Chapter 7 - Lower cranial nerve function
The lower cranial nerves are involved in pharyngeal and laryngeal function as well as in movements of the neck and tongue. Damage usually manifests as problems with speech and swallowing. These nerves arise from the medulla and, in the case of the accessory nerve (CN XI), the spinal cord. These nerves are commonly affected by conditions damaging the medulla, but bilateral damage to corticobulbar connections can create motor problems that affect tongue and pharynx movement and speech.
These two nerves are considered together because they exit from the brain stem side by side, and have similar and frequently side-by-side and overlapping functional and anatomical distributions in the periphery. Also, these nerves connect with many of the same brain stem nuclei (dorsal motor nucleus of the vagus, nucleus ambiguus, nucleus solitarius, spinal nucleus of the trigeminal) and are often damaged together.
The pharynx is innervated by nerves IX and X, with motor and sensory contributions from both. In general, the vagus nerve is motor to the palate elevators and constrictors of the pharynx (as occurs in swallowing and gagging). The glossopharyngeal contains more sensory fibers, including from the posterior part of the tongue and pharynx down to the level of the larynx (where the vagus nerve begins to take over). The entire palate, including the soft palate, has a sensory distribution from the maxillary division of the trigeminal nerve.
Contraction of the paired and fused muscles of both sides of the soft palate causes superolateral movement vectors (Fig. 7-1). The sum vector is an upward, midline movement of the palate to seal the nasopharynx when swallowing and making certain sounds (such as "guh"). When examining palate elevation, look at the point of attachment of the uvula to see if it remains in the midline. Also, if there is deviation, inspect the palate to make sure this is not simply due to scaring of the soft palate due to prior throat surgery. If the vagus nerve of one side is damaged (e.g., by a tumor at the jugular foramen), the palate elevates asymmetrically, being pulled up toward the strong side (i.e., away from the weak side of the palate, Fig. 7-2).
If both sides of the palate are weak, as can occur in certain muscle diseases or if the vagus is damaged bilaterally (such as from invasion by a retropharyngeal carcinoma at the base of the skull), the palate does not elevate normally during phonation and a hypernasal quality is imparted to the voice (especially noted when making a "G" sound). Air usually emanates from the nose when the patient tries to puff up the cheeks and liquid tends to regurgitate into the nose when swallowing. Rarely, a similar finding of bilateral weakness can be seen in patients with bilateral supranuclear lesions (such as by bilateral cortical damage or bilateral damage to the corticobulbar tracts). In this case, the patient will often show signs of "pseudobulbar" affect (see Chapt. 5).
The gag reflex involves a brisk and brief elevation of the soft palate and bilateral contraction of pharyngeal muscles evoked by touching the posterior pharyngeal wall. It is tested on the left and the right sides and the reflex response should be consensual (i.e., the elevation of the soft palate should be symmetrical regardless of the side touched). As with all reflexes, the gag reflex has a sensory and a motor limb. The sensory limb is mediated predominantly by CN IX, the motor limb by CN X. Touching the soft palate can lead to a similar reflex response. However, in this case, the sensory limb of the reflex is the trigeminal nerve. In very sensitive individuals, much more of the brain stem may be involved; a simple gag may enlarge to retching and vomiting in some.
The gag response varies greatly from individual to individual but is relatively constant in any one person. In some individuals, this reflex is under such strong voluntary control that probing causes very little or no response. This could make differentiation of normal suppression of the gag from symmetric pathologic depression of motor and/or sensory function difficult. However, actual damage can usually be determined by asking the patient to count to 10 immediately after rapidly swallowing 4 oz. of water. If there were bilateral sensory and/or motor deficit, one would anticipate that fluid would penetrate into the unprotected larynx, producing a "wet voice" often with choking and coughing. The water-swallowing test is also a useful screen in detecting which patients with neurologic deficit are likely to have trouble eating (neurologic disease is more likely to affect swallowing of thin liquids, like water, than it is to affect the swallowing of pudding consistencies, which are easiest).
