Acoustic tumors are fibrous growths originating from the auditory or balance nerves and are usually not malignant. They do not spread to other parts of the brain other than by direct extension. They constitute approximately 10 percent of all brain tumors and are located between the brain stem and the inner ear, adjacent to vital brain centers. As they grow, they press on surrounding nerves and, over time, can press on vital brain centers, and thereby could be fatal.
In most cases, these tumors grow slowly. The cause of most acoustic neuromas is unknown, although there is a small group of patients who have acoustic tumors as a result of having neurofibromatosis Type II, which is a genetically-based disease. The first symptom from the tumor is usually some disturbance in hearing. The cochlear nerve is more sensitive to pressure than most other cranial nerves, and the ability to hear requires not only an intact nerve, but adequate blood supply to the hearing structure (cochlea).
Diagnosis of Acoustic Neuroma
Many procedures are used to be as certain as possible of an accurate diagnosis and to determine the extent of the tumor growth. When the tumors are small, the mortality rate is less than 1 percent. When they are middle or large sized, the mortality rate is approximately 3 percent. The second objective of surgery is to preserve as many vital structures as possible. In many cases, a completely normal life results following surgery and in others, which are larger and more involved, some permanent handicap may result.
The diagnosis of an acoustic neuroma begins with a complete history, physical examination, and conventional audiologic evaluation. Following this, if there are any indications of tumor, an auditory brain stem response (ABR) test is usually performed by the audiologist. This uses a computerized audiometer to analyze the electrical activity of the hearing nerves on both sides to determine if there is a normal conduction of neural signals to the brain. If the conduction is slowed down on one side, this may indicate a tumor. CAT scanning or Magnetic Resonance Imaging (MRI) scans are used to determine if there is an acoustic neuroma present. At times, an electronystagmogram (test of the balance system) may be used to determine the involvement of the tumor with the balance nerves.
Acoustic Neuroma Treatment
At this time, the primary treatment for acoustic neuroma is surgical removal. With the use of an operating microscope and lasers, the neurologist and neurosurgeon are able to remove the tumor with some preservation of hearing in most cases. The size of the tumor, hearing status, patient’s age, and health determine which surgical approach is utilized.
Another form of treatment is the use of stereotactic radiation therapy (i.e. gamma knife). Stereotactic radiation therapy is based upon the principle that a single high dose of radiation delivered precisely to a small area of tumor could arrest the growth of the tumor and not damage surrounding brain tissue and/or function. There is no incision. Almost all patients are discharged from the hospital the same day or the following day. Life long follow up with MRI scans is currently recommended. Unlike microsurgery, which would remove the tumor, stereotactic radiation therapy “controls” the tumor growth. The tumor does not disappear, but persists (allegedly) in a “harmless” state with a greater degree of facial nerve preservation. Radiation therapy represents an alternative for elderly patients or patients with medical problems, such as cardiac or pulmonary disease, and are at high risk for surgical removal.
With improved surgical techniques and with small tumors, it is possible to remove the tumor and preserve hearing. Even if hearing cannot be maintained in that ear, CROS hearing aids have allowed the transfer of sound from the operated ear side to the functional ear so that one may “hear” from both sides. The earlier tumors are diagnosed and removed, or progress slowed, the less likely the possibility of serious complications.
For further information, visit the Acoustic Neuroma Association’s website at www.anausa.org.