Facial Nerve Paralysis

Anatomy & Pathophysiology:

The facial nerve follows a complex course from its origin at the brainstem, traveling through the temporal bone before exiting the skull base to give off the peripheral branches that innervate the muscles of the face. A detailed discussion of facial nerve anatomy can be found in The Ear. Briefly, the facial nerve originates as the seventh cranial nerve at the pontomedullary junction. It enters the petrous portion of the temporal bone via the internal auditory meatus, which also transmits the auditory nerve (cranial nerve VIII). The first intratemporal segment of the facial nerve is the canalicular segment, which passes through the internal auditory canal (IAC). The second portion of the intratemporal facial nerve is the labyrinthine segment, which passes between the cochlea and vestibular labyrinth. The fallopian canal is narrowest in this region, making the labyrinthine segment particularly sensitive to compression injury (such as from edema or inflammation). At the conclusion of the labyrinthine segment, the facial nerve makes its first genu, or bend, at the geniculate ganglion. After the first genu, the facial nerve enters the tympanic segment, traveling in the middle ear space and making a second genu posterior to the oval window and anterior to the horizontal semicircular canal. The nerve then enters the final intratemporal segment, which is interchangeably referred to as the descending, mastoid, or vertical segment. The chorda tympani, which carries taste and parasympathetic secretomotor fibers, branches off from the facial nerve in the mastoid segment. At the conclusion of this final segment, the nerve exits the temporal bone via the stylomastoid foramen, between the styloid process and the mastoid tip. The nerve then enters the parotid gland, where it divides into its terminal peripheral branches. The five main branches of the facial nerve are the temporal, zygomatic, buccal, mandibular, and cervical branches.

Injury to the facial nerve at any point along its course can result in facial paralysis. Nerve damage can occur due to compression (resulting in ischemia), infiltration by tumor, or blunt or penetrating trauma. Facial nerve injury has been classified into several categories based on the extent of damage and likelihood of functional recovery. The most widely used classification scheme is the Sunderland classification system, which divides injuries into five classes or degrees (Table 29.1). These range from first-degree injury (neuropraxia), in which the affected axon retains function distal to the site of injury, to fifth-degree injury, where the entire nerve is disrupted from endoneurium to epineurium. While patients with first- and second-degree injuries are typically able to recover normal nerve function, third-degree injuries and higher produce irreversible damage.

First-degree injury (neuropraxia)	Axonal transmission is blocked at the site of the lesion, but the axon retains function distal to the lesion. Generally occurs as a result of blunt trauma and subsequent edema.
Second-degree injury (axonotmesis)	Loss of axonal function distal to the lesion with Wallerian degeneration. Can result from progression of a first-degree injury.
Third-degree injury	Disruption of the endoneurium and myelin sheath surrounding the axon. The perineurium and epineurium remain intact.
Fourth-degree injury	Injury extends to disruption of the perineurium. The epineurium remains intact.
Fifth-degree injury	Complete disruption of the nerve fiber, extending to the epineurium.

There is a wide range of possible etiologies for facial nerve injury, including craniofacial trauma, iatrogenic injury (such as during skull base, ear, or parotid surgery), neoplastic involvement, and infectious or inflammatory conditions affecting the brainstem, skull base, middle ear, or face. The majority of cases of facial palsy are unilateral (>98%). Bilateral facial paralysis is most often a result of systemic disease that causes general neurologic dysfunction.

The term Bell's palsy is used to refer to idiopathic acute facial palsy, which accounts for 60-70% of cases of sudden facial paralysis. Although no causative factors are identified in these cases, postmortem histologic studies frequently demonstrate diffuse intratemporal nerve damage that is highly suggestive of an inflammatory neuritis. Theories regarding potential mechanisms for this include viral infection, ischemic injury, and autoimmune reaction. Of these, the strongest evidence points toward herpes simplex virus (HSV) infection as the most likely cause.

