What is an acoustic neuroma?
An acoustic neuroma (also known as neurinoma or vestibular schwannoma) is a benign (non-cancerous) tissue growth, or tumor, that arises on the eighth cranial nerve. The eighth cranial nerve is three separate nerves, one part transmits hearing and the other two send balance information to the brain from the inner ear. These nerves are adjacent to each other as they pass through the bony canal, is about one centimeter (0.4 inches) long, and it is here that acoustic neuromas usually grow from the lining surrounding the eighth cranial nerve. The seventh cranial nerve or facial nerve that controls facial movement also passes through this bony canal along with important blood vessels.
The tumor does not spread to other parts of the body. Instead, this growth causes symptoms by pressing on other nerves and stealing their blood supply. In addition, these tumors can grow enough to place pressure on the brain itself. The natural history for these tumors is continued growth until treatment. Typically, these tumors grow very slowly over a period of years. In others, the rate of growth is more rapid.
Acoustic neuromas account for 6%-10% of all brain tumors. These tumors are not fatal, unless left untreated where they can cause increased pressure on the brain, ultimately becoming fatal. The cause of most acoustic neuromas is unknown. There is a small group of patients who have a condition called neurofibromatosis type 2, which is a genetically determined disorder and is frequently accompanied by one or more acoustic neuromas.
Where is the acoustic nerve and what does it do?
The acoustic nerve arises from a part of the brainstem called the pons (in a region referred to as the cerebellopontine angle). The acoustic nerve and the vestibular nerve run together and are attached to one another. Together they are called the eighth cranial nerve. The seventh cranial nerve controls the facial musculature (hence its other name-the facial nerve) and runs near the eighth nerve for part of its course. The acoustic portion of the eighth nerve goes to the cochlea of the ear and transmits hearing while the vestibular portion goes to the semicircular canals that are responsible for detecting motion (acceleration more technically) and hence are involved in the sense of balance. Most acoustic neuromas actually arise from the vestibular portion of the eighth nerve rather than the acoustic portion, therefore the synonym vestibular schwannoma is preferred by most medical authors.
How do acoustic neuromas grow?
Acoustic neuromas usually grow slowly over a period of many years. They characteristically remain within their lining (encapsulated) and displace normal tissue. The body accommodates to this abnormal growth. An acoustic neuroma first distorts the eighth cranial nerve (hearing and balance nerve), then presses the seventh cranial nerve (facial nerve). The slowly enlarging tumor protrudes from the internal auditory canal into an area behind the temporal bone called the cerebellopontine angle. It then assumes a pear shape, with the small end in the canal. The tumor presses on adjacent nerves, such as the fifth cranial nerve (nerve for facial sensation). Ultimately, the tumor can press on the brainstem and become life threatening.
How often do acoustic neuromas occur?
Non-symptomatic acoustic neuromas have been found during autopsy in 2.4% of the general population. Estimates of symptomatic acoustic neuromas range from 1 in every 3,500 people to 5 in every 1,000,000. Most acoustic neuromas are diagnosed in patients that are between 30 and 60 years old.
What symptoms do acoustic neuromas cause?
Early symptoms are easily overlooked, making diagnosis a challenge. However, there usually are symptoms pointing to the possibility of an acoustic neuroma, which means that persons with “inner ear” hearing problems should be evaluated to eliminate the possibility of an acoustic neuroma.
The first symptom in over 90% of people with an acoustic neuroma is a reduction in hearing in one ear, often accompanied by ear noise or ringing (tinnitus). The loss of hearing is usually subtle and worsens very slowly, although occasionally a sudden loss of hearing does occur. There may also be a feeling of fullness in the affected ear. Unsteadiness and balance problems may occur early in the growth of the neuroma since the balance portion of the eighth cranial nerve is where the tumor arises.
As the tumor presses on other cranial nerves, facial sensation may be affected, with numbness and facial tingling felt constantly or intermittently. Headaches and unsteady gait caused by increased pressure on the brain may occur.
What treatments are available for acoustic neuromas?
There are three main treatment options including observation, stereotactic radiation therapy (radiosurgery), and microsurgical removal. Possible treatment options depend on many factors and an in-depth discussion with our physician will help determine the best option for you.
Acoustic neuromas are occasionally discovered incidentally, in the course of evaluating another problem, or when the tumor is very small with few symptoms. Since acoustic neuromas are benign (non-cancerous) tumors and produce symptoms by pressure on surrounding structures, careful observation over a period of time may be appropriate for some patients. When a small tumor is discovered in an older patient, observation to study the growth rate of the tumor may be indicated if acute symptoms are not present. If it appears that the tumor will not need to be treated during the patient’s normal life expectancy, treatment and its potential complications may be avoided. Another group of patients whom observation is indicated includes patients with a tumor in their only or better hearing ear, particularly when the tumor is of a size that hearing preservation with removal would be unlikely. In this group of patients, MRI is used to follow the growth pattern and treatment is recommended if either the hearing is lost or the tumor size becomes life threatening.