In glossopharyngeal nerve (sensory) involvement, there will be no response when touching the affected side. With vagal nerve damage, the soft palate will elevate and pull toward the intact side regardless of the side of the pharynx that is touched. If both CN IX and X are damaged on one side (not uncommon), stimulation of the normal side elicits only a unilateral response, with deviation of the soft palate to that side; no consensual response is seen. Touching the damaged side produces no response at all.
The vagus nerve is both the sensory and motor innervation of the larynx. Sensory and motor nerve fibers reach the larynx by different courses, with the superior laryngeal nerve being sensory and the recurrent laryngeal nerve being motor. The recurrent laryngeal nerves take a long, circuitous route before reaching the larynx, with the left nerve passing all the way around the aortic arch. Mediastinal lesions (e.g., carcinoma of the esophagus, cancerous lymph nodes or aortic aneurysms) may be first evidenced by hoarseness due to paralysis of the left vocal cord. The same can be true on either side for malignancies in the neck, such as thyroid cancer, since both the left and right recurrent laryngeal nerves pass posterior to that gland to reach the larynx.
Loss of function of one or both recurrent laryngeal nerves causes "hoarseness". Persistent, painless hoarseness should alert the examiner to the possibility of unilateral or bilateral vocal cord weakness or paralysis. This warrants examination of laryngeal appearance and function. This can either be done by fiberoptic laryngoscopy or by indirect laryngoscopy with a simple curved dental mirror and a light source (a bedside lamp shining over the physician's shoulder or a flashlight held by an assistant) (Fig. 7-3). The mirror must be warmed to prevent fogging. The tongue is held protruded with cotton gauze or is depressed with a tongue blade, and the mirror is then placed face down just below the soft palate, not touching the pharyngeal walls to avoid gagging. It is sometimes useful to spray the nasopharynx with a small amount of a weak topical anesthetic, such as 1% Xylocaine. The mirror allows a view of the superior aspect of the larynx covered by the epiglottis. The patient is asked to say "aah." The epiglottis then uncovers the vocal cords, which should be in a relatively open position. The patient then attempts to say "eee," a high pitched sound, the cords should closely appose unless they are paralyzed on one or both sides (see Fig. 7-3). Laryngoscopy (direct or indirect) is not part of a routine bedside examination, however. It should be done only when phonation changes are persistent.
To differentiate between involvement of the peripheral portion of a cranial nerve and the brain stem portions, it is important to consider whether there is associated involvement of other cranial nerves or evidence of damage to cerebellar functions or the tracts that course through the brain stem (corticospinal or the lemniscal or spinothalamic sensory paths). It is unusual for brain stem lesions to involve one or two cranial nerves in isolation, without also affecting the contiguous long-tract and cerebellar system structures. Motor neuron disease (a degenerative condition involving upper and lower motor neurons) is an exception to this rule, as is poliomyelitis (a rare condition today).
Supranuclear motor pathways to the palate, pharyngeal, and laryngeal musculature are bilateral. Therefore, unilateral lesions, even large strokes, rarely produce any persistent problem with lower cranial nerve function (there may be some transient swallowing trouble). Bilateral acute or subacute loss of hemispheric connections to the medullary nuclei causes difficulty with swallowing, phonating and, initially, a depressed gag reflex. In time, the gag reflex may become uncontrollably hyperactive (as do many other skeletal and autonomic reflexes when they are no longer under supranuclear control).
Both the glossopharyngeal and vagus nerves (CN IX and X) have taste and somatic sensory functions that are not routinely examined. However, the taste function in the glossopharyngeal nerve (CN IX) can be examined if there is suspicion of damage to the nerve (vagus nerve taste function cannot be tested). A saturated solution of salt, a substance normally tasted best by the posterior and lateral taste buds (sweet is tasted best by the anterior and midline tastebuds), is used in the testing with the same technique described for the facial nerve (CN VII, see Chapt. 5).
