Neurology: A Clinician's Approach (Cambridge Medicine (Paperback)), 1st Ed.

8. Facial weakness, dysarthria, and dysphagia

Lower brainstem (bulbar) symptoms

The unifying feature of facial movement, speech, and swallowing is that the lower motor neurons that control them all lie within the pons and medulla. Because these motor neurons occupy a small volume within the brainstem, a tiny focus of ischemia, inflammation, or neoplasia often leads to simultaneous facial weakness, dysarthria, and dysphagia.

Facial weakness

Anatomy

Figures 8.1 and 8.2 are rough schematics of the innervation of the muscles of facial expression and associated structures. Fibers derived from the motor cortex descend through the corona radiata, the internal capsule, and the cerebral peduncle to reach the contralateral facial nucleus. Most of the fibers from the lower one-third of the motor cortex project to neurons within the contralateral facial nucleus, while direct cortical projections to the components of the facial nucleus that control the upper part of the face are less robust.1 The facial nerve fascicles travel medially through the pons, curving around the abducens nucleus. They then turn laterally to emerge from the ventrolateral pons as the facial nerve. The nerve courses through the cerebellopontine angle and enters the internal auditory meatus. In both locations, the facial nerve lies in proximity to the vestibulocochlear nerve. The facial nerve then passes from the internal auditory meatus to the facial canal. There are two clinically important branches that arise from the nerve within the facial canal: the nerve to the stapedius, which helps to dampen the vibration of the stapes, and the chorda tympani, which contains taste fibers from the anterior two-thirds of the tongue. It is important to note that the neurons for these taste fibers are actually in the nucleus of the solitary tract, not the facial nucleus. The facial nerve emerges from the facial canal via the stylomastoid foramen to innervate the muscles of facial expression.

Examination of the functions of the facial nerve

Accurate localization of facial weakness requires comprehensive neurological examination of the four clinically important functions of the facial nerve:

1. Examine the upper half of the face by asking the patient to lift their eyebrows, wrinkle their forehead, and close their eyes tightly. In patients with severe upper facial weakness, the sclera will be visible when the patient attempts to close the eye (Bell’s phenomenon).

2. Examine the lower half of the face by asking the patient to show their teeth and hold air in their cheeks.

3. Although the stapedius cannot be tested at the bedside, ask the patient whether loud or high-pitched noises are particularly irritating.

4. Finally, check taste on the anterior two-thirds of the tongue by using a sweetened fruit drink. This works much more effectively than sugar. Pipette a very small volume of the fruit drink into a straw and have the patient close their eyes. Place one drop on the affected side of the tongue first and ask the patient whether they can taste the sweetness. It is important that the patient keeps their mouth open during this study to prevent spread of the fruit drink to the other side of the tongue or to the posterior taste buds. If they do not taste anything on the affected side, place one drop of fruit drink on the normal side of the tongue to verify that taste is intact on that side.

Differentiating between central and peripheral facial weakness

Acute-onset, unilateral facial weakness is a common problem for which emergency room physicians often request neurological consultation. The principal question is whether the facial weakness is due to stroke and requires inpatient evaluation, or to Bell’s palsy,

Figure 8.2

Figure 8.2 Simplified schematic of the facial nerve. CP angle = cerebellopontine angle. See text for details.

Figure 8.1

Figure 8.1 Schematic showing upper motor neuron control of facial movement. The portion of the facial nucleus that innervates the lower half of the face receives predominantly contralateral (solid line) but also some ipsilateral (dashed line) innervation from the lower one-third of the motor cortex. The portion of the facial nucleus that innervates the upper half of the face receives little cortical input.

which may be managed on an outpatient basis. For clinical purposes, the most reliable way to differentiate between central and peripheral lesions is by examining the upper half of the face: cortical lesions do not tend to affect the upper half of the face, as the motor neurons that innervate the upper half of the face receive scant cortical input. Keep in mind, however, that stroke within the pons (a lesion of the CNS that affects the facial nerve fascicles) may produce facial weakness that appears to be peripheral in origin. Neighborhood signs and symptoms, therefore, are often necessary to localize the source of facial weakness:

1. Hand weakness ipsilateral to facial weakness strongly suggests pathology involving the contralateral motor cortex, as the face and hand areas are adjacent to each other on the motor homunculus.

