Tumors of the Facial Nerve

Introduction

Disorders of the facial nerve most commonly develop because of a variety of infectious and inflammatory processes. Involvement of the facial nerve can occur anywhere along its course ranging from the posterior fossa, temporal bone, to parotid gland either by primary or secondary involvement of the facial nerve. Tumors of the facial nerve represent a much less common etiology in patients presenting with facial nerve dysfunction. Consideration for neoplastic involvement of the facial nerve should be routinely considered, particularly in circumstances of slow progressive loss of motor function of the facial nerve or recurrent episodes of paresis or paralysis. The most common tumor involving the facial nerve is the schwannoma. Many other neoplastic processes that can involve the facial nerve have been reported including hemangiomas, paragangliomas, and granular cell tumors. Furthermore, many types of cancer have the capability to metastasize to the temporal bone and may involve the facial nerve.

Schwannomas of the facial nerve arise from Schwann cells that are the myelin-producing cells peripheral to the axon. The histologic appearance of Antoni A (dense array of cells), Antoni B (areas of loose stroma), or combined patterns is identified on histologic examination. A schwannoma can arise anywhere along the course of the facial nerve ( Fig. 139.1 ). These tumors can also exhibit multiple discrete intraneural connections between distant portions of the facial nerve. Hemangiomas of the facial nerve typically occur at the geniculate ganglion, generally believed to arise from the extensive capillary plexus that surrounds the facial nerve at this location. The facial nerve can also become involved with a glomus jugulare or tympanicum, and there have been reports of paragangliomas developing primarily from the Fallopian canal and invading the facial nerve.

Fig. 139.1, Segmental distribution and frequency of facial schwannomas.

Patients with tumors of the facial nerve commonly present with a facial nerve paresis or paralysis that fails to improve with medication, therapy, or time. However, up to 50% of patients with a primary schwannoma of the facial nerve do not present with signs or symptoms of facial nerve dysfunction. Other common symptoms at presentation include hearing loss, tinnitus, or vestibular dysfunction. The diagnosis is typically made after radiologic studies are obtained.

A fundamental consideration regarding management concerns the extent of facial nerve weakness and the prospects for recovery, as the risk of facial nerve injury can be high with surgical management of these tumors. In some patients, consideration for stereotactic radiosurgery (SRS) may play a role in the treatment paradigm.

Key Operative Learning Points

1.
Consideration for facial nerve function should play a key role in the decision as to whether or not to proceed with surgery.
- The fundamental goal is to preserve facial nerve function for the longest duration possible.
2.
Surgical management of tumors that are intimately associated with the nerve will often result in complete facial nerve paralysis.
- Need to compare current facial nerve function with the best conceivable surgical result with optimal reconstruction
3.
It is essential to consider the hearing status in the affected ear and the contralateral ear.
4.
Schwannomas of the facial nerve exhibit a tendency to involve portions of the facial nerve beyond the extent of the visible tumor.
- Intraoperative frozen section should be obtained to ensure clear surgical margins.
- Prepare to use different surgical approaches if additional exposure is required for tumor resection and repair.
5.
SRS may be an effective treatment option for control of tumors of the facial nerve and should be considered in patients with higher surgical risks, good facial function, or only one hearing ear.

Preoperative Period

History

The history should include comprehensive details regarding facial nerve function. Patients commonly present with a progressive facial nerve weakness. A patient who exhibits slowly progressive facial weakness over several months suggests a tumor. The paralysis may also be “recurrent,” although it may never recover completely between episodes. A significant number of patients may have no weakness on initial presentation. The patient may complain of facial twitching or fasciculations and may complain of eye symptoms due to inability to blink or close the eye.

Patients may present with normal hearing or some degree of hearing loss. In some cases, patients may not report significant hearing loss but may only complain of tinnitus. Sensorineural hearing loss may result from tumors that compress the cochlear nerve or invade the otic capsule. Patients may also have conductive hearing loss if the tumor involves the tympanic portion of the facial nerve and impacts the ossicular chain. Patients may report vestibular dysfunction owing to compression of the vestibular nerve or possible erosion into the labyrinth. Patients do not typically complain of pain, but when present, the pain may be indicative of a malignant tumor.

The past medical and surgical history should be thoroughly reviewed. A determination of the patients’ medical status and comorbidities will play a key role in determining which of the potential treatment options would be most suitable for the patient.

Physical Examination

Neurologic examination

Facial nerve examination

1)
Assess extent of weakness of gross and fine motor function.
2)
Attention to signs of synkinesis and mass motion

Categorize the extent of facial nerve function using the House-Brackmann scale ( Table 139.1 ).

TABLE 139.1

House-Brackmann Facial Nerve Grading System

Grade	Description	Characteristics
I	Normal	Normal facial function in all areas
II	Mild dysfunction	Gross: Slight weakness noticeable on close inspection; may have very slight synkinesis At rest: Normal symmetry and tone Motion: Forehead—moderate to good function; eye—complete closure with minimal effort; mouth—slight asymmetry
III	Moderate dysfunction	Gross: Obvious but not disfiguring difference between the two sides; noticeable but not severe synkinesis, contracture, or hemifacial spasm At rest: Normal symmetry and tone Motion: Forehead—slight to moderate movement; eye—complete closure with effort; mouth—slightly weak with maximal effort
IV	Moderately severe dysfunction	Gross: Obvious weakness and/or disfiguring asymmetry At rest: Normal symmetry and tone Motion: Forehead—none; eye—incomplete closure; mouth—asymmetrical with maximal effort
V	Severe dysfunction	Gross: Only barely perceptible motion At rest: Asymmetry Motion: Forehead—none; eye—incomplete closure; mouth—slight movement
VI	Total paralysis	No movement

