Acoustic neuroma

Alternative names

Vestibular schwannoma, acoustic neurinoma, acoustic neurofibroma

Summary

• An acoustic neuroma is a benign tumour of the acoustic or auditory nerve, which leads from the brain to the inner ear.
• One possible cause of acoustic neuroma is a defect in a certain tumour-suppressor gene.
• Acoustic neuromas are relatively uncommon as a whole, but account for 5-10% of all brain tumours.
• Symptoms vary with the size and location of the tumour. Early symptoms include impaired hearing.
• Treatment may include surgical removal of the tumour or radiation therapy to retard its growth.

Definition

An acoustic neuroma is a benign (doesn't spread to other parts of the body) brain tumour that develops in the acoustic or auditory nerve, which is responsible for hearing. The acoustic nerve, also called the 8th cranial nerve or vestibulocochlear nerve, leads from the brain to the inner ear. This nerve has two parts, one part associated with transmitting sound and the other with sending balance information to the brain from the inner ear.

These pathways, along with the facial nerve, lie adjacent to each other as they pass from the skull cavity and enter a bony canal called the internal auditory canal, which leads to the inner ear. This canal is approximately 2cm long and it's here that acoustic neuromas originate from the sheath surrounding the acoustic nerve.

Acoustic neuromas usually grow slowly at their site of origin over a period of years. The slowly enlarging tumour protrudes from the internal auditory canal and eventually forms a pear shape with the small end in the internal auditory canal. Larger tumours can press on the facial nerve. Vital functions to sustain life can be threatened when large tumours cause severe pressure on the brainstem and cerebellum at the base of the brain.

Tumours are typically described as small (less than 1.5cm), medium (1.5cm to 2.5cm) or large (more than 2.5cm).

Cause

The cause is unknown. Acoustic neuromas are thought to arise when there is a defect in a certain tumour suppressor gene. It is unknown, however, why this genetic defect occurs.

Who gets it and who is at risk?

Acoustic neuromas are relatively uncommon as a whole, but account for 5-10% of all brain tumours.

Most acoustic neuromas are diagnosed in patients between the ages of 30 and 60. For unknown reasons they are more common in women than men.

Although there is a high association of acoustic neuromas with an inheritable condition called neurofibromatosis, most acoustic neuromas occur spontaneously without any evidence of an inheritable pattern.

Symptoms and signs

Symptoms vary with the size and location of the tumour, and may include:

• Hearing loss in the affected ear. The first symptom in 90% of people with an acoustic neuroma is reduced in hearing in one ear, often accompanied by tinnitus (ringing) in one ear. The loss of hearing is usually subtle and worsens slowly, although occasionally hearing loss is sudden. The hearing loss may first become apparent as difficulty understanding speech, particularly when using the telephone.
• A feeling of fullness, numbness or pain in the affected ear.
• Unsteadiness, dizziness and balance problems. The remainder of the balance system sometimes compensates for this loss, in which case no imbalance will be noticed.
• Larger tumours can press on the nearby facial nerve, causing facial numbness and tingling or pain. This can progress to facial paralysis, with drooping of the mouth or eyelid on one side of the face and sometimes drooling.
• Vision abnormalities.
• Enlarged, dilated pupil in one eye.
• Increase of intracranial pressure (pressure inside the skull) may result in headaches (particularly on waking), clumsy gait, lethargy and mental confusion. This is a life-threatening complication requiring urgent treatment.
• People who develop acoustic neuroma as part of the disease neurofibromatosis may have bilateral tumours (affecting both sides of the brain).

Diagnosis

Early detection and treatment of an acoustic neuroma is essential for a good prognosis. Initial symptoms are easily overlooked and are sometimes mistaken for normal changes of ageing.

In order for the doctor to plan your treatment, he or she needs to find out as much as possible about the type, position and size of the tumour. This is done through several tests and investigations. A medical history will be taken and a neurological examination performed to assess any effect the tumour has had on your nervous system. Your hearing will be tested and sometimes your sense of balance.

Useful tests used to diagnose an acoustic neuroma and to differentiate it from other conditions include the following:

• Routine auditory tests may reveal a loss of hearing and speech discrimination (the patient can hear sound in that ear but cannot understand what is being said). An audiogram (the result from a test using an instrument called an audiometer) should be performed to evaluate hearing in both ears effectively. This test is important to begin the diagnosis of an acoustic neuroma.
• If you have symptoms of vertigo or dizziness, caloric stimulation (a test for vertigo) or electronystagmography (a test of equilibrium and balance) may be performed.
• Magnetic resonance imaging (MRI) of the head is the diagnostic test that is the most useful (i.e. sensitive and specific) for identifying acoustic neuromas. The image formed clearly defines the presence and location of an acoustic neuroma. The MRI is nearly 100% accurate for this purpose.
• When an MRI is not available or cannot be performed, a computerised tomography scan (CT scan or CAT scan) is suggested in cases of suspected acoustic neuroma.
• Brainstem auditory-evoked response (BAER, BSER, ABR), a test of hearing and brainstem function, may be done in some cases. This test provides information on the passage of an electric impulse along the neural pathways between the inner ear and the brainstem. An acoustic neuroma can interfere with the passage of this electrical impulse. A positive test result implies the possible presence of an acoustic neuroma.
• To confirm the exact type of tumour, a biopsy (tissue sample) is taken from the tumour and examined under a microscope.

