Much of the complexity of the interconnections between cranial nerves V, VII, IX, and X is a result of the evagination of the second branchial arch during the sixth week of development, which causes it to overlap the lower arches. In addition, the embryonic mesodermal tissue of the second arch enters the skin of the face, splitting the elements of the first branchial arch; the underlying muscles of mastication become separated from the overlying cutaneous sensory distribution.²

SUPRANUCLEAR SEGMENT

Facial movement is a complex operation that involves input from both pyramidal and extrapyramidal areas.³ The extrapyramidal circuits involve the premotor cortex, parietal cortex, temporal cortex, cingulate gyrus, hippocampus, amygdala, hypothalamus, basal ganglia, cerebellum, and midbrain tegmentum. These varied connections help explain the tight association of facial movement with emotions and the limbic circuit, as well as the apparent paradox seen in persons who have lost voluntary control of facial movement yet may laugh normally upon hearing a joke.

The cortical area that subserves volitional facial movement is located in the lower third of the precentral gyrus, with representation of the upper face situated rostral to the lower face. Both crossed and uncrossed fibers descend to the upper face, whereas only crossed fibers descend to the lower face (Fig. 1). As a result, unilateral hemispheric lesions spare the forehead and orbicularis musculature.^4,5 These descending motor fibers unite in the internal capsule, decussate in the pons, and terminate in the facial nucleus, which is located in the lateral tegmentum of the caudal pons.⁶

Ascending sensory fibers traveling in the medial lemniscus synapse in the ventral posterior medial nucleus of the thalamus. Medial lemniscal fibers originate in the nucleus solitarius, the gustatory gray of the spinal tract of nerve V, and the main sensory nucleus of nerve V. Much of the primary sensory information is interpreted by the thalamus, while other information is relayed to the postcentral gyrus of the parietal cortex for more complex sensory interpretation. An extensive number of connections within the brain stem are also made, allowing for a richly diverse pattern of reflexes. Lastly, muscle spindle information probably reaches the cerebellum from the mesencephalic nucleus of nerve V in the caudal midbrain.

NUCLEAR SEGMENT

The facial motor nucleus extends approximately 3 to 4 mm in length; its caudal portion is an extension of the nucleus ambiguus, and its rostral portion reaches the abducens nucleus dorsomedially and the motor nucleus of nerve V laterally.⁷ In its rostral extent is found a separate, discrete bunch of large multipolar cells representing the accessory facial nucleus.⁸ Both nuclei control motor function. Running rostromedially and dorsally, these motor fibers sweep around the abducens nucleus to form the internal genu, which bulges into the fourth ventricle. On its course laterally to exit the brain stem, the nerve passes by the superior salivatory nucleus, where visceral motor fibers destined for the submandibular and sphenopalatine ganglia are picked up by the facial nerve.^3,9

Sensory information is transported to the brain stem via small, medium, and large fibers. Each specializes in a different sensory modality, and each synapses on a different sensory nuclei in the brain stem or cervical cord. The terminology of the brain stem can be confusing because many of the tracts and nuclei serve many different cranial nerves (see next paragraph). Yet, these structures are often named after the cranial nerve with the largest input (i.e., descending tract of nerve V).

The descending tract of nerve V and associated gray receive small fiber somatic pain and temperature input from cranial nerves V, VII, IX, and X. The solitary tract and nucleus receives small fiber visceral information from the nasal mucosa and anterior two thirds of the tongue, as well as information from nerves IX and X. The main sensory nucleus of nerve V receives medium fiber somatic input for light touch and vibration from both nerves V and VII. Large fiber muscle spindle information is much less clearly defined, but many believe that the mesencephalic nucleus of nerve V receives this input.^3,10

INFRANUCLEAR SEGMENT: MOTOR ROOT

Within the cerebellopontine angle, the somatic motor component of the facial nerve (motor root) is closely approximated to the nervus intermedius and the auditory nerve. Enclosed in a leptomeningeal sheath that contains an extension of the subarachnoid space, the three nerves enter the internal auditory meatus, traveling laterally.¹¹ At the point of entry, the nervus intermedius is between nerve VIII and the motor root. The motor root and nervus intermedius enter the facial canal (aqueduct of Fallopius) within the substance of the temporal bone and then widen to form the geniculate ganglion. It is in the geniculate ganglion where the first parts of the nervus intermedius (see next section) diverge, while other parts continue with the motor nerve (Fig. 2).

The motor nerve then turns posteriorly at a sharp angle, forming the external genu. After the genu, the nerve enters the tympanic (horizontal) portion of the facial canal. This is located below and medial to the horizontal semicircular canal and above the pyramidal eminence that houses the stapedius muscle. The proximal part of this segment is housed behind a fragile, easily fractured tympanic wall; this is a common site of facial nerve damage in traumatic head injuries. Not infrequently, the nerve prolapses into the oval window niche, partly or completely concealing the footplate of the stapes. This anomaly is of clinical importance when the stapes is removed or manipulated during ear surgery.¹²

At the posterior aspect of the middle ear, the nerve curves downward at which point a branch innervates the stapedius muscle. This minute muscle acts to dampen wide acoustic input surges from the delicate bony transmission system. The downward course takes it into the anterior wall of the mastoid process of the temporal bone. It is within this segment that the chorda tympani arises and marks the final separation of fibers that form the nervus intermedius. The chorda tympani transmits afferent taste sensation and efferent parasympathetic fibers to the submandibular ganglion (also known as the submaxillary ganglion; see Fig. 2).

