CHAPTER 16

The Facial Nerve

Anatomy and Physiology

The facial, or seventh, cranial nerve (CN VII) is a predominantly motor nerve that innervates the muscles of facial expression and the muscles of the scalp and ear, as well as the buccinator, platysma, stapedius, stylohyoid, and posterior belly of the digastric. In addition, it carries parasympathetic secretory fibers to the submandibular and sublingual salivary glands, the lacrimal gland, and to the mucous membranes of the oral and nasal cavities. It has some sensory functions; the most important is to mediate taste from the anterior two-thirds of the tongue. It also conveys exteroceptive sensation from the eardrum and external auditory canal, proprioceptive sensation from the muscles it supplies, and general visceral sensation from the salivary glands and mucosa of the nose and pharynx. Anatomically, the motor division of the nerve is separate from the sensory and parasympathetic portions. In its course from its exit from the pons until its terminal arborizations, several important branches are given off in the following order: the greater (superficial) petrosal nerve, the nerve to the stapedius, and the chorda tympani.

The nerve may be understood as a series of segments: a brainstem or intramedullary segment (from the brainstem nuclei to the exit point), a segment from the exit point to the entrance into the internal auditory canal (IAC) or cisternal segment, a meatal or canal segment (course through the IAC) to the entrance to the facial canal, a labyrinthine segment (from there to the geniculate ganglion), a short horizontal segment (from the geniculate ganglion to the pyramidal eminence of the posterior wall of the tympanic cavity), a mastoid segment (from the pyramidal eminence to the stylomastoid foramen), and an extratemporal or peripheral segment (from the stylomastoid foramen to the pes anserinus). These segments are discussed in more detail below. The associated findings in CN VII palsy often allow identification of the involved segment.

The Motor Portion

The supranuclear innervation to the muscles of facial expression arises from the lower third of the contralateral precentral gyrus in the facial area of the motor homunculus. Fibers descend in the corticobulbar tract through the corona radiata, genu of the internal capsule, medial portion of the cerebral peduncles and into the pons, and then decussate to converge on the facial nuclei. The portion of the nucleus that innervates the lower half to two-thirds of the face has predominantly contralateral supranuclear control; the portion that innervates the upper third to half has bilateral control. The muscles of the lower face may also receive more abundant cortical innervation than the muscles of the upper face and forehead. This scheme applies to voluntary facial movements. Unconscious, emotional, involuntary supranuclear control follows a different pathway. Patients with lesions in certain parts of the nervous system may have different degrees of involvement of the voluntary and involuntary systems (see below).

Studies in humans and nonhuman primates show distributed motor control of facial expression with at least five cortical regions involved: the primary motor cortex, the ventral lateral premotor cortex, the supplementary motor area, and the cingulate cortex. Recent investigations confirm the long established paradigm of bilateral control of the upper face and mainly contralateral control of the lower face. The primary motor cortex, the ventral lateral premotor cortex, and the supplementary motor area are essential for voluntary control of facial expressions. The cingulate cortical areas receive input from different structures of the limbic system and are important for emotional facial expression.

Although most corticobulbar fibers to the facial nuclei decussate in or rostral to the pons, some descend in the aberrant pyramidal tract to medullary levels, decussate there, and ascend contralaterally in the dorsolateral medulla to reach the facial nucleus. The aberrant pyramidal tract is a normal descending fiber tract that leaves the pyramidal tract in the crus cerebri and travels in the medial lemniscus to the upper medulla. Involvement of the aberrant pyramidal tract explains the occurrence of ipsilateral upper motor neuron facial palsy in the lateral medullary syndrome.

A study using transcranial magnetic stimulation to investigate the corticofacial projections found that in the majority of patients, the corticofacial fibers traveled in the base of the pons and crossed at the level of the facial nucleus. But in some individuals, corticofacial fibers formed an “aberrant bundle” in a paralemniscal position at the dorsal edge of the pontine base. In other patients, the corticofacial fibers looped down into the ventral upper medulla, crossed the midline, and ascended in the dorsolateral medullary region ipsilateral to the facial nucleus. The findings suggest that facial paresis because of a brainstem lesion may present as contralateral supranuclear facial paresis by a lesion of the cerebral peduncle, pontine base, the aberrant bundle, and the ventral medulla. Supranuclear facial paresis ipsilateral to the lesion side may result from a lesion in the lateral medulla, and facial paresis of the supranuclear type may be imitated by a lesion of the peripheral facial nerve in the dorsolateral medulla with involvement of the lower pons. The facial nuclei also receive bilateral extrapyramidal, basal ganglia, and hypothalamic innervations that are concerned with maintaining facial muscle tone and with automatic and emotional movements.

The facial nucleus is special visceral efferent, or branchiomotor; it innervates the muscles of the second branchial arch. It lies deep in the tegmentum of the caudal pons, anteromedial to the nucleus of the spinal tract of CN V, anterolateral to the nucleus of CN VI, and posterior to the superior olivary nucleus (Figures 11.6 and 14.8). The facial motor nucleus has lateral, medial, and dorsal subnuclei, arranged in columns. The subnuclear innervation pattern is not as well worked out as for the oculomotor nucleus, but the lateral subnucleus is thought to innervate the lower facial muscles and buccinators; the medial subnucleus the posterior auricular, platysma, and occipital muscles, and probably the stapedius; and the dorsal subnucleus the upper facial muscles via the temporal, orbital, and zygomatic branches. Other schemes of organization have been postulated.

Axons of the facial nerve arise from the dorsal surface of the nucleus and travel dorsomedially, moving up and around to encircle the abducens nucleus and forming the internal genu of the facial nerve. The internal loop of CN VII fibers around the CN VI nucleus forms the facial colliculus, a bump in the rhomboid fossa in the floor of the fourth ventricle, a prominent landmark for surgeons working in the area (Figure 11.4). The facial nucleus is in a somewhat aberrant position more anterolaterally than expected, even considering its branchial arch relationships. In embryonic life, the nucleus is more dorsal and medial, near the CN VI nucleus, but with maturation moves to its adult position trailing its axons behind. In their course, the facial nerve axons run in proximity to the nucleus and fibers of CN VI, the pontine paramedian reticular formation, CN V, and CN VIII as well as the descending and ascending long tracts that course through the pons.

The facial nerve has two components, the motor root, which makes up about 70% of the fibers, and the sensory root, which accounts for 30%. The sensory root forms the nervus intermedius (NI) and contains both sensory and autonomic fibers. The autonomic fibers run near the incoming sensory fibers through the pons. The intrapontine filaments of CN VII thus consist of exiting branchiomotor and parasympathetic fibers, and incoming sensory fibers (Figure 16.1).

FIGURE 16.1 Components of the facial nerve in the pons. (Modified from Kiernan JA. Barr’s The Human Nervous System: An Anatomical Viewpoint. 9th ed. Philadelphia: Wolters Kluwer Health/Lippincott, Williams & Wilkins, 2009, with permission.)

CN VII exits the pons laterally at the pontomedullary junction, just caudal to the roots of CN V between the olive and the inferior cerebellar peduncle (Figure 11.3). The NI is a small bundle that usually leaves the pons closer to CN VIII than CN VII and runs between the larger trunks across the cerebellopontine angle (CPA). In about 20% of specimens, the NI is not identifiable as a separate structure in the CPA. At the entrance to the IAC, the facial nerve motor root lies in a groove on the anterosuperior surface of the vestibulocochlear nerve, with the NI in between. In this segment, CN VII is a paler white color than CN VIII. The facial nerve at this point lies in close proximity to the anterior inferior cerebellar artery (AICA). In some individuals, the AICA loops down into the IAC. As with the vaginal sheaths of the optic nerve, the subarachnoid space extends along the facial nerve to the geniculate ganglion.

