What is an acoustic neuroma?

Acoustic neuroma brain tumour is a sub-type of schwannoma that occurs in the inner ear, wrapping around the vestibular (auditory) nerve, situated in the inner ear. This type of tumour is also called vestibular schwannoma.

The vestibular nerve is one branch of the vestibulocochlear nerve, the other branch being the cochlear nerve. The vestibular nerve is responsible for carrying messages from your inner ear to your brain and is therefore related to your ability to hear, as well as contributing to your sense of balance and perception of body position.

What are the symptoms of acoustic neuroma? 

Symptoms are caused when the tumour grows to a point where it puts pressure on the acoustic nerve or on adjacent nerves. This may include the facial nerve that controls facial muscles, or trigeminal nerves that influence sensation in the face. They may sometimes also press on nearby blood vessels or other structures within the brain.

Symptoms may include one or more of the following:

• Hearing loss that becomes gradually (or occasionally suddenly) worse, usually on one side. This hearing loss may also fluctuate.
• Tinnitus: a buzzing or ringing noise in the ear, usually on one side
• Facial muscle weakness, numbness or pain
• Persistent headaches
• Dizziness
• Vertigo
• Weakness in the arms or legs
• Balance issues, especially if walking in the dark or on uneven ground
• Poor limb co-ordination (ataxia) on one side of the body

What causes an acoustic neuroma?

Most occur spontaneously. The reason why they occur is unclear because there are so many factors that could potentially cause a fault in a gene and hence trigger a tumour. Acoustic neuromas are caused by changes in a gene called NF2 (which is a gene on chromosome 22 in our DNA).

A healthy gene in this position produces a tumour suppressor protein designed to control the growth of Schwann cells, so when it doesn’t work properly, the Schwann cells grow with a lack of control and co-ordination, hence forming a tumour.

Acoustic neuromas can also occur as part of genetic diseases called neurofibromatosis 2 and schwannomatosis. This latter condition tends to occur in early adulthood and can be preceded by chronic pain due to the fact that pain signals originating from nerves where tumours are causing pressure can sometimes be felt at distant sites within the body (where there aren't any tumours).

Only around 7% of people diagnosed with an acoustic neuroma also have neurofibromatosis 2. Anyone diagnosed with any form of neurofibromatosis would be transferred to a specialist team to ensure that patients are cared for by clinical experts in treating this condition.

Are acoustic neuroma low-grade (benign) or high-grade (malignant) brain tumours?

Almost all are classified as grade 1, which is the slowest growing type of tumour. They are often referred to as “benign”, but as the symptoms tend to get worse over time as the tumours increase in size, many patients and clinicians prefer to use the term “slow-growing” or “low-grade”. 

Only very rarely do acoustic neuromas grow quickly (become malignant). The most common low-grade brain tumour in adults is meningioma, followed by pituitary adenoma and acoustic neuroma.

What treatment is available for acoustic neuroma?

Some grow so slowly that they are simply monitored using regular scans, and may not need to be treated. Others can grow rapidly and will be treated more quickly.

Surgery

Many acoustic neuromas can be completely removed by surgery. If the tumour can not be completely removed and slowly grows back, surgery may be offered again to reduce tumour size again.

There are three types of craniotomy that a neurosurgeon would consider when treating an acoustic neuroma: a craniotomy is a type of brain surgery in which a small section of the skull (referred to as a “bone flap”) is removed so that the brain underneath can be accessed. The bone flap is replaced after the surgery, secured using a metal plate and screws, and the skin replaced over the top.

The type of craniotomy depends on factors such as tumour size, position and extent of hearing loss at the time of treatment.

• Translabyrinthine craniotomy is used when there is no useful hearing left in the affected ear, or the tumour is deemed to be in a position where removal takes priority over retaining hearing. The neurosurgeon makes an incision in the scalp behind the ear, then in order to access the tumour has to remove the mastoid bone and the semicircular canals of the inner ear bone, which contain receptors for balance. Although hearing is lost after a translabyrinthine craniotomy, the risk of facial nerve injury may be reduced.
  
  
• Suboccipital craniotomy (also known as retro sigmoid craniotomy) is when a portion of the occipital bone behind the ear is removed in order to access the tumour. This is often used when there is still a good amount of hearing in the affected ear, but a medium or large acoustic neuroma is causing symptoms such as balance issues due to pressure on the brainstem.
  
  
• Middle fossa approaches are useful for small tumours (up to 2cm diameter) in a position where hearing could be preserved. The neurosurgeon approaches the tumour via the temporal bone, located just above the ear canal.
  
  

Intratympanic gentamicin therapy

This therapy is sometimes offered to patients before they have surgery for a tumour that affects hearing or balance, such as an acoustic neuroma. It is most commonly used to treat severe dizziness (vertigo) and tinnitus that people diagnosed with Meniere’s Disease experience.

• Gentamicin is an antibiotic that is used to purposely damage the vestibular (balance) nerve and gradually stop it from working. A low dose of the antibiotic is injected through the eardrum into the inner ear under local anaesthetic, then drained away again after 30 minutes, during which time the patient rests quietly on their side. The procedure may be repeated more one more time, about one month later, if necessary.
  
  
• Intratympanic gentamicin therapy before brain tumour surgery causes a gradual loss of the balance function in one ear, which is easier for patients to cope with than the sudden loss of function that may result from neurosurgery to remove the tumour. It speeds up the recovery process afterwards, including how effectively the brain adjusts to the new way that it gains information about balance. 

Stereotactic radiotherapy

This is a highly targeted form of radiotherapy that may be used to treat small acoustic neuromas (usually less than 3cm diameter), dependent on their position.

Radiotherapy

Radiotherapy is rarely used for slow-growing forms of acoustic neuroma in order to avoid damage to the delicate nerves that they are wrapped around. However for tumours classified as malignant, radiotherapy would be used to help prevent recurrence after surgery.

Chemotherapy

Chemotherapy is rarely used for slow-growing acoustic neuroma tumours because most chemotherapy drugs are designed to target cells that are dividing rapidly.

Molecular profiling

There is a lot of research in this area at the moment to clarify which molecular characteristics can be relied upon to guide treatment decisions. It also enables researchers to explore drugs that can target specific genetic mutations, leading in the future towards more effective, personalised treatment protocols.