The motor nerve continues in the temporal bone until it exits at the base through the stylomastoid foramen. Three branches immediately diverge to innervate the posterior auricular, posterior belly of the digastric, and stylohyoid muscles. It is also in this location that connections are made with nerves IX and X, the auriculotemporal branch of nerve V, and the cervical plexus.

Running anteriorly approximately 2 cm from the stylomastoid foramen the nerve divides into upper and lower divisions, which traverse the superficial portion of the parotid gland (Fig. 3). Pathology of the parotid may impair facial nerve function. Emerging from the parotid gland, the nerve passes over the fascial plane of the masseter muscle, with variable communications between upper and lower divisions. This rich anastomotic network becomes clinically important in cases of aberrant regeneration and in the surgical treatment of synkinetic movements. Eventually the nerve divides further into its five major branches: temporal, zygomatic, buccal, mandibular, and cervical. The distal ends of these branches anastomose with those of the trigeminal nerve; the clinical significance of these communications is open to speculation.¹³

All of the muscles of facial expression are supplied by the facial nerve, with the exception of the levator palpebrae superioris, which is supplied by the oculomotor nerve.

INFRANUCLEAR SEGMENT (NERVUS INTERMEDIUS)

The nervus intermedius of Wrisberg is made up of all the components of the facial nerve except the somatic motor component, as discussed above (see Fig. 2). Broadly categorized, information is transmitted for visceral motor function and for visceral and somatic sensation.

The superior salivatory nucleus is the origin of the visceral motor preganglionic parasympathetic nerve destined for both the submandibular and sublingual glands via postganglionic fibers from the submandibular ganglion. These fibers run in the nervus intermedius to the geniculate ganglion, where they follow the somatic motor nerve through the facial canal. Along its downward course within the temporal bone, the preganglionic fibers separate to run with the chorda tympani. The course of the chorda tympani is complicated, running upward and anteriorly over the incus, under the malleus, across the tympanic cavity, and through the petrotympanic fissure to join with the lingual nerve. Finally these parasympathetic fibers reach their destination in the submandibular ganglion.

The lacrimal nucleus, often considered part of the superior salivatory nucleus, is the origin of the visceral motor preganglionic parasympathetic nerve destined for the lacrimal gland via postganglionic fibers from the sphenopalatine ganglion.¹¹ These fibers run in the nervus intermedius, traverse the geniculate ganglion, and form the greater superficial petrosal nerve. On its course, the greater superficial petrosal nerve joins the deep petrosal nerve carrying sympathetic information from the carotid plexus to form the vidian nerve, which ends in the sphenopalatine ganglion. Postganglionic fibers travel with the zygomaticotemporal branch of cranial nerve V to the lacrimal gland.

Visceral sensory fibers may take two routes on their way back to synapse in the nucleus of solitarius. Regardless of the route, their unipolar cell bodies lie in the geniculate ganglion. Taste from the anterior two thirds of the tongue runs with the lingual nerve, chorda tympani, through the facial canal to the geniculate ganglion, and via the nervus intermedius to the brain stem. Sensation from the palate, nose, and pharynx runs with the maxillary nerve to the sphenopalatine ganglion, vidian nerve, greater superficial petrosal nerve to the geniculate ganglion, and via the nervus intermedius to the brain stem.

Somatic sensory fibers from parts of the tympanic membrane, external auditory canal, and periauricular area have their cell bodies in the geniculate ganglion; via the nervus intermedius, they synapse in the nucleus of the descending tract of nerve V and the main sensory nucleus of nerve V.

MOTOR FUNCTION

The majority of facial nerve functions can be readily assessed by observation:

1. View the patient at rest, during emotionally derived movements, and during volitional facial movements:
   1. With the patient at rest, note the symmetry of the face. Paralysis is associated with drooping of the angle of the mouth and lower lid (Fig. 4). This sagging becomes more apparent as the paresis persists. Mild infranuclear-type weakness may be characterized solely as an asymmetry of blink, an observation that is easily missed if the patient's face is not examined during the initial history taking.
      
   2. The dissociation of emotional and voluntary movements is characteristic of supranuclear lesions; the apparent diplegia of the parkinsonian facies will disappear with volitional movement, and the lower facial weakness caused by a stroke disappears with a smile.^3,5,15
      
   3. In infranuclear lesions, the patient may complain of hyperacusis from weakness of the stapedius muscle.
      
   
   
2. Inspect forced closure of the eyes:
   1. Infranuclear lesions usually result in some difficulty in lid closure; in the extreme, there is lack of orbicularis oculi function (lagophthalmos) and upward-rolling eyes (Bell's phenomenon; see below), with failure of the covering lid to hide the movements.
      
   2. In supranuclear lesions, forehead wrinkling and lid closure are preserved.^3,5
      
   
   
3. Look for involuntary facial movements during volitional movements and with the patient at rest:
   1. The most common involuntary movements during volitional activity are the synkinesias seen following Bell's palsy (see Disorders of Overactivity).
      
   2. Unilateral involuntary movements that occur at complete facial rest represent either facial myokymia, hemifacial spasm, or seizure, each of which have characteristic features (see Disorders of Overactivity section).
      