At the bottom or lateral end of the IAC, the nerve pierces the meninges and enters the facial canal, or fallopian aqueduct. The point of entry is the narrowest portion of the canal. The facial nerve and the NI merge as the nerve enters the canal. In traversing the facial canal, the nerve makes two abrupt, tortuous turns, creating two external genus. In its course through the petrous bone, from its entrance into the facial canal until its exit from the stylomastoid foramen, the nerve has three segments: labyrinthine, horizontal or tympanic, and mastoid or vertical. The labyrinthine segment lies laterally between the cochlea and vestibule, toward the medial wall of the tympanic cavity, running perpendicularly to the long axis of the petrous pyramid. The labyrinthine segment ends at the first external genu where the geniculate ganglion lies. At this point, the nerve turns abruptly and runs horizontally for about 1 cm (the horizontal or tympanic segment), then turns backward and arches downward behind the tympanic cavity (mastoid or vertical) segment. The branch to the stapedius muscle arises from the distal tympanic or upper end of the mastoid segment. At the end of the tympanic segment, the nerve encounters the second external genu as it makes a 90-degree turn to enter the mastoid segment. The mastoid segment then descends toward the stylomastoid foramen, gives off the chorda tympani about 6 mm before its exit, and emerges from the stylomastoid foramen. The tight confines of the bony canal may make the nerve particularly vulnerable to damage from inflammation and edema, a point of possible significance in some CN VII neuropathies (see below). In patients with Bell’s palsy, the involved side usually correlates with the side of the narrower facial canal as determined by high-resolution computed tomography (CT). CN VII runs along with the labyrinthine branch of the AICA, but there is evidence to suggest that it is less well vascularized in its intrapetrous segment, particularly in the labyrinthine segment, than elsewhere along its course. This may also have relevance to the pathologic changes in Bell’s palsy.

There may be anatomical variations in the nerve’s course through the petrous bone. It may split into two or three strands at or distal to the geniculate ganglion. The more proximal the division into strands, the more bizarre the subsequent course. Facial motor fibers may run in an enlarged chorda tympani, diminishing the distal facial nerve into a tenuous strand exiting through a narrowed stylomastoid foramen.

Just after exit, the posterior auricular, digastric, and stylohyoid branches arise. The posterior auricular branch supplies the occipitalis, posterior auricular, and transverse and oblique auricular muscles. The digastric and stylohyoid branches supply, respectively, the posterior belly of the digastric and the stylohyoid. The nerve turns forward and passes into the parotid gland. Within the substance of the parotid, it divides into temporofacial and cervicofacial divisions at the pes anserinus (intraparotid plexus) in the cleft between the superficial and deep lobes of the gland (Figure 16.2). The temporofacial branch crosses the zygoma about 1 cm anterior to the ear, where it is vulnerable to injury.

FIGURE 16.2 Branches and distribution of the facial nerve.

The facial nerve supplies all the muscles of facial expression from the scalp and forehead through the platysma, including the extrinsic and intrinsic muscles of the ear. The muscles of facial expression are responsible for all voluntary and involuntary movements of the face except those associated with movement of the jaws and for all play of emotions upon the face. The muscles innervated by the terminal branches are summarized in Table 16.1.

TABLE 16.1 Muscles of the Face, Their Actions, and Innervations

The Nervus Intermedius

The NI is the sensory and autonomic component of the facial nerve. It runs in a position intermediate between CNs VII and VIII across the CPA, moving ever closer to the main facial nerve trunk as it enters the facial canal. At the external first external genu, the NI fuses with the geniculate ganglion. The sensory cells located in the geniculate ganglion are general somatic afferent (GSA) and special visceral afferent (SVA). The GSA fibers carry exteroceptive impulses from the region of the external auditory canal and tympanic membrane. The SVA fibers convey taste from the anterior two-thirds of the tongue. The autonomic component of the NI consists of preganglionic general visceral efferent parasympathetic fibers from the superior salivatory and lacrimal nuclei, which consist of scattered cells in the reticular formation near the caudal end of the motor nucleus. Their axons are bound for the submandibular gland en route to the sublingual and submaxillary glands, the lacrimal glands, and glands in the nasal mucosa.

Course and Branches of the Facial Nerve

The first branch given off in the facial nerve’s course is the greater (superficial) petrosal nerve, which carries preganglionic parasympathetic fibers (Figure 16.3). These fibers are conveyed by the NI to the geniculate ganglion. They pass through the ganglion without synapsing into the greater petrosal nerve, which goes forward through the hiatus of the facial canal to join the deep petrosal nerve from the carotid sympathetic plexus to form the vidian nerve, or the nerve of the pterygoid canal, which runs to the sphenopalatine ganglion, from where postganglionic fibers proceed to the lacrimal gland.

FIGURE 16.3 Course and branches of the facial nerve.

Distal to the geniculate ganglion, the facial nerve continues to descend. As above, the nerve to the stapedius arises from the distal tympanic or upper mastoid segment and passes forward through a small canal to reach the muscle. Although there is some variability, the chorda tympani usually leaves the main trunk slightly above the stylomastoid foramen; it carries taste and general visceral afferent (GVA) fibers as well as preganglionic parasympathetics. It runs forward and upward in a minute canal in the posterior wall of the tympanic cavity, acquires a mucous membrane investment, and then enters and crosses the middle ear. It is sometimes visible as a small white cord behind the tympanic membrane on otoscopic examination. The chorda tympani runs downward and forward to exit the skull and join the lingual nerve, a branch of the mandibular division of CN V, on its posterior border.

Fibers carrying somatosensory afferents in the chorda tympani have their cell bodies in the geniculate ganglion. The peripheral processes innervate part of the external auditory canal, the tympanic membrane, lateral surface of the pinna, and a small area behind the ear and over the mastoid process. There is a marked individual variation in this distribution. Their central processes terminate in the spinal tract and nucleus of the trigeminal, and the central connections are identical with those of the trigeminal nerve. CN VII may also subserve deep pain and deep pressure from the face.

Taste sensation from the anterior two-thirds of the tongue is carried through the lingual nerve to the chorda tympani, then to the geniculate ganglion. CN VII may also carry taste sensation from the mucosa of the soft palate through the sphenopalatine ganglion. Central processes carrying taste and GVA sensation terminate in the nucleus of the solitary tract. The solitary tract sends communications to the superior and inferior salivatory nuclei, which send parasympathetics to the salivary glands. Other fibers synapse in the reticular formation; next order neurons form a component of the reticulospinal tract bilaterally to synapse with sympathetic neurons in the intermediolateral gray column of the upper thoracic spinal cord. These send sympathetic innervation via the superior cervical ganglion to the salivary glands. Fibers subserving taste sensation ascend with the contralateral medial lemniscus to the thalamus. The primary gustatory cortex, located in the anterior insula and the frontal operculum, mediates the perception of taste. Taste fibers also communicate with the hypothalamus and the olfactory system.

The chorda tympani also carries preganglionic parasympathetic fibers to the submandibular ganglion. Postganglionic fibers convey secretory and vasodilator impulses to the submandibular and sublingual salivary glands and mucous membranes of the mouth and tongue (Figure 16.3). These glands also receive sympathetic innervation through the superior cervical ganglion and the carotid plexus. The parasympathetic fibers cause vasodilation and a copious, thin, watery secretion high in enzymes; the sympathetic fibers cause vasoconstriction and a scant, thick, mucoid secretion low in enzyme content.

Clinical Examination

Examination of the Motor Functions

Examination of facial nerve motor functions centers on assessment of the actions of the muscles of facial expression. A great deal can be learned from simple inspection. At rest the face is generally symmetric, at least in young individuals. With aging, the development of character lines may cause asymmetry that does not indicate disease. Distinguishing minor, clinically insignificant, facial asymmetry from subtle facial weakness is sometimes challenging. Note the tone of the muscles of facial expression, and look for atrophy and fasciculations. Note the resting position of the face and whether there are any abnormal muscle contractions. Note the pattern of spontaneous blinking for frequency and symmetry. A patient with parkinsonism may have infrequent blinking and an immobile, expressionless, “masked” face. Facial dystonia causes an abnormal fixed contraction of a part of the face, often imparting a curious facial expression. The procerus sign is seen most characteristically in progressive supranuclear palsy (PSP) and corticobasal degeneration. There is contraction of the forehead muscles, particularly the procerus and corrugator, with knitting of the brows, raised eyebrows, lid retraction, widening of the palpebral fissures, and reduced blinking. The expression is one of surprise, astonishment, perplexity, or consternation (Figure 16.4). Synkinesis causes abnormal contractions of the face, often subtle, synchronous with blinking or mouth movements. Synkinesis suggests remote facial nerve palsy with aberrant regeneration ( Video 16.1). Spontaneous contraction of the face may be due to hemifacial spasm (HFS) (see below). Other types of abnormal involuntary movements that may affect the facial muscles include tremors, tics, myoclonic jerks, chorea, and athetosis (see below).

