ACOUSTIC NEUROMA EXPLAINED
An acoustic neuroma is a benign (non-cancerous) fibrous tissue growth that arises from the balance nerve (also called the eighth cranial nerve or vestibulocochlear nerve – see FIGURE A) that leads from the brain to the inner ear. Acoustic neuromas are non-malignant, meaning that they do not spread to other parts of the body. An acoustic neuroma is sometimes also called a vestibular schwannoma or neurolemmoma.
Because acoustic neuromas are located deep inside the skull and are adjacent to vital brain centers in the brain stem, as these tumors grow, they affect surrounding structures in the brain that control vital functions. As the acoustic neuroma grows, it typically first affects a patient’s hearing because it protrudes from the internal auditory canal into an area behind the temporal bone. Larger acoustic neuromas can press on another nerve in the area, the trigeminal nerve, which is the nerve affecting facial sensation. When large acoustic neuromas cause severe pressure on the brainstem and cerebellum of the brain, vital functions that sustain life can be threatened.
In the majority of cases, acoustic neuromas grow slowly over a period of years. Usually the symptoms are mild and many patients are not diagnosed until some time after their acoustic neuroma has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans.
Acoustic neuromas account for approximately 6% of all brain tumors. These tumors occur in all races of people and have a slightly higher occurrence in women. In the United States about ten people per million per year are diagnosed with an acoustic neuroma. This translates to roughly 2,500 newly diagnosed acoustic neuromas per year. Most acoustic neuromas occur spontaneously without any evidence of heredity and are diagnosed in patients between the ages of 30 to 60.
SYMPTOMS OF ACOUSTIC NEUROMA
In the majority of patients, the first symptom of an acoustic neuroma is a reduction of hearing in one ear. This hearing loss is often accompanied by tinnitus, sound in the ear that has no externally audible source, and is usually subtle and slow in progression. There may also be a feeling of fullness in the affected ear. These early symptoms are sometimes mistaken for normal changes of aging, delaying diagnosis.
Because the acoustic neuroma originates in the balance portion of the eighth nerve, a patient may experience unsteadiness and problems with balance as the tumor grows. Larger tumors can press on the trigeminal nerve causing facial numbness and tingling, which can be either occasional or constant.
Severe symptoms such as headaches, staggering and mental confusion can indicate an increase of intracranial pressure. Immediate attention is required as this pressure can be a life-threatening complication.
If you are experiencing any of these symptoms, we urge you to consult a physician so that a diagnosis can be established. If an acoustic neuroma is diagnosed, early management is vital to your continued health.
CAUSES OF ACOUSTIC NEUROMA
The exact cause of an acoustic neuroma is not currently known. No specific environmental agents have been identified which cause the development of an acoustic neuroma.
FORMS OF ACOUSTIC NEUROMA
Acoustic neuromas occur in two forms: sporadic and those associated with Neurofibromatosis Type II (NF II).
Approximately 95% of all acoustic neuromas are sporadic cases and are unilateral (affecting one ear).
Patients with sporadic acoustic neuromas tend to begin having symptoms in middle age with the average being around fifty years old at diagnosis.
In contrast, those tumors associated with NF II are bilateral (affecting both ears) and account for approximately 5% of acoustic neuroma patients. NF II is a rare, genetic condition that affects one in 100,000 people in the United States. Patients with NFII develop benign tumors on both auditory nerves and may have to have both hearing nerves severed through tumor removal. Patients with NF II present at a younger age averaging around thirty years old when they first develop symptoms.
DIAGNOSIS OF ACOUSTIC NEUROMA
After patients have received routine auditory tests that show loss of hearing and speech discrimination (hearing sound but not understanding what is being said), the hearing care provider may perform an auditory brainstem response test (ABR, BAER or BSER). This test provides information on the passage of sound information along the path from the ear to the brain. The results may indicate that the acoustic nerve is not functioning optimally. If there is an abnormality in the ABR test, a detailed imaging test such as a CT (also called CAT) Scan or MRI is typically ordered.
A CT (CAT) Scan stands for Computerized (Axial) Tomography. The CT scanning device takes x-rays from many different angles and uses a computer to assemble them into a series of cross sections or ‘slices’ to provide a detailed look at the inside of the body. The CT Scan has proven to be effective in locating acoustic neuromas, although small tumors that are still confined to the internal auditory canal may not show up on standard CT Scans. To assist in providing the clearest possible scan, the patient may be given an injection of ‘contrast medium’ dye before the scan begins to make the images clearer.
MRI stands for Magnetic Resonance Imaging. An MRI Scan is a radiology technique that uses magnetism, radio waves, and advanced computer technology to produce images of body structures. The image and resolution produced by MRI is very detailed and can detect tiny changes of structures within the body. In the case of diagnosis for acoustic neuroma, gadolinium, a contrast agent, is used to increase the accuracy of the images. Gadolinium enhances the tumor making it easier to see.
TREATMENT OF ACOUSTIC NEUROMA
The treatment goal for any benign brain tumor is to eliminate the tumor while preserving neurological function. Because of their location in proximity to delicate brain structures, acoustic neuromas are a complicated treatment problem. The best care for the patient with acoustic neuroma is provided by specialized professionals who have significant and on-going experience in their treatment. An acoustic neuroma is one of the small number or brain tumors that must be attended to by physicians who specialize in and frequently treat this condition.
Because acoustic neuromas are a typically benign, slow-growing tumors, the necessity for treatment is not usually urgent, allowing patients time to research their treatment options and find an experienced team to manage their care. Patients who choose to have their acoustic neuroma monitored rather than have radiation therapy or surgery, generally do not exhibit significant changes in the tumor over time. For those patients who require immediate treatment of their acoustic neuroma either because of its size, its growth or change in shape or the urgency of their symptoms, surgical removal of the tumor is the preferred method of treatment.
Surgical Therapy
Since the early 1960′s, surgeons affilaited with the House Ear and Neurosurgery center have been at the forefront of innovations in the surgical treatment of acoustic neuromas. Many of the most significant advances in surgical approaches to the skull base have originated from House Clinic surgeons. Each year, more than 200 patients with acoustic neuromas are treated by our affiliated members of our surgical team; more than at any other center in the world.
The two main concerns that patients typically have in the surgical treatment of acoustic neuromas is preservation of facial nerve function and of hearing. Normal movement of the face on each side is controlled by the facial nerve. Any disruption leads to a loss of normal muscular tone and movement in that side of the face. The House Ear and Neurosurgery Center affiliated surgeons have a facial nerve preservation success rate of greater than 98%.
As for hearing preservation the size of the acoustic neuroma and how well the patient hears prior to surgery are influential factors. If the results of the patient’s hearing test indicate that the hearing level is sufficient to indicate a reasonable chance of success with saving the hearing during surgery, then a surgical approach is selected that is designed to save hearing. Otherwise, it may be advisable to choose a treatment approach that sacrifices hearing in order to completelly remove the tumor.
Most patients with adequate pre-operative hearing levels have small tumors which are mostly confined to the internal auditory canal. In these cases, the House Ear and Neurosurgery Center routinely recommends and performs a middle fossa approach*. In patients with small tumors who have been operated by the middle fossa* approach by surgeon sffiliated with the House Ear and Neurosurgery Center since 1992, good hearing has been preserved in roughly two thirds of those patients. Any measurable level of hearing was preserved in eighty percent (80%) of those patients.