The nervous system consists of the brain and the spinal cord, and all the connections these have with the various tissues of the body. Most nerves in the body are connected first to the spinal cord, which runs through the space within each vertebra, and then to the brain. There are 12 pairs of nerves which arise directly from the brain and pass through openings in the skull to reach their locations. These are called the cranial nerves.

The eighth cranial nerve (properly known as the 'vestibulocochlear' nerve) is the one which serves hearing and balance. It is really in two parts: one part of the nerve is associated with transmitting sound information and the other with sending balance information to the brain from the hearing and balance organs located within the inner ear. An acoustic neuroma is a benign (non-cancerous) growth that arises from the sheath surrounding the eighth cranial nerve.

The term ‘acoustic neuroma’ is, strictly speaking, incorrect because the tumour arises usually on the part of the eighth nerve which deals with balance rather than sound. It also is a tumour of the covering (sheath) of the nerve rather than the nerve itself. Despite these inaccuracies, the term has stuck. True acoustic neuromas occur in conjunction with a fairly rare hereditary condition called neurofibromatosis 2, in which multiple cranial nerve tumours arise.

Acoustic neuroma is diagnosed only once per year per 100,000 people so from that point of view it is uncommon. However small acoustic neuromas, not causing any symptoms, can be found in up to nearly 3 per cent of elderly people. There is no known cause for these tumours, other than the hereditary type.

Acoustic neuromausually grow slowly over a period of years and when large may press on normal brain tissue. Larger tumours can press on another nerve in the same area (the fifth cranial nerve or 'trigeminal' nerve) which is the nerve concerned with facial sensation.

• Reduction in hearing in one ear which is usually subtle and worsens slowly.
• Ear noise or ‘tinnitus’ in one ear.
• Unsteadiness and balance problems (since the balance portion of the eighth nerve is where the tumour arises).
• Facial numbness and tingling may occur if the tumour is large, and so presses on the fifth cranial nerve.
• Headaches, clumsy gait and mental confusion may be experienced if the pressure on the brain rises.

The pattern of symptoms depends mainly on the exact place along the nerve that the tumour arises, which in turn determines which of the local structures (nerves and brain tissue) are pressed on first. As the tumours keep on growing a young person who develops acoustic neuroma will almost always eventually develop symptoms from it.