FIGURE 16.4 Procerus sign in a patient with progressive supranuclear palsy.

Observe the nasolabial folds for depth and symmetry and note whether there is any asymmetry in forehead wrinkling or in the width of the palpebral fissures with the face at rest. A flattened nasolabial fold with symmetric forehead wrinkles suggests a central (upper motor neuron) facial palsy; a flattened nasolabial fold with smoothing of the forehead wrinkles on the same side suggests a peripheral (lower motor neuron) facial nerve palsy. Eyelid position and the width of the palpebral fissures often provide subtle but important clinical clues. Eyelid position is discussed further in Chapter 14. A unilaterally widened palpebral fissure suggests a facial nerve lesion causing loss of tone in the orbicularis oculi muscle, the eye closing sphincter; this is sometimes confused with ptosis of the opposite eye. It is a common misconception that facial nerve palsy causes ptosis.

Some diseases cause a characteristic abnormality of facial expression that can sometimes be recognized at a glance, either because of facial immobility or some peculiar facial expression. Examples of primarily neurologic conditions include parkinsonism and related extrapyramidal disorders (masked facies), PSP (facial dystonia, procerus sign), Möbius’ syndrome, myotonic dystrophy (hatchet face, myopathic face, Figure 16.5), facioscapulohumeral muscular dystrophy (myopathic face, transverse smile), general paresis (facies paralytica), myasthenia gravis (myasthenic snarl, see below), facial nerve palsy (unilateral or bilateral), and Wilson’s disease (risus sardonicus, Chapter 30). There are of course numerous congenital syndromes that cause distinctively dysmorphic facies.

FIGURE 16.5 Myopathic facies in a newly diagnosed young mother with myotonic dystrophy, holding her hypotonic infant, who has the congenital form. The mother has bilateral ptosis, hollowed temporalis, and a slack lower face. The infant has ptosis and the classic “tented upper lip.” (Reprinted from Campbell WW. Clinical Signs in Neurology: A Compendium. Philadelphia: Wolters Kluwer, 2016, with permission.)

Observe the movements during spontaneous facial expression as the patient talks, smiles, or frowns. Certain upper motor neuron facial palsies are more apparent during spontaneous smiling than when the patient is asked to smile or show the teeth. In infants, facial movements are observed during crying. Have the patient grin, vigorously drawing back the angles of the mouth and baring the teeth. Note the symmetry of the expression, how many teeth are seen on each side and the relative amplitude and velocity of the lower facial contraction. Have the patient close her eyes tightly and note the symmetry of the upper facial contraction. How completely the patient buries the eyelashes on the two sides is a sensitive indicator of orbicularis oculi strength.

Other useful movements include having the patient raise the eyebrows, singly or in unison, and noting the excursion of the brow and the degree of forehead wrinkling; close each eye in turn; corrugate the brow; puff out the cheeks; frown; pucker; whistle; alternately smile and pucker; contract the chin muscles; and pull the corners of the mouth down in an exaggerated frown to activate the platysma. There is no good command for platysma contraction, and the movement must be demonstrated. The platysma can also be activated by having the patient open the mouth against resistance or clinch the teeth. The patient may smile spontaneously after attempting to whistle, or the examiner may make an amusing comment to assess emotional facial movement. Because of their paucity of facial expression, patients with Parkinson’s disease may fail to smile after being asked to whistle: the whistle-smile (Hanes) sign.

Trying to gently push down the uplifted eyebrow may detect mild weakness. It is difficult to pry open the tightly shut orbicularis oculi in the absence of weakness. Vigorously pulling with the thumbs may sometimes crack open a normal eye. If the examiner can force the eye open with her small fingers, then the orbicularis oculi is definitely weak. Likewise, it is difficult to force open the tightly pursed lips in a normal individual. When the orbicularis oris sphincter is impaired, the examiner may be able to force air out of the puffed cheek through the weakened lips. Testing ear and scalp movements is seldom useful, although loss of the ability to wiggle the ear in someone previously able to do so has been cited as a sensitive sign of peripheral facial palsy (PFP). The stylohyoid muscle and posterior belly of the digastric cannot be adequately tested. With stapedius weakness, the patient may complain of hyperacusis, especially for low tones. Other tests of motor function and confirmatory signs of facial paresis are discussed in the following sections. It is important in patients with PFP to examine the ear for vesicles or rash, indicative of zoster infection, and to palpate the parotid to exclude a mass lesion.

Examination of the Reflexes

The corneal and other reflexes mediated largely by CN V are discussed in Chapter 15. Frontal release signs such as the snout, suck, and palmomental reflexes are discussed in Chapter 40. Various other reflexes mediated in large part by CN VII can be obtained but are of little practical value. Some merit brief discussion. They are summarized in Table 16.2.

TABLE 16.2 Facial Reflexes

Wartenberg wrote at length about the orbicularis oculi reflex, which he considered an important reflex, and the “chaos of nomenclature concerned with this reflex.” A reflex contraction of the orbicularis oculi causing an eye blink—the nonfocal orbicularis oculi reflex—can be elicited in different ways ( Video 38.2). The threshold for reflex contraction is very low, and the reaction is very quick. Tapping with a finger or percussing with a reflex hammer at many different sites over the forehead and about the eyes may elicit a reflex eye blink. Wartenberg said the muscle “reacts… easily to…a multitude of external stimuli.” Different names were given to methods of eliciting the reflex by stimulating different areas, all essentially the same response. The most frequently used version currently is the glabellar tap. Patients with Parkinson’s disease are unable to inhibit the reflexive eye blinks (Myerson’s sign; not to be confused with Myerson’s reflex). Despite the widespread use of the eponym, it is in fact difficult to find any clear reference linking Myerson to the glabellar tap reflex.

A more specific orbicularis oculi reflex is the focal “deep muscle” response elicited from one side by a percussion that stretches the muscle ( Video 38.2). A fold of the muscle at the temple is held between the thumb and forefinger and then percussed to stretch it back toward the ear. Wartenberg thought this reflex useful because it may be decreased in PFP in proportion to the severity of the palsy, but it is normal or increased with facial weakness of central origin.

Examination of the Sensory Functions

Testing of CN VII sensory functions is limited to taste. Although Hitselberg described hypesthesia of the posterior wall of the external auditory meatus in proximal facial nerve lesions, there is no reliable way to assess the small sensory contribution the nerve makes to the skin of the external ear region. The peripheral receptors are the taste buds embedded in the tongue epithelium and to a lesser extent in the soft palate and epiglottis. Taste buds respond preferentially, but not solely, to one taste quality. Taste is also carried through CN IX and probably CN X.

There are five primary tastes: bitter, sour, sweet, salty, and umami (delicious or savory). Umami has only recently been added to the list. It is a response to compounds of some amino acids, particularly l-glutamate. Umami is a Japanese term that has no English translation. The many flavors encountered in life are a combination of the primary tastes plus olfaction and oral sensory information (“mouth feel”). Sweet and salty substances are most commonly employed for clinical bedside testing because of their ready availability; sour and bitter are more difficult to come by. Chemosensory referral centers typically use four substances for testing: sucrose (sweet), sodium chloride (salty), quinine (bitter), and citric acid (sour). CN VII only subserves taste on the anterior two-thirds of the tongue. When the tongue is retracted into the mouth, there is rapid dispersion of the test substance outside the area of interest. The tongue must therefore remain protruded throughout testing of an individual substance, and the mouth must be rinsed between tests. If bitter is tested, it should be last because it leaves the most aftertaste.

Some examiners prefer to manually hold the patient’s tongue with a piece of gauze to prevent retraction. Because the patient will be unable to speak with the tongue protruded, instructions must be clear in advance. The patient may raise the hand using some signaling system when taste is perceived, point to words written on paper, or make a similar nonverbal response. A damp applicator stick may be dipped into a packet of sugar, artificial sweetener, or salt and coated with the test substance and then placed on one side of the patient’s tongue and rubbed around. The patient signals whether she can identify the substance. Most patients will identify the test substance in less than 10 seconds. Taste sensation is less on the tip of the tongue, and the substance is best applied to the dorsal surface at about the junction of the anterior and middle third of the tongue. The sweetness of artificial sweeteners such as saccharine and aspartame is more intense, and they may make better test substances than ordinary sugar. For a demonstration of taste testing technique, see Video Link 16.1. More sophisticated methods are available to test for subtle dysfunction in patients who have primary taste and smell complaints. There are many referral centers that specialize in the management of taste and smell disorders (see Chapter 12). There are now commercially available filter paper strips impregnated with sweet, sour, salty, and bitter in different concentrations (taste strips). It is seldom necessary or practical to examine taste on the posterior third of the tongue.

