An acoustic neuroma (also called a vestibular schwannoma, acoustic neurinoma, or neurilemmoma) is a nonmalignant and usually slow-growing tumor caused by an overproduction of Schwann cells, which normally insulate and support nerves. The tumor develops on the sheath of inner ear’s vestibulo-cochlear nerve, which transmits both balance and sound information to brain. Acoustic neuromas are rare; only about 2500–3000 new cases are diagnosed in the United States each year. Symptoms may develop in individuals at any age, but usually occur between the ages of 30 and 60 years. Although people with the genetic disorder neurofibromatosis have bilateral acoustic neuromas, meaning tumors in both ears, the overwhelming majority of acoustic neuromas are unilateral (one-sided). Unilateral acoustic neuromas are not hereditary; however, their actual cause is not yet well understood.
As an acoustic neuroma grows, it compresses the vestibulo-cochlear nerve, usually causing hearing loss, tinnitus, and dizziness or loss of balance. Because the vestibulo-cochlear nerve is located near the trigeminal nerve, which controls facial sensation, large tumors can cause facial numbness. An acoustic neuroma may also exert pressure on nerves controlling the muscles of the face, causing facial weakness or paralysis on the side of the tumor. Vital life-sustaining functions can be threatened when large tumors cause severe pressure on the brainstem and cerebellum.
Diagnosis is sometimes difficult because symptoms may be nonexistent or subtle, and are common to many middle and inner ear problems. However, early detection of an acoustic neuroma is key to preventing its serious consequences. Once symptoms appear, a thorough ear examination and evaluation is critical. In addition to routine hearing tests, an auditory brainstem response test (also called ABR, BAER, or BSER) may also be used to evaluate whether sound information is being successfully passed along the nerve path leading from the ear to the brain. If the ear exam and hearing test reveal problems, a detailed imaging test is typically performed. Magnetic resonance imaging (MRI) is considered the best method of evaluation because it can detect tumors of very small size. When MRI is not an option, a computerized tomography scan (CT scan) is used. These tests help determine a tumor’s location and size and in selecting a treatment plan.
The three treatment options are surgical removal, radiation, and monitoring. Typically, the tumor is surgically removed. The major risk with surgical removal is loss of hearing or facial movement on the side of the affected ear. The exact type of operation selected depends on tumor size and the level of hearing and facial nerve function still intact.
If the tumor is very small, hearing and facial nerve function may be preserved and accompanying symptoms may improve after surgery. As the tumor grows larger, its surgical removal can worsen symptoms of vestibular dysfunction because sections of the nerves themselves may also need to be removed. In this case, vestibular rehabilitation may help promote central nervous system compensation for the inner-ear deficit.
As an alternative to conventional surgical techniques, radiosurgery with a gamma knife or linear accelerator may be employed to reduce the size or limit the growth of the tumor. Alternately, radiation therapy is sometimes the preferred option for patients who are elderly, in poor health, have bilateral acoustic neuromas, or whose tumor is affecting their only hearing ear. In some cases, usually involving elderly or medically infirm patients, it may be preferable to monitor the tumor with repeated MRIs to monitor the tumor for any growth.