Pathology
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).
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.
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