   
   

SENSORY FUNCTION

Although it is a function of overlapping sensory fibers from the trigeminal, glossopharyngeal, vagus, and greater auricular nerves, abnormal sensation along the posterior aspect of the external auditory canal and tympanic membrane may be an early indication of facial nerve dysfunction.¹⁶

Taste is most reliably tested by electrogustometry, which compares the amounts of electrical current applied to the anterolateral aspect of the tongue necessary to produce taste perception. When associated with infranuclear weakness, an increase in the electrical threshold necessary to evoke the perception indicates that the facial nerve lesion is proximal to the origin of the chorda tympani.¹⁷ The simple application of salt or sugar to the anterior aspect of the tongue, although unreliable, may give useful information regarding the nerve's integrity.¹⁸

There are many electrodiagnostic methods capable of evaluating facial nerve disorders, and these are reviewed in detail elsewhere.¹⁷

AUTONOMIC FUNCTION

Salivary and lacrimal function can be quantified and evaluated through comparison to normal controls as well as to the contralateral side. Examples of tests of these autonomic functions are the salivary flow test and Schirmer's test of lacrimal function, respectively. This type of testing may be helpful in the prognosis of facial palsy in select patients, thereby helping to direct more extreme measures of therapy.¹⁹ In general, however, these tests are difficult to quantitate and impractical for use in general practice.

DISORDERS OF UNDERACTIVITY

Supranuclear Lesions

Motor function of the upper face derives from both hemispheres. As a result, supranuclear lesions involving the face will spare the functions of forehead wrinkling and eye closure. Rarely, infranuclear lesions have been described with similar sparing of function, but the accompanying lateralized neurologic findings involving the ipsilateral body usually suggest an obvious supranuclear disturbance.¹² In addition to the physical examination, computerized tomographic (CT) and magnetic resonance imaging (MRI) scans allow prompt identification of these lesions.

Motor strip or corticobulbar tract lesions are characterized by paresis of volitional movement, with preservation of facial tone and involuntary expressions. These involuntary functions are subserved by extrapyramidal input that originates in the basal ganglia and traverses extrapyramidal pathways to the facial nucleus. When these fibers are damaged in disease states involving the basal ganglia or brain stem (e.g., Parkinson's disease), volitional facial movements usually remain normal, whereas little facial tone or emotional expression is displayed.^3,5,15

The list of possible causes of supranuclear facial paresis is large. The major categories of causes are vascular, infectious, demyelinating, and tumorous.

Nuclear Lesions (Including Those of the Intrapontine Fascicle)

The facial nucleus lies in close proximity to the abducens nucleus and the para-abducens (paramedian) reticular formation. Consequently, patients with nuclear lesions often present with abnormalities of contiguous structures. However, a dissociation between the facial nerve modalities of somatic motor, visceral motor, and sensation can be seen at the level of the pons.

FOVILLE'S SYNDROME. Patients with Foville's syndrome display peripheral facial weakness, ipsilateral conjugate horizontal gaze paresis, and contralateral hemiparesis. Lesions associated with these clinical features extend from the facial nucleus to the abducens nucleus and paramedian pontine reticular formation dorsomedially, and into the corticospinal tracts ventrally.²⁰ The paramedian reticular formation can be thought of as the conjugate gaze center of the brain stem, which is why the gaze paresis is conjugate rather than just an ipsilateral abducens palsy.

MILLARD-GUBLER SYNDROME. In Millard-Gubler syndrome there is a combination of contralateral hemiparesis, abducens palsy, and variable facial nerve palsy.²⁰ Lesions associated with these clinical features lie more ventrally in the brain stem than those of Foville's syndrome, and they cause an infranuclear abducens palsy rather than a nuclear disruption. Ipsilateral conjugate gaze paresis, as seen in Foville's syndrome, is absent.

Although Foville's syndrome and Millard-Gubler syndrome are of historical interest, they represent only signatures of brain stem involvement in which there are varying degrees of nuclear and corticospinal tract dysfunction. Clinically, if a patient has both peripheral facial weakness and eye findings, the presence of a central lesion should be considered and signs of contralateral hemiparesis should be inspected. Stroke and infiltrating tumor are the two most common causes; sudden onset suggests the former and subacute progression the latter.

MÖBIUS' SYNDROME. Harlan, writing in 1881, described the first patient. In 1892, Möbius collated a number of cases, bringing greater recognition to the syndrome that now bears his name. Peripheral facial weakness and abduction deficits are central features of this syndrome, and in the majority of cases these findings are bilateral. A host of associated features have been described, including other cranial nerve palsies, ptosis, musculoskeletal abnormalities, cardiac anomalies, craniofacial defects, and mental retardation.²¹

Nuclear agenesis of nerves VI and VII is considered the classic syndrome, but investigators have found cases in which the abnormality was nuclear, neuropathic, or myopathic.^22,23 Furthermore, the etiologies postulated to date include congenital hypoplasias, vasculopathies, genetics, infections, neuronal degeneration, and muscular dystrophy.^24–26 Imaging studies in nuclear cases, when positive, have shown brain stem atrophy and caudal pontine calcifications. At necropsy, the calcifications in one patient correlated with necrotic areas.²⁷

Treatment of the facial diplegia associated with this syndrome has been successful. Treatment involves the use of muscle transplants that are innervated by transposition of either cranial nerves V or XII.²⁸

COMPLICATIONS OF AIDS. Supranuclear, nuclear, and infranuclear lesions can also be found in AIDS patients. The meningeal space is a common site of infranuclear involvement. Depending on the clinical location of disease, CT/MRI, cerebrospinal fluid analysis, and serologic tests are very important ways of differentiating possible causes. Consideration should be given to the presence of toxoplasmosis, lymphoma (intraparenchymal and meningeal), neurosyphilis, tuberculosis, fungus (especially Cryptococcus), and viruses (HIV, cytomegalovirus, and progressive multifocal leukoencephalopathy).

A study by Keane²⁹ found that 50 of 2030 (2.5%) AIDS patients admitted to the hospital had neuro-ophthalmologic pathology; of these 50 patients, 7 had a peripheral-type facial paresis, all due to lesions of the pontine tegmentum.

