An Auditory Brainstem Response (ABR) test will take place for a baby after there have been weak results on an OAE screening test, or if a child is older and there are any behavioural issues that make a visual reinforcement audiometry or other types of hearing test unsuitable. But how does an ABR test work, and what does it mean for your child?
An auditory brainstem response test looks for neurological markers that indicate a response to sound. In an OAE test, sound vibrations are passed along the hair cells in the inner ear, stimulating the cochlea and it’s this vibration that is measured in the OAE, but what happens to the sound after that? Well, the inner ear (cochlea) upon registering a sound, will pass an electrical signal along the nerves, to the brain. It’s this electrical signal that is measured during an ABR test. An ABR can pick up not only issues within the ear itself in terms of conductive or sensorineural hearing loss but can also be effective in detecting possible auditory processing disorders.
For a baby, if an OAE screening test is failed, there will likely be a referral for an auditory brainstem response test. An ABR test is the perfect hearing test for babies because it requires no behavioural cooperation or verbal/active response from the child (besides to be asleep, that is!). Brain activity is measured through electrodes attached to the head, and responses to clicks or tones of differing frequencies are measured on a tracing, to confirm if and when a sound is heard. An ABR test can also be useful for children or even adults who have a type of hearing loss that is present in the brain pathways, rather than in the functionality of the inner ear itself.
For an auditory brainstem response test, a baby should ideally be asleep, but if calm and still enough, the test can occasionally be done while the baby rests. Electrodes will be attached to the babies head, and earphones will be inserted that will play clicking sounds and tones of different frequencies. Sounds are repeated to reconfirm any responses and the brain activity is measured to confirm when a sound is heard. The results display on a tracing, and the brain waves should show increased activity in response to the sounds. For children older than six months, anaesthesia is likely required to perform the test.
Below are three very basic illustrations of what a tracing could look like – the first with an active, awake child, the second a sleeping child, and the third showing the anticipated responses. As you can see, it would be almost impossible to detect responses in already active brainwaves of a sleeping child, but the less divergent lines on the tracing of an asleep child allow any hearing signals sent to the brain to be observed reliably.
An ABR test will show hearing thresholds at different pitches and volumes, so it should give you a good indication of the level of hearing loss your little one is experiencing. If your child registers what looks to be a mild or possibly mild to moderate hearing loss on an ABR test, the audiologist may advise or offer parents an option to delay having hearing aids fitted, or to postpone until a behavioural test reconfirms a hearing loss. The reason for this is that ABR testing is be more reliable but can be less accurate than a visual reinforcement audiometry test (VRA), which can be performed when a child is very slightly older. ABR testing is less sensitive than VRA and PTA testing by between 10 and 20dB and when configuring a hearing aid for a child, it’s very important that the amplification is not too much so as to avoid further damage to hearing. Whether using VRA, PTA or ABR tests to confirm hearing loss, audiologists tend to be on the conservative side when setting hearing aid thresholds, especially for babies and toddlers who cannot vocally express discomfort with sound levels so easily.