatal arch (Figure 18.5). Preserved function of the tensor veli palatini (innervated by CN V) may prevent marked drooping of the palate. On phonation, the median raphe deviates toward the normal side. The palatal gag reflex may be lost on the involved side because of interruption of the motor rather than sensory path.

FIGURE 18.5 Unilateral paralysis of the soft palate.

With bilateral vagus involvement, the palate cannot elevate on phonation; it may or may not droop, depending on the function of the tensor veli palatini. The palatal gag reflex is absent bilaterally. The tendency toward nasal speech and nasal regurgitation of liquids is pronounced. The speech is similar to that of a patient with cleft palate (Chapter 9).

Weakness of the pharynx may also produce abnormalities of speech and swallowing. With pharyngeal weakness, dysarthria is usually minimal unless there is also weakness of the soft palate or larynx. Spontaneous coughing and the cough reflex may be impaired. Dysphagia may occur but without the tendency to greater difficulty with liquids and to nasal regurgitation that occurs with palatal weakness. Dysphagia is marked only in acute unilateral or in bilateral lesions. Examination of the pharynx includes observation of the contraction of the pharyngeal muscles on phonation, notation of the elevation of the larynx on swallowing, and testing the pharyngeal gag reflex. Unilateral weakness of the superior pharyngeal constrictor may cause a “curtain movement” (Vernet’s rideau phenomenon), with motion of the pharyngeal wall toward the nonparalyzed side on testing the gag reflex or at the beginning of phonation. The normal elevation of the larynx may be absent on one side in unilateral lesions and on both sides in bilateral lesions.

CN X innervates the vocal cords. Normal movement of the vocal cords is necessary for three vital functions: breathing, coughing, and talking. During inspiration and expiration, the cords abduct to allow for free airflow; when speaking, the cords adduct and vibrate to accomplish phonation. The cords are also adducted when coughing. Movements of the many small muscles that control the larynx are complex and have different effects on laryngeal function (Table 18.1). The effects of weakness of the different laryngeal muscles are summarized in Table 18.3. A unilateral lesion of the vagus may cause cord weakness or paralysis. Vocal cord dysfunction alters the character and quality of the voice and may produce abnormalities of articulation, difficulty with respiration, and impairment of coughing.

TABLE 18.3 The Effects of Weakness of Muscles of the Larynx

Spasmodic dysphonia is a common focal dystonia that involves the vocal cords and causes characteristic voice changes (Chapter 30; for video, see Video Link 18.2). Spasmodic dysphonia most often causes abnormal adduction spasms of both vocal cords, and the voice is strained and high-pitched. Abductor dysphonia is due to spasmodic contraction of the posterior cricoarytenoid, which causes a failure of normal adduction on phonation; the voice is breathy and hoarse. This type of spasmodic dysphonia is most likely to be confused with a lesion of CN X. Direct and indirect laryngoscopy and videostroboscopy are valuable adjuncts to the routine examination.

The most common cause of vocal cord paralysis is a lesion of one recurrent laryngeal nerve. The paralysis may evolve from mild abduction impairment because of isolated involvement of the posterior cricoarytenoid to complete paralysis with the cord in the cadaveric position. With slight weakness of the vocal cords or pharynx, hoarseness and dysphagia may be apparent only when the head is turned to either side. Occasionally, even severe weakness of a vocal cord causes little appreciable effect on the voice because of preserved movement of the normal cord.

Examination of the Autonomic Functions

The autonomic functions of CN X are summarized above and are discussed in more detail in Chapter 45.

Examination of the Sensory Functions

The somatic sensory elements of CN X are discussed above. They are not clinically important and cannot be adequately tested.

Examination of the Reflexes

CN X plays a part in several autonomic, or visceral, reflexes; loss of these reflexes may follow a lesion of the tenth nerve. In some of these reflexes, such as the sternutatory, sucking, and yawning, the vagus plays a supportive role. The nasal, sneeze, or sternutatory reflex is discussed in Chapter 15. Afferent impulses are carried over CN V to the reflex center in the brainstem and upper spinal cord, with efferent impulses primarily by CN VII with some overflow to CNs IX and X and the phrenic nerve. In other reflexes, such as swallowing, vomiting, and coughing, the vagus is central. These are discussed in Box 18.1.

BOX 18.1

Vagally Mediated Reflexes

The oculocardiac reflex is bradycardia caused by pressure on the eyeball. It may also be induced by painful stimulation of the skin on the side of the neck. The afferent limb is carried by cranial nerve (CN) V and the efferent by CN X. The reflex is inconstant, unstandardized, and influenced by emotion. Usually, the pulse is not slowed more than 5 to 8 beats per minute. The slowing may be accompanied by extrasystoles. The oculocardiac reflex may be absent in lesions involving CN X. It is sometimes used to slow an excessively rapid heart rate, as in tachyarrhythmias.

