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Acoustic neuroma

Acoustic neuroma (or Vestibular Schwannoma) is a benign tumor of the the myelin forming cells called "Schwann cells" of the 8th cranial nerve, known as the acoustic nerve, (or more properly the vestibulocochlear nerve) just after it has left the brainstem, in the pontine angle; also at the point where the peripheral part of the nerve meets the brain part of the nerve called "Hensen's node". more...

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Multiple names used for this type of tumor include acoustic neuroma, acoustic neuronoma and vestibular schwannoma because it attacks the 8th cranial nerve which branches in to the vestibular and acoustic; and under a microscope the tumor resembles schwann cells.

Assessment

Associated symptoms are unilateral sensorineural hearing loss/deafness and vertigo. Additionally more than 80% of patients having acoustic neuromas have reported tinnitus. Larger tumors can compress local structures such as the facial nerve, and lead to local symptoms such as hydrocephalus.

If the tumor grows next to the brain stem and grows large enough, the brainstem may become compressed. Also associated with these nerves is the 7th cranial nerve; this nerve controls the muscles of the face, salivation, tearing, and taste. If a significantly large tumor develops it can involve the 5th cranial nerve controlling the sensation face and eyes.

While most cases occur sporadically, acoustic neuroma may be attributable to neurofibromatosis (type 2) in about 5% to 10% of the cases. If the tumor is caused by Neurofibromatosis a slightly different type of tumor grows, then the tumor often involves the whole nerve rather than particular sections of the nerve as does the schwannoma which can make treatment more a greater challenge. Also the tumor is classically bilateral with this syndrome.

Treatment

Indicated treatments for acoustic neuroma include surgical removal and radiotherapy.

Conservative treatment

Because these neuromata grow so slowly, a physician may opt for conservative treatment beginning with an observation period. In such a case, the tumor is monitored by annual MRI to monitor growth. Records suggest that about 45% of acoustic neuromata do not grow detectably over the 3-5 years of observation. In rare cases, acoustical neuromata have been known to shrink spontaneously. Oftentimes, people with acoustic neruromata die of other causes before the neuroma becomes life-threatening. (This is especially true of elderly people possessing a small neuroma.)

Since the growth rate of an acoustic neuroma rarely accelerates, annual observation is essential.

Acoustic neuromata may cause either gradual or—less commonly—sudden hearing loss and tinnitus. However, the surgical and radiotherapy treatments are even more dangerous to the hearing in the affected ear.

Surgery

The surgery is done by several approaches and is associated with high incidence of complications and quality of life issues - but it often removes the tumor without recurrence. The vestibular nerve is usually removed on the operated side, resulting in severe imbalance, vertigo and dizziness. However, vestibular function improves rapidly due to compensation by the other ear and other balance mechanisms. Steadiness may never be 100% of the pre-surgical level, but patients are usually walking in the first week after surgery. Surgery also has a risk to the facial nerve which is "monitored" during the surgery. Best results (normal or near normal facial function) are most likely with small acoustic neuromas. The larger the tumor, the higher the risks associated with removing it. Three surgical approaches are commonly used. The first is the translabyrinthine, which destroys hearing in the affected ear. Of the surgical approaches, it tends to be the fastest (less anesthesia time) and allows the most complete removal of the tumor (less chance of recurrence). The two other approaches (suboccipital and middle fossa) are hearing preservation approaches, which have a chance of preserving some or all of the hearing in the affected ear. However, all or most of the hearing in the operated ear is lost approximately 50% of the time. In addition, a number of medical reports indicate that surgically preserved hearing in the operated ear is often not stable, but may deteriorate significantly over a period of years. The hearing preservation approaches tend to require longer surgery, have a higher risk of recurrence and both require brain retraction, which carries a low risk of brain damage. Acoustic neuroma surgery is highly technically demanding, and patients are advised to seek out surgical teams with extensive experience.

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Stereotactic radiosurgery failure in acoustic neuroma - Editorial
From Ear, Nose & Throat Journal, 6/1/02

The combined otolaryngologic spring meetings held May 10-14, 2002, in Boca Raton, Florida, included a wide variety of subjects worthy of comment on this page. Of special interest to me, as one involved in the development of neurotologic surgery to routinely preserve normal facial nerve function, was a paper Daniel J. Lee, MID, and colleagues of the Johns Hopkins School of Medicine presented at the American Neurotology Society, entitled "Clinical and histopathological features of recurrent vestibular schwannoma (acoustic neuroma) following stereotactic radiosurgery."

The authors reported four cases of acoustic neuroma that continued to grow after radiosurgery All four patients had cranial nerve neuropathy after radiosurgery; one had trigeminal nerve paresthesia and three had hemifacial spasm. All had salvage microsurgery performed by the authors within two years of the radiosurgery treatment. They observed inconsistent radiation changes in the cerebellopontine angle and internal auditory canal. Fibrosis outside and within the tumor varied markedly, complicating microsurgical dissection. Light microscopy confirmed the presence of viable tumor in all cases.

The authors pointed out that in an effort to avoid complications, the tumor dose of radiosurgery in recent years has been reduced so that long-term evaluation will be required to determine the failure rate with that form of therapy. With both low- and high-dose radiation therapy, there is increased risk of developing neoplasms of the central nervous system. The authors found six cases reported to have malignant degeneration after radiation therapy for a cerebellopontine angle tumor.

There is little question that in some cases radiosurgery (gamma knife or fractionated stereotactic radiosurgery) might be indicated for the treatment of enlarging acoustic neuroma--for example, in a patient in poor medical condition whose risk with surgery is unacceptable. (1,2) On the other hand, the risk of malignant degeneration and other serious complications of radiosurgery clearly make surgical removal by a skilled, experienced neurotologic surgeon the treatment of choice.

JACK L. PULEC, MD

Editor-in-Chief

EAR, NOSE & THROAT JOURNAL

References

(1.) Pulec JL, Giannotta SL. Acoustic neuroma surgery in patients over 65 years of age. Ear Nose Throat J 1995;74:21-7.

(2.) Pulec JL. Acoustic neuroma surgery in geriatric patients. Ear Nose Throat J 1999;78:429-42

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COPYRIGHT 2002 Gale Group

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