Acoustic Neuroma Overview

Figure A – Illustration of the anatomy of the temporal bone and inner ear structures. The vestibulocochlear nerve travels in bony canal, the internal auditory canal, with the facial nerve. The brain stem and its vital structures are adjacent to this region.

Acoustic neuromas, also known as vestibular schwannomas, constitute approximately six percent (6%) of all brain tumors.

These tumors occur in all races of people and have a very slight predilection for women over men. 

Acoustic neuromas are benign fibrous growths that arise from the balance nerve, also called the eighth cranial nerve or vestibulocochlear nerve. (Figure A)

These tumors are non-malignant, meaning that they do not spread or metastasize to other parts of the body.

The location of these tumors is deep inside the skull, adjacent to vital brain centers in the brain stem. As the tumors enlarge, they involve surrounding structures which have to do with vital functions.

In the majority of cases, these tumors grow slowly over a period of years. In other cases, the growth rate is more rapid and patients develop symptoms at a faster pace. Usually, the symptoms are mild and many patients are not diagnosed until some time after their tumor has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans. 

Forms of Acoustic Neuroma

Acoustic neuromas occur in two forms: sporadic and those associated with Neurofibromatosis Type II (NF II).

Approximately ninety-five percent (95%) of all acoustic neuromas are sporadic cases and are unilateral. In contrast, those tumors associated with NF II are bilateral and account for approximately five percent (5%) of acoustic neuroma patients.

Patients with sporadic acoustic neuromas tend to begin having symptoms in middle age with the average being around fifty years old at diagnosis.

Patients with NF II present at a younger age averaging around thirty years old when they first develop symptoms. There is a high degree of variability, however, and patients may begin having symptoms and be diagnosed with an acoustic neuroma during childhood or young adult life, as well when elderly.

Every patient with NF II at some point manifests bilateral acoustic neuromas, while an acoustic neuroma in a patient with NF I is uncommon. Virtually no patients with NF I develop bilateral tumors as do those with NF II.

Patients with NF II also have a propensity to develop benign nerve tumors in other locations which include other nerves that arise from the brain stem, as well as, nerves arising from the spinal cord and located more peripherally in the extremities. Patients with NF II, like all acoustic neuroma patients, benefit from the care of a team of experienced professionals who are capable of dealing with all aspects of their complicated case management.

 

Common Questions About Acoustic Neuromas

1. What is an acoustic neuroma? (vestibular schwannoma)

An acoustic neuroma (sometimes termed a neurolemmoma or schwannoma) is a benign (non-cancerous) tissue growth that arises on the eighth cranial nerve leading from the brain to the inner ear.  This nerve has two distinct parts, one part associated with transmitting sound and the other sending balance information to the brain from the inner ear.  These pathways, along with the facial nerve, lie adjacent to each other as they pass through a bony canal called the internal auditory canal.  This canal is approximately 2 cm (0.8 inches) long and it is here that acoustic neuromas originate from the sheath surrounding the eighth nerve.  The facial nerve provides motion of the muscles of facial expression.

Acoustic neuromas usually grow slowly over a period of years.  They expand in size at their site of origin and when large can displace normal brain tissue.  The brain is not invaded by the tumor, but the tumor pushes the brain as it enlarges.  The slowly enlarging tumor protrudes from the internal auditory canal into an area behind the temporal bone called the cerebellopontine angle.  The tumor now assumes a pear shape with the small end in the internal auditory canal.  Larger tumors can press on another nerve in the area (the trigeminal nerve) which is the nerve of facial sensation.  Vital functions to sustain life can be threatened when large tumors cause severe pressure on the brainstem and cerebellum part of the brain. Tumors are typically described as small (less than 1.5 cm), medium (1.5 cm to 2.5 cm) or large (more than 2.5 cm).

2. Is the tumor benign?

Yes. Acoustic neuromas are benign fibrous growths that are non-malignant, meaning that they do not spread or metastasize to other parts of the body. These tumors are, by nature, very slow growing in general. They affect adjacent nervous structures by creating pressure on these structures.

