About Acoustic Neuroma
An acoustic neuroma, also called a vestibular schwannoma, is a benign tumor that is located on the eighth cranial nerve. The eighth cranial nerve, which traverses from the inner ear to the brain, is also called the auditory nerve or vestibulocochlear nerve.
Acoustic neuromas are typically slow growing tumors that usually develop over a period of years, although growth rates vary. These rounded tumors typically appear as a single mass and are encapsulated (confined to a contained area). At least 95% of acoustic neuromas are unilateral (occur on one side only). Unlike many other types of tumors, acoustic neuromas are benign and do not spread to other parts of the brain or the body.
The auditory nerve (eighth cranial nerve) is made up of two segments: the vestibular nerve segment and the cochlear nerve segment. The vestibular nerve transmits information about equilibrium (balance) from the inner ear to the brain. The cochlear nerve transmits information about sound. The vestibular nerve splits into two branches: the inferior and superior vestibular nerves. These two nerve components lie next to each other and next to the cochlear nerve as they pass through the internal auditory canal, a small bony canal that leads from the inner ear to the brainstem. They also lie next to the facial nerve (seventh cranial nerve). The facial nerve activates the muscles that control facial movement. The vestibular portion of the auditory nerve is the site of origin in up to 95% of the cases of acoustic neuromas. Acoustic neuromas arise with equal frequency on the superior and inferior portion of the vestibular nerve.
The incidence of acoustic neuromas is about one per 100,000 people. According to the National Institutes of Health (NIH), an estimated 2,000 to 3,000 new cases of acoustic neuromas are diagnosed in the United States each year. Reported incidence may be rising, however, due to improved imaging technology that enables smaller tumors to be clearly visualized. Because of the slow growth rate of acoustic neuromas, a large number of cases never become clinically evident, so that the actual number of cases may be much higher.
Though the exact cause of acoustic neuromas is unknown, the only known genetic risk factor is one parent with neurofibromatosis type 2 (NF-2) that accounts for a minority of cases. Other conditions that have been identified that may place individuals at higher risk for developing an acoustic neuroma include:
- Prolonged noise exposure
- Mobile phones
- Childhood exposure to low-dose radiation of the head and neck
- History of a benign tumor of the parathyroid gland
The symptoms of acoustic neuromas often appear so gradually that people are not aware of them or, if there is awareness, it is blunted over time as individuals habituate (become accustomed) to the subtle changes. Typically, people do not seek medical attention until symptoms become severe. In fact, many small, asymptomatic acoustic neuromas are discovered incidentally when individuals are being evaluated for other problems.
Some of the symptoms associated with acoustic neuromas include:
- Unilateral hearing loss or impairment
- Tinnitus (ringing in the ears)
- Impaired speech discrimination
- Vertigo (dizziness)
- Problems with balance
- Headaches
Acoustic neuromas typically remain encapsulated and, as they grow, they slowly displace surrounding tissue and nerves. Tumors first distort the vestibulocochlear nerve from which they develop, and then may eventually press on the facial nerve, which may become stretched as it makes room for the tumor. If left untreated, tumors grow to the point of filling the internal auditory canal and then may grow into the cerebellopontine angle and may begin to erode the surrounding bony structure. As they grow, tumors take on a pear-like shape and may begin to press on the trigeminal nerve, which is responsible for facial sensation. Although tumors do not actually invade the brain, the pressure of the tumors can displace brain tissue. If they compress brain structures that control vital body functions, acoustic neuromas can become life-threatening.
Thanks to advances in imaging techniques, health care providers can often identify small acoustic neuromas when they are still confined to the internal auditory canal. Tumors are typically classified as "small" (up to .05 cm), "medium" (.05 cm to 2 cm), or "large" (2 cm to 4 cm, or larger).
Treatment of acoustic neuromas is based upon an individual's age, symptoms, tumor size, pattern of tumor growth, overall physical health, and level of hearing at the time of diagnosis.
The goals of treatment include:
- Hearing preservation for patients who still have hearing function in the affected ear
- Local control of tumor growth either by removal of the tumor or by stopping its progression
- Avoidance of damage to adjacent nerves (facial and trigeminal nerves) and brain tissue
Hearing preservation is of the utmost importance and often determines which treatment option chosen. The highest success for hearing preservation is achieved if the tumor is treated when it is very small and hearing loss is mild. As MRI technology improves, increasing numbers of acoustic neuromas are being identified at very early stages. Any loss of hearing that occurs before surgery is permanent and is not recovered following surgery. Large tumors may involve permanent total hearing loss on the affected side. If tumors are extremely large and press on the brainstem, the goal of surgery is to relieve the intracranial pressure which results from fluid accumulation in the brain, and hearing preservation is secondary.
Intervention for acoustic neuromas is generally considered for individuals with tumors of any size that are causing symptoms, tumors that are growing quickly, tumors that are growing close to a nerve or are compressing brain tissue, and tumors that are large.
There are three treatment options for acoustic neuromas:
- Observation ("watching and waiting")
- Microsurgery
- Radiosurgery
Thanks to modern imaging tests, the diagnosis of acoustic neuromas can now be made earlier so that many tumors are smaller upon discovery. Therefore, the outlook for patients with an acoustic neuroma does not hold the same threat as it did in past decades. Early in the 20th century, the mortality rate following surgery was extremely high. Advances in microsurgical techniques and intraoperative monitoring of nerve function, as well as stereotactic and functional radiation therapy have greatly reduced the risk of death, facial paralysis, and hearing loss. Today, many tumors can be treated effectively with both surgery and radiation therapy. Some individuals with slow-growing tumors may not need any type of intervention.
The Medifocus Guidebook on Acoustic Neuroma is a unique, comprehensive patient education resource that contains vital information about Acoustic Neuroma that you won't find anywhere else in a single resource. The Guidebook will answer many of your questions about this condition that your healthcare provider may not have the time to answer. To learn more about the Guidebook, please click here