Infranuclear Lesions

When facial weakness is progressive and of a peripheral nature, lesions along the course of the facial nerve must be considered. By testing the various facial nerve functions, these lesions can be clinically localized, and specific neuroradiologic techniques can be used to define further the extent of involvement and suggest a cause. Facial weakness is bilateral in approximately 1% of cases.³⁰

CEREBELLOPONTINE ANGLE LESIONS. As noted above, cranial nerves VII and VIII are enclosed in a common sheath as they leave the brain stem in the cerebellopontine angle on their way to the internal auditory canal. Thus, a combination of progressive facial weakness, tinnitus, hearing loss, dizziness, and periorbital dysesthesias (trigeminal nerve involvement) should alert the clinician to a lesion in this area. Careful evaluation of facial function may also uncover dysfunction of modalities carried by the nervus intermedius.

Tumors of the cerebellopontine angle are generally of a benign histologic character. The most common tumors are acoustic neuroma, meningioma, and epidermal cyst. Extra-axial in location, these tumors grow slowly, which is why dysequilibrium, rather than true vertigo, is most often reported. The advent of MRI has made evaluation of the cerebellopontine angle simple. Initial workup may also include an audiogram and brain stem auditory evoked potentials.³¹

BELL'S PALSY (IDIOPATHIC FACIAL PALSY). Since Sir Charles Bell's classic descriptions of peripheral facial weakness, a voluminous body of literature has developed on the possible etiologies and treatment modalities of the idiopathic variety of facial weakness.^5,18,32–35

Clinical Features. Bell's palsy, by far the most common type of facial palsy, is a disease that typically affects adults 20 to 40 years of age, but persons of any age are susceptible. Men and women are equally affected. It is characterized by an acute unilateral infranuclear facial nerve paresis (see Fig. 4). Maximal deficits are usually reached within 2 to 3 weeks, but the majority are maximally affected by 2 to 5 days. In 50% of patients there will be a complaint of retromastoid pain preceding, or concurrent with, the onset of paresis.³⁵ Many patients complain of either increased or decreased lacrimation; a smaller group of patients report having a subjective feeling of numbness, despite intact sensory testing. The explanation for a complaint of increased lacrimation is not that there is true increased production,³⁶ as demonstrated by normal Schirmer's test results. Rather, laxity of the lower lid prevents normal flow of tears toward the lacrimal duct, and the tears spill over. A subjective feeling of diminished taste or perverted taste (parageusia) on the involved side is reported by 30% of patients.

It is often surprising to find other cranial nerve signs, such as altered facial sensation, corneal hypesthesia, or tongue deviation in a case that is otherwise typical of Bell's palsy. Broadly defined, this might not cause alarm if one considers Bell's palsy to be a viral disease, and thus capable of causing a mononeuritis multiplex.³⁶ If, however, one uses a more narrow definition, patients with more than just facial nerve involvement probably should be diagnosed as having idiopathic cranial polyneuritis.³⁷

Bell's phenomenon, a normal upward deviation of the eye with attempted lid closure, is easily seen when the orbicularis oculi muscles are paralyzed.

Although the cause of Bell's palsy is unknown, an infectious process probably accounts for the majority of cases; vascular and genetic causes account for some cases. It is seen more commonly among diabetics, hypertensives, and pregnant women. Few early pathologic examinations have been done for obvious reasons. Liston³⁸ examined the facial nerve 1 week after onset and found demyelination, axonal changes, and inflammatory cells consistent with a viral cause. Other investigators have not found a cellular infiltrate.

Not all facial paresis is Bell's palsy. In fact, the differential diagnosis is quite large.^39,40 If any other features in the history or examination cannot be explained by seventh cranial nerve dysfunction, alternative diagnoses should be sought. Additional worrisome features include recurrence, bilaterality, subacute progression, otologic findings, history of trauma, or central nervous system dysfunction.

Clinical Course. Overall, 70% to 80% of patients will have complete spontaneous recovery.^19,36,39,41 The majority of the remaining patients will be left with partial paresis, and less than 5% will have no recovery. Several early clinical features predict which patients will have a worse outcome: total paralysis, dry eye, dysacusis, and age greater than 60. The first sign of recovery is gradual return of eyelid closure. Failure to achieve this within 8 to 12 weeks after the onset of symptoms makes complete recovery unlikely.

Electrodiagnostic studies have also been able to make similar predictions as to poor prognosis. Poor outcome is predicted by diminished salivary flow, absence of voluntary motor units on electromyography, decreased compound action potential, and increased nerve excitability threshold.⁴² These studies are rarely done in the routine clinical setting because of the lack of a clearly beneficial treatment modality that will change the course of the disease. A few investigators, however, believe that surgical decompression of the facial nerve in the worst cases is useful.⁴³

Several other problems, both mechanical and cosmetic, can occur in patients who don't obtain full recovery. Contracture of the previously involved side may draw the mouth upward, increase the nasolabial fold, and decrease the palpebral fissure. It may appear as if the unaffected side is now paretic. Synkinesis, “crocodile tears,” and hemifacial spasm are discussed below in the Disorders of Overactivity section.

Treatment. Over the years, steroids have not invariably shown a benefit in the treatment of Bell's palsy. Nonetheless, they are used frequently, and summation of the data points toward a benefit, especially if used early in the course.⁴⁴ They may help prevent progression, hasten recovery, and prevent synkinesis. In patients without a specific contraindication, a typical prednisone course is 1 mg/kg/day for 5 days, followed by a tapered dosage for an additional 5 days. As noted above, surgical decompression cannot be recommended,⁴⁵ although the future may define a special subgroup who will benefit.

