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What is an acoustic neuroma?
An acoustic neuroma (sometimes called a neurinoma or vestibular schwannoma) is a benign (non-cancerous) tissue growth that arises on the eighth cranial nerve. The eighth cranial nerve is three separate nerves, one part associated with transmitting hearing and the other two with sending balance information to the brain from the inner ear. These nerves are adjacent to each other as the pass through the bony canal leading from the inner ear to the brain.
This bony canal, called the internal auditory canal, is about one centimeter (0.4 inches) long, and it is here that acoustic neuromas usually arise from the sheath surrounding the eighth cranial nerve. The seventh cranial nerve or facial nerve that controls facial movement also passes through this bony canal along with important blood vessels.
Where is your acoustic nerve and what does it do?
The acoustic nerve arises from a part of the brainstem called the pons (in a region referred to as the cerebellopontine angle). The acoustic nerve and the vestibular nerve are run together and are attached to one another. Together they are called the eighth cranial nerve. The seventh cranial nerve controls the facial musculature (hence its other name--the facial nerve) and runs near the eigth nerve for part of its course.
The acoustic portion of the eighth nerve goes to the cochlea of the ear and transmits hearing while the vestibular portion goes to the semicircular canals that are responsible for detecting motion (acceleration more technically) and hence are involved in the sense of balance. Most acoustic neuromas actually arise from the vestibular portion of the eighth nerve rather than the acoustic portion, therefore the synonym vestibular schwannoma is preferred by most medical authors.
What causes acoustic neuromas?
The cause of most acoustic neuromas is unknown. There is a small group of patients who have neurofibromatosis type 2, which is a genetically determined disorder and is frequently accompanied by one or more acoustic neuromas.
Acoustic neuroma growth
Acoustic neuromas usually grow slowly over a period of many years. They characteristically remain within their lining (encapsulated) and displace normal tissue. The body accommodates to this abnormal growth. An acoustic neuroma first distorts the eighth cranial nerve, than presses on the seventh cranial nerve.
The slowly enlarging tumor protrudes from the internal auditory canal into an area behind the temporal bone called the cerebellopontine angle. It then assumes a pear shaped, with the small end in the canal. The tumor presses on adjacent nerves, such as the fifth cranial nerve that is the nerve of facial sensation. Ultimately, the tumor can press on the brainstem and become life threatening.
How often do acoustic neuromas occur?
Asymptomatic acoustic neuromas have been found during autopsy in 2.4% of the general population. Estimates of symptomatic acoustic neuromas range from one in every 3,500 people to five in every million people. More women than men are affected. Most acoustic neuromas are diagnosed between the ages of thirty and sixty.
What symptoms do acoustic neuromas cause?
Early symptoms are easily overlooked, making diagnosis a challenge. However, there usually are symptoms pointing to the possibility of an acoustic neuroma, which means that persons with "inner ear" hearing problems should be evaluated to eliminate acoustic neuroma as the cause. It may be that less serious problems, such as Meniere's disorder or a hardening of bone in the middle ear (otosclerosis), are the cause.
The first symptom in over 90% of people with an acoustic neuroma is a reduction in hearing in one ear, often accompanied by ear noise or ringing (tinnitus). The loss of hearing is usually subtle and worsens very slowly, although occasionally a sudden loss of hearing occurs. There may be a feeling of fullness in the affected ear. Unfortunately, since the hearing loss is often mild and there is no pain, some people shift the phone to the opposite ear or make other compromises for the one-sided hearing loss rather than seek medical attention.
Unsteadiness and balance problems may occur early in the growth of the neuroma since the balance portion of the eighth cranial nerve is where the tumor arises. The remainder of the balance system sometimes compensates for this loss, and balance problems may be forgotten.
As the tumor presses on other cranial nerves, facial sensation may be affected, with numbness and facial tingling felt constantly or intermittently. Headaches and unsteady gait caused by increased intracranial pressure may occur.
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