Acoustic Neuroma

Acoustic neuroma (vestibular schwannoma) is a benign tumour of the Schwann cells of the vestibular division of the eighth cranial nerve, arising at the transition zone in the internal auditory canal (IAC) and expanding into the cerebellopontine angle (CPA).

It accounts for about 6 to 8% of intracranial tumours and about 80% of CPA tumours. Clinical incidence is about 1 per 100,000 per year, with more detected through MRI. Peak age is 40 to 60 years.

Most are sporadic unilateral tumours from biallelic NF2 gene inactivation (merlin, chromosome 22q12). Bilateral vestibular schwannomas define neurofibromatosis type 2, an autosomal dominant disease that presents earlier (teens to 30s) with other tumours (meningiomas, ependymomas, spinal schwannomas) and ocular findings (juvenile cataract, retinal hamartoma). High dose ionising radiation to the head raises risk; the evidence for mobile phone use and noise exposure is inconclusive.

Growth is slow, 1 to 2 mm per year on average; 40 to 50% do not grow over several years and some regress. Growth above 2.5 mm per year is significant.

Koos grading:

• Grade I: intracanalicular

• Grade II: extension into the CPA, up to 2 cm

• Grade III: filling the CPA cistern without brainstem displacement, up to 3 cm

• Grade IV: larger than 3 cm with brainstem compression

• Unilateral or asymmetric sensorineural hearing loss (over 90%), gradual, high frequency first, with speech discrimination worse than the pure tone loss predicts; sudden loss in 5 to 10%
• Unilateral high pitched tinnitus (about 70%)
• Imbalance (about 50%); true vertigo is uncommon because of gradual compensation
• Aural fullness
• Larger tumours: facial numbness and paraesthesia with reduced corneal reflex (trigeminal compression), hoarseness and dysphagia (lower cranial nerves), ataxia (cerebellar and brainstem compression), headache, and hydrocephalus with raised intracranial pressure
• Facial weakness is uncommon even with large tumours; its presence suggests a facial nerve schwannoma or another diagnosis
• Signs: reduced corneal reflex, nystagmus (Bruns nystagmus in large tumours), positive head impulse test, dysdiadochokinesis, gait ataxia
• In NF2: spinal cord compression, multiple cranial neuropathies, cutaneous schwannomas, visual loss

Differential diagnosis of a CPA mass: meningioma (10 to 15%), epidermoid cyst, arachnoid cyst, facial nerve schwannoma, metastasis, lipoma, aneurysm. Asymmetric sensorineural hearing loss also arises from Meniere disease, labyrinthitis, vascular loops, autoimmune inner ear disease and sudden idiopathic loss.

1. Pure tone audiometry and speech discrimination: asymmetric sensorineural loss, typically high frequency, with poor word recognition. A difference of 15 dB or more at two adjacent frequencies, or a notable speech score difference between ears, justifies imaging. Hearing is graded from class A to D (good to non-serviceable) or Gardner-Robertson grades 1 to 5.
2. MRI of the internal auditory canals and CPA with gadolinium is the gold standard. A thin section (1 mm) high resolution T2 sequence (CISS or FIESTA) without contrast is an acceptable screening test for unilateral hearing loss. Contrast T1 shows a homogeneously enhancing "ice cream cone" mass with an intrameatal stem; cystic change and heterogeneity occur in larger tumours.
3. CT of the temporal bone shows a widened IAC and is used when MRI is contraindicated.
4. NF2 gene testing (including mosaic analysis) and family screening if tumours are bilateral or onset is young.
5. Auditory brainstem response is no longer a first line test.

Options are observation, microsurgery and radiation (stereotactic radiosurgery or fractionated radiotherapy). Choice depends on tumour size, growth, hearing, symptoms, age, comorbidity and patient preference.

Observation. For small tumours (intracanalicular or under 15 to 20 mm) with minimal symptoms and serviceable hearing, and for elderly or unfit patients. MRI at 6 and 12 months, then yearly for 5 years, then every 2 to 3 years. Intervene for growth above 2 mm per year, growth beyond 20 mm, brainstem compression, or worsening symptoms. About 30 to 50% eventually need treatment, and hearing declines in about half over 5 years.

Stereotactic radiosurgery. Gamma Knife or linear accelerator, single fraction with a marginal dose of 12 to 13 Gy to the 50% isodose (lower than older doses of 16 to 18 Gy, which reduces cranial neuropathy). Tumour control at 5 to 10 years is 92 to 98%. Suitable for tumours up to about 3 cm and for elderly patients, or for residual tumour after surgery.

Hearing preservation is 50 to 70% at 5 years, falling to about 50% at 10 years; facial nerve preservation is above 95%. Transient swelling (pseudoprogression) can occur 6 to 18 months later, with a small risk of hydrocephalus and a very low risk of malignant change.

Larger tumours (above 2.5 to 3 cm) or those touching the brainstem are treated with fractionated radiotherapy (50 to 54 Gy in 25 to 30 fractions) or hypofractionated radiosurgery (for example 18 Gy in 3 fractions or 20 to 25 Gy in 5 fractions).

Microsurgery. For young patients, tumours over 3 cm with brainstem compression or hydrocephalus, symptoms from mass effect, and patients who want complete excision. Approaches:

• Translabyrinthine: sacrifices hearing, for non-serviceable hearing; direct facial nerve control, any size
• Retrosigmoid: hearing preservation possible, any size, with some risk of persistent headache
• Middle cranial fossa: hearing preservation for small, lateral tumours (usually under 1.5 cm)

Gross total resection is achieved in 90 to 95%. Facial nerve function is good (House-Brackmann grade I or II) in 90 to 95% for tumours under 2 cm and 50 to 70% for tumours over 3 cm. Hearing preservation after small tumour surgery is 40 to 60%.

Complications: CSF leak (5 to 10%), meningitis (1 to 2%), persistent headache, imbalance, stroke, brainstem injury, mortality under 1%. Facial nerve monitoring is standard.

For incomplete facial recovery protect the eye (artificial tears, taping, gold weight, tarsorrhaphy) and consider nerve grafting, hypoglossal facial anastomosis or static slings.

Hearing rehabilitation. Hearing aids and CROS systems, bone anchored hearing aids, cochlear implantation after tumour removal, and auditory brainstem implant for bilateral NF2 deafness.

NF2 schwannomatosis. Care in a specialist multidisciplinary centre. Bevacizumab 7.5 mg/kg IV every 3 weeks (or 5 mg/kg every 2 weeks) for growing vestibular schwannomas improves hearing in about 30 to 40% and stabilises or shrinks tumours in 40 to 60%; monitor blood pressure, proteinuria and bleeding. Early radiosurgery and surgery are individualised to preserve hearing as long as possible. Annual MRI of the brain and spine.

Symptomatic treatment. Vestibular rehabilitation, tinnitus management (sound therapy, cognitive behavioural therapy), ventriculoperitoneal shunt for hydrocephalus, and dexamethasone 4 mg every 6 hours for acute brainstem compression pending surgery.