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What is an acoustic neuroma?

An acoustic neuroma (also known as schwannoma or neurinoma) is a benign tumour on the 8th cranial nerve. It grows along the path of the nerve between the inner ear in the pars petrosa and the brainstem inside the skull (Fig.1: MRI of an acoustic neuroma).

Fig.1 AN MRI
The 8th cranial nerve consists of two distinct parts. While the cochlear nerve (i.e. acoustic nerve) is responsible for transmission of acoustic information from the inner ear to the brain stem, the vestibular nerve (i.e. balance nerve) transmits information emanating from the organs of balance to the brain for further processing. These nerves exit the brain stem together and run through the cerebellopontine angle, which is filled with cerebrospinal fluid (CSF),, to the inner ear. They pass the bony inner auditory canal in the pars petrosa (Fig.2). Acoustic neuromas arise here from the sheath of tissue surrounding the cranial nerve, with the majority originating from the vestibular part of the 8th cranial nerve, and rarely from the cochlear. The term vestibular schwannoma is therefore preferred by some authors.

The facial nerve or 7th cranial nerve runs parallel to the 8th cranial nerve and provides motor control for the facial muscles.

How does an acoustic neuroma arise?

The cause of acoustic neurinoma is not yet known. Only a small group of patients with a "type 2 neurofibromatosis" display a genetic defect. This usually leads to multiple neuromas on various cranial nerves and in the spinal cord region. These patients mostly have bilateral acoustic neuromas.

Acoustic neuromas generally grow slowly over a period of many years. During this time they displace the surrounding tissue and destroy bone tissue in the region of the pars petrosa. As the tumour is benign it doesn't form any secondary growths (metastases). Due to the reserve space , in the CSF-r filled cavity in the area of the inner auditory canal and the cerebellopontine angle, no symptoms can initially be detected. The tumour then exerts pressure on the 8th cranial nerve, and later on the facial nerve. The slowly expanding tumour spreads from the inner auditory canal in the region of the pars petrosa towards the cerebello-pontine angle. It appears then as a pear shaped structure, gradually expanding the inner auditory canal. As the tumour increases in size it exerts pressure on neighbouring cranial nerves, such as the trigeminus nerve (responsible for facial sensation or the vagus nerve (responsible for swallowing). Ultimately, the tumour can press on the brain stem, causing possible life threatening impairment to to the cardiovascular system and the circulation of cerebrospinal fluid - hydrocephalus.

How common is acoustic neuroma?

Such tumours occur with a frequency of 0.79 / 100,000 people. 6% of all primary intracranial tumours are acoustic neuromas.
Symptoms:
Most common is a slow onset of deafness, rarely acute in onset. Dizziness and ringing in the ears may also occur; paralysis of facial muscles and loss of feeling or pain in the facial region are uncommon.
Coordination difficulties, symptoms of paralysis and rising pressure on the brain occur only when the tumours are large,but then these can be signs of a life threatening condition.

What examinations are necessary?

The initial appraisal is carried out by an ENT specialist. The essential test is audiometry, with a pure-tone and speech audiogram of an impedance test, with [meaning here unclear, reviser] derivation of the stapedius (acoustic) reflex, and also the objective audiometric procedures.

Fig.2: Axial visualisation of the cerebello-pontine angle
Here the lead deduction of the brainstem auditory evoked potential is important; it can indicate the existence of acoustic neuroma by characteristic delay in conduction times or a conduction block. This method is extremely sensitive although it cannot show the nature of the underlying disorder. In such cases diagnostic imaging with an MRI scan is indispensable. The MRI is the most sensitive and specific investigatory method available. Additional investigation of balance functions and cranial nerves is also of use.

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IG Akustikusneurinom (IGAN)

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