The most common situation calling for assessment of taste is the evaluation of facial nerve palsy. If a patient with a peripheral pattern of facial weakness has impaired taste, the lesion is proximal to the junction with the chorda tympani. A lesion at or distal to the stylomastoid foramen (e.g., in the parotid gland) does not affect taste.

Ageusia is the complete inability to taste. With hypogeusia, taste perception is blunted or delayed. Perversions or abnormal perceptions of taste are parageusias. There is marked individual variation in taste. Complete ageusia is rare unless there is also loss of smell. If there is loss of taste, one should first eliminate the possibility of disease of the tongue. Some causes of disturbed taste are listed in Table 16.3. There are many medications that reportedly alter taste; some commonly used in neurologic practice include the following: carbamazepine, phenytoin, tricyclic antidepressants, dexamethasone, hydrocortisone, penicillamine, lithium, methotrexate, levodopa or levodopa/carbidopa, clozapine, trifluoperazine, baclofen, and dantrolene.

TABLE 16.3 Possible Causes of Disturbed Taste

Modified from Bromley SM. Smell and taste disorders: a primary care approach. Am Fam Physician 2000;61:427–436.

Examination of the Secretory Functions

The secretory functions of CN VII can usually be evaluated by history and observation. Increased tearing is usually apparent; decreased tearing may be determined from the history. Tear production may be quantitated with the Schirmer test. Commercially available filter strips are placed in the inferior conjunctival sac and left in place for 5 minutes. The advancing edge of moisture down the strip is proportional to the moisture in the eye; the results are expressed in millimeters. This test is simple and does not require referral to an ophthalmologist.

The lacrimal reflex is tearing, usually bilateral, caused by stimulating the cornea. The nasolacrimal reflex is elicited by mechanical stimulation of the nasal mucosa or by chemical stimulation using irritating substances such as ammonia. Abnormalities of salivation are usually suggested by the history. Otolaryngologists and oral surgeons can use special techniques to quantitate salivary flow.

Disorders of Function

Motor abnormalities, either weakness or abnormal movements, account for the preponderance of clinical abnormalities of facial nerve function. Changes in sensation, primarily taste, and in secretory function, sometimes occur as a sidebar, but are rarely if ever the major manifestation of disease of CN VII. Changes in these functions can help to localize the lesion along the course of the nerve, although this exercise has little practical value. The major branches in sequence are the greater superficial petrosal, nerve to the stapedius, and chorda tympani, after which the nerve continues to the facial muscles. The mnemonic tear-hear-taste-face may help recall the sequence.

Facial Weakness

There are two types of neurogenic facial nerve weakness: peripheral, or lower motor neuron; and central, or upper motor neuron. PFP may result from a lesion anywhere from the CN VII nucleus in the pons to the terminal branches in the face. Central facial palsy (CFP) is due to a lesion involving the supranuclear pathways before they synapse on the facial nucleus. PFP results from an ipsilateral lesion, whereas CFP, with rare exception, results from a contralateral lesion.

Peripheral Facial Palsy

With PFP, there is flaccid weakness of all the muscles of facial expression on the involved side, both upper and lower face, and the paralysis is usually complete (prosopoplegia). The affected side of the face is smooth; there are no wrinkles on the forehead; the eye is open; the inferior lid sags; the nasolabial fold is flattened; and the angle of the mouth droops (Figure 16.6). The patient cannot raise the eyebrow, wrinkle the forehead, frown, close the eye, laugh, smile, bare the teeth, blow out the cheeks, whistle, pucker, retract the angle of the mouth, or contract the chin muscles or platysma on the involved side. She talks and smiles with one side of the mouth, and the mouth is drawn to the sound side on attempted movement. The cheek is flaccid and food accumulates between the teeth and the paralyzed cheek; the patient may bite the cheek or lip when chewing. Food, liquids, and saliva may spill from the corner of the mouth. The cheek may puff out on expiration because of buccinator weakness. The facial asymmetry may cause an apparent deviation of the tongue (see Chapter 20 and Figure 15.7). A patient with an incomplete PFP may be able to close the eye, but not with full power against resistance. Inability to wink with the involved eye is common. The palpebral fissure is open wider than normal, and there may be inability to close the eye (lagophthalmos). During spontaneous blinking, the involved eyelid tends to lag behind, sometimes conspicuously. Very mild PFP may produce only a slower and less complete blink on the involved side. Attempting to close the involved eye causes a reflex upturning of the eyeball (Bell’s phenomenon). The iris may completely disappear upwardly. This is a normal response, but only visible in the patient with orbicularis oculi weakness. To elicit the levator sign of Dutemps and Céstan, have the patient look down, then close the eyes slowly; because the function of the levator palpebrae superioris is no longer counteracted by the orbicularis oculi, the upper lid on the paralyzed side moves upward slightly. Akin to Bell’s phenomenon is Negro’s sign, where the eyeball on the paralyzed side deviates outward and elevates more than the normal one when the patient raises her eyes (not to be confused with the other Negro’s sign, cogwheel rigidity).

FIGURE 16.6 A patient with a peripheral facial nerve palsy on the right. A. The patient is attempting to retract both angles of the mouth. B. The patient is attempting to elevate both eyebrows.

A sensitive sign of upper facial weakness is loss of the fine vibrations palpable with the thumbs or fingertips resting lightly on the lids as the patient tries to close the eyes as tightly as possible (Bergara-Wartenberg sign). The platysma sign of Babinski is an asymmetric contraction of the platysma, less on the involved side, when the mouth is opened (Figure 16.7). Labials and vowels are produced by pursing the lips; patients with peripheral facial weakness have a great deal of difficulty in articulating these sounds. Articulation of labial sounds is discussed further in Chapter 9. The House-Brackmann scale, Burres-Fisch index, and facial nerve function index may be useful to try to quantitate the degree of weakness.

FIGURE 16.7 On the patient’s right side, there is a clear difference between the appearance of the platysma muscle at rest (view at upper left in composite photograph) and during voluntary effort to retract both corners of the mouth (view at lower left). On the patient’s left side, there is only minimal contraction (views at upper and lower right). In the frontal view, the fully contracting right platysma (arrow) can be directly compared with the paretic muscle on the left (question mark). Note also the incomplete retraction of the left corner of the mouth. (Reprinted from Leon-Sarmiento FE, Prada LJ, Torres-Hillera M. The first sign of Babinski. Neurology 2002;59[7]:1067, with permission.)

Because of weakness of the lower lid sphincter, tears may run over and down the cheek (epiphora), especially if there is corneal irritation because of inadequate eye protection. A lack of tearing may signal very proximal involvement, above the origin of the greater superficial petrosal nerve. With severe weakness, the eye never closes, even in sleep. The involvement of the intrinsic and extrinsic ear muscles, stylohyoid and posterior belly of the digastric cannot be demonstrated by clinical examination. Electromyographic needle examination can sample some of these muscles, particularly the posterior auricular and posterior belly of the digastric. Denervation in these muscles indicates a very proximal lesion and may be of help in some cases, particularly in distinguishing Möbius’ syndrome from birth-related facial nerve trauma. Weakness of the stapedius may produce hyperacusis, especially for low tones that sound louder and higher.

The facial weakness in PFP is obvious on both voluntary and spontaneous contraction. There is no dissociation. With a severe lesion, the passage of time may lead to atrophy of the involved muscles. With PFP, the motor limb of the direct corneal reflex is impaired, but the consensual is intact; in the opposite eye, the direct response is intact and the consensual impaired (Table 15.3); in other words, the involved eye does not blink no matter which side is stimulated, and the normal eye does blink no matter which side is stimulated. The various reflexes that involve motor responses of CN VII supplied muscles are impaired. Some patients with PFP complain of numbness of the face. Sometimes, they are describing the wooden feeling that accompanies immobility, but at other times, patients seem to have slight sensory loss that is real and more than logically expected for a lesion of a predominantly motor nerve. The cause of this is unclear.

