An acoustic neuroma, more correctly called a vestibular schwannoma, is a benign tumor originating from the outer sheath of the balance nerve. They occur at the rate of 1 to 2 cases per 100,000 people and present most often between the ages of 30 and 60 years. There is no male/female predilection.
What causes an acoustic neuroma?
The cause of acoustic neuroma is not well understood. For most acoustic neuromas, the cause at the level of the cellular machinery is the failure of a “governor” gene to exert its effect in suppressing the growth of Schwann cells—those cells responsible for coating nerve fibers with insulation. The result is “wart-like” growth of these cells to produce the neuroma.
The only environmental exposure that has been definitively associated with an increased incidence is radiation exposure to the head. No evidence currently exists for an association between cell phone use and the development of these tumors.
For the most part it is not an inherited disease; however, 5% of cases are associated with a genetic disorder called neurofibromatosis type 2. These individuals demonstrate two-sided vestibular tumors often associated with other tumors around the brain and/or in the spine. The vast majority of acoustic neuromas are sporadic (nonhereditary).
What is the natural history or growth pattern of an acoustic neuroma?
Acoustic neuromas are slow growing benign tumors. The average growth rate is approximately 1.5 millimeters per year. Several studies have demonstrated that as many as 50% of tumors show little to no growth over long periods of time (5 to 10 years). No known environmental or dietary factors are known to influence the growth rate of acoustic neuromas.
What determines the symptoms one might experience with an acoustic neuroma?
Symptoms can be associated with the size of the tumor but this is not always consistent. Many patients experience few symptoms despite the presence of a large tumor. Small tumors are defined at 2 centimeters or less, medium tumors are between 2 and 4 centimeters, and large tumors are over 4 centimeters. Smaller tumors can often be associated with few symptoms whereas large compressive tumors can be life threatening.
Why do acoustic neuromas cause symptoms?
As described above, acoustic neuromas arise from the balance or vestibular nerves in an area called the cerebellopontine angle. The tumors often arise within the internal auditory canal which contains the facial nerve, the cochlear or auditory nerve, and the two balance or vestibular nerves. Larger tumors project inward from the internal auditory canal and cause displacement of the adjacent cerebellum and brainstem.
Although acoustic neuromas arise from the balance nerve, patients rarely suffer from severe dizziness but may experience unsteadiness. The most common symptom of acoustic neuroma is single-sided hearing loss and fullness often associated with tinnitus or head noise. Unless very large, the facial nerve, providing the necessary input to the facial muscles for facial expression, is rarely affected.
What do I do if I have an acoustic neuroma?
Discuss your symptoms with your primary care physician or specialist and seek their help in directing you to a center with a team of physicians and nurses that have extensive experience in the care and treatment of patients with acoustic neuromas. As discussed below, this step can be arduous.
What can a team of experienced healthcare professionals at a center do for me?
The first and most important step is the establishment of a cooperative and trusting relationship with your team. The treatment team must be multidisciplinary and include Neurotology, Neurosurgery, Radiation Internal Medicine, Nursing, Neuroradiology, Anesthesia, Ophthalmology, Otolaryngology, Plastic and Reconstructive Surgery, Audiology, and Physical Therapy.
There are three treatment options available:
1) A “wait-and watch” strategy can be a good start for you if your tumor is small (<2 cm) and you are experiencing few symptoms. This approach generally involves a second MRI in 6 months and annually provided there is no growth, and/or symptoms have not changed.
2) Microsurgery or tumor removal has historically been the treatment of choice for patients and healthcare providers. This approach is often indicated for tumors over 2.5 centimeters, smaller tumors when long-term hearing preservation is desired, or when dizziness or disequilibrium is present.
3) Radiation is the third option. This is a nonsurgical approach often done in an outpatient setting. Radiation can be given as a single dose or in multiple or fractionated doses. This approach can be used for tumors under 2.5 centimeters with minimal contact with the brainstem and in patients that are not healthy enough for surgery. In general, we do not recommend radiation for people under the age of 65 as the long-term efficacy and potential risks are still not clearly defined.
Can “wait-and-watch” be good for me?
This is an accepted strategy for smaller, asymptomatic tumors. For smaller tumors, there may be no growth for periods of time, even over several years. However, it is important to understand that hearing often declines over time despite the absence of observable growth tumor. Provided you are in the care of an experienced physician, and your hearing or balance do not become problematic, ”wait and watch” is a viable option.
