What is an ABR? Auditory Brainstem Response Explained.

Auditory Brainstem Response (ABR) is a type of evoked potential. Evoked meaning it’s been triggered by something (an audiologist!) and potential meaning an electrical charge/change.

So, we trigger an ABR by playing a sound into the ear, and measure the electrical responses of the brain that happen whenever we hear sound. By looking at the expected waveforms of these responses, we can see:

1) Whether you heard the sound, even if you cannot tell us.

2) If your brain responded as expected - by measuring your results against what we know is normal.

Additionally we can work out:

3) If there is a problem in the chain of expected responses (seen above as peaks in the waveform), WHERE that problem is.

4) The threshold of your hearing. By playing sound at different pitch and volume, then looking for responses, we can figure out how loud they have to be before you ‘make waves’.

ABR are used when the patient is unable to respond to a traditional test. They might be used to verify the hearing test, in that interesting case of Non Organic Hearing Loss or Compensation Payout Deafness.

Generally though, we use ABR for working out the source of a person’s hearing difficulties. The typical hearing test and audiogram is excellent for telling us how much amplification you need / how much hearing loss you have. But it can only tell us if the problem is in the middle ear system (conductive), the inner ear/nerve (sensorineural) or a mix of both.

Once you get to the cochlea - the inner ear organ responsible for translating sound energy into electrical signals - it’s very hazy and we take an educated guess. Is it ageing, noise damage, viral? Who knows?! Well the ABR can tell us roughly where on the electrical pathway there is a fault.

Faults can occur at:

• The cochlea itself, the hair cells within it may be damaged

• The synapses (junctions) between nerve cells

• The nerves themselves

You may have a lesion (damaged area) anywhere along the long and winding path that electrical signals have to travel between your cochlea and the patch of brain tissue called the auditory cortex.

This cortex is on the opposite side of your head to the ear that hears the sound, and all signals are routed first to the Brainstem (base of your skull) before the majority travel over to the opposite cortex. Some nerve fibres go to both sides from each ear

The place where the wave falters, therefore, can tell us the approximate location of the lesion, if there is one.

There are many different types of tests for hearing. Babies have an Otoacoustic emission test, which measures sound that reflects back from the cochleae. You might be asked to have several, so that the audiologist can compare them with each other for additional information on the site and cause of your hearing difficulties. All the tests are painless and relatively easy to do.