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An acoustic neuroma is a benign (non-cancerous) growth. It originates in the sheath of the vestibular nerves (the nerves that carry balance signals from the inner ear to the brain) and runs next to the acoustic nerve (which carries hearing signals) and the facial nerve (which carries the signals that move the face muscles).
The condition is misnamed. When the tumor was originally described over a century ago, technology was not sufficiently accurate to determine exactly which nerve these tumors came from and, thus, they were called "acoustic" rather than "vestibular." The most correct name is vestibular schwannoma — vestibular refers to the vestibular nerve, and schwannoma refers to the Schwann cell, the cell from which the tumor arises.
Acoustic neuromas are most commonly diagnosed in people in their late-40s and early-50s. However, with the advent of more accurate scanning technology, they are being discovered at a smaller size and, therefore, at a younger age than just 20 years ago.
There is no known cause of acoustic neuromas. The tumor itself often shows a genetic mutation, but it is usually not a genetic mutation that was inherited from a parent. Instead, the genes change locally and the control of the growth of the nerve sheath cell is lost. There is an unusual condition, called neurofibromatosis type 2 or "acoustic neurofibromatosis" which is inherited and is associated with multiple tumors of nerve sheaths and of the membranes that line the skull ("meningiomas"). If a person has an acoustic neuroma on both sides, or an acoustic neuroma and a meningioma, this diagnosis must be considered.
Making the Diagnosis:
Frequently a patient's first experience relating to an acoustic neuroma may be episodes of dizziness. Because those symptoms are usually brief, it is often suspected to be nothing more than an inner ear infection. The next symptom is typically partial hearing loss. When hearing starts to decline, a person usually seeks additional medical help and requests further evaluation. When noted through an audiogram (a hearing test) that the hearing loss is only in one ear — a sign of acoustic neuroma — a physician's referral for an MRI scan is indicated. It is that scan which determines whether or not a tumor is present.
While there are currently no medications that are effective in treating acoustic neuromas, there is a wide spectrum of treatments — with a wide spectrum of risks. Treatment ranges from "watchful waiting" (where no action is taken, other than regular scans and audiograms, unless a significant change is noted) to surgical removal.
Success of the treatment depends on several factors:
- Watchful waiting is the treatment commonly chosen for a patient with a small tumor and good hearing.
- Watchful waiting is also frequently chosen for a person with a small tumor where hearing is poor, because there is little to gain from treatment (particularly in older patients).
- For patients with small tumors and good hearing, there is an opportunity to preserve the hearing with surgery to remove the tumor. This type of surgery is successful in 50 to 70 percent of cases, depending on various circumstances.
- Surgical removal for a large tumor is necessary if the tumor is pushing on the brain, because it may cause secondary, life-threatening problems, such as buildup of spinal fluid.
Another way of treating acoustic neuromas is with stereotactic radiosurgery. In this procedure, radiation is directed at the tumor with pinpoint precision. The entire treatment is planned and delivered by computer to limit the radiation to the exact site of the tumor. Unlike traditional surgery, the tumor is not removed, yet the radiation has the potential to slow or stop the growth of the tumor.
Stereotactic radiosurgery (sometimes called Gamma Knife® radiosurgery) may be an adequate treatment for a patient who is not experiencing problems related to the tumor but wishes some form of treatment or who will be satisfied if the tumor does not get much larger. It may also be the best choice for people who are on blood thinners or have other health conditions that make surgery unusually dangerous.
The average growth of acoustic neuromas is estimated to be about one to two millimeters in diameter per year. Because they are generally benign and slowly growing, the outlook for people with acoustic neuromas is usually very good.
If ignored and not treated in a young person, the tumor can eventually grow to such a size that becomes life threatening. In young people, tumor removal cures the patient in about 98 percent of the cases. The lifetime recurrence risk if the tumor is completely removed is about one to two percent. For the younger, healthier person, surgery is frequently appropriate, in order to eliminate not only the problem but the worry as well.
In older patients, if acoustic neuromas are discovered to be small, they may not grow enough to ever be a problem. The growth of this type of tumor is somewhat unpredictable therefore occasionally they grow much more rapidly. Either way, if they are discovered and treated before reaching a life-threatening size, then the long-term outlook is excellent.