Other viral infections have been documented to cause facial nerve dysfunction, including mumps virus, mononucleosis (caused by Epstein-Barr virus), poliomyelitis, human immunodeficiency virus (HIV), influenza, and coxsackievirus. Notably, herpes zoster oticus (also known as Ramsay-Hunt syndrome), which is caused by varicella zoster virus (VZV) infection, causes entrapment neuropathy of the facial nerve along with other characteristic symptoms (such as a blistering rash of the external ear) that distinguish it from Bell's palsy. Herpes zoster oticus accounts for approximately 10-15% of cases of acute facial palsy.

Bacterial infections causing facial paralysis are most commonly those originating in the ear or intracranially, such as otitis media, mastoiditis, or meningitis. Malignant otitis externa is an infection seen most often in immunocompromised patients. It is most commonly caused by infection with Pseudomonas aeruginosa and can involve the facial nerve with extension to the temporal bone. Other systemic bacterial infections can also produce facial neuropathy, such as Lyme disease (infection with Borrelia burgdorferi).

Finally, traumatic injury or neoplasm affecting the brainstem, skull base, middle ear, or face can also involve the facial nerve through disruption, extrinsic compression, infiltration, or stimulation of edema or inflammatory response. Some of these conditions, such as facial trauma and skull base tumors are covered in other chapters.

Epidemiology:

Bell's palsy, which accounts for the majority of cases of acute facial palsy, has an estimated incidence of around 20 cases per 100,000 individuals. Incidence increases with increasing age, with those over 65 at highest risk. Bell's palsy is rare in young children. There is a female predilection in the adolescent and young adult population and a slight male predilection among middle-aged patients; in all other age groups males and females are affected equally.

Natural History:

The likelihood of spontaneous recovery of facial nerve injury depends on the extent of nerve damage. In cases of Bell's palsy, up to 90% of patients experience full recovery. Rates of recovery are highest in patients who experienced only partial nerve dysfunction without complete paralysis. These cases most likely represent first- or second-degree nerve injury.

Presentation:

By definition, Bell's palsy is an acute facial nerve paralysis where no cause has been found. The degree of facial immobility can vary in severity from mild paresis to complete paralysis. (See subsequent section on patient evaluation for grading of facial palsy severity.) Typically, disease progression is complete within 48 hours of symptom onset. Frequently, patients also exhibit mild dysfunction of other cranial nerves, such as cranial nerves V, VIII, IX, and X. Some patients experience pain of the ear, tongue, or face, as well as changes in taste sensation.

Patients with herpes zoster oticus typically present with otalgia and characteristic cutaneous lesions in addition to facial palsy. The cutaneous lesions (referred to as varicelliform lesions) form a blistering rash that commonly occurs in the preauricular area, on the auricle, or in the external ear canal. In some patients, lesions may also be found in the mouth, on the tongue, or on the face. Cranial nerves V, VIII, IX, and X may also be involved in this condition. Involvement of cranial nerve VIII is particularly common in herpes zoster oticus, resulting in a high incidence of concurrent auditory and vestibular symptoms (such as hearing loss and vertigo).

Patients with facial palsy resulting from other causes, such as trauma or neoplasm, may present with additional symptoms related to the specific etiology. In some cases of neoplasm, facial paralysis may be the first indicator of disease. Facial palsy resulting from tumor infiltration or compression may be either sudden or gradual in onset.

Differential Diagnosis of Facial Paralysis:

Evaluation:

History

The patient history should include timing of palsy onset and progression, presence of associated otolaryngologic and systemic symptoms, and history of craniofacial trauma or surgery.

Physical Examination

The physical exam includes an evaluation of facial nerve function, including assessment of each major peripheral branch of the nerve. The temporal branch of the facial nerve can be tested by having the patient raise the eyebrows, the zygomatic branch by asking the patient to close the eyes tightly, the buccal branch by having the patient purse the lips, and the mandibular branch by asking the patient to smile. In cases of unilateral palsy, symmetry between the two sides of the face is disrupted when these actions are performed. In addition, significant denervation of the facial muscles can produce facial asymmetry at rest, such as loss of the nasolabial fold on the side of injury. The most commonly used grading system for facial nerve function is the House-Brackmann scale (Table 29.2), which classifies function into six grades, from normal function (grade I) to complete paralysis (grade VI). The ability of the patient to close the eye distinguishes House-Brackmann grade III (complete eye closure) from House-Brackmann grade IV (incomplete eye closure) and has the greatest functional significance.