3. What are symptoms of acoustic neuroma?

Early symptoms are easily overlooked, thus making diagnosis a challenge.  However, there usually are symptoms pointing to the possibility of an acoustic neuroma. The first symptom in 90% of those with a tumor is a reduction in hearing in one ear, often accompanied by ear noise or tinnitus.  The loss of hearing is usually subtle and worsens slowly, although occasionally a sudden loss of hearing is noted.  There may be a feeling of fullness in the affected ear.  These early symptoms are sometimes mistaken for normal changes of aging, and diagnosis is often delayed.

Since the balance portion of the eighth nerve is where the tumor arises, unsteadiness and balance problems may occur during the growth of the neuroma.  The remainder of the balance system sometimes compensates for this loss, and no imbalance will be noticed.  Larger tumors can press on the trigeminal nerve, causing facial numbness and tingling, constantly or intermittently.  Increase of intracranial pressure may be experienced with headaches, clumsy gait and mental confusion.  This is a life-threatening complication requiring urgent treatment.

4. If I have an Acoustic Neuroma, are my children at risk for developing it, too?

The majority of acoustic neuromas are unilateral and are not hereditary. Ninety-five percent (95%) of cases are sporadic and only involve one side. This is in contrast to those tumors which are associated with a hereditary disease called Neurofibromatosis Type II (NF II). In these patients, the acoustic neuromas are bilateral. This condition is hereditary. NF II is a rare disease and it is found in approximately one person per one hundred thousand population in the United States. If someone is diagnosed with NF II, they should undergo genetic counseling.

5. Why me? What causes these tumors to develop?

The exact cause of acoustic neuromas is not currently known. There is ongoing research at the many centers to try and determine the genetic defects that occur in the tumor cells. However, no specific environmental agents have been identified which causes the development of an acoustic neuroma.

6. Does my acoustic neuroma have to be treated?

The majority of patients who present with an acoustic neuroma do have treatment of the tumor. However, these are benign, very slow growing tumors in the vast majority of cases. Therefore, it should be understood that it is rarely an emergent situation to undergo treatment once this tumor is diagnosed. Patients have time to research their options for treatment and find an experienced team to manage their care. Those who do elect to be observed and not undergo surgery or radiation therapy for the tumor, in general, do not exhibit significant changes in the tumor over time as documented by frequent MRI scans. In a recently reviewed series of one hundred and nineteen (119) mostly elderly patients followed conservatively at the House Ear Clinic over a two and a half (2 1/2) year average follow-up time, seventy percent (70%) of the patients did not exhibit any growth of their tumor. Those that did exhibit growth or developed new symptoms went on to have surgical removal of the tumor.

7. Are there support organizations I can contact for information?

Yes. The "Acoustic Neuroma Association" ( www.anausa.org) is an excellent source of information. The organization provides educational material regarding acoustic neuromas and the various forms of treatment and can put you into contact with its local support group organizations.

8. How is acoustic neuroma identified?

Advances in medicine have made possible the identification of small acoustic neuromas; that is, those still confined to the internal auditory canal.  Routine auditory tests may reveal a loss of hearing and speech discrimination (the patient can hear sound in that ear, but cannot understand what is being said).

An audiogram should be performed to effectively evaluate hearing in both ears.  This test is important to begin the diagnosis of acoustic neuroma.  Magnetic resonance imaging (MRI) is the diagnostic test that is preferred for identifying acoustic neuromas.  Gadolinium, an enhancing material, is required to reveal the tumor.  The image formed clearly defines an acoustic neuroma if it is present. Currently, this imaging study is the preferred test for identifying acoustic neuromas.

An auditory brainstem response test (ABR, BAER, or BSER) may be done in some cases.  This test provides information on the passage of an electric impulse along the circuit from the inner ear to the brainstem pathways.  An acoustic neuroma can interfere with the passage of this electrical impulse through the hearing nerve at the site of tumor growth in the internal auditory canal.  This implies the possible diagnosis of an acoustic neuroma when the test is positive.

When a MRI is not available or cannot be performed, a computerized tomography scan (CT scan), with contrast, is suggested for patients in whom an acoustic neuroma is suspected.  The CT scan and audiogram can provide valuable information to determine the presence of an acoustic neuroma.  However, the MRI is nearly 100% accurate in identifying the presence of an acoustic neuroma and should be performed.

