Acoustic neuromas, also known as vestibular schwannomas, are non-malignant tumors of the 8th cranial nerve. Most commonly they arise from the covering cells (Schwann cells) of the inferior vestibular nerve (Komatsuzaki and Tsunoda, 2001; Krais, 2007). They can also arise within the labyrinth (Neff et al, 2003).
Acoustics comprise about 6 percent of all intracranial tumors (Anderson et al, 2000), about 30% of brainstem tumors, and about 85% of tumors in the region of the cerebellopontine angle -- another 10% are meningiomas. The 6% number is probably much too high as meningiomas and pituitary tumors are underreported.
Only about 10 acoustic tumors are newly diagnosed each year per million persons (Evans et al, 2005), corresponding to between 2000 and 3000 new cases each year in the US. Another way of looking at this is that an average person has a risk of about 1/1000 of developing an acoustic neuroma in their lifetime (Evans et al, 2005). In Denmark, the annual incidence was estimated to be 7.8 patients operated/year (Tos et al, 1992). As technology has improved, more small tumors have been diagnosed, resulting in a similar estimate of about 10 tumors/million/year.
In patients with hearing asymmetry, it is believed that only about 1 in 1000 has acoustic neuroma (source: NIH), although some report prevalences as high as 1-2.5% (Stewart et al, 1975; Baker et al. 2003). The higher prevalence figures do not correspond to our clinical experience in our practice in Chicago, or the findings of others. Lin et al (2005) suggested that only 2/10,000 persons have acoustic neuromas, based on an imaging database of 46414 people. In our practice in Chicago, where we see roughly 800 new patients/year with dizziness or hearing loss, we typically diagnose ourselves only about one new acoustic/year. As we have a symptomatic population that presumably would be more likely to have acoustics than individuals with no hearing or dizziness symptoms, this implies that the prevalence in the general population is probably much lower than 1%. Of course, this does not really answer the question as to the # of persons with hearing asymmetry that have AN.
Acoustic neuromas occur largely in adults -- they are very uncommon in children. Only 39 cases in children had been reported in the literature as of 2001 (Pothula et al, 2001).
Causes of Acoustic Neuroma
What exactly causes acoustic neuroma is unknown. Certain risk factors -- such as age and a family history of neurofibromatosis type 2 -- may make a person more likely to develop this condition. However, people with risk factors do not always get acoustic neuroma, and people without risk factors can get it as well.
No one knows the exact cause or causes of acoustic neuroma. Doctors cannot always explain why one person gets acoustic neuroma and another does not. However, research has shown that people with certain risk factors are more likely than others to develop acoustic neuroma. A risk factor is anything that increases a person's chance of developing a disease.
While not an acoustic neuroma "cause," studies have found a couple of factors that may increase a person's risk of developing it. Specific risk factors for acoustic neuroma include:
- Age
- Family history of neurofibromatosis type 2.
Age
Most people develop acoustic neuroma between the ages of 30 and 60. The average age at diagnosis is 50.
Neurofibromatosis Type 2
Neurofibromatosis type 2 (NF-2) is a rare inherited disorder characterized by the development of acoustic neuromas on both auditory nerves. The disease is also characterized by the development of malignant central nervous system tumors. Half of all affected individuals have inherited neurofibromatosis from an affected parent, and half seem to have a mutation for the first time in their family. Each child of an affected parent has a 50 percent chance of inheriting the disorder.
People who have known risk factors for the condition do not always get acoustic neuroma. On the other hand, many who do get it have none of the risk factors for acoustic neuroma. Research scientists do not know the causes of acoustic neuroma, but they continue to search (see Acoustic Neuroma Gene).
People who think they may be at risk for acoustic neuroma should discuss this concern with their healthcare provider. The healthcare provider may suggest ways to reduce the risk and can plan an appropriate schedule for checkups. People over age 60 are at highest risk and should get an ear exam at least every two years.
The signs and symptoms of acoustic neuroma develop from the tumor pressing on the adjacent nerves, or nearby blood vessels or brain structures.
As the tumor grows, it may be more likely to cause signs and symptoms, although tumor size doesn't always determine effects. It's possible for a small tumor to cause significant signs and symptoms.
Signs and symptoms may include:
- Hearing loss, usually gradual — although in some cases sudden — and occurring on only one side or more pronounced on one side
- Ringing (tinnitus) in the affected ear
- Unsteadiness, loss of balance
- Dizziness (vertigo)
- Facial numbness and weakness
Exams and Tests
The health care provider may suspect an acoustic neuroma based on your medical history, an examination of your nervous system, or tests.Often, the physical exam is normal when the tumor is diagnosed. Sometimes, the following signs may be present:
- Decreased feeling on one side of the face
- Drooping on one side of the face
- Unsteady walk
- Hearing test (audiology)
- Test of equilibrium and balance (electronystagmography)
- Test of hearing and brainstem function (brainstem auditory evoked response)
Treatment
Treatment depends on the size and location of the tumor, your age, and your overall health. You and your health care provider must decide whether to watch the tumor (observation), use radiation to stop it from growing, or try to remove it.
Many acoustic neuromas are small and grow very slowly. Small tumors with few or no symptoms may be followed, especially in older patients. Regular MRI scans will be done.
If they are not treated, some acoustic neuromas can damage the nerves involved in hearing and balance, as well as the nerves responsible for movement and feeling in the face. Very large tumors can lead to a buildup of fluid (hydrocephalus) in the brain, which can be life-threatening.
Removing an acoustic neuroma is more commonly done for:
- Larger tumors
- Tumors that are causing symptoms
- Tumors that are growing quickly
- Tumors that are pressing on the brain
Stereotactic radiosurgery focuses high-powered x-rays on a small area. It is considered to be a form of radiation therapy, not a surgical procedure. It may be used:
- To slow down or stop the growth of tumors that are hard to remove with surgery
- To treat patients who are unable to have surgery, such as the elderly or people who are very sick
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