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An acoustic neuroma, known as a vestibular schwannoma, is a benign (non-cancerous) growth that arises on the eighth cranial nerve leading from the brain to the inner ear. This nerve has two distinct parts, one part associated with transmitting sound and the other with sending balance information to the brain from the inner ear. The eighth nerve, along with the facial or seventh cranial nerve, lie adjacent to each other as they pass through a bony canal called the internal auditory canal. This canal is approximately 2 cm (0.8 inches) long and it is generally here that acoustic neuromas originate from the sheath surrounding the eighth nerve. The seventh or facial nerve provides motion to the muscles of facial expression.
Acoustic neuromas usually grow slowly over a period of years. They expand in size at their site of origin and when large can displace normal brain tissue. The brain is not invaded by the tumor, but the tumor pushes the brain as it enlarges. The slowly enlarging tumor protrudes from the internal auditory canal into an area behind the temporal bone called the cerebellopontine angle. The tumor now assumes a pear shape with the small end in the internal auditory canal. Larger tumors can press on another nerve in the area (the trigeminal nerve) which is the nerve of facial sensation. Vital functions to sustain life can be threatened when large tumors cause severe pressure on the brainstem and cerebellum. Tumors are typically described as small (less than 1.5 cm), medium (1.5 cm to 2.5 cm) or large (more than 2.5 cm).
Figure 1. The normal anatomy of the ear. The outer ear funnels sound to the eardrum, which vibrates three tiny bones called ossicles (malleus, incus and stapes). The spiral-shaped cochlea is filled with liquid, which moves in response to vibrations. As the fluid moves, thousands of hair cells are stimulated, sending signals along the cochlear nerve (responsible for hearing) to the brain. Attached to the cochlea are three semicircular canals positioned at right angles to each other. The three canals are able to sense head position and posture. Electrical signals from the semicircular canals are carried to the brain by the superior and inferior vestibular nerves (responsible for balance). The cochlear and vestibular nerves form a bundle inside the bony internal auditory canal. Inside the canal, the vestibulocochlear nerve lies next to the facial nerve (responsible for facial movement). (Printed with permission of the Mayfield Clinic – www.mayfieldclinic.com)
Figure 2. An acoustic neuroma expands out of the internal auditory canal displacing the cochlear, facial and trigeminal nerves located in the cerebellopontine angle. Eventually the tumor can compress the brainstem. (Printed with permission of the Mayfield Clinic – www.mayfieldclinic.com)
Figure 3. Acoustic neuromas are classified according to their size. MRI scans and correlative illustrations of small (intracanalicular), medium and large acoustic neuromas. (Printed with permission of the Mayfield Clinic – www.mayfieldclinic.com)
Although there is an heritable condition called Neurofibromatosis Type 2 (NF2) which can lead to acoustic neuroma formation in some people, most acoustic neuromas occur spontaneously without any evidence of family history (95%).
Most recent publications suggest that the incidence of acoustic neuromas is rising. This is because of advances in MRI scanning both on incidental scans and for patients experiencing symptoms. Studies in Denmark published in 2004 show the incidence is 17.4 per million or close to 2 persons per 100,000. Most acoustic neuromas are diagnosed in patients between the ages of 30 and 60.
There is a growing body of evidence that sporadic defects in tumor suppressor genes may give rise to these tumors in some individuals. Other studies have hinted at exposure to loud noise on a consistent basis. One study has shown a relationship of acoustic neuromas to prior exposure to head and neck radiation, and a concomitant history of having had a parathyroid adenoma (tumor found in proximity to the thyroid gland controlling calcium metabolism). There are even controversies on hand-held cell phones. It remains to be seen whether or not the radiofrequency radiation has anything to do with acoustic neuroma formation. To date, no environmental factor (such as cell phones and diet) has been scientifically proven to cause these tumors. ANA does recommend that frequent cell phone users use a hands-free device to enable separation of the device from the head. Please refer to cell phone statement on our website.
NF2, a genetic disorder, occurs with a frequency of 1 in 30,000 to 1 in 50,000 births. The hallmark of this disorder is bilateral acoustic neuromas (an acoustic neuroma on both sides). This creates the perplexing problem of the possibility of complete deafness if the tumors are left to grow unchecked. Preventing or treating the complete deafness that may befall individuals with NF2 requires complex decision making. The trend at most academic U.S. medical centers is to recommend treatment for the smallest tumor which has the best chance of preserving hearing. If this goal is successful, then treatment can also be offered for the remaining tumor. If hearing is not preserved at the initial treatment, then usually the second tumor, in the only-hearing ear, is just observed. If it shows continued growth and becomes life-threatening, or if the hearing is lost over time as the tumor grows, then treatment is undertaken. This strategy has the highest chance of preserving hearing for the longest time possible.
There are now several options to try to rehabilitate deafness in NF2 patients. Implanting the hearing part of the brainstem (Auditory Brainstem Implant) can help restore some sound perception to these patients. Also cochlear implants can be used if the cochlear nerve is preserved following surgery. Radiosurgery may be an option although stereotactic radiosurgery may not have the effect on the NF2 patient as in patients with unilateral sporadic tumors.There are some centers using radiation therapy for NF2 with mixed results. The risk of malignant transformation after radiation is higher in this group. Recent studies have shown that these individuals may have more tumors that are resistant to radiation, due to the cell type. These cases should be handled in centers with very experienced skull base teams.