ACOUSTIC NEUROMA-THE BASIC FACTS Origin and Cause

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What is an acoustic neuroma?

An acoustic neuroma (sometimes also termed a neurinoma or vestibular schwannoma) is a benign or non-cancerous growth that arises from the 8th or vestibulo-cochlear nerve. The 8th nerve is actually 2 separate nerves - the vestibular nerve and the cochlear nerve. The vestibular nerve is responsible for balance while the cochlear nerve is responsible for hearing. The vestibular nerve has 2 parts-the superior vestibular nerve (SVN) and the inferior vestibular nerve (IVN). These nerves lie adjacent to each other as they pass through a bony canal, from the inner ear to the brainstem. This bony canal is called the internal auditory canal (IAC) and it varies in length from 0.4 to 1.2 cm. We have two figures of a temporal bone (that part of the skull which has the ear in it) dissection to the right. The first figure is a view from the top showing the middle ear and the internal auditory canal (IAC) with the nerves passing through it. The organ of hearing (cochlea) and the dura lining the IAC can be seen clearly. The second figure is a magnification of the IAC region showing the different nerves passing through it. This figure also demonstrates clearly, the cochlear nerve supplying the cochlea. Acoustic neuromas usually arise from the cells of the VIII nerve within the internal auditory canal (third figure). The third figure is a schematic drawing showing an acoustic neuroma arising from the vestibular nerve within the IAC. The facial or 7th nerve which is responsible for facial movement, along with important blood vessels, also passes with the 8th nerve in the canal (figures). The cause of acoustic neuroma is unclear. There is a genetic defect that leads to the appearance of the tumor. An abnormal protein called Merlin is produced. A small percentage of individuals have neurofibromatosis type 2 (NF-2) which is a hereditary condition. These patients may have an acoustic neuroma on both sides with an aggressive growth pattern and often involve adjacent nerves. Most people cannot pass the gene defect to their children unless they have NF-2. No one knows why this gene waits until age 48 (on average) to appear but it does.

What is the growth pattern?

Acoustic neuromas usually grow very slowly over a period of many years. Once the tumor fully occupies the internal auditory canal, it often begins to erode the walls of the canal and enlarges it which can be detected on X-rays or by MRI. This bony erosion however, does not always occur. They typically remain within their capsule or lining and displace the surrounding nerves and brain tissue very slowly. This is why the body has ample time to accommodate the abnormal growth. The tumor first distorts the 8th nerve, then presses on the adjacent 7th nerve. The 7th nerve is gradually stretched into a ribbon like structure over the enlarging tumor (figure; cross section of the 7th nerve is shown in the right half of the figure). As the tumor slowly enlarges towards the brain, it protrudes from the internal auditory canal into an area of the skull called cerebello-pontine angle. The tumor is now pear or mushroom shaped with the smaller end within the canal and the larger part towards the brain (figure). It is at this stage that the tumor presses adjacent nerves like the trigeminal or 5th nerve responsible for facial sensation. Ultimately, with increasing tumor size, it can press on the brainstem which can be life threatening. There is no relationship with hearing loss and size. Tumors tend to grow from the ear side toward the brain and not the other way around. The growth rate does not appear to be consistent. That is to say that the tumor might grow for a while and appear to stop, only to start again later.

How often do acoustic neuromas occur?

Acoustic neuromas have been known to occur in all areas of the world without any predilection for individuals of any ethnic background. Small AN's without any symptoms, have been found on autopsy in 2.5% of the general population. Estimates of AN which may produce symptoms range from 1 in every 3,500 to 5 in every million people. It appears that women are more affected than men and most AN's are diagnosed between the ages of 30 & 60 years.

For more information, you may visit the Acoustic Neuroma Association website

Department of Otolaryngology, University of Minnesota

University of Minnesota Physicians - Clinic and Appointment Information