4194 · 4 Brainstem
Acoustic Neuroma
As already stated, the common (indeed almost universal) designation “acoustic neuroma” is actually a misnomer for a schwannoma arising from the vestibular fibers of the vestibulocochlear nerve. The tumor destroys these fibers first, slowly and progressively impairing the excitability of the vestibular organ on the affected side; patients rarely suffer from vertigo, because this deficit can be compensated for at higher levels of vestibular processing, but the asymmetric excitability can be demonstrated by caloric testing. Depending on whether the tumor grows rapidly or slowly, irritation and/or compression of the fibers of the cochlear nerve leads sooner or later to clinically evident high-frequency hearing loss. The diagnosis of acoustic neuroma is supported by the finding of high-frequency hearing loss by audiometry, and of a prolonged conduction time by measurement of brainstem auditory evoked potentials (BAEP); it can be confirmed by MRI. There is, however, no direct and reliable relationship between the size of the tumor and the severity of the hearing loss that it causes.
Further growth of the tumor can compress neighboring structures (brainstem, facial nerve, trigeminal nerve), leading to further cranial nerve deficits (e. g., impaired lacrimation and taste due to dysfunction of the chorda tympani) and, finally, to symptomatic compression of the brainstem and cerebellum.
Patients with bilateral acoustic neuroma most likely suffer from neurofibromatosis type II (also called bilateral acoustic neuromatosis).
The treatment of acoustic neuroma is currently the subject of intense discussion among neurosurgeons. Many lesions that previously could only have been treated by open surgery can now be treated with as good or better results by stereotactic radiosurgery (i.e., with the Gamma Knife or a stereotactic linear accelerator).
Vagal System (CN IX, X, and the Cranial Portion of XI)
Glossopharyngeal Nerve (CN XI)
The glossopharyngeal nerve shares so many of its functions with the nervus intermedius, the vagus nerve, and the cranial portion of the accessory nerve that these nerves can be considered together as a single “vagal system” to avoid making the presentation unnecessarily repetitive. These nerves are all mixed (sensory and motor) nerves, and some of their components arise from common brainstem nuclei (the nucleus ambiguus and nucleus solitarius) (cf. Table 4.1 and Figs. 4.2 and 4.3).
Anatomical course and distribution (Fig. 4.48). The glossopharyngeal, vagal, and accessory nerves exit the skull together through the jugular foramen,
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Cranial Nerves · 195 4
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Fig. 4.48 Distribution and central connections of the glossopharyngeal and vagus nerves
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4196 · 4 Brainstem
which is also the site of both ganglia of the glossopharyngeal nerve, the superior (intracranial) ganglion and the inferior (extracranial) ganglion. After leaving the foramen, the glossopharyngeal nerve travels between the internal carotid artery and the jugular vein toward the stylopharyngeus muscle. It continues between the stylopharyngeus and styloglossus muscles and onward to innervate the root of the tongue, the pharyngeal mucosa, the tonsils, and the posterior third of the tongue. Along its course, it gives off the following branches:
The tympanic nerve runs from the inferior ganglion to the tympanic cavity and tympanic plexus (of Jacobson), and then onward in the lesser petrosal nerve, by way of the otic ganglion, to the parotid gland (Fig. 4.38). It supplies sensation to the mucosa of the tympanic cavity and eustachian tube.
Stylopharyngeal branches to the stylopharyngeus muscle.
Pharyngeal branches, which, together with branches of the vagus nerve, form the pharyngeal plexus. This plexus supplies the striated muscles of the pharynx.
Branches to the carotid sinus, which run with the carotid artery to the carotid sinus and carotid body.
Lingual branches conveying gustatory impulses from the posterior third of the tongue.
Lesions of the Glossopharyngeal Nerve
Isolated lesions of the glossopharyngeal nerve are rare; the vagus and accessory nerves are usually involved as well.
The causes of glossopharyngeal nerve lesions include basilar skull fracture, sigmoid sinus thrombosis, tumors of the caudal portion of the posterior fossa, aneurysms of the vertebral or basilar arteries, iatrogenic lesions (caused, e. g., by surgical procedures), meningitis, and neuritis.