Grade I	No impairment (normal function)
Grade II	Normal tone and symmetry at rest Slightly perceptible weakness with movement Good to moderate forehead movement Complete eye closure with minimal effort Slight asymmetry of mouth with movement
Grade III	Normal tone and symmetry at rest Obvious asymmetry with movement but not disfiguring Slight to moderate forehead movement Complete eye closure with effort Slight asymmetry of mouth with maximal effort
Grade IV	Normal tone and symmetry at rest Obvious disfiguring asymmetry with movement No perceptible forehead movement Incomplete eye closure Moderate or severe asymmetry of mouth with maximal effort
Grade V	Asymmetrical facial appearance at rest Minimal perceptible facial movement No forehead movement Incomplete eye closure Slight movement of mouth with effort
Grade VI	Complete paralysis (no facial function)

Other elements to evaluate on physical examination include the presence of cutaneous lesions (as may be seen in herpes zoster oticus or Lyme disease), hearing acuity, balance, and palpation of the parotid gland. A cranial nerve examination should also be performed and documented.

Imaging Studies

Radiographic imaging is indicated in cases of suspected neoplasm and is typically ordered in the setting of trauma (such as temporal bone fracture). Recurrent episodes of Bell's palsy, lack of recovery within 3 months, and significant dysfunction of other cranial nerves are also indications for imaging. Magnetic resonance imaging (MRI) is generally the modality of choice, as it provides superior soft tissue resolution. However, computed tomography (CT) may be useful for assessment of bony deficits affecting the fallopian canal in cases of trauma.

Electrophysiologic Studies

The use of electrophysiologic testing can provide further information regarding the extent of facial nerve injury and prognosis by determining the proportion of nerve fibers that have degenerated to those that are still functional. The process of Wallerian degeneration occurs over time; it takes approximately 48-72 hours for degeneration to reach the extratemporal portion of the facial nerve after intratemporal injury has occurred. Since nerve conduction tests can only assess nerve function in the extratemporal region (there is no way to stimulate the nerve within the temporal bone), testing should be delayed until at least 72 hours after a known injury to obtain accurate results. One method of electrophysiologic testing is the nerve excitability test (NET), which determines the minimum (threshold) amount of stimulation necessary to elicit a facial twitch. As axons degenerate, an increasing amount of stimulation is necessary to produce a response. With complete degeneration or disruption of the nerve fiber, no amount of stimulation will be able to generate a response. NET is most useful when performed in cases of complete facial paralysis in the first 2-3 weeks after symptom onset. A threshold difference of 3.5 mA or more between the affected and unaffected side is an indication of severe axonal degeneration and predicts incomplete recovery in 80% of cases.

Treatment:

In the context of acute facial palsy, corticosteroid and antiviral therapy are frequently given empirically. There is good evidence to support the use of steroids in terms of improving final outcome. The goal of steroid administration is to reduce inflammation, thereby preventing progression to complete denervation, decreasing recovery time, and preventing synkinesis (formation of aberrant neural connections during regeneration). Acyclovir, which is active against herpes simplex virus (hypothesized to be the cause of Bell's palsy) and varicella zoster virus, has only been shown to be of benefit in patients with herpes zoster oticus. If steroid or antiviral medications are to be given, they should be started as soon after the onset of symptoms as possible, preferably within the first 24 hours.

