Describe acoustic neuroma with its clinical features and management.

Nature and Presentation

• The name IS doubly misleading, and saying so shows understanding: it is not acoustic, arising from the vestibular (superior vestibular) division; and it is not a neuroma, being a schwannoma. Vestibular schwannoma is the correct term
• It is the commonest tumour of the cerebellopontine angle, accounting for 80% of lesions there

Investigation and Management

• Pure tone audiometry shows an asymmetric sensorineural loss, characteristically worse in the high frequencies
• Speech audiometry shows disproportionately poor discrimination for the pure tone thresholds — a roll-over phenomenon in which discrimination worsens as the sound is made louder. This is the classical retrocochlear pattern and distinguishes nerve from cochlear disease
• Other retrocochlear signs — absent stapedial reflexes or reflex decay; absent or reduced otoacoustic emissions with a normal cochlea is not expected — in a retrocochlear lesion the OAEs may be present despite a poor audiogram.
• BERA (auditory brainstem response) shows prolonged wave V latency and an increased interaural latency difference; it was formerly the screening test but is insensitive for small intracanalicular tumours
• Caloric testing shows reduced or absent response on the affected side
• Gadolinium-enhanced MRI IS the investigation OF choice and the gold standard, detecting tumours of only a few millimetres. A normal MRI excludes the diagnosis, and no other test does
• CT is inadequate for small tumours and is used only where MRI is contraindicated
• Three management options, and the choice depends on the tumour, the hearing and the patient:
• 1. Observation ("watch, wait and rescan") with serial MRI — appropriate for small tumours, elderly or unfit patients, and where hearing is still useful, given that many do not grow. This is now the commonest initial management
• 2. Stereotactic radiosurgery (gamma knife) — for small to medium tumours (under 3 cm), achieving growth control in a high proportion with better facial nerve preservation than surgery; hearing preservation is variable and declines with time, and there is a small risk of malignant transformation
• 3. Microsurgical excision — for large tumours, brainstem compression, hydrocephalus, or documented growth in a young patient
• The three surgical approaches, chosen by tumour size and whether hearing is worth preserving:
• Translabyrinthine — sacrifices hearing completely, but gives excellent early identification of the facial nerve at the fundus and requires no cerebellar retraction. Used where hearing is already poor
• Retrosigmoid (suboccipital) — hearing may be preserved; suits tumours of any size, but requires cerebellar retraction and access to the lateral end of the canal is limited
• Middle fossa — hearing may be preserved; best for small, mainly intracanalicular tumours, but involves temporal lobe retraction and the facial nerve lies superficial to the tumour, so it is more at risk
• Facial nerve monitoring is used throughout, and facial nerve preservation is now the principal measure of surgical success alongside complete excision
• Complications — facial palsy; total hearing loss; CSF leak and meningitis; headache; cerebellar injury; lower cranial nerve palsies; and persistent imbalance, which is common and frequently the patient’s chief long-term complaint