In comatose or otherwise uncooperative patients, facial movements can be elicited by painful pressure over the supraorbital nerves or by other painful stimuli applied to the face to elicit an avoidance response. Pinprick marks on a comatose patient’s face are best avoided. The jab of a broken applicator stick is usually sufficient and causes less tissue damage. The groove between the nostrils and the cheek is particularly sensitive for these purposes.

Minimal facial weakness on one side must be differentiated from a facial contracture on the opposite side, which can cause the normal nasolabial fold to appear flattened in comparison. Bona fide facial weakness must also be differentiated from developmental asymmetry, facial hemiatrophy, character lines, and habitual emphasis on the use of one side of the mouth (“Brooklyn facial”). Unequal palpebral fissures from ptosis on one side may be confused with facial weakness on the opposite side causing widening of the fissure; the usual error is the reverse.

Localization of Peripheral Facial Nerve Palsy

PFP can occur from a lesion involving the facial nerve nucleus in the pons or at any point along the infranuclear segment. The weakness of the muscles of facial expression is the same with lesions anywhere along the course of the nerve. Diagnostic localization depends on the associated findings, such as hyperacusis, decreased tearing, impaired taste, and involvement of neural structures beyond CN VII. Table 16.4 summarizes the localization and differential diagnosis of PFP. The most common cause of PFP by far is Bell’s palsy.

TABLE 16.4 Differential Diagnosis of Lesions of the Facial Nerve

CSF, cerebrospinal fluid; CT, computed tomography; EMG, electromyography; IAC, internal auditory canal; MRI, magnetic resonance imaging.

Bell’s Palsy

Most cases of idiopathic facial paralysis (Bell’s palsy, for Sir Charles Bell [Box 16.1]) are likely due to Herpes simplex virus activation, but there is no simple method of confirming this mechanism in clinical practice. Polymerase chain reaction (PCR) testing suggests viral reactivation leading to inflammation and demyelination. Herpes zoster, Ramsay Hunt syndrome (see below), is probably the second most common viral infection associated with PFP. Other viruses implicated include cytomegalovirus, Epstein-Barr virus, human herpes virus 6, and coxsackie. Unfortunately, antiviral treatment has not proved particularly efficacious. An inactivated intranasal influenza vaccine was associated with an increased incidence of Bell’s palsy and subsequently withdrawn from the market. Pathologically, abnormalities are present throughout the bony course of the nerve, but nerve damage is concentrated in the narrow labyrinthine part of the facial canal, probably because of compression related to edema and the tenuous blood supply in that segment. Ischemia has long been thought to play a role in the development of Bell’s palsy. There may be a genetic predisposition in some cases. Bell’s palsy is more prevalent in women who are pregnant or have recently given birth. The risk is three times greater during pregnancy, especially in the third trimester or in the first postpartum week.

BOX 16.1

Sir Charles Bell

Bell’s palsy is named for Sir Charles Bell, a Scottish surgeon, anatomist, and artist. Early in his career, he published a book on anatomy of facial expression for artists. Among his many contributions (Moritz Romberg proclaimed Bell to be the “Harvey of our century”) was a description of the nerve supply to the muscles of the face. He described facial palsy of various etiologies, including one patient who was gored in the face by an ox. He provided the illustrations for his own dissections. It is fitting that the Mona Lisa syndrome refers to the facial synkinesis that sometimes follows Bell’s palsy, hypothesized to be the basis for the enigmatic Gioconda half smile in da Vinci’s painting.

Certain criteria should be fulfilled to confirm a diagnosis of Bell’s palsy. There should be diffuse PFP, onset over a day or 2, paralysis reaching a maximum within 3 weeks, and full or partial recovery within 6 months. A prolonged, progressive course suggests a tumor, as does distal involvement of only some branches or the presence of a parotid mass. Involvement of individual distal branches can also occur from trauma, as by obstetrical forceps.

Symptoms often begin with pain behind the ear, followed within a day or 2 by facial weakness. The pain may rarely precede the paralysis by up to 2 weeks. There is peripheral facial weakness involving both upper and lower face. The paralysis is complete in approximately 70% of patients. Some authorities contend there are often subtle or subclinical abnormalities of other CNs. About 25% of patients report some degree of facial numbness that is often dismissed as an odd sensation related to the immobility. Depending on the relationship of the lesion to the geniculate ganglion, to the takeoff of the chorda tympani, and to the takeoff of the branch to the stapedius, patients may note loss of taste sensation on the ipsilateral anterior two-thirds of the tongue, dryness of the eye, or hyperacusis for low tones. The most common symptoms accompanying Bell’s palsy are increased tearing, pain in or around the ear, and taste abnormalities. Trying to localize the lesion by testing taste and lacrimation are not very accurate and of little practical value. In patients studied at surgery, only 6% of lesions were distal to the geniculate ganglion.

Dysgeusia occurs in about 60% of patients, ageusia in about 10%. There may be drooling and difficulty speaking because of the slack facial muscles. Patients are often unable to close the eye, liquids and saliva may drool from the affected corner of the mouth, and tears may spill down the cheek. The majority of patients with Bell’s palsy, Lyme disease, and geniculate herpes will show enhancement of the facial nerve on gadolinium magnetic resonance imaging (MRI). Some enhancement may be seen in normals, but enhancement of the distal intrameatal and labyrinthine segments appears specific for facial nerve palsy.

Age-adjusted incidence rates are higher in the elderly. About 1% of cases are bilateral. About 80% of patients recover fully within 6 months; some have persistent synkinesis because of aberrant regeneration, and the rare patient is left with complete permanent paralysis. The prognosis is age related: best in children, worst in patients over 55. The condition may recur in 6% to 7% of patients. Those without enhancement may have a better prognosis.

Aberrant regeneration is common after Bell’s palsy and after traumatic nerve injury. Axons destined for one muscle regrow to innervate another, so that there is abnormal twitching of the face outside the area of intended movement. On blinking or winking, the corner of the mouth may twitch. On smiling, the eye may close ( Video 16.1). These synkinesis can be prominent in some patients; more often, they are subtle, such as a slight twitch of the orbicularis oris synchronous with blinking of the eye. When misdirection is conspicuous, the main effect of smiling on the involved side of the face may be eye closure. The automatic closure of one eye on opening the mouth, the Marin Amat sign, or inverted or reversed Gunn phenomenon (inverse jaw winking), has been explained as a trigeminofacial associated movement. However, it occurs primarily in patients who have had a peripheral facial paralysis and is probably an intrafacial synkinesis.

Aberrant regeneration may also involve autonomic and taste fibers. The syndrome of crocodile tears is a gustatory-lacrimal reflex, characterized by tearing when eating, especially highly flavored foods. It is due to misdirection of salivary axons to the lacrimal gland. Frey auriculotemporal syndrome is similar, but with sweating and flushing over the cheek rather than lacrimation (Chapter 15). In the chorda tympani syndrome, there is unilateral swelling and flushing of the submental region after eating.

Other Causes of Peripheral Facial Weakness

There are numerous other causes of PFP. Common processes involving the motor neurons of the CN VII nucleus in the pons include motor neuron disease and Möbius’ syndrome. Clinical involvement of facial muscles is more likely in progressive bulbar palsy than in classical sporadic amyotrophic lateral sclerosis (ALS); needle electromyography may show subclinical changes. In spinobulbar muscular atrophy (Kennedy’s syndrome), facial fasciculations and facial weakness are often prominent. Facial nerve paralysis, unilateral or bilateral, may be congenital. Möbius’ syndrome (congenital oculofacial paralysis) is the association of congenital facial nerve palsy with paralysis of the extraocular muscles, especially the lateral rectus because of hypoplasia or aplasia of the CN nuclei (Figure 16.8). For a courageous and dramatic video of Möbius’ syndrome by an affected patient, see Video Link 16.2. Other CN-innervated muscles may be involved, and there may be other developmental defects. The condition is sporadic. Reportedly, involvement of facial nerve motoneurons can be the only manifestation of an acute attack of paralytic poliomyelitis. PFP has been reported in hereditary neuropathy with liability to pressure palsies.

FIGURE 16.8 Child with Moebius syndrome with typical masklike facies and downslanted oral commissures. (Reprinted from Thorne C, Chung KC, Gosain A, et al., eds. Grabb and Smith’s Plastic Surgery. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2014, with permission.)