The glossopharyngeal and vagus nerves (along with the facial nerve) supply tiny sensory branches to the external auditory canal. This extensive overlap (which also includes some contributions from the trigeminal nerve and the 2nd cervical nerve) precludes detecting loss of sensation caused by lesions of any one of these nerves. However, pain in the ear may be a prominent early symptom of irritation of any one of these cranial nerves. If the vagus or glossopharyngeal nerve is involved, the pain often extends into the pharyngeal region, helping to differentiate from the pain of seventh-nerve irritation (which would be confined to the ear and mastoid region). If facial weakness is present, this would be a clue to facial nerve irritation, while depression of the gag reflex would suggest vagus or glossopharyngeal nerve involvement. Trigeminal involvement is differentiated by pain in the face and deficits in sensation in the trigeminal distribution; involvement of the upper cervical nerves is indicated by hypoesthesia or pain in the scalp and upper back of the neck.
The baroreceptor reflex is mediated by sensory fibers in the glossopharyngeal nerve and motor fibers in the vagus nerve. The normal reflex detects increased blood pressure in the carotid sinus, triggering a slowing of the heart and lowering of blood pressure. Because the receptor works as a mechanical transducer, any kind of distortion of the carotid sinus can cause slowing of the pulse and hypotension. Firm massaging of the carotid bifurcation while monitoring pulse and blood pressure is the bedside technique for testing the reflex. However, this is hazardous due to the potential for excessive slowing of the heart and for disrupting any atherosclerotic plaque that might be in the carotid sinus region (potentially producing embolic stroke).
Technically, the accessory nerve (CN XI) has two components: (1) a central branch arising from the medullary nuclei, and (2) a spinal accessory branch arising in the first five to six cervical spinal segments from the lateral portion of the ventral horn. The central branch joins the vagus immediately after leaving the brain stem and is involved in innervation of the laryngeal musculature. We typically consider this component with the vagus nerve and have discussed examination of the larynx and pharynx (above).
The spinal accessory branch has an unusual course. It arises from motor neurons in the upper 6 cervical segments. These neurons send their nerve roots to exit the spinal cord laterally (not with the ventral motor nerve root). The nerve roots that comprise the spinal accessory nerve ascend the vertebral canal adjacent to the lateral side of the spinal cord and they enter the skull by passing upward through the foramen magnum. This nerve then turns laterally to pass through the jugular foramen along with cranial nerves IX and X. The spinal accessory nerve provides the motor innervation of the sternocleidomastoid (SCM) muscle before passing through the posterior triangle of the neck to reach the trapezius muscle (which it also innervates). Cervical nerves provide sensory innervation of these muscles.
When examining the SCM muscle, the bulk and outline of the muscle should be observed. Atrophy is common in damage to the nerve and fasciculations may be seen especially if the motor neurons are diseased. The SCM muscle rotates the head away from the side of contraction. Testing entails having the subject turn their head against the examiner's hand, which is pressed against the patient's chin (Fig. 7-4). The bulk of the muscle is then easily seen and palpated, and its strength can be determined. Having the patient attempt to bring their chin toward their chest can test the left and right SCM as they work together in this action. Paralysis of this muscle will produce weakness, although not complete loss of ability to rotate the head away from the lesion. This is because there are other muscles that are able to partially compensate. For this same reason, the resting head position is usually not affected by isolated SCM paralysis. Rarely, the patient will hold their head turned slightly toward the side of the lesion. The two sternocleidomastoids contracting together will flex the head toward the chest. Bilateral weakness may prevent the patient from lifting their head off a pillow and the head may be inclined posteriorly for lack of flexor tone. Bilateral weakness suggests muscle or neuromuscular disease.
Spasmodic torticollis is a condition that often affects the tone of the SCM muscle, although it can affect several other cervical muscles as well. In this condition, there is an excessive activity of unknown etiology in one (rarely both) of the sternocleidomastoids. This results in an obvious deviation of head position. The subject's head is spasmodically turned away from the involved muscle, which usually shows hypertrophy. One rather striking observation is that the patient can often terminate the spasm by simply touching the opposite side of the chin or cheek. The head drifts back into its dystonic position once the touch is removed.
The spinal accessory nerve innervates the trapezius muscles, which elevate the shoulders and rotate the scapula upward during abduction of the arm. Denervation is evidenced by atrophy and often fasciculations. The shoulder droops on the side of the weak muscle and there is downward displacement of the scapula posteriorly. Shrugging the shoulders against resistance is the standard way of testing the upper trapezius (Fig. 7-4).