The mainstay of therapy remains close ophthalmologic observation for corneal exposure and the use of ocular emollients in patients with a poor or absent blink. Although ocular shields that moisturize the eye may be quite helpful, patching or closing the eye with tape at night, unless performed with meticulous care, adds little to the instillation of emollients and may actually lead to pressure breakdown of the corneal epithelium. If corneal breakdown appears likely, a soft contact lens or temporary tarsorrhaphy produced either surgically or with botulinum toxin is indicated to provide ocular protection.

In patients who have experienced a poor recovery, muscle feedback training and nerve transfers have been successful.⁴⁶

IDIOPATHIC CRANIAL POLYNEURITIS. As the name suggests, multiple cranial nerve palsies are present at the same time in this condition. The abducens nerve is most frequently affected, but any combination is possible with the exception of the olfactory nerve. In a retrospective review by Juncos and Beal,³⁷ the facial nerve was found to be involved in 4 of 14 cases. Face or head pain was almost invariable, and in one case this finding preceded any nerve deficits by more than 3 months. The disease is self-limited, but it tends to recur. Steroids are the mainstay of treatment.

Idiopathic cranial polyneuritis should be distinguished from Guillain-Barré syndrome (see Acute Idiopathic Polyneuritis section), carcinomatous meningitis, and identifiable inflammatory or infectious causes.

TRAUMATIC FACIAL NERVE PALSY. Trauma to the head may result in facial paresis, which can often be hard to detect if accompanied by facial swelling. Cases of both immediate and delayed paresis (up to days after the trauma) are seen. The natural history is for both groups to do well, although the delayed group probably fares a little better.⁴⁷ CT scanning to look for temporal bone fractures should be performed. Longitudinal fractures are more common than transverse and seem to have a better prognosis. Although controversial, surgical exploration in cases of immediate-onset total paralysis together with a transverse fracture should be seriously considered.⁴⁸ Electrodiagnostic studies can be helpful, as in some cases of Bell's palsy.⁴²

RAMSAY HUNT SYNDROME (GENICULATE HERPES, OTITIC HERPES). The association of facial paresis with herpetic eruptions along the ipsilateral external auditory meatus constitutes the Ramsay Hunt syndrome^49,50 (Fig. 5). Patients frequently give a history of a recent viral syndrome and auricular pain that preceded the facial weakness and vesicular eruption. The extent of the herpetic involvement may not be limited to the distribution of the facial nerve. Other cranial nerve palsies in a mononeuritis multiplex fashion can be seen. Vesicles can involve any aspect of the ipsilateral face indicating trigeminal nerve involvement, and auditory and vestibular symptoms are frequent. The pain associated with this viral eruption is typically severe and often persists for weeks.

The malignant nature of this condition is highlighted by reports showing incomplete recovery in 40% to 70% of patients who received either no treatment or only steroid treatment.^51–54 The accuracy of these findings, however, is questionable because large prospective trials have not been done. Also, a significant percentage of patients develop a postherpetic neuralgia after resolution of the skin lesions.

Currently recommended treatment is a combination of steroids and acyclovir, although a definitive prospective randomized trial has not been done.⁵² Several case studies^53–55 have shown better results with acyclovir compared to the outcome of historical controls. The dose, route, and length of treatment has not been worked out, but a course of prednisone as used for Bell's palsy cases is reasonable. Whether a dosage of 5 mg/kg intravenous acyclovir every 8 hours for 5 to 7 days, followed by 800 mg acyclovir PO 5 times a day for 7 days, should be given, or whether the whole course can be given orally, is not clear.

Zoster sine herpete is a condition in which varicella zoster titers rise, but vesicles never develop.⁵⁶ One approach would be to treat all patients with facial palsy and severe auricular pain with acyclovir, so as not to miss treatment of this condition.

MELKERSSON-ROSENTHAL SYNDROME. MelkerssonRosenthal syndrome comprises a triad of recurrent infranuclear facial paralysis, orofacial edema (predominately of the lips), and lingua plicata.^57,58 Not all three features need be present; only very rarely do they appear in combination. Onset can be at any age. The swelling may be bilateral, despite unilateral facial paresis, or it may occur independent of the facial paresis, with the edema antedating the weakness by months to years.³⁵ Cheilitis granulomatosis is seen on lip biopsy and helps confirm the diagnosis. Several causes have been put forth, such as inheritance,⁵⁹ infection, autoimmunity, and allergic reaction.^60,61 Treatment includes several drugs, such as clofazimine and steroids, as well as the surgical reduction of granulomatous tissue.⁵⁸

UVEOPAROTID FEVER (HEERFORDT'S DISEASE). As the name suggests, in its full presentation uveoparotid fever is characterized by uveitis, parotitis, and mild pyrexia.⁶² The facial nerve is the most commonly involved cranial nerve in sarcoidosis,⁶³ and it is affected in 50% of cases of uveoparotid fever. Although frequently asymmetric in extent, the facial weakness can be bilateral.^29,64 The site of facial nerve inflammation is often within its path through the parotid gland, although it can be anywhere along its course, as demonstrated by some patients with parageusia and reduced lacrimation.^62,65 The cause is probably nerve infiltration with noncaseating granulomatous material. The diagnosis is made on the basis of the appearance of bilateral, asymmetric, peripheral facial paresis accompanied by other systemic signs or symptoms of sarcoidosis as well as a positive tissue biopsy. An elevated serum angiotensin-converting enzyme (ACE) level can be helpful.