Lesions involving the facial nerve fibers in the pons may cause PFP. There are usually, but not always, associated findings to indicate the lesion is intramedullary. Fascicular lesions may or may not involve tearing and taste. Many disorders may affect the intrapontine fibers of CN VII (Table 16.4). Ischemic lesions are common. Millard-Gubler syndrome is ipsilateral PFP and contralateral hemiparesis, which may be due to pontine stroke, hemorrhage, or tumor. A CN VI palsy is often but incorrectly included as part of Millard-Gubler syndrome (Chapter 21, Box 21.1). Foville syndrome is ipsilateral PFP and horizontal gaze palsy with contralateral hemiparesis (Chapter 21, Box 21.1). The “eight and a half syndrome” is a one-and-half syndrome (Chapter 14) in association with a facial palsy due to a pontine lesion. Combined PFP and abducens palsy in isolation, without a hemiparesis, has been reported with an infarction of the caudal pontine tegmentum. A PFP has also been reported in Wallenberg’s syndrome because of extension of the infarct into the caudal pons.

Other processes that may affect CN VII fibers in the pons include abscess, syringobulbia, demyelinating disease, and trauma. Because of the proximity of the nucleus and fibers of CN VII to the nucleus and fibers of CN VI, pontine lesions frequently cause both an ipsilateral facial paralysis and an ipsilateral lateral rectus paralysis. Pontine lesions are discussed further in the Chapter 21.

Mass lesions in the CPA, such as acoustic neuroma and meningioma, commonly extend to involve CN VII, the NI, CN VIII, CN V, the cerebellar peduncles, and the cerebellum. Because of the associated hearing loss, there may not be hyperacusis even though the lesion is proximal to the branch to the stapedius. There is usually hearing loss, facial sensory changes, ipsilateral ataxia, and nystagmus. CPA syndromes are discussed further in Chapter 17.

In Ramsay Hunt syndrome (herpes zoster oticus, Hunt syndrome, geniculate herpes), the PFP is due to a reactivation of varicella zoster virus (VZV) involving the geniculate ganglion. Geniculate herpes is one of five conditions eponymically tied to James Ramsay Hunt (Box 16.2). Because of the very proximal involvement, the facial weakness is accompanied by taste impairment, hyperacusis, and diminution of salivary and lacrimal secretion. Pain in and behind the ear may be prominent. There may be vesicles on the tympanic membrane, in the external auditory canal, on the lateral surface of the pinna, and in the cleft between the ear and mastoid process (Figure 16.9). Occasionally, the herpetic eruption may also involve the anterior faucial pillar of the palate or the neck. Hunt described two types: an otalgic form with pain in the ear and a prosopalgic form with pain deep in the face, primarily in the posterior orbit, palate, and nose. The latter may result from involvement of sensory fibers in the greater superficial petrosal nerve.

FIGURE 16.9 Vesicles in the external ear canal in a case of geniculate herpes (Ramsay Hunt syndrome).

BOX 16.2

Ramsay Hunt Syndromes

Other conditions sometimes referred to as Ramsay Hunt syndrome are deep palmar branch ulnar neuropathy, dyssynergia cerebellaris myoclonica, juvenile Parkinson’s disease, and dentatorubropallidoluysian atrophy.

Some patients develop facial paralysis without ear or mouth rash but associated with serologic or DNA evidence of VZV infection (zoster sine herpete, zoster sine zoster). Preherpetic neuralgia refers to pain and dysesthesias preceding the development of rash. In one study% of patients developed vesicles only after the onset of facial weakness. It is likely that some patients with Bell’s palsy have Ramsay Hunt syndrome without a herpetic eruption. It has been estimated that up to one-third of idiopathic PFP cases may be due to zoster sine herpete. Imaging and virologic studies have shown that extensive viral attack beyond the facial nerve occurs frequently. Tinnitus, hearing loss, nausea, vomiting, vertigo, and nystagmus from involvement of CN VIII are common. Rarely, cochleovestibular symptoms outweigh the PFP, presumably because of VZV reactivation in the ganglia of CN VIII. Other CNs may be affected as well. Compared with Bell’s palsy, patients with Ramsay Hunt syndrome often have more severe paralysis at onset and are less likely to recover completely.

Patients with diabetes mellitus have a four- to fivefold increased risk of developing acute PFP, and diabetes is present in about 5% to 10% of patients with PFP. Diabetes is particularly likely in older patients, and those with recurrent or bilateral PFP. Slowly progressive facial weakness can occur with neoplasms involving either the pons or the facial nerve peripherally. Both HIV infection and Lyme disease can occasionally present with facial neuropathy. Lyme disease may cause 10% to 25% of cases of Bell’s palsy in hyperendemic areas; there may be no history of tick bite or erythema migrans, and some patients are not seropositive initially. The cerebrospinal fluid (CSF) is often but not invariably normal. PFP because of Lyme disease is particularly prone to be bilateral.

Fractures of the petrous bone because of closed head injury may injure the facial nerve. The fracture may occur longitudinally down the long axis of the petrous pyramid or transversely across it. The facial nerve may be injured in either type. With the more common longitudinal fractures, the facial palsy is usually due to edema, does not occur immediately, and tends to resolve spontaneously. With transverse fractures, the nerve is often lacerated, contused, or severed; the facial palsy comes on immediately and may be permanent. Rupture of the ear drum and bleeding from the ear suggest longitudinal fracture. The tympanic membrane appears bright red, dark red, brown or bluish depending on the color of the fluid in the middle ear (Figure 16.10). CSF otorrhea is more common with transverse fractures (Chapter 17).

FIGURE 16.10 Hemotympanum due to a left temporal bone fracture. (Reprinted from Chung EK, Atkinson-McEvoy LR, Lai N, et al. Visual Diagnosis and Treatment in Pediatrics. 3rd ed. Philadelphia: Wolters Kluwer, 2015, with permission).

Melkersson syndrome (Melkersson-Rosenthal syndrome) is characterized by recurrent attacks of facial palsy, nonpitting facial and lip edema, and a congenitally furrowed and fissured tongue (lingua plicata, scrotal tongue); it is sometimes familial and usually begins in childhood. Its cause is unknown.

Bilateral facial palsy (facial diplegia) refers to bilateral PFP; it is much less common but much more ominous than unilateral PFP. Bilateral facial weakness can also occur because of neuromuscular disorders, including myasthenia gravis, bulbospinal neuronopathy, and muscle disease. Myasthenia gravis may cause marked facial weakness, with difficulty in both closing and opening the eyes. The pattern of perioral muscle involvement is capricious. In some patients, the smile looks like a weak, halfhearted effort, no matter the underlying jocularity, and may be more vertical than horizontal (Figure 16.11). The vertical myasthenic smile may look more like a snarl and is not without social consequences (myasthenic smile, myasthenic snarl). Ectropion, worse in the afternoon and responsive to anticholinesterase agents, is a rare manifestation of myasthenic weakness of the orbicularis oculi (Figure 16.12). Some myopathies are particularly likely to involve the facial muscles. Myopathic facies are particularly typical of facioscapulohumeral muscular dystrophy (Landouzy-Dejerine syndrome). The eyelids droop, but the eyes cannot be tightly closed. The lips cannot be pursed, but protrude and droop tonelessly, leaving an involuntary protrusion of the upper lip (bouche de tapir). On smiling, the risorius pulls at the angle of the mouth, but the zygomaticus is unable to elevate the lips and the smile is transverse, see Video Link 16.3.

FIGURE 16.11 The vertical myasthenic smile or snarl.

FIGURE 16.12 Myasthenia gravis. A. Asymmetric ectropion due to orbicularis oculi weakness.B. Improvement after administration of neostigmine. (Reprinted from Solé G, Perez F, Ferrer X. Teaching NeuroImages: reversible ectropion in myasthenia gravis. Neurology 2009;73[16]:e83, with permission.)