2. Paralysis of ipsilateral conjugate gaze or of ipsilateral eye abduction places the lesion in the pons where the facial nerve fascicles cross the abducens nucleus and nerve. Contralateral limb weakness due to a lesion of the adjacent corticospinal tract is often present.

3. The combination of facial weakness and dysphagia point to a lesion involving both the pons and medulla.

4. Ipsilateral hearing loss points to a lesion involving the facial nerve in combination with the vestibulocochlear nerve in the cerebellopontine angle (Chapter 23) or internal auditory meatus.

5. Ipsilateral hyperacusis (intolerance of high-pitched sounds) accompanies facial nerve lesions proximal to the departure of the nerve to the stapedius, provided that the vestibulocochlear nerve is spared.

6. Loss of taste to the anterior two-thirds of the tongue accompanies lesions within the cerebellopontine angle, internal auditory meatus, or facial canal proximal to the takeoff of the chorda tympani. This is the most common pattern of deficits in Bell’s palsy.

7. Isolated weakness of the muscles of facial expression of both the upper and lower halves of the face points to a lesion of the nerve in the facial canal distal to the takeoff of the chorda tympani.

Causes of facial weakness

Supranuclear lesions

Cortical lesions, because of the organization of the motor homunculus, usually produce contralateral facial weakness accompanied by hand weakness. Lesions within the corona radiata or internal capsule are usually accompanied by arm and leg weakness. In both cases, the upper half of the face should be spared. Strokes are the most common cause of supranuclear facial lesions.

Pontine lesions

Pontine lesions (most commonly strokes, tumors, or demyelinating diseases) cause flaccid facial palsy that mimics Bell’s palsy. Although the lesion is in the brainstem, fascicular lesions within the pons are technically peripheral in nature. As noted above, the facial nerve fascicles first run in proximity to the abducens nucleus and then the corticospinal tract. Clues suggesting that pontine stroke is the source of facial weakness therefore include deviation of the eyes to the opposite side, ipsilateral lateral rectus palsy, or contralateral hemiparesis. Because the fascicles of the facial nerve in the pons do not contain taste fibers (which run in the chorda tympani), finding preserved taste in patients with fascicular lesions and impaired taste in those with peripheral nerve lesions is an important clinical tool to differentiate between the two lesion sites.

Bell’s palsy and other lesions of the facial nerve proper

Bell’s palsy is a facial nerve disorder characterized by profound unilateral facial weakness that develops over the course of several hours. A patient with Bell’s palsy may note difficulty with closing their eye and may slur their speech or lose food from the corner of their mouth. They may also note reduced taste, or feel that loud noises are particularly intense. Although the patient may report that their face feels numb, this abnormal sensation is actually caused by difficulty moving the face rather than by specific involvement of sensory fibers in the trigeminal nerve. Bell’s palsy is considered an idiopathic disease, but most cases are likely caused by herpes zoster or herpes simplex virus infections of the facial nerve. Indicators of “nonidiopathic” Bell’s palsy include headache, fever, involvement of other cranial nerves, and bilateral facial palsies, all of which should prompt further investigation including lumbar puncture and testing for Lyme disease, sarcoidosis, HIV, and Sjögren’s syndrome.

The mainstays of treatment of Bell’s palsy are steroids and antiviral agents such as acyclovir. The most recently published large study, however, suggests that steroids improve recovery in patients with Bell’s palsy, but that the addition of valacyclovir does not offer an additional benefit.2 Treat patients diagnosed within 3 days of symptom onset, therefore, with prednisone 60–80 mg for 1 week. Provide artificial tears and an eye patch to prevent corneal abrasions for patients who have difficulty with closing the eye. Prognosis of Bell’s palsy depends on the initial degree of deficits: approximately 60% of patients with complete paralysis make a full recovery, compared with 95% with incomplete paralysis at diagnosis.3 Younger patients and those whose recovery begins within 3 weeks of onset also have better outcomes.

