A Discussion of Acoustic Neuroma
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% of all brain tumors. They are located between the brain stem and the inner ear, adjacent to vital brain centers. As they grow, they cause additional involvement by pressing on surrounding nerves. If they are allowed to grow over a long period time, they 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. 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.
Acoustic tumors usually displace normal tissue as they grow, yet remain within their lining (encapsulated). An acoustic neuroma first distorts the eighth nerve, and then presses on the seventh (facial) nerve. The slowly enlarging tumor protrudes from the internal auditory canal and as mentioned will ultimately press into the brainstem. The tumor also may press on adjacent nerves, such as the fifth, or trigeminal nerve, which is the nerve providing facial sensation
Many diagnostic 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 (i.e., loss of life) is less than 1%. When they are middle or large sized, the mortality rate is approximately 3%. 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 first symptom from the tumor is usually some disturbance in hearing. This is due to pressure on the cochlear portion of the eighth nerve but also due to involvement of the blood supply to the hearing structure. The cochlear nerve is more sensitive to pressure than most other cranial nerves. The ability to hear requires not only an intact nerve but adequate blood supply to the hearing structure (cochlea). The blood supply to the cochlear nerve and to the hair cells in the cochlea comes from the internal auditory artery.
DIAGNOSIS OF ACOUSTIC NEUROMA
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 brainstem response (ABR) test, is usually perfomred by the audiologist. This uses a sophisticated 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 sophisticated imaging methods used to determine (by picture) the cerebellopontine angle region of the brain to determine if there is an acoustic neuroma present. At times, an electronystagmagram (test of the balance system) may be used to determine the involvement of the tumor with the balance nerves.
At this time, the primary treatment used to cure a patient with an acoustic neuroma is surgical removal. With the use of an operating microscope and lasers, the Neurotologist 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 which has recently developed is the use of stereotactic radiation therapy (i.e., gamma knife). Stereotactic radiation
therapy is a technique 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 patients who are elderly, or have medical problems such as cardiac or pulmonary disease and are at high risk for surgical removal.
Increasingly 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.