In facial hemiatrophy (progressive facial hemiatrophy, Parry-Romberg syndrome, Wartenberg syndrome), there is either congenital failure of development or a progressive atrophy of the skin, subcutaneous fat, and musculature of one half of the face, sometimes with trophic changes in the connective tissue, cartilage, and bone (Figure 16.13). Loss of tongue muscle occurs in some patients. The disorder may be a form of localized scleroderma. Accompanying changes may include trophic changes in the hair, with loss of pigmentation and circumscribed alopecia and vitiligo. The facial atrophy may be accompanied by classic linear scleroderma lesions on the face or elsewhere. Rarely, there is hemihypertrophy instead of hemiatrophy. The disease may be a neural crest migration disorder.

FIGURE 16.13 Facial hemiatrophy (Parry-Romberg syndrome) with atrophy of the skin, subcutaneous fat, and musculature of one half of the face.

When bilateral facial weakness is due to disease of CN VII, the differential diagnosis includes bilateral Bell’s palsy, sarcoidosis, Lyme disease, diabetes, head trauma, HIV infection, Guillain-Barré syndrome, the Fisher variant of Guillain-Barré syndrome, carcinomatous or lymphomatous meningitis, tuberculous or fungal meningitis, pontine tumor, Melkersson-Rosenthal syndrome, pseudotumor cerebri, Möbius’ syndrome, and a long list of other conditions. Leprosy may cause bilateral facial paralysis with greater involvement of the upper face. In Keane’s series of inpatients with facial diplegia, the most common causes were Bell’s palsy, Guillain-Barré syndrome, meningeal tumor, prepontine tumor, idiopathic cranial polyneuropathy, intrapontine tumor, brainstem encephalitis, and syphilis. Bilateral PFP must be differentiated from other causes of bifacial weakness, such as myopathies and myasthenia gravis.

In its course across the middle ear, the chorda tympani may be damaged during middle ear surgery. Interestingly, disturbed taste after middle ear surgery is usually transient, even when the chorda tympanis are sectioned bilaterally. However, bilateral chorda tympani lesions may lead to severe and persistent xerostomia because of damage to the autonomic fibers. A syndrome of paroxysmal otalgia because of neurovascular compression of the chorda tympani has been described, with evidence of compression of the nervus intermedius by a branch of the AICA in the IAC demonstrated by magnetic resonance angiography (MRA).

Facial Weakness of Central Origin

In a supranuclear, upper motor neuron or CFP, there is weakness of the lower face, with relative sparing of the upper face. The upper face has both contralateral and ipsilateral supranuclear innervation, and cortical innervation of the facial nucleus may be more extensive for the lower face than the upper. The paresis is rarely complete.

A lesion involving the corticobulbar fibers anywhere prior to their synapse on the facial nerve nucleus will cause a CFP. Lesions are most often in the cortex or internal capsule. Occasionally, a lesion as far caudal as the medulla can cause a CFP because of involvement of the aberrant pyramidal tract. There is considerable individual variation in facial innervation, and the extent of weakness in a CFP may vary from the lower half to two-thirds of the face. The upper face is not necessarily completely spared, but it is always involved to a lesser degree than the lower face. There may be subtle weakness of the orbicularis oculi, the palpebral fissure may be slightly wider on the involved side, and there may be a decrease in palpable lid vibrations. However, involvement of the corrugator and frontalis is unusual, and the patient should be able to elevate the eyebrow and wrinkle the forehead with no more than minimal asymmetry. Inability to independently wink the involved eye may be the only demonstrable deficit. Occasionally, a patient with incompletely developed Bell’s palsy will have relative sparing of the upper face, causing confusion with a CFP.

Even if there is some degree of upper facial involvement in a CFP, the patient is always able to close the eye, Bell’s phenomenon is absent, the corneal reflex is present, and the orbicularis oculi reflex may be exaggerated. In CFP, the lower face is weak, the nasolabial fold is shallow, and facial mobility is decreased. However, the lower face weakness is never as severe as with a PFP, which suggests that there may be some direct cortical innervation to the lower face as well as the upper. Separating CFP and PFP is rarely difficult. CFP is typically part of a more extensive paralysis because of a lesion of the upper motor neuron pathways. Rarely, it may occur in isolation without other neurologic abnormalities; this pattern has been reported with a lacunar lesion of the contralateral basis pontis.

There are two variations of CFP: (a) volitional, or voluntary; and (b) emotional, or mimetic. In most instances of CFP, the facial asymmetry is present both when the patient is asked to smile or show the teeth, and during spontaneous facial movements such as smiling and laughing. However, spontaneous movements and deliberate, willful movements may show different degrees of weakness (Figure 16.14). When asymmetry is more apparent with one than the other, the facial weakness is said to be dissociated. Facial asymmetry more apparent with spontaneous expression, as when laughing, is called a mimetic, emotive or emotional facial palsy (EFP) (see Figure 16.14C); weakness more marked on voluntary contraction, when the patient is asked to smile or bare her teeth, is called a volitional facial palsy (VFP) (see Figure 16.14E). With VFP, automatic or spontaneous movements may not only be preserved, but at times exaggerated. VFP may result from a lesion involving either the cortical center in the lower third of the precentral gyrus that controls facial movements, or the corticobulbar tract. The lesion thus may be either in the cortex or in the subcortical corticobulbar pathways as they go through the internal capsule, the cerebral peduncle, or the pons above the facial nucleus. The dissociation may be due to bilateral supranuclear innervation for lower facial spontaneous, emotional movements not present for volitional movements. In EFP, the weakness is most marked with spontaneous facial movements, and the patient can contract the lower facial muscles on command without difficulty. The anatomical explanation for EFP is unclear. Facial weakness seen only with emotional movements most commonly results from thalamic or striatocapsular lesions, usually infarction, rarely with brainstem lesions. It has been described in lesions of the frontal lobe anterior to the precentral gyrus involving the supplementary motor area. The fibers that mediate the emotional response travel through pathways other than the corticobulbar tracts. Facial asymmetry has been described in patients with temporal lobe seizure foci; the weaker side is usually contralateral to the lesion.

FIGURE 16.14 Patient with left thalamic tumor with face at rest (A), on voluntarily baring the teeth (B), and on reflex smiling (C); there is right facial paresis on smiling but not on voluntary contraction, an emotional facial palsy. Patient with a lesion of the corticobulbar fibers in the genu of the left internal capsule with face at rest (D), on voluntarily baring the teeth (E), and on reflex smiling (F); there is right facial paresis on voluntary contraction but not on smiling, a volitional facial palsy. (From Ross RT, Mathiesen R. Images in clinical medicine. Volitional and emotional supranuclear facial weakness. N Engl J Med 1998;338[21]:1515. Copyright © 1998 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.)

Abnormal Facial Movements

Some conditions involving the face produce abnormal movements rather than weakness. Common disorders causing abnormal facial movements include aberrant regeneration because of facial nerve palsy, blepharospasm, HFS, and facial myokymia.

Hemifacial Spasm

Facial synkinesis may progress to a stage of HFS. More often, HFS arises de novo, because of intermittent compression by an ectatic arterial loop in the posterior circulation, most often a redundant loop of the AICA. The compression is usually near the anterior aspect of the root exit zone. The pathophysiology is similar to that in some cases of trigeminal neuralgia (Chapter 15). The arterial pulsations are thought to cause demyelination and focal nerve damage leading to ephaptic transmission and ectopic excitation. Combined studies using MRI and MRA may demonstrate the neurovascular compression. An MRI study using 3D reconstruction confirmed the AICA as the most common causative vessel, with the posterior inferior cerebellar artery, vertebral artery, internal auditory artery, and veins occasionally causing facial nerve compression at the root entry zone. However, radiographic studies using a 3T MRI has shown that some contact between the facial nerve and nearby vessels, even enough to cause mild nerve deviation, is the rule rather than the exception.

Microvascular decompression is sometimes done and may effectively halt the movements. The lateral spread response is an electrophysiologic phenomenon seen in HFS. Stimulation of the mandibular branch of the facial nerve may cause a compound muscle action potential to appear in the orbicularis oculi. This response does not occur in normals. The lateral spread response is objective evidence of ephaptic transmission from one facial nerve branch to another. During microvascular decompression, the lateral spread response may disappear when the offending vessel is lifted off the nerve, and the status of the response may be used as an indicator of the effectiveness of the decompression. HFS may also occur with other extra-axial or intra-axial lesions, including aneurysm, tumor, multiple sclerosis, or basilar meningitis.

