Acoustic neuroma

What is acoustic neuroma?

An acoustic neuroma is a type of benign (noncancerous) brain tumor that grows on the vestibular nerve as it travels from the inner ear to the brainstem. It is one of the most common types of benign brain tumors. The first sign of one is usually hearing loss.

The cochleo-vestibular nerve (also called the eighth cranial nerve) is made up of three nerves that connect the inner ear to the brain. One branch — the cochlear nerve — carries hearing information. The other two branches — the inferior and superior vestibular nerves — carry balance information to the brain. The nerves are wrapped in a layer of specialized cells called Schwann cells. An acoustic neuroma — also called a vestibular schwannoma — is a tumor of those cells. If an acoustic neuroma is not diagnosed or treated, it can grow large enough to press on important structures in the brainstem and cause serious problems.

The main symptoms of an acoustic neuroma are hearing loss and tinnitus (ringing in the ears). They are caused by the tumor pressing on the auditory nerve. Although the tumor actually grows on the balance nerve, imbalance is usually mild or absent. Since we have two balance systems, the opposite side can compensate for the slow progressive loss caused by the tumor.

These tumors have been linked to a mutation in a protein that regulates tumor suppression. In most cases the tumor grows only on one side of the head and is diagnosed between the ages of 30 and 50. Acoustic neuromas in children are very rare. People with a hereditary disease called neurofibromatosis type 2 develop bilateral acoustic neuromas because they lack the tumor suppressor protein merlin. About 10% of all acoustic neuromas occur in people with neurofibromatosis.

Symptoms of acoustic neuroma

Acoustic neuromas generally grow slowly so the symptoms develop gradually and are easy to miss or misinterpret. The earliest and most common symptoms of an acoustic neuroma are:

• Loss of hearing in one ear — This usually is gradual, but can occur suddenly in 10% of cases.
• Tinnitus, a ringing, buzzing or noisy sound in the ear when there is no external sound present.

Less common symptoms include:

• dizziness or loss of balance
• numbness in the face
• headache
• facial weakness
• mental confusion.

Diagnosing acoustic neuroma

Because the symptoms of acoustic neuroma are often subtle and slow to develop, they can be missed easily in their early stages. Gradual hearing loss, especially if it occurs only in one ear, always should be checked by a physician.

If your doctor suspects that you have an acoustic neuroma, he or she will examine you to look for other conditions that can cause similar symptoms. This examination usually will include:

• looking in your ears with a lighted magnifying lens
• using tuning forks to test your hearing
• examining your nose, throat and neck
• testing the nerves in your face
• checking your balance.

Your doctor also may recommend a formal hearing test (audiogram) to determine the type and amount of the hearing loss.

Sometimes your doctor may recommend an auditory brain-stem response test, also called evoked potentials or evoked responses. In this test, electrodes are placed on the scalp to measure the brain's electrical responses to various noises. The test measures the speed that the sound is transmitted through the brain. This test will be abnormal and show a delay in the transmission if a tumor is pressing on the nerve that carries signals from the ear to the brain (the cochlear nerve).

If an examination and hearing testing indicate a possible acoustic neuroma, your doctor may order additional tests to confirm the diagnosis. Most commonly, he or she will recommend a magnetic resonance imaging (MRI) scan. MRI uses magnetic waves to create pictures of structures inside the body. These pictures can show whether you have an acoustic neuroma, how big the tumor is, and where it is located. An MRI can detect tumors as small as 2 mm.

Expected duration of acoustic neuroma

Most acoustic neuromas grow slowly, taking years before they become large enough to cause symptoms. The average growth rate is 2 mm/year. A few acoustic neuromas do grow at a faster rate. At least 10% of all acoustic neuromas, once found, do not show further growth. There is no way to tell the growth rate of any tumor except by periodic MRI scans.

Preventing acoustic neuroma

There is no known prevention for acoustic neuromas.

Treating acoustic neuroma

There are three ways to treat acoustic neuromas — observation, radiation and surgery.

At least 10% of acoustic neuromas do not show signs of growth after they are found. Since the tumor is very slow-growing and benign, having a follow-up MRI scan and an audiogram in six and 12 months is a safe alternative to immediate intervention. If no changes are found, yearly checkups afterward are adequate to monitor the tumor. If the tumor does not show signs of growth, intervention is not necessary. The risk of this approach is that further permanent hearing loss can occur during this observation period.

If the tumor shows signs of growth or is pressing on the brainstem, radiation or surgery are necessary. The choice between the two depends upon a lot of factors best discussed with your surgeon and radiation oncologist. Factors such as size and location of the tumor, related health issues, age, and hearing loss all need to be considered.

If surgery is necessary it is usually performed by a team consisting of a neurosurgeon and an otologist. The neurosurgeon removes the part of the tumor around the brain and the otologist removes the part of the tumor in the ear. Hospitalization is usually for four to seven days after surgery. Possible complications of surgery include loss of hearing and injury to the facial nerve, the nerve that supplies motion to the face.

Radiation is an alternative to surgery. It does not remove the tumor, but many times it can stop the tumor growth or cause the tumor to shrink. Radiation can be delivered in a number of different ways — gamma knife, stereotactic radiosurgery, proton beam radiation, and fractionated stereotactic surgery. The choice is made after discussion with the radiation oncologist. Possible complications of radiation include loss of hearing, facial nerve injury, and continued growth of the tumor.

When to call a professional

See your physician if you develop new hearing loss or tinnitus, particularly if the hearing loss or tinnitus is only on one side.

Prognosis

Acoustic neuromas are not cancerous (malignant) and do not spread to other parts of the body. Timely diagnosis and proper treatment can minimize the losses that they cause and prevent any shortening of ones lifespan.

Additional info

Acoustic Neuroma Association
https://www.anausa.org/

National Institute on Deafness and Other Communication Disorders
https://www.nidcd.nih.gov/

American Academy of Otolaryngology — Head and Neck Surgery
https://www.entnet.org/