Acoustic neuroma

An acoustic neuroma is a benign tumour that can lead to hearing loss. It usually comes from the Schwann cells of the vestibular nerve (which sends information about balance to the brain), and in rare cases from the Schwann cells of the cochlear nerve (which carries sound information to the brain). Together the vestibular and cochlear nerves make up the vestibulocochlear nerve.

This type of tumour is also known as a vestibular schwannoma. The tumor comes from an overproduction of Schwann cells—the cells that normally wrap around nerve fibers like onion skin to help support and insulate nerves.This electrical insulation layer around the vestibulocochlear nerve ensures for a faster transfer of the stimulus, or “information”, that is relayed via nerves. When a tumour is formed it creates pressure on the nerve causing the transfer of stimuli to be less effective, leading to symptoms like hearing difficulties. The acoustic neuroma is a very rare tumour, but an increased incidence is associated with age.

As the vestibular electrical schwannoma grows, it affects the hearing and balance nerves, usually causing unilateral (one-sided) or asymmetric hearing loss, tinnitus (ringing in the ear), and dizziness/loss of balance. As the tumor grows, it can interfere with the face sensation nerve (the trigeminal nerve), causing facial numbness. Vestibular schwannomas can also affect the facial nerve (for the muscles of the face) causing facial weakness or paralysis on the side of the tumor. If the tumor becomes large, it will eventually press against nearby brain structures. Normally, it grows very slowly and depending on where it forms, two types are distinguished.

Medial tumour

This type of acoustic neuroma grows in the so-called cerebellopontine angle, which is a very tight space between parts of the cerebellum (lower part of the brain) and the pons (part of the brainstem). This small niche harbours central parts of many cranial nerves (which relay information between the brain and other parts of the body). An acoustic neuroma may result in pressure damage to certain nerves. Depending on which nerves are affected, different symptoms may appear.

Lateral tumour

This type of tumour is located in the internal auditory meatus, which is a canal within the petrous part of the temporal bone. Besides the vestibular and cochlear nerves (nervus vestibulocochlearis), this canal also channels the facial nerve and the gustatory nerve (that has to do with taste). Again, the space-occupying growth of the acoustic neuroma can result in pressure damage of nerves and, consequently, varying symptoms.

How is an acoustic neuroma diagnosed?

Generally, the diagnosis can be confirmed by combining three different examination methods:

1. A hearing test
During a pure-tone audiometry test, patients are exposed to tones at different frequencies and intensity levels. They indicate when they can just about hear a tone and an audiogram records their threshold levels. If there’s a rollover on speech discrimination results (20 dB drop from where the patient heard 100% of the words correctly) it is an indication of a possible Acoustic neuroma and the patient needs to be referred to an ENT for further testing.

Brain-stem-evoked response audiometry (BERA) allows the specialist to determine which part of the acoustic nerve is damaged by the tumour. Electrodes are attached to the head and records how the neural pathways respond to sound stimuli.

2. An examination of the vestibular system

If the vestibular system in the inner ear gets damaged, it can lead to dizziness and rapid eye movements. In the majority of cases, the tumour arises from the vestibular portion of the eighth nerve, yet it causes deafness and tinnitus more often than vertigo. This is because it is usually a slow growing tumour and the gradual loss of vestibular function is compensated by central mechanisms.

Electronystagmography (ENG) is a sensitive, reliable, objective and valuable method of investigating the vestibular system in a patient suspected to have an acoustic neuroma. Pendular eye tracking, tests for spontaneous, gaze and positional nystagmus, and the bithermal caloric test are all tests that could be performed to provide more information in the diagnosis of an Acoustic neuroma.

3. An MRI scan

A medical examination performed using magnetic resonance imaging can confirm an acoustic neuroma with absolute certainty.

How is an acoustic neuroma treated?

Usually, acoustic neuromas larger than 2,5 cm are removed surgically if the general condition of the patient allows it. If the tumour is smaller than 1,5 cm, there are two options: The first is to wait and regularly examine the tumour until it’s large enough for surgical removal. The second is stereotactic radiosurgery (single high dose of radiation), using radiation beams to destroy the tumour.

Surgically removable options include: 

1. Retro – sigmoid Craniotomy

Starting with a line incision behind the mastoid, the neuro-otologist will remove a piece of bone from the canal allowing him access to the tumor. The neurosurgeon will attempt to peel and free the tumor from the nerves which may become damaged due to the pressure of the mass. The surgeon will try to preserve the cochlear nerve if there’s a chance that the patient’s hearing will improve.

2. Translabyrinthine Craniotomy

With this method there is no attempt to save the clients hearing. This is the better procedure for a patient that has no serviceable hearing left.

3. Middle fossa craniotomy

This procedure does not necessarily sacrifice hearing but it is not as successful in preserving hearing as retro-sigmoid craniotomy. Middle fossa is not done very frequently.