Auditory Brainstem Response
Auditory brainstem response is an electrical test that records the activity of the auditory nerve and brainstem in response to clicks or tone bursts. It needs no active response from the patient, so it is used to estimate hearing thresholds in babies and in people who cannot cooperate, and to look for disease of the auditory nerve and brainstem.
Small electrodes are placed on the scalp (high forehead or vertex), on each mastoid or earlobe, and on the forehead as a ground. Sounds are delivered through insert earphones, and the tiny electrical signals are averaged over 1000 to 2000 or more repeated sounds to separate them from background activity. Five waves appear in the first 10 milliseconds after a click.
Wave I arises from the distal auditory nerve, wave III from the lower brainstem (cochlear nucleus and superior olivary region), and wave V from the upper brainstem (lateral lemniscus and inferior colliculus). Wave V is the most robust and is used to find the threshold.
The test takes 30 to 90 minutes. Babies are tested in natural sleep, and older children and some adults need sedation or light anaesthesia. Click stimuli test the 2000 to 4000 Hz region, and tone bursts at 500, 1000, 2000 and 4000 Hz give frequency-specific thresholds. Threshold estimates agree with behavioural thresholds within about 10 to 15 dB in most patients. Hearing loss affects 1 to 3 in 1000 newborns, and the test is the main diagnostic step after a failed newborn screen.
Indications.
- Diagnostic testing of infants who fail newborn hearing screening, to confirm loss and estimate thresholds.
- Children and adults who cannot do behavioural audiometry (developmental delay, learning disability, non-cooperation).
- Suspected auditory neuropathy spectrum disorder (present otoacoustic emissions with abnormal or absent responses).
- Asymmetric sensorineural hearing loss, unilateral tinnitus, or vertigo, to look for retrocochlear disease when MRI is not possible.
- Intraoperative monitoring of the auditory nerve during skull base and posterior fossa surgery.
- Brainstem assessment in coma, multiple sclerosis, and after head injury or stroke.
- Suspected non-organic hearing loss, where behavioural thresholds are inconsistent.
- Hearing assessment before cochlear implantation, and in neonatal jaundice with risk of nerve damage. Not a test of speech understanding, and not a full test of hearing, since it samples the tract up to the brainstem only.
Contraindications and precautions.
- Blocked ear canal, wax, discharge, or middle ear fluid: these reduce the sound reaching the cochlea and are treated or noted, since they cause a conductive effect on thresholds.
- Sedation or anaesthesia carries risks. It is avoided or done with monitoring in babies with airway problems, heart disease, or breathing difficulty, and fasting rules are followed.
- Skin infection or broken skin at electrode sites.
- Patients with a pacemaker or implanted device: the device information is checked before electrode placement.
- Movement, muscle tension, and electrical noise ruin the recording, so the patient is warm, fed, relaxed, and asleep.
- Equipment is calibrated, and electrode impedance is kept low (under 5 kilo-ohms).
Preparation and technique.
Otoscopy and tympanometry are done first. Skin is cleaned and electrodes are attached. For a baby, the test is done after a feed, in a dark quiet room, during natural sleep, with the baby wrapped. Clicks at a high level (80 to 90 dB nHL) are presented first to each ear to confirm that waves I, III and V are present and to measure their delay (latency).
The level is then lowered in 10 to 20 dB steps until wave V disappears, and the lowest level with a repeatable wave V is the threshold. Both air conduction and bone conduction are tested when a conductive problem is suspected. Tone bursts are then used to give thresholds at each frequency. Each condition is repeated at least twice to confirm repeatability.
