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Acoustic
Neurinoma
What is acoustic neurinoma?
An acoustic neurinoma is a benign tumor which may develop on the
hearing and balance nerves near the inner ear. The tumor results
from an overproduction of Schwann cells—small sheet-like
cells that normally wrap around nerve fibers like onion skin and
help support the nerves. When growth is abnormally excessive,
Schwann cells bunch together, pressing against the hearing and
balance nerves, often causing gradual hearing loss, tinnitus or
ringing in the ears, and dizziness. If the tumor becomes large,
it can interfere with the facial nerve, causing partial paralysis,
and eventually press against nearby brain structures, becoming
life-threatening.
How is acoustic neurinoma diagnosed? 
Early diagnosis of an acoustic neurinoma is key to preventing
its serious consequences. 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. Therefore, once the symptoms
appear, a thorough ear examination and hearing test are essential
for proper diagnosis. Computerized tomography (CT) scans and
magnetic resonance imaging (MRI) are helpful in determining
the location and size of a tumor and also in planning its removal.
How is acoustic neurinoma treated?
If an acoustic neurinoma
is surgically removed when it is still very small, hearing may be preserved
and accompanying symptoms may go away. As the tumor grows larger, surgical
removal is often more complicated because the tumor may become firmly
attached to the nerves that control facial movement, hearing and balance.
The removal of tumors attached
to hearing, balance or facial nerves can make the patient's symptoms
worse because sections of these nerves must also be removed with the
tumor. As an alternative to conventional surgical techniques, radiosurgery
may be used to reduce the size or limit the growth of the tumor. Radiosurgery,
utilizing carefully focused radiation, is sometimes performed on the
elderly, on patients with tumors on both hearing nerves, or on patients
with a tumor growing on the nerve of their only hearing ear. If the
tumor is not removed, MRI is used to carefully monitor its growth.
What are the two types of acoustic neurinoma?
There are two types
of acoustic neurinomas: unilateral and bilateral. Unilateral acoustic
neurinomas affect only one ear and account for approximately 8 percent
of all tumors inside the skull. Symptoms may develop at any age, but
usually occur between the ages of 30 and 60 years.
Bilateral acoustic neurinomas,
which affect both ears, are hereditary. Inherited from one's parents,
this tumor results from a genetic disorder known as neurofibromatosis-2
(NF2). Affected individuals have a 50 percent chance of passing this
disorder on to their children. Unlike those with a unilateral acoustic
neurinoma, individuals with NF2 usually develop symptoms in their teens
or early adulthood. Because NF2 patients usually have multiple tumors,
the surgical procedure is more complicated than the removal of an unilateral
acoustic neurinoma. Further research is needed to determine the best
approach in these circumstances.
In addition to tumors arising
from the hearing and balance nerves, NF2 patients may develop tumors
on other cranial nerves associated with swallowing, speech, eye and
facial movement and facial sensation. NF2 patients may also develop
tumors within the spinal cord and from the brain's thin covering.
Scientists believe that
both types of acoustic neurinoma form following a loss of the function
of a gene on chromosome 22. A gene is a small section of DNA responsible
for a particular trait like hair color or skin tone. Scientists believe
that this particular gene on chromosome 22 suppresses the growth of
Schwann cells. When this gene malfunctions, Schwann cells can grow out
of control. Scientists also think that this gene may help suppress other
types of tumor growth. In NF2 patients, the faulty gene on chromosome
22 is inherited. For individuals with unilateral acoustic neurinoma,
however, some scientists hypothesize that this gene somehow loses its
ability to function properly as a result of environmental factors.
What
is being done about acoustic neurinoma?
Once the gene that
suppresses Schwann cell growth is "mapped" or located, scientists
can begin to develop gene therapy to control the overproduction of these
cells in individuals with acoustic neurinoma. Also, learning more about
the way genes help suppress acoustic neurinoma may help prevent brain
tumors and lead to a treatment for cancer.
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