Neurofibromatosis should be considered in all patients newly diagnosed with an acoustic neuroma. If found, genetic evaluation and counselling should be provided for relevant family members.

Treatment

Surgery

Surgery is the treatment of choice for the majority of patients with symptoms of acoustic neuroma.

Goals of surgical treatment are removal of the tumour and prevention of facial paralysis, and, if possible, preservation of hearing - although this is more difficult. If a tumour is removed when it is very small, hearing may be preserved. Any hearing lost prior to surgery will not be regained.

Large tumours usually result in total loss of hearing on the affected side and may also compress nerves important for facial movement and sensation. These tumours are usually safely removed but the surgery often results in paralysis of some facial muscles. Often exercises and massage can help to stimulate facial movement but these should be done under medical supervision.

Extremely large tumours may additionally compress the brainstem, threatening other cranial nerves and preventing the normal flow of cerebrospinal fluid (fluid that cushions the brain and spinal cord). This can lead to a buildup of fluid in the head (hydrocephalus) which can cause potentially life-threatening increased intracranial pressure. Goals of surgery in these cases are treatment of the hydrocephalus and decompression of the brainstem.

For people with bilateral tumours and the risk of total hearing loss, surgery may be delayed if at all possible. In such cases, regular scans will be done to check for any growth of the tumours.

Radiation therapy

Radiation therapy (radiotherapy or radiosurgery), the use of high-energy rays to destroy cancer cells, is occasionally used to treat an acoustic neuroma. It may be used if surgery is not possible, for example in elderly or sick patients who are unable to tolerate brain surgery.

Sometimes during brain surgery to treat acoustic neuromas, not all of the tumour can be safely removed, and some residual tumour must be left behind. Radiotherapy is often used post-operatively to treat residual tumour tissue in these cases. The goal of radiotherapy is to slow or stop the tumour growth, not to cure or remove the tumour.

Radiotherapy is only appropriate for small tumours so that radiation damage to surrounding tissues can be minimised. Like brain surgery, radiotherapy can sometimes result in facial paralysis or loss of hearing.

Observation

Since acoustic neuromas usually grow very slowly, small tumours that have minimal or no symptoms (asymptomatic) can be safely observed with regular MRI scans to ensure they are not growing to a dangerous size. Very often elderly patients will die of other natural causes before small, slow growing tumours become symptomatic.

Treatment of hearing loss

Unfortunately, hearing loss in the affected ear is unavoidable in many cases. Often a hearing aid that diverts sounds from the affected ear to the good ear can be fitted. Your doctor will be able to give you more information about this. Hearing therapists (contactable through your GP) can help with special equipment for people with hearing problems and provide counselling and help with communication.

Outcome

Acoustic neuromas are benign, i.e. they do not spread (metastasize) to other body systems. However, they may continue to grow at their site of origin and, when large enough, can compress vital structures within the skull. The brain is not invaded by the tumour but the tumour pushes against the brain as it enlarges. Continued tumour growth can cause permanent hearing loss and brain damage and be life-threatening.

The likely outcome of treatment is as follows:

• Brain surgery results in complete removal of the tumour in more than 95% of cases with less than 1% mortality.
• About 95% of patients with small tumours will have no permanent facial paralysis following surgery. However, roughly two-thirds of patients with large tumours will have some permanent facial weakness following surgery. Facial palsy can cause problems with facial movements, such as chewing and blinking. Damage to the facial nerve, which controls blinking, may lead to eye problems, such as dry eyes or difficulty in closing the eye properly.
• If a tumour is removed when it is very small, hearing may be preserved. Approximately one half of patients with small tumours will retain useful hearing in the affected ear following surgery. A large tumour may be safely removed, but may result in total deafness of the affected ear.
• There may be delayed radiation effects following radiosurgery, including nerve damage, loss of hearing and facial paralysis.

When to call the doctor

• Consult your doctor if hearing loss occurs or worsens.
• Seek immediate medical attention if you experience any symptoms of increased intracranial pressure, such as headaches, clumsiness and mental confusion. This is a life-threatening condition requiring urgent treatment.

Prevention

Because the cause of an acoustic neuroma is currently unknown, ways to prevent these tumours from arising are unknown.

Reviewed by Dr Andrew Rose-Innes, neurologist, Orgeon Clinic, Portland, United States, 2010