9. What are the chances I will lose my hearing?

The chances of losing hearing prior to treatment depend upon many factors. Patients lose hearing due to these tumors from pressure effects of the tumor on the auditory nerve, as well as, invasion of the auditory nerve by the tumor. Similarly, the tumor can cause an obstruction of blood flow to the auditory nerve and the cochlea which results in hearing loss. Most patients present with hearing loss as the first symptom of their acoustic neuroma.

The chances of losing hearing completely after surgery are dependent upon the size of the tumor and the level of hearing before treatment. Patients with poor hearing before treatment have a very low chance, with any therapy, of having their hearing preserved. Patients with better hearing have a much better chance of having their hearing preserved. As noted above, in a recent series of over three hundred and eighty (380) patients with small tumors treated by the middle fossa approach here at the House Ear Clinic, roughly two thirds (2/3) of patients had their hearing preserved at a functional level. Eighty percent (80%) of patients maintained at least some measurable hearing which, in many cases, was good enough for them to use the telephone with that ear.

10. What is the chance of losing facial nerve function?

An occasional patient presents with facial nerve weakness as the first symptom of their acoustic neuroma. These patients tend to be those with larger tumors. The risk of losing facial nerve function as a consequence of surgical treatment is dependent upon the size of the tumor. Patients with small tumors have an excellent chance of having excellent facial nerve function after surgery. Patients with small tumors can expect a ninety-five (95%) chance of excellent nerve function, as judged by our recent review of patients with small tumors treated at the House Ear Clinic with a middle fossa approach. Only five percent (5%) had what is considered good facial nerve function, which translates to a grade 3 on the House/ Brackmann scale. The chances of losing facial nerve function with stereotactic radiosurgery is dependent upon the dose of radiation delivered to the nerve. Preservation of good facial nerve function with radiosurgery has varied in reports in the neurosurgical literature from seventy to ninety-five percent (70-95%).

11. What is the chance the unilateral and not hereditary acoustic neuroma will come back after treatment ?

The chance of the unilateral and not hereditary acoustic neuroma recurrence after surgical complete resection is extremely low. A study of over three thousand patients with acoustic neuromas who had their surgery at the House Ear Clinic demonstrated that the recurrence rate after total resection was 0.2% in these patients. Recurrence rates in other surgical series have, in general, been in the zero to two percent (0-2%) range after total resection. Recurrence after stereotactic radiosurgery is still undetermined at this point for patients with lower radiation doses, as are being utilized at the present time. The radiosurgical series "tumor control" rates over the short-term (i.e. approximately 2.5 years) are ninety-five percent (95%). However, it is still unknown whether "tumor control" is equivalent to what is observed after total surgical resection (i.e. less than 1% chance of tumor recurrence).

12. What type of doctor should I be consulting about my acoustic neuroma?

Ideally, a neurosurgeon or neuro-otologic surgeon who has experience with the treatment of acoustic neuromas should be consulted. Many specialized teams exist at several centers around the country who have the necessary experience and frequently treat patients with acoustic neuromas. These are difficult problems that are best handled by an experienced, multidisciplinary team in order to provide the best chance for a good outcome. It is important to find out the level of experience of the physician that is consulted. Patients should not be bashful in asking how many acoustic neuromas the doctor has treated within the past year, or in the past five to ten years, and what the results of that treatment were. Patients should expect to receive specific answers based on a review of the physician's own personal data. It is important to find out the physician's personal results with preservation of facial nerve function, hearing preservation, and the incidence of other major complications with either surgery or radiosurgery. Patients should probably be wary if a surgeon does not have easily produced statistical results (this usually means that he/she does not frequently treat these tumors). Likewise, patients should be wary of being treated by a physician who only occasionally treats this complicated problem.

13. Is this a tumor that any general neurosurgeon or otolaryngologist should be treating?

Again, the experience of the surgeon and sub-specialty training influences outcome to a tremendous degree. Patients should seek out an experienced team who frequently treats these tumors and can give sound advice based upon adequate experience.

Source :

-          Web Site- www.hei.org  

- Web Site – www.anausa.org

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