An acoustic neuroma (also known as a “vestibular schwannoma”) is a benign, slow-growing tumor of the nerve leading from the brain to the inner ear (the eighth cranial nerve or auditory nerve). This nerve is associated with balance and 
hearing transmission to the brain. The nerve pathway passes through a bony canal called the internal auditory canal, and it is here that acoustic neuromas originate from the sheath that surrounds the nerve. Eventually the tumor will protrude from the internal auditory canal into an area behind the temporal bone called the cerebellopontine angle. This area is located at the base of the brain where the nerve leaves the skull cavity and enters the bony structure of the inner ear.
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Incidence
Acoustic neuromas constitute about 8% of all primary intracranial (inside the skull) tumors and are said to affect approximately one out of every 100,000 people each year. Symptoms may develop at any age but usually occur between the ages of 30 and 60 years. The tumor comes from an overproduction of Schwann cells—the cells that normally wrap around nerve fibers like onion skin to help support and insulate nerves.
Acoustic neuromas are thought to arise when there is a defect in a certain tumor suppressor gene which normally prevents tumors from growing. The cause of the genetic defect is not known, but acoustic neuromas are not thought to be hereditary. However, acoustic neuromas are often linked with the genetic disorder neurofibromatosis type 2.
Symptoms
Although acoustic neuromas grow slowly over a period of years, when they become large, they can displace normal brain tissue. As the tumor grows, it usually causes one-sided hearing loss, ringing in the ears (tinnitus), and dizziness or loss of balance (vertigo). When the tumors press on the trigeminal nerve (another cranial nerve), facial sensation can be affected. If the tumor presses on the brainstem (the lower part of the brain that connects to the spinal cord) or cerebellum (the lower back part of the brain that helps coordinate movement), vital functions to sustain life can be threatened.
Less common symptoms of acoustic neuromas include headache (especially aggravated by lying down, coughing, sneezing, or straining), numbness in the face or one ear, and vision abnormalities.
Diagnosis
Hearing loss, ringing in the ears, and loss of balance or dizziness are early signs of an acoustic neuroma. Unfortunately, early detection of the tumor is sometimes difficult because the symptoms may be subtle and may not appear in the beginning stages of growth. Also, hearing loss, dizziness, and tinnitus are common symptoms of many middle and inner ear problems. Once the symptoms appear, a thorough ear examination and hearing test are essential for proper diagnosis. CT scans and MRI are critical in the early detection of an acoustic neuroma and are helpful in determining the location and size of the tumor. Of the two types of imaging, MRI is currently the preferred test for identifying acoustic neuromas.
Treatment
There are three treatment options available to patients with acoustic neuromas. These options are observation, microsurgical removal of the tumor, and radiation using radiosurgery (or Gamma Knife) or radiotherapy. Choosing the best treatment option is a decision that must be made by the patient and physician after careful review of the patient’s age, physical health, tumor size, and location. The skill and experience of the treating physician should also be considered.
Observation
Acoustic neuromas are benign tumors and generally very slow-growing. The symptoms are produced by pressure on surrounding nerves and brain tissue and not because the tumor has “invaded” the normal brain tissue. When a small tumor is discovered in an older patient, observation may be indicated rather than aggressive treatment. If the growth rate of the tumor is slow enough, the tumor may not be expected to cause problems during the patient’s normal life expectancy, and treatment and potential complications may be avoided. If, however, the tumor shows progressive increase in size, treatment may become necessary.
Observation may also be indicated in patients with tumors in their only hearing ear. Sometimes hearing cannot be preserved with removal of the tumor, and it may be desirable to retain the hearing for as long as possible. For these patients, treatment is recommended if either the hearing is lost or the tumor size becomes life-threatening.
Microsurgical removal
Acoustic neuromas may be removed either partially or totally during surgery. Partial removal may be advised in order to reduce the risk of complications, with the realization that further surgery may be needed in the future. Partial removal may also be advised for patients who have large tumors in their only hearing ear. This approach may reduce the tumor size, so that threat to the patient’s health may be reduced during his or her normal life expectancy, and the risk of facial nerve dysfunction may be reduced. There is still a risk for hearing loss even with partial removal, and periodic MRI studies are important to follow the growth of the residual tumor.
Many acoustic neuromas can be removed entirely by surgery. Preservation of the facial nerve is the primary objective during this procedure to prevent permanent facial paralysis. Preservation of hearing in the affected ear is also an important goal for patients who still have functional hearing.
Surgical removal of an acoustic neuroma is a complex and delicate process. In general, the smaller the tumor at the time of surgery, the fewer complications. The hospital stay after tumor removal ranges from four to seven days, with approximately four to six weeks suggested for recovery. Patients routinely spend at least one night after surgery in the intensive care unit for care and observation. The time after surgery can be filled with days or perhaps weeks of new sensations. There is usually head discomfort and fatigue. Even when tumor removal has been accomplished, there is a small chance of tumor recurrence. Follow-up MRI imaging should be performed to monitor the possibility of regrowth.
