Auditory Brainstem Response (ABR), sometimes called Brainstem Auditory Evoked Response (BAER), measures the 8th (auditory) nerve and brainstem’s response to sound. This testing can be used several ways, including newborn hearing screening, estimation of hearing sensitivity in very young patients, estimation of hearing in difficult-to-test populations, neurodiagnostics to rule out retrocochlear pathology and, sometimes, is used during surgical procedures to monitor the auditory brainstem status.
Evergreen Speech & Hearing Clinic, Inc. uses ABR to estimate infant hearing sensitivity and to rule out retrocochlear (8th nerve or brainstem) pathology in adults and children.
Surface electrodes are placed on the head to measure brainstem level EEG activity in response to a clicking and/or brief tone sound. The responses to the signals are averaged across numerous stimulus presentations, revealing a characteristic waveform. The time required for the signal to travel along the nerve is calculated, and compared to age appropriate normative data. In cases of estimation of hearing levels, the stimulus intensity is reduced till the characteristic waveform can no longer be recorded. The lowest level at which reliable wave components can be identified is considered the ABR threshold. The ABR threshold is highly correlated with the behavioral hearing threshold.
Test time is approximately 40-60 minutes for adults and children and 60-90 minutes for infants.
This procedure requires the patient to be quiet and still (or sleeping) for up to 60 minutes. For this reason, when estimating infant hearing, the younger the child the better.
This testing is appropriate for adults and children for whom require testing to rule out 8th nerve and/or brainstem pathology. It is also used to estimate frequency specific hearing thresholds in infants and difficult to test populations.
At high stimulus intensities (80 – 90 dBnHL), the resulting waveform is characterized by five distinct peaks (labeled by roman numerals I – V) that, in normal auditory systems, occur approximately one millisecond apart. The absolute, interpeak and interaural latencies are calculated and compared to normative data. Additionally, the waveform’s morphology and repeatability is evaluated.
For the estimation of hearing loss, Wave V is identified, using a latency-intensity function, down to threshold for both click and tone burst (frequency specific) stimuli. If the nerve and brainstem do not respond to respond to stimuli at normal hearing levels, it is assumed a hearing loss is present at the frequency of the stimulus. The lowest level at which a reliable and repeatable Wave V can be recorded is the ABR threshold.
Tympanometry should be utilized to rule out middle ear pathology as a contributing factor to abnormal ABR results. At high intensities, waveform morphology and interweave latencies remain intact, however absolute latencies may be longer. Additionally, the click and tone burst stimuli may be presented via bone conduction. To further differentiate sensorineural hearing loss versus conductive hearing loss.
Otoacoustic Emissions (OAEs) testing should be used in the case of an abnormal ABR to rule out Auditory Disyncrony (also called Auditory Neuropathy).
Due to the need for quiet during the testing, the patient should dress in comfortable clothing and relax as much as possible for the duration of the recording. For infant testing, it is best to schedule the procedure during the child’s natural sleep time. In order to help ensure the infant sleeps through this appointment, please refer to the “Infant Hearing Assessment Preparation” link on the procedure page.
Hall, JW III & Mueller, H.G. Audiology Desk Reference, Vol. 1 (365-381) Singular Publishing Group, 1997
Hall, JW III Handbood of Auditory Evoked Responses, Allyn and Bacon (1992)