Acoustic Neuroma
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A vestibular schwannoma (also known as acoustic neurinoma,
acoustic neurinoma, or acoustic neurilemoma) is a benign, usually
slow-growing tumor that develops from the balance and hearing nerves
supplying the inner ear. The tumor comes from an overproduction of Schwann
cells--the cells that normally wrap around nerve fibers like onion skin
to
help support and insulate nerves. As the vestibular schwannoma grows, it
presses against the hearing and balance nerves, usually causing unilateral
(one-sided) or asymmetric hearing loss, tinnitus (ringing in the
ear), and dizziness/loss of balance. As the tumor grows, it can interfere
with the face sensation nerve (the trigeminal nerve), causing facial
numbness. Vestibular schwannomas can also press on the facial nerve (for
the muscles of the face) causing facial weakness or paralysis on the side
of the tumor. If the tumor becomes large, it will eventually press against
nearby brain structures (such as the brainstem and the cerebellum),
becoming life-threatening.
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Unilateral/asymmetric hearing loss and/or tinnitus and loss of
balance/dizziness are early signs of a vestibular schwannoma.
Unfortunately, early detection of the tumor is sometimes difficult because
the symptoms may be subtle and may not appear in the beginning stages of
growth. Also, hearing loss, dizziness, and tinnitus are common symptoms of
many middle and inner ear problems (the important point here is that
unilateral or asymmetric symptoms are the worrisome ones). Once the
symptoms appear, a thorough ear examination and hearing test (audiogram)
are essential for proper diagnosis. Computerized tomography (CT) scans,
enhanced with intravenous dye (contrast), and magnetic resonance imaging
(MRI) are critical in the early detection of a vestibular schwannoma and
are helpful in determining the location and size of a tumor and in
planning its microsurgical removal.
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Early diagnosis of a vestibular schwannoma is key to preventing its
serious consequences. There are three options for managing a vestibular
schwannoma: (1) surgical removal; (2) radiation; and (3) monitoring.
Typically, the tumor is surgically removed (excised). The exact type of
operation done depends on the size of the tumor and the level of hearing
in the affected ear. If the tumor is very small, hearing may be saved and
accompanying symptoms may improve. As the tumor grows larger, surgical
removal is more complicated because the tumor may have damaged the nerves
that control facial movement, hearing, and balance and may also have
affected other nerves and structures of the brain.
The removal of tumors affecting the hearing, balance, or facial nerves
can make the patient's symptoms worse because sections of these nerves may
also need to be removed with the tumor.
As an alternative to conventional surgical techniques, radiosurgery
(that is, radiation therapy--the "gamma knife" or LINAC) may be used to
reduce the size or limit the growth of the tumor. Radiation therapy is
sometimes the preferred option for elderly patients, patients in poor
medical health, patients with bilateral vestibular schwannoma (tumor
affecting both ears), or patients whose tumor is affecting their only
hearing ear. In some cases, usually elderly or medically infirm patients,
it may be reasonable to "watch" the tumor for growth. Repeat MRI over time
is used to carefully monitor the tumor for any growth.
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Unilateral vestibular schwannomas affect only one ear. They account for
approximately 8 percent of all tumors inside the skull; one out of every
100,000 individuals per year develops a vestibular schwannoma. Symptoms
may develop at any age but usually occur between the ages of 30 and 60
years. Unilateral vestibular schwannomas are not hereditary.
Bilateral vestibular schwannomas affect both hearing nerves and are
usually associated with a genetic disorder called neurofibromatosis type 2
(NF 2). Half of affected individuals have inherited the disorder 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. Unlike those with a unilateral vestibular
schwannoma, individuals with NF2 usually develop symptoms in their teens
or early adulthood. In addition, patients with NF2 usually develop
multiple brain and spinal cord related tumors. They also can develop
tumors of the nerves important for swallowing, speech, eye and facial
movement, and facial sensation. Determining the best management of the
vestibular schwannomas as well as the additional nerve, brain, and spinal
cord tumors is more complicated than deciding how to treat a unilateral
vestibular schwannoma. Further research is needed to determine the best
treatment for individuals with NF2.
Scientists believe that both unilateral and bilateral vestibular
schwannomas form following the loss of the function of a gene on
chromosome 22. (A gene is a small section of DNA responsible for a
particular characteristic like hair color or skin tone). Scientists
believe that this particular gene on chromosome 22 produces a protein that
controls the growth of Schwann cells. When this gene malfunctions, Schwann
cell growth is uncontrolled, resulting in a tumor. Scientists also think
that this gene may help control the growth of other types of tumors. In
NF2 patients, the faulty gene on chromosome 22 is inherited. For
individuals with unilateral vestibular schwannoma, however, some
scientists hypothesize that this gene somehow loses its ability to
function properly.
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Scientists are working to better understand how the gene works so they
can begin to develop gene therapy to control the overproduction of Schwann
cells in individuals with vestibular schwannoma. Also, learning more about
the way genes help control Schwann cell growth may help prevent other
brain tumors.
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NIDCD Information Clearinghouse
Acoustic Neuroma Association
(ANA)
American Academy of Otolaryngology-Head
and Neck Surgery (AAO-HNS)
Neurofibromatosis, Inc. (NF,
Inc.)
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