Both the SCM and the trapezius muscles are under voluntary control, requiring some input from the corticospinal system. The projections from the cerebral cortex to the motor neurons innervating the SCM are bilateral. Therefore, even large unilateral lesions do not produce weakness of the SCM or any deficits in head turning. However, in the case of corticospinal innervation of trapezius motor neurons there is usually a contralateral predominance. This contributes to mild to moderate contralateral weakness of shoulder elevation following large, unilateral injuries of corticospinal systems. This is rarely very severe, however.
The hypoglossal nerve (CN XII) has an entirely motor function, innervating the muscles of the tongue. It originates from the columns of motor neurons located near the midline in the dorsal aspect of the medulla. The nerve exits the ventral side of the medulla as a row of small nerve rootlets adjacent to the pyramid. After a short course through the subarachnoid space, the rootlets come together as a single nerve that passes through the hypoglossal foramen in the base of the skull. Ultimately it reaches the tongue and innervates the intrinsic and extrinsic tongue muscles.
Of course, the tongue is under voluntary control. Accordingly, corticobulbar pathways activate hypoglossal motor neurons. As with most cranial nerves, these corticobulbar projections are bilateral, although there is a slight contralateral predominance. Therefore, large lesions to the corticobulbar system, such as large strokes, can produce slight weakness of the contralateral tongue.
Weakness of the tongue manifests itself as a slurring of speech. The patient complains that their tongue feels "thick", "heavy", or "clumsy." Lingual sounds (i.e., l's, t's, d's, n's, r's, etc.) are slurred and this is obvious in conversation even before direct examination.
Examination of the tongue first involves observation for atrophy and fasciculations. With supranuclear lesions, weakness, frequently mild, is not accompanied by loss of muscle mass or fasciculations. Lesions of the nerve (e.g., hypoglossal neurolemmoma, nasopharyngeal tumor along the base of the skull, basal skull fracture) or of the nucleus in the brain stem (e.g., medullary stroke, motor neuron disease or bulbar poliomyelitis) the tongue displays weakness, atrophy and, possibly, fasciculations on the side of the involvement (Fig. 7-5). Atrophy and fasciculations in combination suggest disease or damage to the motor neurons of the brain stem, but can be seen with peripheral nerve damage as well. Fasciculations are fine, random, multifocal twitches of muscle. They are evaluated by observing the tongue while it is at rest in the floor of the mouth. They are best seen along the lateral aspect of the tongue. Protrusion frequently causes a fine tremor in the normal tongue, which can obscure or mimic fasciculations. Simply having the patient protrude their tongue in the midline tests strength of the tongue. The normal vectors of protrusion are illustrated in Figure 7-5. When one side of the tongue is weak, it protrudes toward the weakened side (Fig. 7-5). A repetitive or complex lingual sound (e.g., "la la la la" or "Methodist artillery") often shows impediment when any part of the vocal apparatus is affected (e.g., Broca's region, motor cortex, basal ganglia, cerebellum, brain stem, nucleus, or nerve).
The most common process causing major involvement of the hypoglossal nerve is motor neuron disease (amyotrophic lateral sclerosis). This is a degenerative disease that has a predilection for early and severe involvement of the hypoglossal motor neurons. The involvement is almost always bilaterally symmetrical. Unilateral damage of the hypoglossal nerve can be produced by tumors or trauma involving the base of the skull, whereas stroke can damage corticobulbar projections and is the usual cause of unilateral supranuclear dysfunction.
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Define the following terms:dysarthria, dysphonia, dysphagia.
7-1. What are the main functions of the glossopharyngeal nerve?
7-2. What would be the effect on soft palate movement of unilateral damage to the vagus nerve?
7-3. What would be the effect of unilateral damage to the vagus nerve on larynx function?
7-4. Describe the course of the spinal accessory nerve.
7-5. What does the spinal accessory nerve innervate?
7-6. What does the hypoglossal nerve innervate?
7-7. What would be the findings in unilateral damage to the hypoglossal nerve?
7-8. What would be the effect of a large stroke in the motor cortex on tongue movement?
7-9. What is the reflex pathway of the gag reflex?
7-10. What is the reflex pathway of the cough reflex?
7-11. What is the reflex pathway of the baroreceptor reflex?