Facial Nerve Schwannoma

Facial nerve schwannoma is a rare benign tumour of the Schwann cells of the facial nerve (the seventh cranial nerve). It can arise anywhere along its course, from the cerebellopontine angle to the parotid gland.

It accounts for under 1 percent of temporal bone tumours and about 0.8 percent of intratemporal tumours, with an incidence of about 0.8 per million per year. Most are sporadic and solitary, while multiple lesions occur in NF2-related schwannomatosis.

The most common sites of involvement are the geniculate ganglion region, the tympanic segment, the mastoid segment, the internal auditory canal, and the cerebellopontine angle. Rarely, the extratemporal segment in the parotid is involved. Tumours often involve multiple segments (about 50 percent).

It is a slow growing tumour (usually below 1 to 2 mm per year). Facial function may remain normal for years because the nerve fascicles are displaced rather than invaded.

The tumour spreads along the nerve and into adjacent structures (middle ear, mastoid, petrous bone, middle cranial fossa). It causes bone remodelling and expansion of the fallopian canal.

• Facial weakness (progressive, fluctuating or sudden, in about 50 to 70 percent; slowly progressive over months to years, or sudden onset mimicking Bell palsy but without recovery), facial twitching (hemifacial spasm), or a long history of recurrent episodes of facial palsy.
• Hearing loss (conductive from middle ear mass or ossicular involvement, or sensorineural from the internal auditory canal and cerebellopontine angle location), tinnitus, and aural fullness.
• Vertigo, and imbalance with cerebellopontine angle extension.
• A middle ear mass behind an intact tympanic membrane (retrotympanic mass), otorrhoea, and otalgia.
• Parotid mass, with facial paralysis for extratemporal segment lesions.
• Facial numbness, headache, and cerebellar signs with large tumours; asymptomatic cases found incidentally on imaging.
• Features of NF2-related schwannomatosis in some.

• Examination of facial function using the House-Brackmann grading (grade I to VI) and electromyography. Use electroneuronography (percent of degeneration) to follow nerve function.
• Audiometry: conductive, sensorineural or mixed hearing loss.
• High resolution CT of the temporal bone: a smooth, expansile soft tissue lesion enlarging the fallopian canal (in the geniculate ganglion, tympanic or mastoid segment). There is bone erosion at the geniculate fossa, labyrinthine segment, or in the ossicles and middle ear. The lesion has a "tubular" or "dumbbell" shape across segments.
• MRI with gadolinium (the key imaging study): an enhancing, well defined mass along the facial nerve course. It is hypointense or isointense on T1, hyperintense on T2, and has intense homogeneous enhancement (cystic components may be present). The tumour follows the nerve.
• On MRI, identify the segments involved and the extension intracranially. Differentiate from vestibular schwannoma, meningioma, haemangioma (usually geniculate fossa with intratumoral bone spicules), paraganglioma, cholesteatoma and perineural spread of cancer.
• Biopsy is not usually needed. Histology after removal shows Antoni A and B patterns, and S100 positivity.
• Genetic assessment for NF2-related schwannomatosis if multiple tumours.

Management is individualised according to the facial function, tumour size, hearing, age and comorbidity. It balances disease control and facial function preservation.

• Observation (wait-and-scan strategy): for patients with normal or near normal facial function (House-Brackmann I to II), small tumours, and no hearing or brainstem compromise. Perform MRI at 6 and 12 months and then every 1 to 2 years.
• Surgery is done after facial nerve function deteriorates to House-Brackmann grade III to IV, or with tumour growth (greater than 2 mm per year), brainstem compression or severe hearing loss. Surgical outcomes are similar for patients operated at grade III compared with grade IV.
• Stereotactic radiosurgery (gamma knife, 11 to 13 Gy to the margin; or fractionated stereotactic radiotherapy 50.4 Gy in 28 fractions) for tumours less than 3 cm with preserved facial function. Tumour control is about 90 to 95 percent. Facial nerve function is preserved or maintained in most (about 80 to 90 percent). Risks are hearing loss or facial dysfunction, and long term follow up is needed.
• Microsurgical resection: for large tumours, progressive deterioration, brainstem compression, or poor facial function (House-Brackmann V to VI). The goal is complete removal and facial nerve reconstruction in the same sitting.
• The approach depends on location and hearing: translabyrinthine (non-serviceable hearing), transmastoid extended middle ear (for tympanic and mastoid segments), middle cranial fossa (geniculate ganglion and labyrinthine segment with hearing preservation), retrosigmoid, or combined (transmastoid middle fossa) approaches. Use parotidectomy for extratemporal tumours.
• Facial nerve reconstruction after resection: interposition cable graft (great auricular nerve or sural nerve), with a result of House-Brackmann grade III in most patients (typically grade III at 1 to 2 years). Use end-to-end anastomosis if there is a small gap. Use hypoglossal facial nerve jump or end-to-side anastomosis when the proximal stump is unavailable. Surgery within 1 to 2 years of paralysis gives better results.
• Subtotal resection with nerve preservation (intracapsular debulking or capsule sparing) is an option when facial function is good. It leaves a small remnant that is observed or irradiated.
• Facial rehabilitation: facial reanimation (nerve transfers, free gracilis muscle transfer, static slings and gold weight implants for the upper lid), physiotherapy, and botulinum toxin for synkinesis and hemifacial spasm. Protect the eye (lubricants, taping, tarsorrhaphy, gold weight) for incomplete eye closure.
• Hearing rehabilitation with hearing aids, bone anchored hearing devices or cochlear implants for hearing loss.
• Follow up: MRI at 6 to 12 months after treatment, then every 1 to 2 years; facial function assessment. Prognosis is excellent for tumour control, with variable facial function recovery.