The eighth cranial nerve is involved in both hearing (the auditory or acoustic component of the nerve) and balance (the vestibular component of the nerve). Like all cranial nerves, the eighth cranial nerve (also called the acoustic or auditory nerve) is paired, meaning that there is one on each side of the body. Each eighth cranial nerve runs from the inner ear to the brain, passing through a bony canal called the internal auditory canal. This canal is shared with the seventh cranial nerve, the facial nerve.
Like many nerve fibers, the eighth cranial nerve is wrapped in a sheath composed of specialized Schwann cells that serve to speed the transmission of information along the nerve. When the Schwann cells grow in an uncontrolled fashion, they can develop into a tumor, called schwannoma or neuroma. Although a vestibular schwannoma is not malignant (cancerous), it can still result in serious symptoms caused by pressure on the eighth cranial nerve or on surrounding tissues or the adjacent facial nerve. Most cases of vestibular schwannoma are unilateral; that is, only one of the two eighth cranial nerves is affected.
About 100,000 people in the United States develop vestibular schwannoma. Most people who develop a vestibular schwannoma are between the ages of 30 and 50; children rarely develop vestibular schwannoma. Women are slightly more likely than men to develop a vestibular schwannoma.
There is an increased risk of developing a vestibular schwannoma in individuals who have a disease called neurofibromatosis. In these cases, the tumors tend to develop on both sides (bilaterally). In fact, about 10% of all cases of vestibular schwannoma occur in individuals who have neurofibromatosis. People with neurofibromatosis who develop vestibular schwannoma may do so at a younger age, sometimes in their teens or early adulthood.
Causes and symptoms
No one knows exactly why some people develop a vestibular schwannoma. Most seem to occur sporadically, with no identifiable cause. There is an increased risk of developing a vestibular schwannoma in individuals with neurofibromatosis, and some research has suggested that individuals who are chronically exposed to loud noise may have an increased risk of developing a vestibular schwannoma.
The initial symptoms of vestibular schwannoma are caused by pressure on the eighth cranial nerve, and include gradually progressive one-sided hearing loss, buzzing in the ears (tinnitus), dizziness, and difficulty with balance. In particular, the hearing impairment greatly affects the ability to understand speech (speech discrimination). When the vestibular schwannoma puts pressure on the seventh cranial nerve, pain and numbness in the face may develop. Eventually, the facial muscles may become paralyzed. The individual may also experience difficulty chewing and/or swallowing, ear pain, and headache. When left untreated, hearing impairment may eventually lead to complete deafness in the affected ear. If the tumor begins to encroach on other brain tissues, the person may experience nausea, vomiting, fever, vision changes, and difficulty walking.
A careful neurologic examination will reveal the deficits that are characteristic of vestibular schwannoma. Computed tomography (CT) or magnetic resonance imagaing (MRI) scan may help pinpoint the tumor. Audiometry and brain stem auditory evoked potential tests are performed to establish the degree of hearing deficit prior to treatment. Audiometry assesses hearing acuity by evaluating the ability to hear various volumes and tones. A brain stem auditory evoked potential test evaluates brain wave responses to clicking sounds, in order to assess the functioning of the auditory (hearing) pathways in the brain.
When an individual is suspected of having a vestibular schwannoma, an otorhinolaryngologist and/or neurologist may be consulted to arrive at a diagnosis. An otorhinolaryngologist will be called upon if surgery is required.
Surgery is nearly always necessary to treat vestibular schwannoma. There are several different types of surgery that are used to remove a vestibular schwannoma, classified by the anatomical pathway used to reach the tumor (called the "approach"). The surgeon will choose the approach based on tumor size, preoperative hearing acuity, and the patient's ability to tolerate surgical risk. In some cases, it is not possible to remove the entire vestibular schwannoma without considerable risk of damage to adjacent structures. In these cases, only part of the vestibular schwannoma may be removed, and the rest may be left in place (called "partial resection").
When a patient is medically frail, the surgeon may choose to simply monitor the growth of the vestibular schwannoma, delaying surgery until it becomes absolutely necessary. Occasionally, very small vestibular schwannoma may be treated with radiation therapy; when partial resection is necessary, surgery may be followed by radiation treatment.
Newer treatment techniques are called stereotactic radiosurgery or gamma knife surgery. Three-dimensional imaging allows the exact location of the tumor to be defined. The patient's head is held in a frame that allows high-dose radiation to be delivered from multiple angles directly at the tumor site.
Recovery and rehabilitation
In patients for whom the hearing impairment is not total, a hearing aid may be helpful. In patients who have completely lost hearing in one ear, a system called contralateral routing of sound (CROS) sends sound from the deaf ear through a microphone to the hearing ear, improving overall hearing acuity.
Without treatment, vestibular schwannoma will nearly always result in permanent deafness. Although surgery carries a high risk of hearing loss and facial nerve impairment, about 66% of patients who have small-to medium-sized vestibular schwannoma will have improved hearing acuity following surgery.
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National Institute on Deafness and Other Communication Disorders, National Institutes of Health. 31 Center Drive, MSC 2320, Bethesda, MD 20892-2320. (301) 496-7243; Fax: (301) 402-0018. email@example.com. <http://www.nidcd.nih.gov/health/hearing/acoustic_neuroma.asp>.
Acoustic Neuroma Association. 600 Peachtree Pkwy, Suite 108, Cumming, GA 30041-6899. (770) 205-8211; Fax: (770) 205-0239. ANAUSA@aol.com. <http://anausa.org/>.
Rosalyn Carson-DeWitt, MD