An acoustic neuroma is a benign (non-cancerous) fibrous tissue growth that arises from the balance nerve (also called the eighth cranial nerve or vestibulocochlear nerve - see FIGURE A) that leads from the brain to the inner ear. Acoustic neuromas are non-malignant, meaning that they do not spread to other parts of the body. An acoustic neuroma is sometimes also called a vestibular schwannoma or neurolemmoma.

Because acoustic neuromas are located deep inside the skull and are adjacent to vital brain centers in the brain stem, as these tumors grow, the affect surrounding structures in the brain that control vital functions. The brain is not invaded by the acoustic neuroma, but the tumor pushes on the brain as it enlarges. As the acoustic neuroma grows, it typically first affects a patient's hearing because it protrudes from the internal auditory canal into an area behind the temporal bone. Larger acoustic neuromas can press on another nerve in the area, the trigeminal nerve, which is the nerve affecting facial sensation. When large acoustic neuromas cause severe pressure on the brainstem and cerebellum of the brain, vital functions that sustain life can be threatened.

In the majority of cases, acoustic neuromas grow slowly over a period of years. In other cases, the growth rate is more rapid and patients develop symptoms at a faster pace. Usually the symptoms are mild and many patients are not diagnosed until some time after their acoustic neuroma has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans.

Acoustic neuromas account for approximately 6% of all brain tumors. These tumors occur in all races of people and have a slightly higher occurrence in women. In the United States about ten people per million per year are diagnosed with an acoustic neuroma. This translates to roughly 2,500 newly diagnosed acoustic neuromas per year. Most acoustic neuromas occur spontaneously without any evidence of heredity and are diagnosed in patients between the ages of 30 to 60.