Acoustic neuroma
Also called vestibular schwannoma, schwannoma,
acoustic neurinoma, or neurolemmoma
Definition
An acoustic neuroma is a benign (noncancerous) tumor on the 8th cranial nerve, the vestibulocochlear nerve. This nerve leads from the brainstem to the ear and is involved in hearing and maintaining balance. The more precise term for this tumor is “vestibular schwannoma,” because the tumor involves the vestibular portion of the nerve, not the acoustic (cochlear) portion, and it arises from Schwann cells (which help provide myelin insulation to axons of peripheral nerves) rather than from neurons.
Acoustic neuromas are usually solitary tumors that grow slowly over a period of years, expanding in size at their site of origin. A large tumor can displace normal brain tissue. Acoustic neuromas typically extend into the posterior fossa to occupy the angle between the cerebellum and the pons. Because of the tumor’s position, it may also compress the 5th, 7th, and (less often) the 9th and 10th cranial nerves. Later, it may compress the pons and lateral medulla, causing obstruction of the cerebrospinal fluid and increased intracranial pressure.
Most cases are diagnosed in patients between the ages of 30 and 60. Men and women are affected equally. Approximately 3,000 cases are diagnosed each year in the United States. Worldwide, there are about 1 to 2 cases for every 100,000 people.
Causes
Acoustic neuromas mainly occur sporadically, with no known cause. Less often, they can occur as part of von Recklinhausen neurofibromatosis. In these cases, the neuroma may take one of two forms: In neurofibromatosis type I, a schwannoma may incidentally involve the 8th nerve, usually in adult life, but may involve any other cranial nerve or the spinal root. Bilateral acoustic neuromas, a hallmark of neurofibromatosis type II, are typically present before age 21. Neurofibromatosis type I and type II are both inherited disorders.
Symptoms
The earliest symptoms of acoustic neuroma include gradual hearing loss and tinnitus (ringing) in the affected ear. Balance problems often arise as the tumor grows. These manifestations can be attributed to the disruption of normal vestibulocochlear nerve function. Larger tumors that compress the adjacent brainstem may affect other local cranial nerves, leading to facial weakness, numbness and tingling, and impaired sensation in the face and mouth on the involved side. At advanced stages, the tumor may be large enough to cause an increase in cranial pressure, which can produce headaches, vomiting, and altered consciousness.
Diagnosis
Tests to confirm the presence of an acoustic neuroma may include the following:
- audiogram: a test to evaluate hearing in both ears.
- auditory brainstem response test (ABR, BAER, or BSER): This test measures the rate of electric impulses traveling from the inner ear to the brainstem. A slowed or absent impulse may indicate the presence of an acoustic neuroma.
- electronystagmography: a test for balance. Cold and warm water or air are inserted into the ear canal, and the resulting dizziness and rapid eye movement (nystagmus) are recorded.
- MRI scan with gadolinium enhancement: This is the preferred test for confirming the presence of an acoustic neuroma; it will reveal an image of even very small tumors.
- computed tomography (CT) scan: This test can be used to create images of tumors larger than 2 cm across.
Treatment
Treatment depends on the age and general health of the patient as well as the tumor’s size, location, and rate of growth. Treatment options include:
observation: If the tumor is very small and if there are no serious symptoms, the physician may decide simply to monitor its growth. This conservative route is common among people over age 70.
microsurgical removal: As the tumor grows, or if hearing becomes impaired, removal of all or part of the tumor may be necessary. The surgical approach depends on the size and location of the tumor. Whatever the approach, neurophysiological monitoring of cranial nerves, especially the 7th cranial nerve (the facial nerve) is always performed. Three surgical approaches are commonly used:
- The translabyrinthine approach destroys hearing in the affected ear. Therefore, this approach is often used in cases where the patient has already had a substantial hearing loss because of the tumor. This approach is often considered the safest route to remove the tumor. Not all tumors can be removed through this approach.
- The middle fossa approach is used to remove tumors typically less than 2 cm in diameter and is employed primarily to preserve hearing. A small window of bone is removed above the ear canal. This exposes the tumor from the upper surface of the internal auditory canal beyond the inner ear.
- The retrosigmoid approach creates an opening in the skull behind the ear, allowing the surgeon to see the posterior surface of the tumor and its relation to the brainstem. This approach can be used for larger tumors and also offers the possibility of hearing preservation.
radiation therapy: Radiation is expected to prevent further growth of the tumor and may be used when tumors are small. This method may preserve hearing. Radiation therapy is done in a variety of ways, but mainly by two methods:
- radiosurgery (like Gamma Knife). In the Gamma Knife approach, beams of gamma radiation are focused on the tumor in a single session.
- fractionated stereotactic radiotherapy. This form of therapy involves a course of several sessions during which a beam of ionizing radiation is directed at the tumor. The beam is wider and less accurate than that of the Gamma Knife, and the total dose is much higher. What spares normal tissue in this treatment method is the drawn-out timing in delivering that dose.
Radiosurgery or radiotherapy can also be used as supplementary treatment for patients who had surgery with partial or subtotal resection of the tumor.
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