The clinical syndrome of a glossopharyngeal nerve lesion is characterized by:
Impairment or loss of taste (ageusia) on the posterior third of the tongue
Diminution or absence of the gag and palatal reflexes
Anesthesia and analgesia in the upper portion of the pharynx and in the area of the tonsils and the base of the tongue
A mild disturbance of swallowing (dysphagia)
Impaired salivation from the parotid gland
Glossopharyngeal neuralgia is approximately 1% as common as trigeminal neuralgia; like trigeminal neuralgia, it is characterized by intense, paroxysmal pain. The painful attacks generally begin suddenly in the pharynx, neck, tonsils, or tongue, and last a few seconds or minutes. They can be provoked by swal-
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Cranial Nerves · 197 |
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lowing, chewing, coughing, or speaking. The patient is afraid to eat because of the pain and rapidly loses weight. This syndrome usually resolves spontaneously within six months of onset. Persistence suggests a possible anatomical cause, such as a tumor in the pharynx, which must be ruled out by radiological study. In analogous fashion to trigeminal neuralgia, it is generally treated medically with carbamazepine or gabapentin at first. In refractory cases, a neurosurgical procedure called microvascular decompression can be considered (Jannetta 1977); this involves opening the posterior fossa and moving a loop of the vertebral or posterior inferior cerebellar artery away from the ninth cranial nerve.
Vagus Nerve (CN X)
Like the glossopharyngeal nerve, the vagus nerve also possesses two ganglia, the superior (jugular) ganglion and the inferior (nodose) ganglion, both of which are found in the region of the jugular foramen.
Anatomical course. The vagus nerve is derived from the fourth and lower branchial arches. Below the inferior (nodose) ganglion, it follows the internal carotid and common carotid arteries downward, and then passes through the superior thoracic aperture into the mediastinum. Here, the right vagal trunk crosses over the subclavian artery, while the left trunk runs behind the hilum and past the aortic arch. Both then become applied to the esophagus, with the fibers of the right vagal trunk running on its posterior side, and those of the left vagal trunk on its anterior side. The terminal vagal branches then accompany the esophagus through the esophageal hiatus of the diaphragm into the abdominal cavity.
Branches of the vagus nerve. Along its way to the abdominal cavity, the vagus nerve gives off the following branches (Figs. 4.48, 4.49; and Fig. 6.14, p. 290):
Dural branch: running from the superior ganglion back through the jugular foramen to the dura mater of the posterior fossa.
Auricular branch: from the superior ganglion of the vagus nerve to the skin on the posterior surface of the external ear and the inferoposterior portion of the external auditory canal. This is the only cutaneous branch of the vagus nerve.
Pharyngeal branches: these accompany the fibers of the glossopharyngeal nerve and the sympathetic chain into the pharyngeal plexus to supply the muscles of the pharynx and soft palate.
Superior laryngeal nerve: from the inferior ganglion to the larynx. This nerve splits into two branches of its own. The external branch gives off branches to the pharyngeal constrictor muscle and then goes on to innervate the cri-
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4198 · 4 Brainstem
cothyroid muscle. The internal branch is a sensory nerve supplying the laryngeal mucosa as far downward as the vocal folds, as well as the mucosa of the epiglottis. It also contains gustatory fibers for the epiglottis and parasympathetic fibers innervating the mucosal glands.
Recurrent laryngeal nerve: This branch runs around the subclavian artery on the right side and the aortic arch on the left (Fig. 4.49), then proceeds upward between the trachea and the esophagus toward the larynx. It supplies motor innervation to the internal laryngeal musculature, with the exception of the cricothyroid muscle, as well as sensory innervation to the laryngeal mucosa below the vocal folds.
Superior cervical cardiac branches and thoracic cardiac branches: these accompany sympathetic fibers to the heart, by way of the cardiac plexus.
Bronchial branches: these form the pulmonary plexus in the wall of the bronchi.
Anterior and posterior gastric branches, and hepatic, celiac, and renal branches: these travel, by way of the celiac and superior mesenteric plexuses, and together with sympathetic fibers, to the abdominal viscera (stomach, liver, pancreas, spleen, kidneys, adrenal glands, small intestine, and proximal portion of large intestine). In the abdominal cavity, the fibers of the right and left vagus nerves become closely associated with those of the sympathetic nervous system and can no longer be clearly distinguished from them.
Syndrome of a Unilateral Lesion of the Vagus Nerve
The soft palate hangs down on the side of the lesion, the gag reflex is diminished, and the patient’s speech is nasal because the nasal cavity can no longer be closed off from the oral cavity. Paresis of the pharyngeal constrictor muscle causes the palatal veil to be pulled over to the normal side when the patient phonates.