The vomiting reflex produces reverse peristalsis in the esophagus and stomach, with forceful ejection of material from the stomach. The reflex center is in the region of the dorsal efferent nucleus. Vomiting occurs for many reasons. Stimulation of the pharynx, palate, esophagus, stomach, duodenum, or lower gastrointestinal tract may activate the reflex. The afferent limb is carried by CN X, probably to the solitary tract; from there, the impulse is relayed to the dorsal efferent nucleus and also down the spinal cord to contract the diaphragm and abdominal muscles, relax the cardiac sphincter, and contract the pyloric sphincter. The swallowing reflex is caused by stimulation of the pharyngeal wall or back of the tongue. Afferent impulses travel through CNs V, IX, and X and efferent impulses through CNs IX, X, and XII. The cough reflex is activated by stimulation of the mucous membrane of the pharynx, larynx, trachea, or bronchial tree. Stimulation of the tympanic membrane or external auditory canal can also elicit a cough response (mitempfindung). The afferent limb of the reflex is carried through CNs IX and X to the solitary tract, and the efferent impulses descend to the pharyngeal muscles, tongue, palate, and larynx and to the diaphragm, chest, and abdominal muscles.

Hiccup (singultus) is a sudden reflex contraction of the diaphragm causing a forceful inspiration. Associated laryngeal spasm causes the glottis to snap shut, causing sudden arrest of the inspiration and the characteristic sound. The phrenic nerves are the major pathway, but CN X contributes. Yawning is a complex respiratory reflex with deep, prolonged inspiration, usually involuntary, through the open mouth. It typically occurs during sleepiness and fatigue but may also be brought on by suggestion or boredom. Yawning can occur in neurologic disease as well.

The carotid sinus reflex is produced by stimulation of the carotid sinus or the carotid body by pressure at the carotid bifurcation. It causes slowing of the heart rate, a fall in blood pressure, a decrease in cardiac output, and peripheral vasodilatation. When the response is exaggerated, there may be syncope. The afferent limb of the reflex is carried over CN IX and the efferent over CN X. The carotid sinus reflex is discussed further under CN IX.

Disorders of Function

A unilateral vagal lesion causes weakness of the soft palate, pharynx, and larynx. Acute lesions may produce difficulty swallowing both liquids and solids and hoarseness or a nasal quality to the voice. The only definite sensory change is anesthesia of the larynx because of involvement of the superior laryngeal nerve. It is seldom possible to demonstrate loss of sensation behind the pinna and in the external auditory canal. The gag reflex is absent on the involved side. Autonomic reflexes (vomiting, coughing, and sneezing) are not usually affected. Tachycardia and loss of the oculocardiac reflex on the involved side may occur, but usually there are no cardiac symptoms. Gastrointestinal disturbances are inconspicuous. Bilateral complete vagal paralysis is incompatible with life. It causes complete paralysis of the palate, pharynx, and larynx, with marked dysphagia and dysarthria; tachycardia; slow, irregular, respiration; vomiting; and gastrointestinal atonia. Lesions of individual vagal branches are rare except for involvement of the recurrent laryngeal nerve.

The primary effect of increased vagal activity is bradycardia. The term vasovagal refers to the effects of the vagus nerve on the blood vessels. Vasovagal attacks (fainting, syncope) are characterized by bradycardia, hypotension, peripheral vasoconstriction, and faintness, sometimes with loss of consciousness. Vasovagal attacks are typically induced by strong emotion or pain. The bradycardia and projectile vomiting that occur with increased intracranial pressure may be vagally mediated. Cheyne-Stokes, Biot, and Kussmaul breathing; respiratory tics; forced yawning; and other abnormalities of breathing may be vagally mediated as well. Spasm of pharyngeal muscles can occur in certain central nervous system disorders. Other conditions in which there is increased activity in the vagal system are seldom of primary neurologic origin.

Rhythmic movements of the palate (palatal myoclonus, palatal microtremor, or palatal nystagmus) can occur with a lesion of the brainstem, usually vascular ( Video 30.7). The movements are mediated by CN X. Palatal myoclonus is discussed further in Chapter 30. The very rare syndrome of superior laryngeal neuralgia causes lancinating pains that radiate from the larynx to the ear.

