Acoustic neuroma (vestibular schwannoma)

An acoustic neuroma is a benign (non-cancerous) tumour that grows from the vestibulocochlear nerve, cranial nerve VIII. The name is slightly misleading, because the tumour almost always starts on the vestibular (balance) part, which is why it is properly called a vestibular schwannoma. Schwann cells wrap around nerve fibres and insulate them, and a schwannoma is a tumour of these cells.

The vestibulocochlear nerve carries signals from the inner ear to the brain. Its vestibular part carries balance information, and its cochlear part carries hearing from the cochlea (the snail-shaped hearing organ). Both parts leave the skull together through the internal acoustic meatus (a canal in the temporal bone), so the tumour can press on the neighbouring cochlear fibres. The result is slowly worsening hearing loss in one ear, often with tinnitus and unsteadiness. Tinnitus is a ringing or buzzing heard without an outside sound.

The hearing loss is sensorineural, meaning the fault lies in the cochlea or the nerve. In conductive loss, by contrast, sound is blocked in the outer or middle ear. Because the tumour usually grows on the vestibulocochlear nerve of one ear only, the loss is typically in one ear. A hearing test chart called an audiogram usually shows an asymmetric sensorineural loss, which prompts an MRI scan. The hearing loss can occasionally begin abruptly or fluctuate (come and go), so a sudden loss in one ear still needs assessment and does not exclude the tumour.

As the tumour enlarges it can press on the facial nerve and, in larger tumours, the trigeminal nerve (which supplies sensation to the face and the muscles used for chewing). These nerves are neighbours that are pressed on, not the nerve of origin.