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How not to miss.... an acoustic neuroma

ENT consultant Mr Andy Bath advises on the key signs and pitfalls in diagnosing an acoustic neuroma

ENT consultant Mr Andy Bath advises on the key signs and pitfalls in diagnosing an acoustic neuroma

Worst outcomes if missed

The worst-case scenario occurs when a patient presents with a large acoustic neuroma causing compression on the brainstem and raised intracranial pressure. This may cause headaches, visual disturbance or diminution in level of responsiveness. Fortunately, this rarely occurs but may result in death.

The outcome of treatment is generally dependent on the size of the neuroma at presentation.

Early diagnosis offers patients a range of management options, which may significantly reduce morbidity.

The options include watchful waiting with interval MRI scans, surgery and stereotactic radiosurgery.

Epidemiology

The term ‘acoustic neuroma' is a misnomer as it actually arises from the vestibular nerve but because this term is in such common use, it has been retained.

Acoustic neuromas account for 6% of all intracranial tumours and are the most common form of cerebellopontine angle tumour.

They are benign, slow-growing tumours, which arise from the Schwann cells that surround the hearing and balance nerves.

About 95% of cases are unilateral and occur sporadically. Some 5% are associated with neurofibromatosis type II and are bilateral.

The incidence is about 1:100,000 newly diagnosed cases per year.

Symptoms and signs

41214922Patients generally present with a variety of cochlear symptoms such as:

• 90% – gradual hearing loss in one ear

• 5% – sudden hearing loss

• 70% – tinnitus in one ear.

But 3% have normal hearing at presentation.

Patients may also admit to experiencing mild balance disturbance, which may seem insignificant.

Less common presentations include:

• facial numbness or pain from irritation of the trigeminal nerve

• lack of co-ordination caused by cerebellar compression

• earache.

Notably, facial nerve weakness is an uncommon presentation.

Rarely, large neuromas may present with symptoms of raised intracranial pressure.

MRI scans have led to the diagnosis of symptomless acoustic neuromas in patients who have undergone scans for unrelated conditions.

Differential diagnosis

41214921Acoustic neuromas account for 85% of cerebellopontine angle (CPA) tumours. Other diagnoses include:

• meningiomas – benign but can be locally aggressive

• epidermoids – epithelial cysts that develop from rest cells in the temporal bone or CPA

• lower cranial nerve neuromas – these arise from the Schwann cells from any of these nerves

• arachnoid cysts – thin-walled sacs that are congenital abnormalities.

First-line investigations

All patients with suspected unilateral or asymmetric hearing loss should be referred for pure tone audiometry.

Speech discrimination and auditory brainstem responses are not used any more to investigate these patients, as they are not sensitive enough and can miss small acoustic neuromas.

Second-line investigations

MRI is the gold standard investigation. This has been refined so that fine 1mm cuts are used and neuromas can be detected down to 6mm in size – roughly the same size as the nerve itself.

High-resolution CT scans can be useful if the patient is not willing to undergo an MRI scan because of implanted medical devices such as a pacemaker or for reasons of claustrophobia.

Mr Andrew Bath is a consultant otolaryngologist at the Norfolk and Norwich University Hospital

Competing interests None declared

5 key questions 5 red herrings Acoustic neuroma (shown in green) Acoustic neuroma (shown in green)

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