Incidental facial weakness

Incidental mild facial asymmetry is a frequent finding on neurological examination. The question arises as to whether this facial weakness is a component of the process that brings the patient to neurological attention (which becomes especially relevant in patients with suspected multifocal disease such as multiple sclerosis) or whether it is a preexisting, incidental finding. Asking the patient or available family members whether the facial weakness is old is frequently unhelpful. The first way to determine whether facial asymmetry is new or old is to examine old pictures such as a driver’s license photo or family portrait. The second is to take advantage of synkinesis: axon sprouts that form during healing of facial nerve injuries often innervate both the orbicularis oculi and orbicularis oris, so that when a patient with an old facial nerve palsy blinks, the ipsilateral corner of the mouth rises.

Dysarthria

Dysarthria is defined as a disorder of the mechanical production of speech. Although it is distinct from aphasia, it is common in patients with nonfluent aphasias such as Broca’s aphasia and transcortical motor aphasia (Chapter 3). The following are the most important subtypes of dysarthria4:

Spastic (upper motor neuron) dysarthria

The upper motor neurons for speech are found in the mouth area of the motor cortex in the precentral gyrus. Fibers descend through the subcortical white matter and internal capsule to reach the lower motor neurons in the pons and medulla. Spastic dysarthria has a harsh, strained, mechanical quality. Words are spoken slowly and with great effort. On examination, the jaw jerk and gag reflex may be brisk. The three main causes of spastic dysarthria are amyotrophic lateral sclerosis (Chapter 10), progressive supranuclear palsy (Chapter 13), and the pseudobulbar state caused by chronic ischemic disease of the subcortical white matter (see below).

Flaccid (lower motor neuron) dysarthria

Patients with flaccid dysarthria have thick, muddy speech, usually secondary to generalized disorders of the motor neuron, neuromuscular junction, or muscle. Common causes include amyotrophic lateral sclerosis, bulbar myasthenia gravis, oculopharyngeal muscular dystrophy, and inflammatory myopathy. Individual neuropathies of the facial, hypoglossal, and vagus nerves may also cause flaccid dysarthria, but are less common than these generalized disorders. The standard technique to differentiate among the mononeuropathies that lead to dysarthria is to test labial, lingual, and guttural sounds:

• The facial nerve innervates the lips and other muscles of facial expression. Facial nerve lesions cause the greatest difficulties producing labial sounds such as “puh.”

• The hypoglossal nerve innervates the tongue. Hypoglossal nerve lesions lead to difficulties producing lingual sounds such as “tuh.”

• The vagus nerve innervates the laryngeal muscles. Lesions of this nerve produce difficulties with making guttural sounds such as “kuh” or “guh.” Injury to the vagus nerve, and more specifically its recurrent laryngeal branch, however, more often leads to hoarseness.

Extrapyramidal dysarthrias

Tremor, myoclonus, chorea, and tics are manifestations of extrapyramidal disease that may affect speech in fairly predictable ways. Moderate to severe Parkinson’s disease is usually accompanied by slow, hypophonic (reduced in volume), and monotonous speech. Spasmodic dysphonia is a dystonic disorder of the laryngeal muscles: in adductor spasmodic dysphonia, speech is cut off and choppy, while in abductor spasmodic dysphonia, speech is breathy and whispery. Spasmodic dysphonia, similar to the other focal dystonias, may be treated with botulinum toxin injections. Multisystem atrophy may produce a fairly unique, high-pitched quivering dysarthria.

Scanning (cerebellar) dysarthria

This is the classic speech pattern in patients with disease of the cerebellum and its connections. The speech has a halting, uncoordinated pattern with awkward volume modulations and separations between words and phrases. The best-known cause of scanning speech is advanced multiple sclerosis.

“Slurred” speech

Unfortunately, many patients with acute dysarthria do not fit neatly into one of the four common subtypes described in this section, and are most appropriately labeled as having “slurred speech.” This is common in patients with substance intoxication or metabolic disturbances. Although many patients with stroke have slurred speech, it is rarely an isolated problem, as it is usually accompanied by facial or hand weakness or by a language disturbance.