ACUTE IDIOPATHIC POLYNEURITIS ( GUILLAIN-BARRÉ SYNDROME). Although the typical presentation of Guillain-Barré syndrome is an ascending paresis with depressed tendon reflexes, there are several variants that involve the cranial nerves to a greater extent.⁶⁶ In 1956, Fisher⁶⁷ described the cases of three patients with ophthalmoplegia, ataxia, and areflexia. Another less common variant is facial diplegia with distal paresthesias. The acute or subacute onset of any combination of these signs should alert the clinician to this potentially fatal disorder, since respiratory and autonomic involvement may occur. Classically, motor nerve conduction studies reveal slow responses, and cerebrospinal fluid examination shows cytoalbuminologic dissociation.

PROGRESSIVE HEMIFACIAL ATROPHY (PARRYROMBERG SYNDROME). Progressive hemifacial atrophy is clearly a syndrome, rather than a specific disease. The unifying characteristic of all cases over the past 150 years is acquired hemifacial atrophy, which must be distinguished from congenital forms⁶⁸ and bilateral lipodystrophy. Additional manifestations have included hyporeflexia, seizures, trigeminal anesthesia, and dementia as well as the ophthalmologic manifestations of enophthalmos, lid atrophy, tonic or irregular pupils, Horner's syndrome, Fuchs's heterochromic cyclitis, retinal vascular abnormalities, scleral melting, extraocular muscle imbalance and palsies, and bony defects of the inferior orbital rim and floor.^69–73 One case of prolonged follow-up for 43 years has been reported.⁷⁴

Typically, patients present in the first or second decade of life, when they notice that their face appears lopsided (Fig. 6). This early facial asymmetry progresses for 2 to 10 years to variably involve the skin, subcutaneous tissue, muscle, cartilage, and bone. A common coup de sabre deformity in the forehead (not shown in Fig. 6) represents the demarcation between normal and abnormal tissue. Rarely, the ipsilateral body is also affected.

Several explanations for the progressive atrophy have been put forth, and all of them may be correct. These include trauma, sequelae of irradiation, infection, scleroderma, sympathetic dysfunction,⁷⁵ lupus erythematosus,⁷⁶ trigeminal neuropathy, and lymphocytic neurovasculitis.^73,77–79 Recent microscopic and biochemical analyses of connective tissue are beginning to find clues.^77,79 MRI data were reported recently by Terstegge and associates.⁸⁰ In cases in which MRI abnormalities were found, a high majority of these abnormalities were ipsilateral to the hemifacial atrophy, supporting the lymphocytic neurovasculitis theory. Cerebral hemiatrophy was the most common finding; gliosis, cortical calcification, and meningeal enhancement were also seen. These findings help explain why seizures, when present, are most often contralateral to the facial atrophy. The overwhelming majority of MRI findings were seen in patients with symptoms involving the central nervous system; most asymptomatic patients had normal scans.

Currently, treatment is limited to either chemotherapy, if an underlying autoimmune cause is found, or surgical reconstructive techniques after progression of the atrophy has halted.⁸¹ Surgical options include free flaps and lipofilling.

DISORDERS OF OVERACTIVITY

In any evaluation of unusual facial movements, the synkinetic movements seen after aberrant regeneration and the reflex grimacing movements created by trigeminal irritation must be considered first.

Synkinetic Movements

Synkinesis is defined as an unintentional movement following the initiation of volitional movement. The term is most often applied to the mass movements that follow incomplete recovery from Bell's palsy. (It also has been used, perhaps inaccurately, to describe hemifacial spasms that can occur spontaneously, without prior volitional movement.)

Axonal compression or disruption along the course of the facial nerve may lead to involuntary or synkinetic movements. Several theories have been put forth, including facial nuclear reorganization, aberrant regeneration, ephaptic (false synapse) transmission, and kindling. All explain some components of involuntary facial movements, and electrophysiologic studies provide support for each theory.^82–86 No single theory, however, is able to explain all aspects of this complicated subject. Only a simple overview will be provided here (please see the appropriate references for more details).

Facial nuclear reorganization refers to the process whereby deafferentation changes nuclear inhibitory connections, thereby “unmasking” reflex movements.⁸⁴ The complex nature of synkinesis is best explained by a nuclear origin. Aberrant regeneration refers to the resprouting of axons down incorrect myelin sheaths after nerve disruption. Ephaptic transmission refers to the concept of an “artificial synapse” between contiguous nerves, caused by the lateral spread of extra-axonal current through the interstitium.^82,83 Current spread may occur between efferent fibers, or from afferent to efferent fibers, and the impulse may be transmitted in either direction along the nerve. Ultimately, misdirected neural firing leads to the synchronous contraction of unassociated muscle groups. Finally, kindling incorporates the concepts of both ephaptic transmission and nuclear reorganization, whereby an antidromic impulse from the ephapse of the nerve activates the facial nucleus, which then coordinates facial muscle contraction.⁸⁶

As noted in the section on Bell's palsy, several complications involving the above mechanisms can occur after peripheral facial nerve disruption. Eye closure when eating or smiling as well as elevation of the corner of the mouth upon blinking are the most common forms of synkinesis, and become apparent within weeks to months of the insult. Unilateral lacrimation while eating is a complication referred to as crocodile tears and is an example of aberrant regeneration. Fibers of the nervus intermedius destined originally for the submandibular ganglion, become misdirected to the lacrimal gland via the greater superficial petrosal nerve; this suggests involvement of the facial nerve prior to the geniculate ganglion.⁸⁷

