An acoustic neuroma is a benign tumour of the eighth cranial nerve. As such they are neither ‘acoustic’ nor ‘neuromas’, but the term was coined before the microscopic details of the lesion were known and is easier than the etymologically correct ‘vestibular schwannoma’. The tumour arises from the lateral end of the eighth nerve in the internal auditory canal and is surrounded by bone, except medially where the internal auditory canal joins the posterior cranial fossa. Growth can only be inwards to the brain stem. They become ‘brain tumours’ when at 1cm growth they are long enough to reach the posterior fossa. They are never cancerous and never metastasise, yet if very large they can cause death by increased intracranial pressure, although this is rare.
Incidence is one per 100,000 per year. They are slow growing, at an average rate of 1.5mm per year, but some grow faster and many reach a state when the rate of growth equals the rate of cell death and they remain stable.
Presentation and investigation
When patients present with symptoms (hearing loss or tinnitus and occasionally vertigo or imbalance) these symptoms have usually been present for years. As tiny tumours do not tend to cause symptoms, such patients do not present with a facial nerve paralysis. If this is present, it is usually down to another lesion. Lesions can be primary or rarely secondary metastases from breast, prostate or other malignancies.
In patients with otological symptoms (either an inner-ear hearing loss or tinnitus) with trigeminal, facial or lower cranial nerve signs, an MRI scan of the posterior fossa will identify an abnormality (such as acoustic neuroma) in 10% of cases. Similarly, in cases of sudden inner-ear hearing loss, 10% of patients will have an identifiable cause on MRI (such as an acoustic neuroma, multiple sclerosis or a vascular lesion).
Unilateral tonal non-pulsatile tinnitus without other symptoms represents less than 1% of all acoustic neuromas and these are usually smaller than average. Patients with no hearing asymmetry but unilateral tinnitus may need no investigation except a hearing test. Those presenting with only isolated vertigo and no hearing asymmetry need no imaging. If audiogram shows significant asymmetry of inner-ear hearing, the patient needs an MRI scan.
Most acoustic neuromas are managed with scans and no active treatment. If tumours do not grow, scans are repeated after one, two, three, five, seven and 10 years. The gamma knife can treat even large tumours as a day case with no risk to the facial nerve. Very large tumours or those that grow require surgery.
Most patients with an inner-ear hearing loss and tinnitus in one ear have an idiopathic hearing loss. Possible causes are vascular and viral. Other tumours, such as meningiomas and facial schwannomas, are rarer. A patient with a sudden hearing loss might have a benign acoustic neuroma, but will need a haematological and neurological workup as they might be at imminent risk of stroke.
Mr Gerard Kelly is a consultant ENT surgeon at the Leeds Teaching Hospitals NHS Trust and an honorary lecturer in otolaryngology at the University of Leeds