Hoarseness results from paresis of the vocal folds (lesion of the recurrent laryngeal nerve with paresis of the internal muscles of the larynx, with the exception of the cricothyroid muscle).
Further components of the syndrome are dysphagia and occasionally tachycardia, and cardiac arrhythmia.
Causes. Many diseases can cause a central vagal lesion, including malformations (Chiari malformation, DandyWalker syndrome, etc.), tumors, hemorrhage, thrombosis, infection/inflammation, amyotrophic lateral sclerosis, and aneurysms. Peripheral vagal lesions can be caused by neuritis, tumors, glandular disturbances, trauma, and aortic aneurysms.
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Cranial Nerves · 199 4
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Inhibition
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Fig. 4.49 Distribution and central connections of the vagus nerve. a Overview. b Topographic relations of the recurrent laryngeal nerve.
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4200 · 4 Brainstem
Extrapyramidal fibers to the |
Pyramidal |
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nucleus of the accessory n. |
tract |
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Jugular foramen Superior ganglion
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cleidomastoid and trapezius mm.
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Fig. 4.50 Distribution and central connections of the accessory nerve
Cranial Roots of the Accessory Nerve (CN XI)
The accessory nerve has two sets of roots, cranial and spinal (Fig. 4.50). The neurons giving rise to the cranial roots lie in the nucleus ambiguus next to the neurons whose processes run in the vagus nerve. This portion of the eleventh cranial nerve is best considered a functional component of the vagus nerve, as its functions are essentially the same as those of the portion of the vagus nerve
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Cranial Nerves · 201 4
that arises in the nucleus ambiguus. (The spinal roots of the accessory nerve, on the other hand, have an entirely different function.) The cranial roots separate off from the spinal roots within the jugular foramen to join the vagus nerve. This portion of the accessory nerve thus belongs to the “vagal system.” The spinal roots and their function will be discussed below.
Common Nuclear Areas and Distribution of CN IX and X
Nucleus Ambiguus
The nucleus ambiguus is the common motor nucleus of the glossopharyngeal and vagus nerves and of the cranial portion of the accessory nerve (Figs. 4.48, 4.49, and 4.50). It receives descending impulses from the cerebral cortex of both hemispheres by way of the corticonuclear tract. Because of this bilateral innervation, unilateral interruption of these central descending fibers does not produce any major deficit in the motor distribution of the nucleus ambiguus.
The axons that originate in the nucleus ambiguus travel in the glossopharyngeal and vagus nerves and the cranial portion of the accessory nerve to the muscles of the soft palate, pharynx, and larynx, and to the striated muscle of the upper portion of the esophagus. The nucleus ambiguus also receives afferent input from the spinal nucleus of the trigeminal nerve and from the nucleus of the tractus solitarius. These impulses are the afferent limb of the important reflex arcs by which mucosal irritation in the respiratory and digestive tracts produces coughing, gagging, and vomiting.
Parasympathetic Nuclei of CN IX and X
The dorsal nucleus of the vagus nerve and the inferior salivatory nucleus are the two parasympathetic nuclei that send fibers into the glossopharyngeal and vagus nerves. The superior salivatory nucleus is the parasympathetic nucleus for the nervus intermedius, as discussed above (Figs. 4.48 and 4.49).
Dorsal nucleus of the vagus nerve. The efferent axons of the dorsal nucleus of the vagus nerve travel as preganglionic fibers with the vagus nerve to the parasympathetic ganglia of the head, thorax, and abdomen. After a synaptic relay, the short postganglionic fibers convey visceromotor impulses to the smooth musculature of the respiratory tract and of the gastrointestinal tract as far down as the left colic flexure, as well as to the cardiac muscle. Stimulation of the vagal parasympathetic fibers causes slowing of the heartbeat, constriction of the bronchial smooth muscle, and secretion from the bronchial glands. Peristalsis in the gastrointestinal tract is promoted, as is secretion from the glands of the stomach and pancreas.
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The dorsal nucleus of the vagus nerve receives afferent input from the hypothalamus, the olfactory system, autonomic centers in the reticular formation, and the nucleus of the tractus solitarius. These connections are important components of the reflex arcs for the control of cardiovascular, respiratory, and alimentary function. Impulses from the baroreceptors in the wall of the carotid sinus, which reach the dorsal nucleus of the vagus nerve through the glossopharyngeal nerve, serve to regulate arterial blood pressure. Chemoreceptors in the glomus caroticum participate in the regulation of the partial pressure of oxygen in the blood. Other receptors in the aortic arch and para-aortic bodies send afferent impulses to the dorsal nucleus of the vagus nerve by way of the vagus nerve, and have similar functions.