Hemifacial spasm is a rare, if at all existent, complication of facial nerve palsy, and may be very hard to distinguish from the synkinetic movements that follow facial nerve palsy.⁴⁵ In addition to the history of previous palsy, close observation may help with this distinction, since the synkinesias accompanying Bell's palsy are invariably produced only after voluntary movement, whereas hemifacial spasms can be self-perpetuating, as noted above.⁸⁸

In the more mild forms of synkinesis, reassurance is the best treatment. When the contractures are cosmetically upsetting, botulinum toxin is the treatment of choice.^89,90

Reflex Blepharospasm

Irritation of any branch of the trigeminal nerve may lead to a bilateral orbicularis oculi reflex spasm. This may be incorrectly labeled as essential blepharospasm initially (see Essential Blepharospasm section), resulting in potentially treatable underlying disorders going unrecognized. Patients with anterior segment ocular inflammation or meningeal inflammation (caused by subarachnoid hemorrhage or meningitis) may present with referred pain in the first trigeminal division and reflex blepharospasm. Eye irritation and photophobia may be initial complaints.

Patients with reflex blepharospasm do, however, have volitional control over their squinting. Because the lid closure is secondary rather than primary, they prefer to keep their eyes closed; however, in the disorders of overactivity to be discussed next, patients present with a primary complaint of eyelid closure.

Supranuclear Disorders

HABIT SPASM OF THE FACE (NERVOUS TWITCH, FACIAL TIC). Although frequently confused clinically with blepharospasm or hemifacial spasm, habit spasm of the face can be easily identified. The onset typically occurs in childhood and is characterized by stereotypical, repetitive facial movements that are reproducible and can be promptly inhibited on command. Motor tics can occur as an isolated disorder, or as a component of Tourette's syndrome. Treatment may be as simple as reassurance, or it may require more aggressive drug therapy.

FOCAL CORTICAL SEIZURES. Rarely, epileptiform discharges arising from the facial cortex of the motor homunculus can manifest as gross clonic movements of the contralateral face. These movements, when closely inspected, are seen to involve contiguous cortical areas that serve a distribution beyond the facial nerve. Postictally there can be a supranuclear type of paresis (e.g., Todd's paresis), representing exhaustion of the cortical tonic input. These patients should be considered to have focal cortical disease, and prompt neuroanatomic studies should be carried out to direct the appropriate treatment course.

ESSENTIAL BLEPHAROSPASM. Essential blepharospasm is a form of cranial dystonia limited to the orbicularis oculi muscles. Age of onset is between 45 and 60 years and is more prevalent in women than in men; excessive blinking is the usual first symptom.^91–93 This blinking gradually intensifies in character, insidiously becoming a spasm of the eyelid that is not under volitional control (Fig. 7). Although involvement may appear unilateral in early stages or far into the disease course, causing some diagnostic confusion with habit spasm or hemifacial spasm, bilateral impairment is always found eventually. As the disease progresses, the eye closure may become so frequent and prolonged that the patient is functionally blind and may withdraw from all social contact.

Because essential blepharospasm and more widespread dystonic conditions such as Meige's syndrome probably are part of a spectrum of the same disease (cranial dystonia), clinical features, treatment, and pathophysiology will be discussed together in the next section.

MEIGE'S SYNDROME (BLEPHAROSPASM-OROMANDIBULAR DYSTONIA, OROFACIAL-CERVICAL DYSTONIA, BRUEGHEL'S SYNDROME). Dystonic involvement of the lower cranial muscles (mouth retraction, jaw opening or closing, facial grimacing), neck, vocal cords (spastic dysphonia), and limbs is often referred to as Meige's syndrome.⁹⁴ Also, in a study of 100 patients by Jankovic and Ford,⁹⁵ a full 30% had tremor. Although the most frequent presenting sign is blepharospasm, it is not uniformly present.^95,96 Full expression of the syndrome often takes many years.

Initial complaints in cranial dystonia are often dry eyes, ocular pain, and photophobia. Exacerbating features include stress, bright lights, social interaction, driving, reading, watching television, and wind. Relief may be found in the morning or after rest, and by using sensory “ricks” such as touching the side of the eye, singing, humming, yawning, sucking, chewing, drinking alcohol, extending the neck, and wearing dark glasses.^93,95,97 Cranial dystonia can be confused with tardive dyskinesia, which has a more choreiform movement rather than the sustained postures of dystonia, although overlap does occur.

The presumed cause of cranial dystonia is an upset in the normal dopamine balance in the basal ganglia and brain stem.^98–100 The reason for differences in the extent of cranial and limb involvement is not understood. Evidence for the dopamine hypothesis comes from a complicated body of literature.^{93,95,97,101–103} These studies have reported on (1) different responses to pharmacologic agents that exert their effect on the basal ganglia; (2) associated conditions that affect the basal ganglia and can produce secondary dystonia, including Wilson's disease, encephalitis lethargica, and levodopa or neuroleptic use; (3) associated conditions that affect the upper brain stem, including strokes and multiple sclerosis; and (4) pathologic reports of cases in which abnormalities when evident have inconsistently found gliosis and cell loss in the caudate, putamen, substantia nigra, locus ceruleus, midbrain tectum, and dentate nuclei.

Satisfactory treatment for blepharospasm is best accomplished with botulinum A toxin injected into the muscles around the eye. Other muscles of the face and neck can be injected, depending on the extent of dystonia, but a satisfactory response is less likely than for pure essential blepharospasm.^90,104 The presynaptic nerve terminal is the site of action, where botulinum toxin binds to acetylcholine-containing vesicles to prevent exocytosis. Muscle paralysis is the result, with a peak effect at 5 to 7 days and a duration of effect of between 10 and 16 weeks. Side effects occur in approximately 25% of patients and include ptosis, lagophthalmos, entropion, epiphora, diplopia, bruising, and lower facial weakness.⁹⁰ Changes at the neuromuscular junction have been reported by several investigators,^105,106 but the importance of these changes and whether or not they are reversible is debatable.