Dysphagia

Swallowing is divided into oral, pharyngeal, and esophageal phases. The first important task in evaluating dysphagia, therefore, is to determine which phase is dysfunctional by asking the patient where the food gets stuck. If they point to the mouth (oral phase) or back of the throat (pharyngeal phase), the dysphagia may be due to neurological dysfunction. If they point instead to the sternum, they have an esophageal problem and should be referred to a gastroenterologist. The other piece of important information that must be determined from the history is which consistencies are difficult for the patient to swallow: dysphagia for liquids or nasal regurgitation during swallowing strongly suggests neurological disease. In some cases, it may be difficult to determine whether dysphagia is caused by oral or pharyngeal phase dysfunction, and video swallowing studies are needed to localize the problem. Many of the causes of dysphagia discussed below are irreversible, and patients must be treated with dietary restrictions or, when dysphagia is severe, feeding tubes.

Oral phase dysphagia

Trigeminal nerve lesions

The muscles of mastication are innervated by the trigeminal nerve. Because trigeminal nerve lesions rarely produce dysphagia, they will not be discussed further.

Hypoglossal nerve lesions

Supranuclear control of tongue movement is derived from the lower one-third of the precentral gyrus. The lower motor neurons are located in the dorsomedial medulla (Chapter 21Figure 21.2). These motor neurons give rise to the hypoglossal nerve fascicles, which course anteriorly through the medulla, passing through the corticospinal tracts to emerge from the medulla. After emerging from the hypoglossal canal, the nerve takes a long course through the neck, looping around the internal carotid artery, external carotid artery, and internal jugular vein before supplying the muscles of the tongue. Most tongue muscles have bilateral cortical innervation, while the genioglossus has predominantly contralateral innervation.

When examining a patient with suspected tongue weakness or oral phase dysphagia, first observe the tongue as it rests in the floor of the mouth. Look for atrophy, particularly scalloping at the edges. Also look for fasciculations or wriggling movements of the tongue. Both atrophy and fasciculations may suggest motor neuron disease. Next, ask the patient to stick their tongue out straight. Deviation to one side suggests either an ipsilateral hypoglossal nuclear or nerve lesion or a contralateral hemispheric lesion. Finally, ask the patient to push their tongue into their cheek. Look for asymmetries in resistance as you press the tongue inwards through the cheek.

The first common localization of tongue weakness is the hypoglossal motor neurons themselves, as in amyotrophic lateral sclerosis. Although tongue weakness is common in this disorder, it is almost always accompanied by other signs, as discussed in Chapter 10. Infarction of the hypoglossal nerve fascicles within the medial medulla is the next common cause of tongue weakness. Because the fascicles cross through the corticospinal tract, patients with fascicular lesions usually have contralateral hemiparesis (medial medullary syndrome of Dejerine). Hypoglossal nerve lesions within the posterior fossa are often due to trauma or to mass lesions, and affect the glossopharyngeal, vagus, and accessory nerves simultaneously. Finally, lesions of the hypoglossal nerve within the neck may occur as a result of carotid endarterectomy or retropharyngeal mass.

Pharyngeal phase dysphagia

Supranuclear lesions (the pseudobulbar state)

Because the motor neurons of the nucleus ambiguus (see below) receive supranuclear inputs from both hemispheres, unilateral lesions do not produce dysphagia. Bilateral lesions, however, are quite common in patients with the “pseudobulbar state” characterized by dysphagia, spastic dysarthria, and emotional incontinence. A patient with emotional incontinence laughs or cries in situations that are not congruent with their actual emotional state, so-called pathological laughter and crying. For example, they may burst out laughing when given devastating news or start to cry unexpectedly when they are told the time. Attacks are frequent and spontaneous. Although the patient is not emotionally bothered by the symptoms, attacks may pose challenges in social settings. One examination clue to the diagnosis of the pseudobulbar state is the presence of a hyperactive gag reflex. The pseudobulbar state has two basic causes. The first is bilateral subcortical white matter disease caused by ischemia or demyelination. The second is upper motor neuron degeneration secondary to amyotrophic lateral sclerosis or primary lateral sclerosis. Unfortunately, treatments for dysphagia related to supranuclear lesions are limited. Pathological laughter and crying, however, may improve with antidepressants.

