Acoustic neuromas

Synonyms: acoustic neurilemoma, acoustic neurinoma, vestibular schwannoma

What is acoustic neuroma?

Acoustic neuromas are tumours of the vestibulocochlear nerve (eighth cranial nerve), arising from the Schwann cells of the nerve sheath. Most arise from the vestibular portion and only a few arise from the cochlear (auditory) division. They are typically benign and slow-growing, but can cause symptoms through mass effect and pressure on local structures, eventually becoming life-threatening. Patterns of growth can vary and a small number may grow rapidly (doubling size in six months). Considering the possibility can enable earlier diagnosis, can increase management options and may decrease morbidity.

Within the cerebellopontine angle, tumours can grow as big as 4 cm in diameter and slow growth allows stretching and accommodation of growth without affecting function. However, tumours within the internal auditory canal produce symptoms much earlier with hearing loss (the most common presenting symptom) or vestibular disturbance.

How common are acoustic neuromas?¹

Acoustic neuromas represent the third most common intracranial non-malignant tumour after meningiomas and pituitary adenomas. They are the most common extra-axial posterior fossa tumours in adults, comprising over 80% of tumours in the cerebellopontine angle.

In most cases, the tumours present unilaterally; bilateral are a hallmark of neurofibromatosis type 2.

Risk factors for acoustic neuroma

Confirmed risk factors include:

• Neurofibromatosis.²

• High-dose ionising radiation (children who received radiation for benign conditions of their head and neck - for example, to decrease the size of their tonsils and adenoids - were at increased risk of developing an acoustic neuroma much later in life.)³ However, the medical use of low-dose ionising radiation, such as used in imaging, has not been established as a risk.⁴

• Many epidemiological studies have failed to find a link between acoustic neuroma and mobile phone use. A 2009 systematic review and meta-analysis evaluating long-term mobile phone use found a 60% increase in risk for acoustic neuroma following ten years of ipsilateral mobile phone usage, but the methodology used by this study has been questioned⁵

Symptoms of acoustic neuromas

Clinically, most patients present with unilateral sensorineural hearing loss (94%) and tinnitus (83%). The frequency of the vestibular symptoms vertigo and unsteadiness varies widely (17‒75% of patients), but they are probably underreported. Large tumours may cause trigeminal and facial neuropathies as well as brainstem compression and hydrocephalus.¹

• A small minority of patients will experience sudden and complete unilateral hearing loss.

• Hearing may also fluctuate.

• A few patients will have normal hearing at presentation.

• Most patients have quite subtle balance disturbance at presentation.

• The severity of tinnitus has been shown to correlate with tumour size.⁶

As the tumour spreads, there is an increase in hearing loss and disequilibrium, and symptoms due to compression of other structures may occur:

• Facial pain or numbness due to involvement of the trigeminal nerve.

• Facial weakness is uncommon despite the tumour pressing on the facial nerve.

• Earache.

• Ataxia due to cerebellar compression.

• Severe brainstem compression can produce hydrocephalus with visual loss and persistent headache and even decreased level of consciousness.

Acoustic neuroma may be diagnosed incidentally and earlier as a result of investigations for unrelated problems.

Associated diseases

Bilateral acoustic neuroma occurs in neurofibromatosis-type 2 (NF2). NF2 is an autosomal dominant disorder (ie has a 50% risk of transmission from a parent) but also shows high levels of mosaicism. 7% of patients with acoustic neuroma also have NF2.⁷ Acoustic neuroma due to NF2 tends to present earlier, typically around 30 years old. Genetic screening for NF2 in patients presenting with sporadic, unilateral acoustic neuroma is usually only productive in cases of very early onset (younger than 20 years)/⁸ NF2 patients are predisposed not only to developing acoustic neuroma but also schwannomas of other cranial nerves.⁹

Increasing symptoms associated with acoustic neuroma increase the likelihood of clinically significant anxiety and depression.¹⁰

Differential diagnosis

Other CPA tumours include meningiomas, epidermoids, lower cranial nerve schwannomas and arachnoid cysts.

Investigations⁹

Audiology

If hearing impairment is present, the patient should be referred for audiometric testing

Diagnostic imaging¹

Gadolinium-enhanced MRI is considered the gold standard, but one review found that non-enhanced MRI was more cost-effective. CT scanning is an alternative but is more expensive and less accessible.

Treatment for acoustic neuromas

Current treatment options include observation, surgical resection, fractionated radiotherapy, and radiosurgery. The choice of treatment depends on clinical presentation, tumour size, and expertise of the treating centre. In small tumours, observation has to be weighed against radiosurgery; in large tumours surgical decompression is mandatory, potentially followed by fractionated radiotherapy or radiosurgery. Except for bevacizumab in neurofibromatosis type 2, there is no role for pharmacotherapy.¹

Conservative management⁵

For patients with small neuromas and good preserved hearing, the most appropriate course of action may be to watch and wait with serial scans to monitor growth. Growth usually manifests within the first three years after presentation, and a recommended protocol is serial MRI scans at six-monthly intervals for two years, and another scan two years later, followed by scans every five years with lifelong follow-up. There appears to be no associated increase in mortality. Where growth is detected, more active treatment is usually advocated since risk of complications with surgery and ability to preserve hearing are related, in part, to tumour size. There is no agreed size of tumour triggering active treatment across centres.

Surgical treatment

In the UK, most patients receiving active treatment undergo microsurgery. The surgical approach taken depends on the location of the tumour, its size and the relative importance of hearing preservation. Complete removal is possible in most cases. The risks of surgery include:¹¹

• Mortality (risk about 1%).

• CSF leak and meningitis.

• Cerebellar injury.

• Stroke.

• Epilepsy.

• Facial paralysis (either partial or complete).

• Hearing loss.⁵

• Balance impairment.¹²

• Persistent headache.⁵

Radiotherapy⁵

There are three forms of radiotherapy. stereotactic radiosurgery (SRS), fractionated stereotactic radiotherapy (FSRT), and proton beam therapy.

Stereotactic radiosurgery (SRS)
This is the most common method. SRS targets a tumour with a single large dose of radiation using convergent beams of high-energy X-rays, gamma rays ('gamma knife radiosurgery') or charged particles and is an alternative, emergent treatment. The aim of treatment is to control (either slow or stop) the growth of the tumour. Stereotactic radiation may be associated with significantly better long-term hearing preservation outcome rates than microsurge.

Salvage surgery for neuromas post-radiotherapy is technically more difficult than primary surgery. Other potential longer-term risks associated with stereotactic radiosurgery and stereotactic radiotherapy include:

• Radiation-induced brain necrosis.¹³

• Radiation-related cranial nerve injury.

• Malignant change (for example, to a glioblastoma multiforme).¹⁴

Fractionated stereotactic radiotherapy (FRST)
Unlike SRS, FRST requires numerous sessions of radiotherapy in an attempt to target the tumour at the most radiation-sensitive phase of the cell cycle but is more readily available in hospitals and can be used for larger lesions.

Proton beam therapy
This is less available than the other two modalities and the evidence supporting efficacy is limited.

Emerging treatments⁵

Targeted biological therapies are emerging as potential treatments - eg, bevacizumab, everolimus, and lapatinib. There may also be a role for aspirin in slowing tumour growth.