Oral medications have become a second-line treatment, which is fortunate considering that most patients respond incompletely or not at all to this treatment. Several different medications have been tried, such as tricyclic antidepressants, anticholinergics, neuroleptics (including clozapine), dopamine depleters (reserpine, tetrabenazine), levodopa, cholinergics, clonazepam, baclofen, and lithium.^92,95,97,107 One reason why such a varied number of drugs have been used in an attempt to treat cranial dystonia is that this disorder was initially considered a psychiatric illness. For the few patients who do not respond to pharmacotherapy, surgical options include orbicularis myectomy, peripheral facial nerve avulsion, and peripheral facial neurectomy.¹⁰⁸

Nuclear Disorders

FACIAL MYOKYMIA. The pathophysiology of facial myokymia needs to incorporate the variety of conditions known to be associated with it, including brain stem tumors, pontine tuberculoma, cerebellopontine angle tumors, carcinomatous meningitis, sarcoidosis, syringobulbia, subarachnoid hemorrhage, multiple sclerosis, Guillain-Barré syndrome, cysticercosis, timber rattlesnake envenomation, hypoparathyroidism, and cardiopulmonary arrest.^109–115 Unknown specific changes in the microenvironment of the motor neuron or its axon due to edema, demyelination, toxins, ischemia, destruction, or metabolic alterations are proposed.^{110,111,113–115} This may be due to direct damage or disruption of regulatory neurons. Pathologic studies have documented cases of nuclear edema or damage, but other studies have also documented cases of disease above or below the facial nucleus without direct involvement.^114,116

Myokymia is a continuous, undulating, involuntary movement of the facial muscles involving predominantly the periocular and orbicularis oris musculature (Fig. 8). It is usually unilateral. This movement disorder is perceived by the patient as a “nervous twitch” and rarely arises in isolation of other neurologic signs or symptoms. Spastic paretic facial contracture may accompany myokymia in intraparenchymal brain stem cases.^111,117 In the absence of other obvious causes, the most likely causes are pontine glioma in children less than 10 years old and multiple sclerosis in persons older than 15. Facial myokymia should be differentiated from the transient twitches of extreme fatigue, the rhythmic contractions seen in hemifacial spasm, facial tics, focal seizures, and the synkinetic movements that follow facial palsy.

Treatment of this condition should be aimed at the underlying pathology. Symptomatic pharmacologic treatment options include carbamazepine, phenytoin, and most recently, botulinum toxin.⁹⁰

Infranuclear Disorders

HEMIFACIAL SPASM. Rhythmic, intermittent, unilateral facial twitching, which begins insidiously around the orbicularis oculi and spreads slowly over 1 to 5 years to involve all the muscles of facial expression, is characteristic of hemifacial spasm (Figs. 9 and 10). These bursts of clonic activity may last only seconds or eventually may become tonic and continue for periods of minutes to hours. The prolonged, severe contractions of facial musculature lead to an annoying, and frequently socially disfiguring, grimacing appearance with partial eyelid closure. Although this is usually painless, patients may complain of mouth or neck pain during severe contractions as well as the occasional sensation of oscillopsia and auditory discomfort during lid and tensor tympani spasm.¹¹⁸ Often, voluntary movements such as smiling, eating, talking, or raising the eyebrows may precipitate involuntary spasms. Spasms can be exacerbated by stressful situations and may occur during sleep. Ultimately, progression is the rule; however, the course is variable, some patients having spontaneous exacerbations and remissions.^93,97 Mild facial weakness can be seen, and because of the facial nerve's proximity to cranial nerve VIII, hearing loss, tinnitus, and vertigo may also occur.^85,119

The varied causes that can produce hemifacial spasm have in common the ability to produce ephaptic transmission at some point along the nerve. By far the most common site of disruption is at the root exit zone of the facial nerve. In most cases, aberrant vascular loops of the basilar, posterior inferior cerebellar, anterior inferior cerebellar, vertebral, or internal auditory artery compress the facial nerve at its root (Fig. 11). Other known causes include cerebellopontine tumors, aneurysms, arteriovenous malformations, and aberrant venous structures; in some reported cases, the cause was undetermined.^88,120,121 Only rarely is there an antecedent history of ipsilateral facial palsy. Tic convulsif is the term given to the combination of trigeminal neuralgia and hemifacial spasm, and a common structural cause is often found. MRI and MR angiography have greatly increased our ability to document the cause of hemifacial spasm prior to surgery.

Treatment. Surgical decompression of the facial nerve, in which a nonabsorbable sponge is placed between the nerve and the offending artery, is the only permanent treatment and has an 85% success rate.^88,120,121 Occasionally a second operation is needed. Some authors have suggested that this procedure can be beneficial even in the absence of an offending vessel, suggesting that the real benefit comes from manipulation of the nerve and subsequent fibrosis.¹²² Unfortunately the long-term complication rate is 16% and includes facial palsy and hearing loss.¹²¹ Nonsurgical management is most successful with botulinum toxin, and given its lack of permanent complications, one could argue that a therapeutic course with this agent should be tried in all patients before surgery is considered.^90,123 Alternative drug trials, which are effective 25% of the time, include carbamazepine, phenytoin, baclofen, clonazepam, and anticholinergics.^85,93

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