As most cases of acute facial palsy (particularly if paralysis is incomplete) achieve complete spontaneous recovery, patients are usually observed for a period of up to 12 months before surgical rehabilitation options to restore facial function are considered. During this time, if the facial palsy is House-Brackmann grade IV or higher, eye care is the primary concern. Incomplete eye closure causes corneal drying and can lead to exposure keratitis and corneal ulceration. Patients should be instructed on preventative care, such as the use of ophthalmic lubricants and taping the affected eye shut at night while sleeping. The placement of a gold weight in the upper eyelid is effective for patients with poor compliance or who are likely to have permanent paralysis. In this procedure, a weight is placed in a small pocket superficial to the tarsus, allowing gravity to close the eyelid when not opposed by the action of the levator muscle .

Surgical decompression of the facial nerve within the fallopian canal may be considered in a subset of patients with complete paralysis and poor expected outcome based on results of electrophysiologic testing once degeneration has reached 90%. Decompression may be performed via either a transmastoid or middle cranial fossa approach; however, the procedure is rarely attempted, as the evidence for its efficacy is poor.

For patients with permanent facial palsy, several rehabilitative procedures may be considered. These can be divided into dynamic and static treatments, depending on whether innervation to the facial muscles is restored. In general, the order of preference of treatment options is direct repair of the facial nerve (neurorrhaphy), followed by cable grafting, nerve transposition, dynamic muscle transposition, and, lastly, static procedures (such as static facial slings). The choice of intervention depends on several factors, including the time elapsed since injury, the nature of the injury, status of the facial muscles, and availability and status of potential donor nerves. For example, if the paralysis is long-standing due to a remote injury, substantial muscle atrophy may have occurred, resulting in inability of the muscles to be reinnervated.

Neurorrhaphy produces the best outcome, but is limited to cases where enough viable (and accessible) nerve is available proximal and distal to the injury to effect a repair. In cases where insufficient nerve is present, cable grafting is the next best viable option. The most common donor nerves used for cable grafting the facial nerve are the great auricular nerve, sural nerve, cervical plexus nerves, and medial antebrachial cutaneous nerve. In some cases, insufficient proximal facial nerve is present to allow for a cable graft, such as if injury or tumor occurs close to the stylomastoid foramen. Under these circumstances, a nerve transfer can be considered. The most common of these procedures is a hypoglossal nerve transfer, in which the hypoglossal nerve is transected distally and anastomosed to the distal facial nerve stump.

Dynamic muscle transfers may be used to augment nerve transfers or in cases where nerve transfer is not feasible. Muscle transfer is also preferable in cases where severe denervation atrophy of the facial muscles has occurred. Examples of dynamic muscle transfers include masseter transfer to correct sagging of the oral commissure and temporalis transfer, which can be used for either oral or orbital rehabilitation. Free tissue transfer of the gracilis muscle is also increasing in popularity.

Finally, static procedures produce a less optimal functional outcome than dynamic options, but may be considered in patients who do not have available donor nerve or muscle. Static suspension involves the elevation and fixation of oral commissure soft tissue, improving facial symmetry and reducing symptoms of drooling and impaired articulation. Static facial slings involve the placement of an implant that is secured to the orbicularis oris and then fixed to the temporalis fascia or the zygomatic periosteum. Other static techniques, such as blepharoplasty and brow lift, may also be useful in restoring resting facial symmetry.

Complications, Prognosis & Follow-Up:

Available studies show conflicting data regarding the efficacy of surgical decompression, but are limited in number and scope. While some have demonstrated a significant improvement in restoration of facial function in the subset of patients with substantial nerve degeneration, others suggest no clear benefit compared to observation alone. Complications of surgical decompression include hearing loss, injury to the labyrinth and vestibular dysfunction, meningitis, and intracranial hemorrhage.

In appropriately selected patients, dynamic rehabilitative procedures (such as nerve grafts and transfers) can achieve remarkable success in restoring facial appearance and function. A complication of reinnervation is aberrant nerve regeneration. Patients may experience undesirable muscle activity, including spasm and synkinesis. In such cases, botulinum toxin injection may be administered to counteract these hyperkinetic effects. Additionally, long-term studies suggest that aberrant activity after nerve transfer gradually decreases over time.