Acoustic Neuroma

A benign Schwann cell tumour arising from the vestibular division of the vestibulocochlear nerve. Vestibular schwannoma is the accurate term, since it arises from the vestibular rather than the cochlear nerve and is not a neuroma. It accounts for around 8 percent of intracranial tumours and 80 to 90 percent of cerebellopontine angle tumours.

Origin and growth

Most arise from the superior or inferior vestibular nerve at the Obersteiner Redlich zone, the transition from central to peripheral myelination near the porus acusticus. Growth is typically slow, averaging 1 to 2 mm per year, but is highly variable: roughly 30 to 50 percent of small tumours show no growth over several years, a proportion grow, and a small number regress. Growth rate is unpredictable from size alone, which is the basis for surveillance rather than reflexive intervention.

As the tumour enlarges it expands the internal auditory canal, extends into the cerebellopontine angle, then compresses the trigeminal nerve, the cerebellum and brainstem, and finally obstructs the fourth ventricle producing hydrocephalus.

Genetics

• Sporadic in around 95 percent, unilateral, presenting in the fifth to sixth decades, with biallelic inactivation of the NF2 gene on chromosome 22q12 within the tumour.
• Neurofibromatosis type 2 in around 5 percent, characterised by bilateral vestibular schwannomas, presenting in the second to third decades, with meningiomas, ependymomas and other schwannomas. Bilateral tumours are diagnostic. Management differs fundamentally, since hearing preservation across both ears becomes the dominant concern and auditory brainstem implantation may be required.

Koos grading, which guides management:

• Grade I: intracanalicular only.
• Grade II: extension into the cerebellopontine angle without brainstem contact, up to 2 cm.
• Grade III: occupying the cerebellopontine angle and touching the brainstem without displacement.
• Grade IV: brainstem displacement and compression.

• Unilateral progressive sensorineural hearing loss in around 90 percent, typically high frequency and gradual.
• Sudden sensorineural hearing loss in 10 to 20 percent, which is why every patient with sudden loss requires magnetic resonance imaging. A partial recovery with steroid does not exclude a tumour.
• Unilateral tinnitus, often the presenting symptom and often more troublesome than the hearing loss.
• Speech discrimination that is disproportionately poor for the pure tone thresholds, which is the classic retrocochlear pattern.
• Disequilibrium and unsteadiness rather than true vertigo, because the vestibular loss is gradual and centrally compensated as it develops. Acute vertigo is uncommon and its presence suggests another diagnosis.
• Facial hypoaesthesia and a reduced corneal reflex, indicating trigeminal involvement and a tumour usually exceeding 2 cm. The corneal reflex is often the earliest trigeminal sign.
• Facial weakness, which is a late and uncommon feature. Early facial palsy suggests a facial nerve schwannoma or a malignant lesion rather than a vestibular schwannoma.
• Facial twitching or hemifacial spasm.
• Headache, ataxia, diplopia and lower cranial nerve symptoms in large tumours.
• Raised intracranial pressure with headache, vomiting and papilloedema in very large tumours with hydrocephalus.

Examination

• Normal otoscopy.
• Weber lateralising to the unaffected ear, Rinne positive bilaterally.
• Reduced corneal reflex, which should be tested in every patient with unilateral audiovestibular symptoms.
• Cerebellar signs including dysdiadochokinesis, past pointing and gait ataxia in large tumours.
• Nystagmus, sometimes with the Bruns pattern of a coarse low frequency nystagmus on gaze toward the tumour and a fine high frequency nystagmus on gaze away.

