by Steven Seung, M.D., Ph.D. An acoustic neuroma (also called vestibular schwanoma) is a tumor that arises from the nerve sheath that envelops the vestibular nerve. They are usually benign and slow growing, and very few acoustic tumors threaten the patient's general health initially. The symptoms are hearing loss, vertigo (imbalance), or tinnitus (ringing in the ears). Less commonly, patients experience facial weakness or numbness. The rationale for treating the tumor is to prevent serious health problems down the road if left alone to grow. Unlike microsurgery, Gamma Knife treatments carry very low short-term and long-term risks. The first successful removal of an acoustic neuroma was performed in 1894; however, the mortality following surgery at the turn of the century was at least 80%. The results improved gradually but were still far from satisfactory in the early 1960s, when microsurgical techniques were gradually introduced into this field. Virtually all of the about 3000 new acoustic neuroma patients that are diagnosed in the United States every year are candidates for Gamma Knife radiosurgery. However, for tumors larger than 3cm in diameter, there is a greater risk that these tumors, even before any treatment, interfere with the circulation of the cerebrospinal fluid (CSF), causing hydrocephalus. Temporary swelling induced by the Gamma Knife treatment may occasionally result in hydrocephalus not present earlier. Thus, surgical removal of large tumors is recommended. If the patient is not an operable candidate, a shunt may be placed prior to the Gamma Knife procedure for these larger tumors. Acoustic neuromas sometimes increase in size temporarily as a reaction to the Gamma Knife treatment. This is actually a favorable sign indicating a brisk response. Such swelling usually is most obvious between 6 and 18 months after the procedure. It should not be confused with increase due to lack of response. A definite assessment should be made two years after the treatment. Gamma Knife treatment can be repeated without increased risks if the acoustic neuroma did not respond to the first treatment. Microsurgery can also be selected, depending on the patient's preference. At experienced Gamma Knife centers, the incidence of temporary facial and trigeminal nerve dysfunction among acoustic neuroma patients is less than 2-3%. Preservation of useful hearing currently is achieved in 55-75% in different series with the better results usually in smaller tumors. Hearing tends to remain stable when the first one to two years have elapsed after treatment. The tinnitus (spontaneous noise) so frequently associated with the hearing loss in acoustic neuroma patients is usually not affected, for better or worse, by Gamma Knife treatment initially.
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