Stereotactic Radiation Therapy (RADIOSURGERY):
Conventional radiation of most tumors involves multiple doses of radiation delivered over three to six weeks. Stereotactic radiation therapy, which is sometimes referred to as “radiosurgery,” does not involve surgery. Instead, this is a technique based on the principle that a single relatively high dose of radiation delivered precisely to a small area will often arrest the tumor, while minimizing injury to surrounding nerves and brain tissue and/or function. The source of the radiation used in radiosurgery is from either radioactive cobalt or a linear accelerator. The radiation is called a gamma ray when it comes from a cobalt source (such as the Gamma Knife Unit) and an x-ray when it comes from a linear accelerator (LINAC). Radiation, even at relatively high doses such as those used in radiosurgery, does not kill or injure cells immediately. Following, radiation, some tumor cells die in a matter of weeks, but others die more gradually, generally six to eighteen months after treatment. This treatment often arrests the growth of the tumor and some tumors will shrink, but the tumor does not disappear. Follow-up studies are important because some tumors will continue to grow after this treatment or at some time in the future. A tumor which has been irradiated and grows may be more difficult to remove than an unradiated tumor. The possible cancerous effects of radiating an acoustic tumor have not been fully defined and there are published reports of typical acoustic tumors becoming malignant (cancerous) following radiation treatment.
Symptoms such as dizziness and disturbances of balance are improved earlier after microsurgical treatment than after radiation treatment because the effects of radiation treatment may require up to eighteen months. Many patients have some residual dizziness and balance disturbances after either radiation or microsurgical treatment, but this is commonly less after microsurgical treatment. In patients with useful hearing before radiation therapy, about 50% still have useful hearing two years after irradiation. Among patients with normal facial movement and sensation before treatment, 10% will develop some degree of facial weakness or numbness that usually recovers in several months. These early statistics compare very favorably with microsurgical tumor removal.
Microsurgical removal remains the treatment of choice for acoustic neuromas. An experienced microsurgeon, operating on tumors can produce results comparable to radiosurgery, but the long term tumor cure of control rate appears more certain with microsurgery. At this time, the only treatment that can cure the patient with an acoustic neuroma is microsurgical removal. Within the last three decades, microsurgical techniques have been refined so that the risk of total tumor removal has been greatly reduced. Microsurgical instruments and the operating microscope are routinely employed, Damage to the surrounding nerve tissue has been markedly decreased and the mortality rate is extremely low. Facial nerve function is routinely monitored during surgery, which has reduced the frequency and severity of facial nerve injury. Cochlear nerve monitoring is also employed during surgery where it appears feasible to preserve hearing. According to the 1991 NIH Consensus Statement, “the best published surgical outcomes in the treatment of vestibular schwannoma are from medical centers that have highly organized and dedicated teams with specific internest in these tumors and sufficient continuing experience to develop, refine, and maintain proficiency.” Our specially trained audiologists participate in the surgery by monitoring facial function and hearing. There are several surgical approaches which may be used to remove an acoustic tumor. The route depends upon the location and size of the tumor, as well as the presence of residual hearing. The approaches are as follows:
- Middle Fossa Approach – In this approach, the bone is opened above the ear and the bone overlying the tumor is removed. This approach may be selected for small tumors with good hearing. It is usually limited to tumors that extend our of the internal auditory canal no further than one centimeter toward the brainstem.
- Retrosigmoid Approach – In this approach, the bone is opened behind the mastoid and inner ear and the tumor is approached from behind. The approach also allows for the possibility of hearing preservation and may be used for both small and large tumors.
- Translabyrinthine Approach – In this approach, the incision is located behind the ear and the mastoid bone is removed. This approach involves removing inner ear structures, and thus destroys hearing. It is, therefore, only used for those cases where hearing loss is already severe or the tumor is so large that hearing conservation is not a realistic goal.
Regardless of the approach, the patient is observed in the intensive care unit for one or two days with careful monitoring. Headache, nausea, vomiting, and decreased mental alertness may occur. Other postoperative problems include cerebrospinal fluid leakage and meningitis. The cerebrospinal fluid leak may be treated by drainage of the spinal fluid through the lower back, or in rare cases, secondary surgery. Meningitis is an infection in the fluid surrounding the brain and is treated with antibiotics. Each of the surgical approaches has advantages and disadvantages. Excellent results have been achieved using each of the above approaches. This decision is individually based on patient preference, hearing capabilities and tumor characteristics. Hospital stay after microsurgery ranges from 4-7 days with approximately 4 weeks suggested for recovery. Regrowth rates after microsurgery are less than 5%
Sometimes partial microsurgical removal may be indicated. Incomplete removal of an acoustic neuroma is elected by some patients and our physicians in order to reduce the risk of complication with the realization that further surgery may be needed in the future. Occasionally, because of disturbances of the patient’s vital brain centers during surgery, the operation is terminated before the tumor is totally removed. In that case, the residual tumor is followed with MRI scans and, if tumor growth is shown, further surgery to totally remove the tumor may be necessary. On the other hand, if the tumor shows no growth, continued observation can take place. Partial tumor removal has been advocated in some patients who have a tumor in their only hearing ear. This is primarily in patients who have Neurofibromatosis Type 2. Unfortunately, partial removal may result in substantial hearing loss in these patients and this risk must be considered.