Nuclear lesions

The nucleus ambiguus within the lateral medulla contains the motor neurons that mediate swallowing. Isolated lesions at this level are rare, and adjacent brainstem structures including the vestibular nuclei, Horner’s tract, the spinal trigeminal tract and nucleus, and the spinothalamic tract are often involved simultaneously. The classic lesion of the lateral medulla is Wallenberg’s syndrome (Chapter 21). Other causes of nuclear lesions include amyotrophic lateral sclerosis, tumors, demyelinating disease, and syringobulbia.

Glossopharyngeal and vagus nerve lesions

The glossopharyngeal and vagus nerves contain motor, sensory, and parasympathetic fibers. They are often discussed together because they arise from shared brainstem structures, lie in proximity to each other as they exit the brainstem, and perform similar functions. The swallowing fibers that contribute to both nerves are derived from the nucleus ambiguus. The nerves may be affected in isolation or in combination:

• Because the stylopharyngeus is the only pharyngeal muscle innervated by the glossopharyngeal nerve, isolated nerve lesions produce only mild, if any, dysphagia. Clinical signs of glossopharyngeal nerve lesions include loss of taste on the posterior one-third of the tongue and a decreased gag reflex.

• Isolated vagus nerve lesions lead to severe dysphagia, as the vagus nerve innervates all of the pharyngeal muscles with the exception of the stylopharyngeus. Because the vagus nerve also innervates the laryngeal muscles, the patient also develops a hoarse dysarthria. Clinical signs of vagus nerve lesions include decreased ipsilateral palate elevation and a decreased gag reflex.

• After emerging from the lateral medulla, the glossopharyngeal and vagus nerves exit the skull with the accessory nerve through the jugular foramen. In addition to the signs and symptoms of glossopharyngeal and vagus nerve lesions, jugular foramen processes lead to signs of accessory nerve dysfunction including weakness of the ipsilateral sternocleidomastoid (weakness of head turning in the contralateral direction) and trapezius (weakness of ipsilateral shoulder shrug).

Neuromuscular junction lesions

Bulbar myasthenia gravis is an important, treatable cause of dysphagia that often leads to urgent hospitalization. While the classic temporal profile of myasthenia gravis is one of fluctuating deficits, patients with severe bulbar myasthenia gravis usually progress rapidly over hours to days. Agents such as oral pyridostigmine are obviously ineffective for bulbar myasthenia gravis, as the patient cannot swallow their medications. Intravenous pyridostigmine (1–2 mg) may be used as a temporizing measure, but almost all patients with severe bulbar myasthenia gravis should be treated with intravenous immunoglobulin or plasmapheresis as if they were in myasthenic crisis (Chapter 12). The other important cause of pharyngeal dysphagia secondary to neuromuscular junction dysfunction is botulism (Chapter 12). Lambert–Eaton myasthenic syndrome does not usually impair swallowing.

Myopathic lesions

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominantly inherited disorder characterized by dysphagia and extraocular movement weakness. It is most common in people of French–Canadian background. Unlike most muscular dystrophies, it typically begins in middle age. The diagnosis is straightforward when there is a positive family history. If this is not available, muscle biopsy showing distinctive rimmed vacuoles or genetic testing may help to establish the diagnosis. Although cricopharyngeal myotomy may help some patients with OPMD, the treatment of this condition is largely supportive.

Inflammatory myopathies and mitochondrial myopathies may on occasion present with dysphagia as the first symptom.

Extrapyramidal lesions

Bradykinesia of swallowing is often a later feature of extrapyramidal disorders such as Parkinson’s disease. It may be present earlier in the course of atypical parkinsonian syndromes such as multisystem atrophy and progressive supranuclear palsy.

References

1. Jenny ABSaper CB. Organization of the facial nucleus and corticofacial projection in the monkey: a reconsideration of the upper motor neuron facial palsy. Neurology 1987;37:930–939.

2. Engstrom MBerg TStjernquist-Desatnik A, et al. Prednisolone and valaciclovir in Bell’s palsy: a randomised, double-blind, placebo-controlled, multicentre trial. Lancet Neurol 2008;7:993–1000.

3. Peitersen E. Bell’s palsy: the spontaneous course of 2,500 peripheral facial nerve palsies of different etiologies. Acta Otolaryngol 2002;S549:4–30.

4. Darley FLAronson AEBrown JRMotor Speech Disorders. Philadelphia: W. B. Saunders Company; 1975.