• Pure tone audiometry with masked bone conduction and speech audiometry with word recognition. Rollover on the performance intensity function and disproportionately poor discrimination indicate retrocochlear disease.
• Referral criteria for imaging in asymmetric hearing loss vary, and a widely used threshold is an interaural difference of 15 dB or more at two adjacent frequencies, or 20 dB at any single frequency. Any unilateral tinnitus, unilateral sudden loss, or unexplained unilateral vestibular symptom also warrants imaging.
• Acoustic reflexes: absent or showing reflex decay, defined as a reduction of the reflex amplitude by 50 percent or more within 5 seconds of a sustained 500 or 1000 Hz stimulus.
• Auditory brainstem response, showing an interaural wave V latency difference above 0.2 to 0.4 milliseconds or an absent response. Sensitivity is around 90 percent for tumours over 1 cm but falls to 60 to 70 percent for intracanalicular tumours, so it is not an adequate substitute for imaging where imaging is available.
• Gadolinium enhanced magnetic resonance imaging of the internal auditory meati is the definitive investigation, detecting tumours of 2 to 3 mm. The tumour is isointense to hypointense on T1, hyperintense on T2, and enhances avidly, often with an ice cream cone appearance where the intracanalicular portion is the cone.
• High resolution T2 sequences such as CISS or FIESTA provide a non contrast screening alternative with high sensitivity and lower cost.
• Computed tomography is inadequate for diagnosis but is used for surgical planning and shows widening of the internal auditory canal in larger tumours.
• Vestibular testing with caloric and video head impulse testing, which localises the nerve of origin and provides a baseline predicting postoperative imbalance.
• Assess for neurofibromatosis type 2 where the patient is young, has bilateral tumours, other tumours, café au lait spots, posterior subcapsular lenticular opacities, or a family history. Genetic testing and imaging of the whole neuraxis are then required.

Differential diagnosis of a cerebellopontine angle lesion

• Meningioma: broad dural base, dural tail, calcification, eccentric to the internal auditory canal, often with less hearing loss for its size.
• Epidermoid cyst: does not enhance, restricts diffusion on diffusion weighted imaging, insinuates around structures.
• Arachnoid cyst: follows cerebrospinal fluid signal on all sequences, no enhancement, no diffusion restriction.
• Facial nerve schwannoma: early facial weakness or twitching, extension along the labyrinthine segment and geniculate ganglion.
• Metastasis and lymphoma: rapid progression, multiple cranial neuropathies.
• Aneurysm and vascular loop.

Three options exist: observation with serial imaging, stereotactic radiosurgery, and microsurgical resection. The decision depends on tumour size, growth, hearing, age, comorbidity, symptoms and patient preference.

Observation with serial imaging

• Appropriate for small intracanalicular and Koos grade I to II tumours, elderly patients, patients with significant comorbidity, and asymptomatic incidental findings.
• Protocol: magnetic resonance imaging at 6 months, then annually for 5 years, then at lengthening intervals, with audiometry at each visit.
• Around half of small tumours show no growth over 5 years.
• Intervene for documented growth exceeding 2 mm per year, progressive symptoms, brainstem compression, or hearing deterioration in a patient who wishes to attempt hearing preservation.
• Counsel that hearing frequently deteriorates during observation regardless of growth, which is an argument for earlier intervention in patients for whom hearing preservation is the priority.

Stereotactic radiosurgery

• Delivered as single fraction gamma knife or linear accelerator treatment at a marginal dose of 12 to 13 Gy to the tumour margin, or as fractionated stereotactic radiotherapy.
• Indicated for tumours up to approximately 2.5 to 3 cm, in patients unfit for or declining surgery, in the elderly, and for residual or recurrent tumour after surgery.
• Tumour control at 5 to 10 years is 90 to 95 percent. Control means arrest of growth, not disappearance; transient post treatment swelling in the first 6 to 18 months is common and should not be misinterpreted as failure.
• Hearing preservation at 5 years is around 50 to 70 percent in patients with good pretreatment hearing, and declines with time.
• Facial nerve preservation exceeds 95 percent, and trigeminal dysfunction occurs in around 2 to 5 percent.
• Risks: malignant transformation, which is very rare at around 1 in 1000 or less; hydrocephalus; and the technical difficulty of subsequent surgery through irradiated adherent tissue.
• Not suitable for large tumours with brainstem compression, where swelling after treatment could be catastrophic.