4)
Keep photographs and video documentation to illustrate any progression of facial weakness over time.

b.
Hearing examination
- 1)
  Weber and Rinne tuning fork testing to ascertain the type of hearing loss
  - Negative Rinne may result when ossicular chain movement is impeded by the tumor.
- 2)
  Formal audiogram
  - Acoustic reflexes—This can give insight into compromise of facial nerve function with ipsilateral and contralateral stimulation.
c.
Full cranial nerve examination
- 1)
  Trigeminal nerve
  - Dysfunction can be evaluated by testing of the corneal reflex.
- 2)
  Lower cranial nerves
  - Close attention to voice, cough, swallow, and vocal cord function
d.
Clinical examination of the vestibular system to ascertain underlying balance disorders
- 1)
  Electronystagmography or videonystagmography may be useful in determining the extent of vestibular dysfunction.
- 2)
  Fistula testing in cases in which the integrity of the labyrinth is suspect

2.
Otoscopic examination
- a.
  Essential, but generally normal
- b.
  If the tumor involves the tympanic segment of the facial nerve, then a retrotympanic mass may be visualized.
- c.
  May present as a swelling in the external auditory canal if the descending or mastoid segment of the facial nerve is involved
3.
Head and neck examination
- a.
  The parotid gland and neck should be examined to search for evidence of tumor or cervical lymphadenopathy.
4.
Ophthalmologic examination
- a.
  Essential to assess corneal protection
- b.
  If the patient demonstrates a poor Bell’s phenomenon and is unable to adequately protect the cornea, then the patient may need a gold or platinum eyelid weight.
- c.
  If ectropion is present the patient may need a lower lid-tightening procedure.
5.
Electrophysiologic facial nerve tests
- a.
  May be useful in some cases
- b.
  Electroneuronography may show a reduction in amplitude that supports the compromise of neural integrity.
- c.
  Facial electromyography prototypically shows simultaneous denervation (fibrillation potentials) and re-innervation (polyphasic voluntary motor unit potentials).
  - This combination of potentials is more commonly encountered in slowly progressive pathologic processes.

Imaging

1.
High-resolution computed tomography (CT) and magnetic resonance imaging (MRI) should be used to demonstrate the presence and extent of a facial nerve tumor.
2.
Imaging characteristics of selected tumor types:
- a.
  Facial nerve schwannomas
  - 1)
    MRI
    - Typically enhance with administration of contrast
    - Solitary lesions or may have skip lesions with intervening segments of normal appearing facial nerve (“beads on a string”)
  - 2)
    CT
    - May exhibit widening of the Fallopian canal
- b.
  Hemangiomas
  - 1)
    Found predominantly at the geniculate ganglion
  - 2)
    MRI
    - Heterogeneous hyperintensity on T2-weighted sequences with punctate foci of hypointense signal
  - 3)
    CT
    - Irregular borders with remodeling of surrounding bone or evidence of intratumor bony spicules
- c.
  Paragangliomas
  - 1)
    MRI
    - Salt and pepper appearance on T1-weighted sequences with contrast as well as hyperintensity on T2-weighted sequences

Indications

Treatment is typically reserved for patients who exhibit evidence of progressive tumor size and/or neurologic sequelae. Some tumors may appear stable from a radiologic perspective, but they can result in worsening facial function, hearing loss, vestibular dysfunction, or other neurologic changes. Before undertaking surgery, it is essential to consider the site and the extent of the lesion and the hearing status of both ears. It is generally difficult to recommend treatment if the patient exhibits normal facial movement or only subtle weakness. However, patients may require active management in cases of hearing loss, unrelenting pain, pulsatile tinnitus, or atypical vestibular complaints.

1.
Facial nerve weakness
- a.
  Surgery is typically not undertaken until facial nerve function has worsened to greater than House-Brackmann grade 3.
  - If resection of the facial nerve tumor results in loss of facial nerve continuity, the best facial nerve result attainable with reconstruction is a House-Brackmann grade 3.
- b.
  Residual facial nerve function
  - Patients with long-standing facial nerve paralysis may have worse results after facial nerve repair.
2.
Hearing status
- a.
  Good contralateral hearing
  - Full complement of surgical options is available
- b.
  Poor contralateral hearing
  - 1)
    In patients with normal hearing, attempted removal of tumor should generally be avoided if injury to the ipsilateral cochlear nerve or otic capsule is likely.
  - 2)
    Tumor decompression should be considered unless the size or location of the tumor presents no other options.

Contraindications

1.
Surgery should not be performed in patients with severe underlying medical conditions that preclude their ability to undergo a surgical procedure. SRS may be an option for these patients.
2.
Patients with small tumors or with little or no neurologic sequelae should be strongly considered for watchful waiting with serial clinical and radiologic examinations.
3.
Surgical intervention should be avoided in patients with facial nerve tumors in an only hearing ear.

Preoperative Preparation

1.
Medical clearance should be obtained prior to surgery. Standard avoidance of medications or supplements that may promote bleeding and inhibit coagulation should be stressed to the patient.
2.
Coordination with electrophysiology monitoring services should be arranged in advance of the procedure date.
3.
Review the diagnostic imaging studies and audiometric findings of both ears.
4.
If facial nerve repair is to be performed by another service, reconfirm their planned participation.

Operative Period

Anesthesia

1.
All standard surgical techniques require general anesthesia.
2.
Careful coordination and discussion with the anesthesia service regarding the extent of usage of paralytic medications.

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