HFS usually develops in older patients, and the condition is twice as common in women than men. Twitching usually begins in the orbicularis oculi, less often in the oris. Initially, the twitching may be subtle and difficult to distinguish from facial synkinesis. HFS may involve the entire facial nerve distribution, or only certain nerve branches; it may propagate from one branch to another. Over months to years, HFS usually spreads to involve all of the facial muscles on one side, but it remains strictly limited to the muscles supplied by the facial nerve. As HFS worsens, it may involve the auricular muscles even when the patient cannot deliberately wiggle the ears; the platysma may also be affected. Fully developed HFS causes repetitive, paroxysmal, involuntary, spasmodic, tonic and clonic contractions of the muscles innervated by the facial nerve on the involved side of the face. The mouth twists to the affected side, the nasolabial fold deepens, the eye closes, and there is contraction of the frontalis muscle ( Video 16.2).

The spasms may persist in sleep and are often exacerbated by chewing or speaking. Synkinesis following PFP may cause movements resembling HFS. The essential difference is that synkinesis is provoked by a voluntary movement, whereas HFS is a spontaneous, involuntary contraction. HFS is commonly associated with some degree of facial weakness because of underlying nerve damage. Rare patients may have both HFS and trigeminal neuralgia, with lancinating pain accompanying the facial spasms (tic convulsif). Brissaud-Sicard syndrome is HFS with contralateral hemiparesis because of a lesion in the pons.

Babinski’s brow lift sign is seen only in HFS and consists of cocontraction of the frontalis and orbicularis oculi muscles causing simultaneous eye closure and paradoxical elevation of the eyebrow during a spasm. This movement is impossible to execute voluntarily and does not occur in blepharospasm, tic or psychogenic movement disorders. The brow lift sign has been referred to as the “other Babinski sign,” but at least nine signs bear Babinski’s name and this designation has been used for other signs as well, most notably Babinski’s platysma sign.

Blepharospasm (nictitating spasm) causes involuntary twitching that primarily involves the orbicularis oculi and frontalis muscles. Blepharospasm is most often idiopathic or “essential” and is a form of focal dystonia (Chapter 30, Video 30.5). Blepharospasm is always bilateral and fairly symmetric. Meige’s syndrome is the association of blepharospasm with oromandibular dystonia. Patients with central nervous system (CNS) Whipple’s disease may have an oculofacial, more often an oculomasticatory, myorhythmia (see Video Link 16.4).

Tic, or habit spasm, can cause a movement resembling HFS or blepharospasm. Tic often causes retraction of the angle of the mouth, contraction of the orbicularis oculi or platysma, or eye blinking. The movements are somewhat more bizarre and purposeful, and other muscles not innervated by CN VII may be brought into action. Bizarre grimacing movements of the face are usually habit spasms. The movements in HFS and essential blepharospasm are stereotyped. The patient with tic can suppress the movements, at least temporarily, while the movements of HFS and blepharospasm are totally beyond volitional control and cannot be suppressed or imitated.

Spastic Paretic Facial Contracture

Instead of spasm, there may be a facial contracture causing a fixed expression with wrinkling of the forehead, narrowing of the palpebral fissure, drawing up or twisting of the angle of the mouth, and increased depth of the nasolabial fold. A facial contracture may give the faulty impression of weakness on the opposite side. Facial contracture may follow a facial paralysis, or occur de novo. Careful testing may reveal that the affected muscles are still paretic, even though in a state of contracture. This type of spastic paretic facial contracture may occur with a progressive lesion of the pons and is suspicious for neoplasm. When facial myokymia and spastic paretic contracture occur together, the likelihood of pontine neoplasm is very high.

Facial Myokymia

Facial myokymia is a continuous, involuntary muscular quivering that has a rippling, wormlike, appearance (Chapter 30). It is usually unilateral. Facial myokymia has been reported with numerous conditions, most intrinsic to the brainstem. It is a classic feature of multiple sclerosis but may also occur with pontine tumor, CPA tumors, Guillain-Barré syndrome, facial nerve compression, rattlesnake envenomation, subarachnoid hemorrhage, meningeal neoplasia, basilar invagination and in association with high titers of voltage-gated K⁺ channel antibodies (see Video Link 16.5). Facial myokymia may occur after cardiac arrest, even in some patients with brain death. With intraparenchymal lesions, the facial nucleus itself is usually intact, but the process disrupts its connections, possibly disinhibiting some neural generator. Mild, usually fleeting, myokymia is common, especially in the orbicularis oculi, and of no clinical significance. These movements often worsen with fatigue and with hyper caffeinism. Patients often require reassurance.

Other Abnormal Facial Movements

Focal seizures involving the face may occur with seizure foci in the motor cortex. Facial seizures may be part of a versive seizure or Jacksonian march. Disease of the basal ganglia or extrapyramidal system may involve the facial muscles causing hypokinesia or hyperkinesia (Chapter 30). Parkinson’s disease causes hypokinesia. Forms of facial hyperkinesias include dyskinesias, choreiform, athetoid, dystonic, grimacing, and myoclonic movements and tremors. Oral-facial dyskinesias are common, most often as a tardive manifestation of psychoactive drug use. Facial muscles, especially the platysma, may sometimes be involved in palatal myoclonus, which is a persistent, rhythmic movement in contrast to other forms of myoclonus (Chapter 30). Facial myoclonus can occur with dolichoectasia of the vertebral artery, with hypocalcemia, serotonin syndrome, and other conditions. Facial fasciculations may occur in any motor neuron disease; perioral and chin fasciculations are frequent in Kennedy’s disease.

Sensory Involvement

Except for disturbances of taste, sensory abnormalities are not a common part of facial nerve lesions. Taste may be affected with lesions of the facial nerve proximal to the takeoff of the chorda tympani. Permanent taste disturbances may follow Bell’s palsy. Disturbances of taste and smell often occur together. Taste abnormalities are usually due to olfactory dysfunction (Chapter 12). Dysgeusia may be a direct or indirect effect of malignancy. Hypergeusia and parageusias may occur in psychoses and conversion disorder. Gustatory hallucinations may occur with complex partial seizures and with tumors involving the uncus or parietal operculum. Gustatory and olfactory hallucinations often occur together. Elderly patients sometimes develop dysgeusia of obscure origin that may lead to anorexia and weight loss. Increased taste sensitivity occurs in patients with Addison disease, pituitary deficiency, and cystic fibrosis.

Geniculate neuralgia causes paroxysmal pain deep in the ear, sometimes radiating to the face. “Tic douloureux of the chorda tympani” has also been described. Lesions of the lingual nerve may cause loss of taste together with loss of exteroceptive sensation on the involved side of the tongue; there is also usually subjective numbness.

Secretory Changes

CN VII is involved in lacrimation and salivation; lesions of the nerve at or proximal to the geniculate ganglion can cause abnormalities of these functions. Absence of salivation occurs only with bilateral lesions. Central lesions, especially those involving the hypothalamus or the autonomic connections, may cause changes in secretory function. Changes in lacrimal and salivary flow are more often the result of systemic processes. Anticholinergic drugs often cause an unpleasantly dry mouth. Keratoconjunctivitis sicca, which occurs in Sjögren’s syndrome and other connective tissue disorders, causes deficient secretion of the lacrimal, salivary, and mucosal glands. This in turn causes dryness of the eyes, mouth, and upper respiratory tract. Sialorrhea (ptyalism) is an excess of saliva. It occurs in Parkinson’s disease and when patients are unable to swallow, such as in bulbar involvement with motor neuron disease.

An increase or decrease in lacrimal or salivary secretion may occur on a psychogenic basis. Lacrimation, of course, is most frequently the result of an emotional stimulus. Salivation may occur from the smell, taste, sight, or thought of food. Xerostomia is common in depressed and anxious patients.

Video Links

Video Link 16.1. Demonstration of taste testing technique. http://www.youtube.com/watch?v=ldkpd88KSUA&feature=mfu_in_order&list=UL

Video Link 16.2. Möbius’ syndrome. http://www.youtube.com/watch?v=3FJPvBcMNAE

Video Link 16.3. Myopathic facies. http://neurosigns.org/wiki/Myopathic_facies

Video Link 16.4. Oculomasticatory myorhythmia in CNS Whipple’s disease. http://neurosigns.org/wiki/Oculomasticatory_myorhythmia

Video Link 16.5. Facial myokymia after rattlesnake bite. https://www.youtube.com/watch?v=KaM3-qy8uqU

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