Microsurgical resection

Indicated for large tumours, particularly Koos grade III and IV with brainstem compression, for tumours causing hydrocephalus, for growth after radiosurgery, and for younger patients where lifelong tumour control is the goal.

Approaches:

• Translabyrinthine: sacrifices hearing entirely, so it is used where hearing is already non serviceable or where the tumour is large. Provides early identification of the facial nerve at the fundus and requires no cerebellar retraction, giving the best facial nerve outcomes for large tumours. Risks include cerebrospinal fluid leak, which is managed by fat obliteration of the mastoid and blind sac closure of the canal.
• Retrosigmoid or suboccipital: permits hearing preservation, gives good access to the cerebellopontine angle including large tumours, but requires cerebellar retraction and provides limited access to the most lateral part of the internal auditory canal, risking residual tumour at the fundus. Postoperative headache is more common with this approach.
• Middle fossa: for small, predominantly intracanalicular tumours in patients with good hearing, giving the best hearing preservation rates, but requires temporal lobe retraction and gives limited access to the cerebellopontine angle. Facial nerve is superior to the tumour in the canal and is at risk during exposure.

Intraoperative and perioperative principles:

• Continuous facial nerve monitoring, and auditory brainstem response or direct cochlear nerve action potential monitoring where hearing preservation is attempted.
• Subtotal or near total resection with planned radiosurgery to the residue is an increasingly used strategy for large tumours, prioritising facial nerve function over complete removal.
• Perioperative dexamethasone, and antibiotic prophylaxis.
• Careful sealing of opened air cells with bone wax and fat to prevent cerebrospinal fluid leak.

Outcomes:

• Complete resection achieves cure in the great majority, with recurrence under 5 percent.
• Facial nerve preservation, defined as House Brackmann grade I or II at 1 year, exceeds 90 percent for tumours under 2 cm and falls to 50 to 70 percent for tumours over 3 cm.
• Hearing preservation with hearing preserving approaches is achieved in around 30 to 50 percent for small tumours with good preoperative hearing, and is unlikely for tumours over 1.5 cm or with poor preoperative discrimination.
• Complications: cerebrospinal fluid leak in 5 to 15 percent, meningitis, headache, cerebellar injury, lower cranial nerve deficits, stroke from vascular injury particularly to the anterior inferior cerebellar artery, and mortality under 1 percent in experienced centres.

Postoperative and long term care

• Eye care is the priority where the facial nerve is weak: ocular lubricants hourly, ointment and taping at night, and early ophthalmology involvement. Consider an upper eyelid weight or tarsorrhaphy where corneal sensation is also reduced, since the combination of facial weakness and trigeminal sensory loss puts the cornea at high risk.
• Vestibular rehabilitation from the early postoperative period, since unilateral vestibular loss is deliberately created and compensation must be trained. Older patients compensate more slowly.
• Facial rehabilitation: neuromuscular retraining, botulinum toxin for synkinesis, and reanimation procedures including hypoglossal to facial or masseteric to facial nerve transfer within 12 to 18 months where the nerve is anatomically disrupted, or free muscle transfer thereafter.
• Hearing rehabilitation: contralateral routing of signal aids or bone conduction devices for single sided deafness, and cochlear implantation where the cochlear nerve is anatomically preserved, which is increasingly offered after hearing preserving surgery and after radiosurgery.
• Auditory brainstem implantation in neurofibromatosis type 2 where both cochlear nerves are lost.
• Serial magnetic resonance imaging at 1 year, then at 3 and 5 years after complete resection, and more frequently after subtotal resection or radiosurgery.
• Multidisciplinary skull base team management, including neurotology, neurosurgery, radiation oncology, audiology, physiotherapy and ophthalmology.