These are benign tumours of the VIII nerve and usually arise from the vestibular nerve. Their other name is vestibular schwannoma. Benign tumours do not invade or spread to other places but they do get bigger.
In every million people there will be 20 acoustic neuromas and patients with them present to their doctors between the ages of 30 to 60, usually. Men and women are equally affected and sometimes, rarely, they run in a family.
Acoustic neuromas grow in the internal acoustic meatus and, as they get bigger, they grow towards the brain stem. The IAM is bony and the tumour does not invade it. Instead it grows in the only direction that it can – medially.
The diagram below shows the opening of the internal acoustic meatus.
Inside the IAM there are four big nerves: the cochlear and facial anteriorly and the superior and inferior vestibular posteriorly. The tumour grows slowly on one of the vestibular nerves. As it enlarges it starts to press on the cochlear nerve and this causes sensory deafness and tinnitus.
VII - Facial nerve
C - Cochlear nerve
SV - Superior vestibular nerve
IV - Inferior vestibular nerve
The acoustic neuroma grows on one of the vestibular nerves.
Acoustic neuromas are usually only present none side although there are some patients who have them on both sides due to a problem in their genetic structure. Symptoms are:
1. Sensory deafness - unilateral
2. Tinnitus - unilateral
3. Mild imbalance
4. Later, when the tumour is very big and is pressing against the brainstem it can cause drowsiness, headaches, vomiting and coma.
Once the patient has attended and a unilateral sensory deafness has been diagnosed an MRI scan is performed. This is the gold standard investigation.
ABR testing can be used. It shows slowed conduction within the cochlear nerve and this is related to the size of the tumour. So large tumours can be found this way but small ones may be missed.
CT with contrast is an alternative but it is not as good as MRI.
There are three options for management:
1. Do nothing and watch the tumour
2. Remove the tumour surgically
3. Use radio-surgery
Tumours are benign and grow slowly mostly. On average they grow about 1mm a year and there is plenty of room for them to do this before they reach the brainstem. The majority don't seem to grow at all once they have been diagnosed. All that is required is that the patient have scans to see if the tumour has grown.
The tumour can be removed through the mastoid bone if necessary or can be taken out through the occipital bone.
Choosing the right option for the patient depends on the size of the tumour, the symptoms that it causes, the wishes of the patient, the patient's other illnesses and on local expertise.
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