Acoustic Neuromas

Synonyms: Acoustic neurilemoma, Acoustic neurinoma, Vestibular schwannoma

Acoustic neuromas (AN) are tumours of the vestibular branch of the eighth cranial nerve (the cochlear nerve), arising from the Schwann cells of the nerve sheath. They are typically benign and slow growing, but can cause symptoms through mass effect and pressure on local structures, eventually becoming life-threatening. Considering the possibility can enable earlier diagnosis, increase management options and may decrease morbidity.

AN represents 6-10% of primary intracranial tumours but is the most common form of cerebellopontine angle (CPA) tumour. Sporadic tumours are unilateral and account for 95% of cases, whilst those associated with neurofibromatosis are bilateral and account for the other 5%.

Incidence

There are approximately 13 newly diagnosed cases per million people in the general population per annum. Danish studies showed a rise in incidence between the 1970s to 1990s from 7.8 to 12.4 per million, but this is thought to reflect better diagnosis rather than a true rise in incidence.

Prevalence

Estimates of prevalence are based on autopsy (8000 cases per million) and radiological series (700 per million, based on MRI) suggest that the vast majority of AN cases go undiagnosed.

Risk factors

Confirmed risk factors include:

• Neurofibromatosis
• 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 much later in life² but the medical use of low dose ionising radiation such as used in imaging has not been established as a risk.³)

Hayfever has been suggested as a risk factor although not confirmed.⁴
Occupational noise exposure has not been shown to be a risk factor⁵ although some other epidemiological studies have suggested a link.⁴ Similarly risk due to radiofrequency exposure from mobile phone use remains contentious: the large Interphone case-control study suggested no increased risk in the first decade of use but the risk of longer term use is unknown.⁶

Classic presentation of AN confined to the internal auditory canal, involves unilateral progressive hearing loss (HL), vestibular dysfunction and tinnitus:

• 90% have unilateral progressive HL and 70% tinnitus.
• About 5% of patients will experience sudden and complete unilateral HL.
• Hearing may also fluctuate.
• 3% will have normal hearing at presentation.
• Most patients have quite subtle balance disturbance at presentation.

As the tumour spreads, hearing loss and disequilibrium worsen 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 tumour pressing on the facial nerve.
• Earache
• Ataxia due to cerebellar compression.
• Severe brain stem compression can produce hydrocephalus with visual loss and persistent headache and even decreased level of consciousness.

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

Acoustic neuromas account for 85% of CPA tumours. Other CPA tumours include:¹⁰

• Meningiomas (3-13%)
• Epidermoids (2-6%)
• Lower cranial nerve schwannomas (1-2%)
• Arachnoid cysts (1%)

Audiology

All patients with unilateral or asymmetric HL should be referred for audiological assessment to quantify and clarify the loss as sensorineural.

Diagnostic imaging

MRI has largely superseded CT scanning as the investigation of choice for suspected AN.
See reference¹¹ for examples of audiograms and MRI scans from patients with AN.

There are three basic treatment options for AN: watch-and-wait, surgical removal and stereotactic radiosurgery. There are no studies systematically comparing the different treatment modalities and even evidence for the different treatments is often low grade.^12,13 In this environment of uncertainty and with each option associated with particular risks and benefits, it is critical that the patient is fully counselled and consented to their course of treatment.¹⁴ Consideration needs to be given to future quality of life and symptom relief as well as tumour control, facial nerve function and hearing preservation.

Conservative management

The natural history of acoustic neuromas is not fully understood: in one study looking at small neuromas, with a mean follow-up of 40 months, 66% showed no growth, 24% showed slow growth, 4% showed rapid growth and 3% regressed.¹⁵ Therefore, in patients with small neuromas and good preserved hearing, the wisest course of action is often to watch-and-wait with serial scans (usually annually) to monitor growth. Where growth is detected, more active treatment is usually advocated since risk of complications with surgery and ability to preserve hearing is related, in part, to tumour size. There is some evidence that quality of life is reduced in untreated patients.¹² There is no agreed size of tumour triggering active treatment across centres.

Surgical treatment

In the UK, the majority of patients receiving treatment for AN 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 about 95% of cases. Risks of surgery include:

• Mortality (risk about 1%)
• CSF leak and meningitis
• Cerebellar injury
• Stroke
• Epilepsy
• Facial paralysis (either partial or complete)
• Hearing loss¹⁶
• Balance impairment¹⁷
• Persistent headache¹⁸

Stereotactic radiosurgery

Stereotactic radiosurgery targets 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. A variant, fractionated (smaller, multiple dose) stereotactic radiotherapy is advocated by some centres. Neither of these treatments is conceived to remove neuromas but rather to control (either slow or stop) their growth.
One small prospective cohort study suggests that for unilateral, small (<3cm) tumours, early outcomes (facial movement, hearing, general health status) for stereotactic radiosurgery were better than for surgical resection with equivalent tumour control.¹⁹ However, longer term follow-up is required to indicate that tumour progression is not worse in the stereotactic radiosurgery group. Other potential longer-term risks associated with stereotactic radiosurgery and stereotactic radiotherapy include:

• Radiation-induced brain necrosis²⁰
• Radiation-related cranial nerve injury
• Malignant change (for example, to a glioblastoma multiforme²¹)
• Salvage surgery for neuromas post-radiotherapy is technically more difficult than primary surgery