An acoustic neuroma (also referred to as vestibular schwannoma or neurilemmoma) is a benign tumour of the eighth cranial nerve. It grows along the nerve between the inner ear in the temporal bone and the auditory brainstem inside the brain.
The eighth cranial nerve (called the vestibulocochlear nerve or auditory vestibular nerve) consists of two parts. The cochlear nerve (auditory nerve) relays the auditory information from the inner ear to the brainstem. The vestibular nerve (balance nerve) transmits information from the organ of balance to the brain. Most acoustic neuromas originate in the vestibular part of the eighth cranial nerve (the tissue sheath); in rare cases they arise in the cochlear nerve.
The facial nerve – the seventh cranial nerve – runs parallel to the eighth cranial nerve and controls the facial muscles.
The cause of acoustic neuroma is unknown. Only in a small group of patients – those who suffer from a condition called neurofibromatosis type 2 – is a genetic defect present. This generally leads to multiple neuromas at different cranial nerves and in the region of the spinal cord. These patients tend to have acoustic neuromas on both sides.
Acoustic neuromas usually grow slowly over a period of several years, displacing the surrounding tissue and destroying the temporal bone in the process. As the tumour is benign (noncancerous), it does not form secondary tumours (metastases). In some cases symptoms may occur later in the tumour’s development because the subarachnoid space is relatively large.
These tumours occur in 0.79 out of 100,000 people. Six per cent of all primary intracranial tumours (i.e. those within the skull) are acoustic neuromas.
The main symptom is gradually progressive hearing loss. Acute hearing occurs only infrequently. Other symptoms include dizziness and tinnitus. Paralysis of the facial muscles and numbness or pain in the face are only rarely present. Coordination disorders and symptoms of paralysis or increasing intracranial pressure are found only where tumours are large, and are a sign of a life-threatening condition.
The initial examination is usually carried out by an ENT specialist. The most effective approach involves audiometric tests including pure-tone and speech audiograms, impedance tests with recording of the stapedius reflex and objective audiometric tests.
The main objective audiometric test is the measurement of acoustically evoked brainstem potentials, which indicate the occurrence of an acoustic neuroma if the brainstem responses exhibit a characteristic delay or are completely absent. In addition, imaging techniques such as magnetic resonance imaging are essential to confirm a suspected diagnosis of acoustic neuroma. Vestibular diagnostics (i.e. balance tests) and examinations of other cranial nerves are also recommended.
Acoustic neuroma treatment depends on the size of the tumour, the state of hearing and the patient’s age and general health. The primary aim is complete surgical removal of the neuroma while at the same time ensuring functional preservation of the facial nerve and hearing ability. This can be achieved in most cases, thanks to today’s microsurgical techniques and the use of surgical microscopes, intraoperative monitoring and special neuronavigation systems, which allow exact localisation of the tumour. As a rule, priority is usually given to the surgical solution. However, there are other treatment options, which must be considered on a case-by-case basis and weighed up against surgery.
Information on these alternative options and various surgical approaches is given here:
Radiotherapy using the gamma knife
The wait-and-see approach is also an option. This is justified if the tumour is very small and has not yet affected the hearing, or in older patients where the tumour has not yet resulted in displacement of the brainstem. In any case, annual MRI scans are required to monitor the tumour’s growth and – in the event that signs of compression are observed – to determine precisely when surgery is necessary.