How do microsurgeons remove these tumors?
There are three different ways that an experienced microsurgical team can safely remove your tumor. Each of these approaches requires the use of state-of-the-art microsurgical instruments that include a binocular microscope. The translabyrinthine approach is performed through an incision behind the ear. The surgeon opens the mastoid bone and inner ear structures to identify the tumor within the internal auditory canal and cerebellopontine angle. This is the favored approach for larger tumors in patients with poor hearing or in cases where hearing preservation is unlikely (tumors >2 cm). This approach allows early identification of the facial nerve and has a very low incidence of postoperative headache.
The retrosigmoid approach involves an incision slightly farther back behind the ear. This approach can be used for hearing preservation as it does not involve removal of the inner ear structures. We reserve this approach for patients with useful hearing and tumors that are 2 centimeters or less in diameter and do not extend to the farthest reaches of the internal auditory canal. This approach has a low but finite risk of chronic headache and is not recommended for patients with a history of troubling headaches.
The middle cranial fossa approach involves an incision above the ear. The internal auditory canal is approached from above and an experienced surgeon can expose the tumor without compromising the structures of the inner ear. This approach is reserved for patients with useful hearing and tumors smaller than 1.7 centimeters. This approach is associated with the highest likelihood of long-term hearing preservation.
When would I consider radiation as a treatment?
Radiation therapy (more than one session) or radiosurgery (single session) are useful treatment methods in appropriate circumstances. What those circumstances are is debatable and confusing at best. In general, radiation induces strand breaks in DNA, the genetic material of the cell. This theoretically leads to cell death and/or growth arrest in some fraction of the tumor. Although there are many studies described in our professional literature, each suffers from a lack of complete and long-term follow up. With that said, any patient with an acoustic neuroma can be treated with some form of radiation.
We will discuss radiation thoroughly with every patient as each member of our team has extensive experience with this approach. In general, we avoid using radiation in young patients and/or patients with tumors 2.5 centimeters or greater in size. Radiation is a good option for older patients and patients that are not medically fit for surgery. We await solid evidence of efficacy and tolerable long-term side effects before recommending it as a first line in most cases.
How do I make such a difficult decision about how I wish to be treated?
It is essential that a multidisciplinary team with varied perspectives and areas of expertise manage your case. These teams should meet on a regular basis, as a group, to discuss each case and provide the most thoughtful recommendation. This team should include you as an active participant. Rarely is treatment urgent.
What are the risks of treatment?
There are risks that are common to all three methods of management and they include, to varying degrees, hearing loss, tinnitus, dizziness, facial numbness, and headache. Hearing loss is a common outcome after surgery and radiation treatment as is the accompanying tinnitus. Dizziness, although worse immediately after surgery, ultimately is comparable whether one has surgery or radiation. Headache is not uncommon with both approaches. Hydrocephalus or brain swelling can occur with either surgery or radiation.
There are also risks more specific to each approach. Facial nerve weakness or paralysis has historically been more common after microsurgery but can occur after radiation treatment. Our microsurgeons are more apt to do partial (>90%) or near total (>98%) removal when necessary to protect the integrity and function of the facial nerve. After near total removal, the risk of further treatment being necessary is less than 5%.
Spinal fluid leak, bleeding, and stroke are very rare complications that are specific to surgery. Chronic disequilibrium, although common to both, can be more severe after radiation if dizziness is a problem prior to treatment. As one might assume, there is a finite and very low risk of mortality from surgery and radiation, both being about 1 in 1000.
What happens after treatment?
For the “wait-and watch” group of patients our team will follow your MRI scans at regular intervals and will be available to discuss any new symptoms or concerns.
For the surgery and radiation groups, the after care begins prior to the treatment. Our physician and nursing staff will make contact with your doctors and physical therapists at home informing them of the process and your individual needs for after care. The Keck Medical Center of the University of Southern California’s acoustic neuroma team will be an integral part of your care with specialized nursing care and physical therapy to prepare you for your return to normalcy.
How do I learn more?
The Acoustic Neuroma Association (anausa.org) is an invaluable resource for education and developing a network of caring friends and supporters. The Acoustic Neuroma Association has published printed and online materials that can introduce you to many of the concepts discussed here.