The acoustic (eighth cranial) nerve, also known as the vestibulocochlear nerve, has two parts — the cochlear nerve, which transmits sound from the inner ear to the brain, and the vestibular nerve, which facilitates balance. A benign (noncancerous) tumor that develops in the vestibulocochlear nerve is commonly called an acoustic neuroma, but is also known as a vestibular schwannoma or neurilemoma. Acoustic neuromas begin in Schwann cells, which form a substance called myelin that insulates nerves. The vast majority of these tumors arise in the vestibular portion of the nerve, but a few begin in the cochlear nerve. Between 3,000 and 5,000 people, usually between the ages of 30 and 60, are diagnosed with an acoustic neuroma each year in the United States.
Most acoustic neuromas affect only one ear and have no apparent risk factors. Rarely, some people — usually teens and young adults — develop acoustic neuromas in both ears in association with a hereditary disorder called neurofibromatosis type 2 (NF2).
Most acoustic neuromas grow very slowly, some more rapidly, and others not at all. As an acoustic neuroma expands, it compresses nearby nerves, blood vessels, and the surface of the brainstem and cerebellum, threatening neurological function.
The first symptoms of an acoustic neuroma usually involve hearing and balance. As the tumor grows, it may compress the brainstem and other cranial nerves, leading to a variety of progressive symptoms including:
- hearing loss (usually gradual) occurring on one side
- ringing (tinnitus) in the ear
- dizziness (vertigo)
- loss of balance
- facial numbness
- double vision
- facial weakness
- nausea and vomiting
- hydrocephalus (a blockage in the flow of cerebrospinal fluid that surrounds the brain and spinal cord)
Diagnosing an acoustic neuroma may be difficult, because symptoms such as hearing loss, ringing in the ear, and dizziness are similar to those of other ear-related or neurologic conditions. Initially, the doctor may perform an ear examination and hearing tests. If an acoustic neuroma is suspected, a radiologist or other specialist may use an MRI scan to determine the location and size of the tumor. Prior to imaging, a contrast dye is administered intravenously to obtain a more detailed picture of the tumor.
Once an acoustic neuroma has been identified, the patient may undergo additional functional tests to evaluate hearing and balance.