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What is Acoustic Neuroma?
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Important Points To Know About an Acoustic Neuroma:
- An acoustic neuroma, also called a vestibular schwannoma, is a rare benign tumor of the balance or hearing nerves.
- It is usually slow growing and expands at its site of origin.
- The most common first symptom is hearing loss in the tumor ear.
- The cause is generally unknown, although may be genetic in some cases.
- If an acoustic tumor becomes large it will push on the surface of the brainstem but not really grow into brain tissue.
- Continued tumor growth may threaten neurological function and even life.
- The treatment options are observation, surgical removal or radiation.
What is an Acoustic Neuroma?
An acoustic neuroma (sometimes termed a vestibular schwannoma or neurolemmoma) is a benign (non-cancerous) 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 with sending balance information to the brain
from the inner ear. The eigth nerve, along with the facial or seventh cranial 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 generally here that acoustic neuromas originate from the sheath surrounding the eighth
nerve. The seventh or facial nerve provides motion to the muscles of facial expression.
Small
Acoustic Neuroma
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Medium
Acoustic Neuroma |
Large
Acoustic Neuroma
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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. 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).
Are Acoustic Neuromas hereditary?
No. Although there is an inheritable condition called neurofibromatosis type 2 which can lead to acoustic
neuroma formation in some people, most acoustic neuromas occur spontaneously without any evidence of an inheritable pattern.
How often do Acoustic Neuromas occur?
Most recent publications suggest that the incidence of acoustic neuromas is rising.
This is because of advances in MRI scanning both on incidental scans and for
patients experiencing symptoms. Studies in Denmark published in 2004 show the
incidence is 17.4 per million or close to 2 persons per 100,000. Most acoustic
neuromas are diagnosed in patients between the ages of 30 and 60.
Cause or Etiology of Acoustic Neuroma
Many patients inquire about the cause of the tumor. There is a growing body of evidence that sporadic defects
in tumor suppressor genes may give rise to these tumors in some individuals. Other studies have hinted at exposure to loud noise on a consistent basis.
One study has shown a relationship of acoustic neuromas to prior exposure to head and neck radiation, and a concomitant history of having had a parathyroid adenoma
(tumor found in proximity to the thyroid gland controlling calcium metabolism). There are even controversies on hand held cellular phones. Whether or not the
radiofrequency radiation has anything to do with acoustic neuroma formation, remains to be seen. To date, no environmental factor (such as cell phones and diet) has been scientifically proven to cause these tumors.
Neurofibromatosis (NF2)
NF2, a genetic disorder, occurs with a frequency of 1 in 30,000 to 1 in 50,000 births. The hallmark of this
disorder is bilateral acoustic neuromas (an acoustic neuroma on both sides). This creates the perplexing problem of the possibility of complete deafness if the
tumors are left to grow unchecked. Preventing or treating the complete deafness that may befall individuals with NF2 requires complex decision making. The trend
at most academic U.S. medical centers is to recommend treatment for the smallest tumor which has the best chance of preserving hearing. If this goal is successful,
then treatment can also be offered for the remaining tumor. If hearing is not preserved at the initial treatment, then usually the second tumor, in the only-hearing
ear, is just observed. If it shows continued growth and may become life-threatening, or if the hearing is lost over time as the tumor grows, then treatment is
undertaken. This strategy hopefully, has the highest chance of perserving hearing for the longest time possible.
There are now several options to try to rehabilitate deafness in NF2 patients. Implanting the hearing part of the brainstem (Auditory Brainstem Implant) can help
restore some sound perception to these patients. Also cochlear implants can be used if the cochlear nerve is preserved following surgery. Radiosurgery may be an
option although stereotactic radiosurgery may not have the effect on the NF2 patient as in patients with unilateral sporadic tumors. Recent studies have shown that
these individuals may have more tumors that are resistant to radiation, due to the cell type. These cases should be handled in centers with very experienced skull base teams.
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