Inferior salivatory nucleus. The parasympathetic fibers arising in the inferior salivatory nucleus and traveling by way of the glossopharyngeal nerve to the parotid gland have already been discussed (pp. 175 and 196).
Visceral Afferent Fibers of CN IX and X
Special visceral afferent fibers. The perikarya (cell bodies) of the afferent gustatory fibers of the glossopharyngeal nerve (pseudounipolar neurons) are found in the inferior (extracranial) ganglion, while those of the vagus nerve are found in the inferior (nodose) ganglion. Both groups of fibers convey gustatory impulses from the epiglottis and the posterior third of the tongue. The glossopharyngeal nerve is the main nerve of taste. Its central processes travel in the tractus solitarius to the nucleus of the tractus solitarius, which also receives gustatory impulses from the anterior two-thirds of the tongue, conveyed by the nervus intermedius (Fig. 4.37). From the nucleus of the tractus solitarius, gustatory impulses ascend to the ventral posteromedial nucleus of the thalamus (VPM) and then onward to the gustatory cortex at the lower end of the postcentral gyrus (Fig. 4.37).
Visceral afferent fibers of the glossopharyngeal nerve belong to the pseudounipolar cells of the superior (intracranial) ganglion, while those of the vagus nerve are derived from its inferior ganglion. These fibers conduct sensory impulses from the mucosa of the posterior third of the tongue, the pharynx (CN IX), and the thoracic and abdominal viscera (CN X) (Figs. 4.48 and 4.49).
Somatic Afferent Fibers of CN IX and X
Pain and temperature fibers. Nociceptive and probably also temperature-re- lated impulses from the posterior third of the tongue, the upper portion of the pharynx, the eustachian tube, and the middle ear travel by way of the glos-
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Cranial Nerves · 203 4
sopharyngeal nerve and the superior (intracranial) ganglion to the nucleus of the spinal tract of the trigeminal nerve. Impulses of this type from the lower portion of the pharynx, the skin behind the ear and in part of the external auditory canal, the tympanic membrane, and the dura mater of the posterior fossa arrive at the same brainstem nucleus by way of the vagus nerve and its superior ganglion (the jugular ganglion).
Fibers for touch perception (somatosensory fibers) from the areas just named probably terminate in the principal sensory nucleus of the trigeminal nerve. Somatosensory impulses ascend from this nucleus in the medial lemniscus to the thalamus, and thence to the postcentral cortex.
Spinal Roots of the Accessory Nerves (CN XI)
The spinal portion of the accessory nerve is purely motor and arises in a cell column in the ventrolateral portion of the anterior horn, extending from C2 down to C5 or C6 (Fig. 4.50). The root fibers climb one or two segments in the lateral funiculus and then exit the spinal cord between the anterior and posterior roots, just dorsal to the denticulate ligament. They then ascend in the subarachnoid space and join with root fibers from higher levels to form a common trunk, which enters the skull through the foramen magnum and unites, over a short stretch, with the cranial roots of the accessory nerve. As the accessory nerve passes through the jugular foramen, the spinal portion splits off again as the external branch (ramus externus), while the cranial portion joins the vagus nerve. The external branch then descends into the nuchal region to innervate the sternocleidomastoid and trapezius muscles. It is joined along its course by spinal somatic efferent fibers from C2 through C4.
The literature offers conflicting views regarding the relative importance of the accessory nerve and spinal nerves C2 through C4 in the innervation of the trapezius muscle. Some authors assert that the accessory nerve mainly supplies the lower portion of the muscle, others that it mainly supplies the upper portion. Lesions of the accessory nerve are followed by atrophy mainly affecting the upper portion of the trapezius muscle.
The external branch also contains a few afferent fibers that conduct proprioceptive impulses toward the brainstem.
Lesions Affecting the Spinal Roots of the Accessory Nerve
Causes. The most common cause of a peripheral extracranial accessory nerve palsy is iatrogenic injury as a complication of surgical procedures in the lateral triangle of the neck (e. g., lymph node biopsy), followed by pressureand radia-
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