Radiosurgery (Gamma Knife)
In 1951, the Swedish neurosurgeon Lars Leksell presented the idea of converging a large number of beams of ionizing radiation to crossfire at one target in the brain. He coined the term "radiosurgery" to describe this concept, because the technique differed greatly from conventional radiotherapy. He suggested radiosurgery for the treatment of deep-seated brain tumors and functional disorders.
The first device for routine clinical use based on this idea was the prototype Gamma Knife constructed in 1967. Dr. Leksell treated the first acoustic neuroma with the Gamma Knife in June of 1969 at Karolinska Hospital in Stockholm, Sweden. Since then, many acoustic neuroma patients have been treated with this technique worldwide.
Stereotactic radiation therapy, referred to as “radiosurgery” or “radiotherapy”, is a technique based on the principle that radiation delivered precisely to the tumor will arrest its growth while minimizing injury to surrounding nerves and brain tissue. During the Gamma Knife procedure, 201 small beams of radiation are aimed at the acoustic neuroma. This results in a high dose of radiation to the tumor and very little radiation to the surrounding brain structures. Radiosurgery is delivered as a one-time, outpatient treatment. Many patients have been treated this way with high success rates. Facial weakness or numbness occurs in only a small percentage of cases and is usually temporary. Hearing can be preserved in many cases.
Obvious advantages of radiosurgery are its non-invasive nature, its shortened immediate recovery time, its preservation of hearing in many cases, and its value as an alternative for patients unable or unwilling to undergo surgery. On the other hand, radiosurgery is limited to small or medium tumors, usually less than 3 cm. The patient should be aware that radiosurgical treatment is a means to seek to control the tumor’s growth without removing it.
The source of radiation used in radiosurgery with the Gamma Knife is radioactive cobalt. The radiation is called a gamma ray when it comes from a cobalt source. The treatment team consists of a neurosurgeon, a radiation oncologist, a medical physicist, and a nurse. The physicians and physicists work together to develop a treatment plan based on the size and shape of the tumor.
In radiosurgery patients, tumor cell growth is not arrested immediately. Some tumor cells die in a matter of weeks, but others do so more gradually, generally six to eighteen months after treatment. The treatment usually stops the growth of the tumor, and some tumors will shrink in size, but the tumor does not disappear. Tumor growth is controlled in a high percentage of cases, but some tumors continue to grow despite the treatment. It is not possible to determine which tumors will grow larger and which will not. Periodic MRI imaging is necessary to monitor this possibility.
In May of 2004, a study by Dr. L. Dade Lunsford, et al., published by the American Association of Neurological Surgeons, stated that “radiosurgery provides a low risk, minimally invasive treatment option for patients with newly diagnosed or residual acoustic neuromas.” According to the report (which summarized the experience of the Gamma Knife center at the University of Pittsburgh for 829 cases), tumor control rates at ten years were 97% with “no additional treatment needed.”
Residual problems after treatment
Some, but not all, patients experience short- and long-term problems after conventional surgery for acoustic neuromas, and the patient should be aware that complications exist. Besides hearing loss, the most common problems are excessive eye dryness, balance difficulties, ringing or “noise” in the ears (tinnitus), facial weakness, and headaches. Some patients experience cerebrospinal fluid leak through the incision or nose, and this should be reported to the neurosurgeon immediately.
Radiosurgery, because it is an outpatient treatment performed under local anesthesia, is not associated with most of the complications of open surgery—such as infection, cerebrospinal fluid leak, stroke, or systemic problems. Occasionally patients develop facial numbness, facial weakness or deafness of the side of treatment. These symptoms typically occur between six and eighteen months after treatment, and are usually temporary.
For more information, please contact:
Rebecca O. Heitkam, RN, BSN, CCRN, coordinator
Gamma Knife Center
Saint Joseph's Hospital
404-851-5513
or toll free at 1-866-SJGAMMA
or email
rheitkam@sjha.org
More information on acoustic neuromas
Several organizations provide information on acoustic neuromas and other health-related topics. Please contact the following for more information on this topic:
- Acoustic Neuroma Association (ANA)
600 Peachtree Parkway, Suite 108
Cumming, GA 30041-6899
Voice: 770-205-8211
Fax: 770-205-0239
Email: anausa@aol.com
Internet: http://www.anausa.org/
- International RadioSurgery Association
3005 Hoffman Street
Harrisburg, PA 17110
Phone: 717-260-9808
Fax: 717-260-9809
- National Institute on Deafness and Other Communication Disorders
National Institutes of Health
31 Center Drive, MSC 2320
Bethesda, MD USA 20892-2320
E-mail: nidcdinfo@nidcd.nih.gov
- American Association of Neurological Surgeons
5550 Meadowbrook Drive
Rolling Meadows, IL 60008
847.378.0500 or 1.888.566.AANS (2267)
Fax: 847.378.0600
E-mail: info@aans.org