Unilateral supranuclear lesions generally cause no dysfunction because of bilateral innervation; dysphagia from a unilateral lesion can occur but is rare. Bilateral supranuclear lesions, as from pseudobulbar palsy, cause dysphagia and dysarthria (Chapter 21). Extrapyramidal disorders may produce difficulty with swallowing and talking. Patients with Parkinson’s disease typically have a hypokinetic dysarthria (Chapter 9). Laryngeal spasm with stridor may occur in Parkinson’s disease and other extrapyramidal disorders. The voice is commonly affected by essential tremor.

Nuclear lesions of the nucleus ambiguus can occur with any intrinsic brainstem disease. A slowly progressive nuclear lesion, such as in bulbar ALS, syringomyelia, and some neoplasms, may cause fasciculations in the palatal, pharyngeal, and laryngeal muscles. The speech disturbances are discussed in Chapter 9. Lesions of the nucleus ambiguus or intramedullary fibers of CNs IX and X commonly occur with vascular disease, for example, lateral medullary (Wallenberg) syndrome. Nuclear lesions are usually associated with involvement of other CN nuclei and long motor or sensory tracts. Because of the somatotopic organization, lesions limited to the rostral portion of the nucleus ambiguus may produce only weakness of the palate and pharynx, sparing laryngeal functions.

Infranuclear involvement may occur with lesions at the base of the brain, in the cerebellopontine angle, in the jugular foramen, or along the course of the vagus nerves. Extramedullary, intracranial involvement can occur in processes involving the meninges, extramedullary tumors, aneurysms, trauma, sarcoidosis, and skull fractures. Other lower CNs are usually involved as well (Chapter 21). Lesions at the jugular foramen or in the retroparotid space usually involve some combination of CNs IX, X, XI, and XII and the cervical sympathetics. These lower CN syndromes are discussed in Chapter 21. Isolated or multiple lower CN palsies can be a manifestation of dissecting aneurysm of the cervical internal carotid artery or occur as a complication of carotid endarterectomy. Isolated CN IX palsy has been reported as a complication of traumatic internal maxillary artery dissection.

The main trunk of the vagus may be injured in the neck or thorax by trauma, carotid aneurysms, or other mass lesions. Vocal cord and diaphragmatic weakness occur in some forms of Charcot-Marie-Tooth disease. Individual vagal branches may be involved by disease processes in the neck, upper mediastinum, thorax, and abdomen. The recurrent laryngeal nerve is the most frequently affected; the left is more often damaged than the right because of its longer course. The recurrent laryngeals may be damaged by tumors in the neck, especially carcinoma of the thyroid, cervical adenopathy, metastatic lesions, Hodgkin disease, lymphosarcoma, aortic aneurysms, mitral stenosis with enlargement of the left atrium, pericarditis, mediastinal and apical tumors, stab wounds in the neck, or accidental trauma during a thyroidectomy or other surgical procedure. Recurrent laryngeal weakness causes a flaccid dysphonia with breathiness and mild inspiratory stridor; palatopharyngeal functions are preserved. Diplophonia may occur because of unbalanced vocal cord vibration frequency. Compression of the left recurrent laryngeal nerve between the aorta and the pulmonary artery because of a variety of cardiovascular disorders may cause hoarseness (cardiovocal or Ortner’s syndrome). Bilateral recurrent laryngeal palsies cause abduction impairment and leave the vocal cords approximating each other in the midline; the most common cause is thyroid surgery. This results in dyspnea and inspiratory stridor. The superior laryngeal and pharyngeal branches may be involved in trauma, or in neoplasms or abscesses in the neck, but clinical dysfunction is scant because of the primarily sensory function of the nerve; there may be mild hoarseness because of weakness of the cricothyroid muscle. Metastatic breast cancer infiltrating behind the carotid sheath at C6 has been reported to produce a combination of recurrent laryngeal and phrenic nerve dysfunction with an accompanying Horner syndrome. Hoarseness and voice fatigue because of laryngeal involvement may be prominent in rare patients with myasthenia gravis.

Syncope, sometimes associated with paroxysmal neck pain, may occur because of neoplasms involving the carotid sinus nerve. The mechanism is probably similar to that seen in syncope due to glossopharyngeal neuralgia. Swallow syncope results from dysfunction, usually because of metastatic disease, of CNs IX and X. The patient develops bradycardia and hypotension because of involvement of the baroreceptor nerves.

Video Links

Video Link 18.1. Palatal deviation. http://neurosigns.org/wiki/Palatal_deviation

Video Link 18.2. Spasmodic dysphonia. http://neurosigns.org/wiki/Spasmodic_dysphonia

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