What is Auditory Brainstem Response?
Auditory Brainstem Response (ABR) is an electrophysiological test, is non-invasive and examines the functioning of the central auditory nervous system (CANS).
The ABR does not test the cortex or cochlea. The Otoacoustic Emission (OAE) tests the cochlea or inner ear.
The ABR is an evoked response test that requires a stimulus and assumes that the cochlea is good.
By using 100 micro second clicks or tone bursts, a stimulus is sent into the ear of the patient.
The CANS responds by synchronously firing neurons which create the output voltage.
The ABR machine then generates the waveforms by reading the voltages output from the CANS.
The ABR machine is only capable of reading voltage when all the neurons fire together, time locked to the stimulus tone bursts.
The dog is not required to respond or participate during the testing. The patient is kept calm and quiet and may even sleep during the ABR test.
How are captured waves interpreted?
The negative trough after Wave I reflects activity arising from the region of CN VIII as it exits the internal auditory canal at the cerebellopontine angle.
Wave II is likely generated by the proximal portion of CN VIII as it enters the brainstem.
Wave III is thought to arise from the activity of second-order neurons beyond CN VIII in the cochlear nucleus, within the negative trough that follows wave III arising from the trapezoid body.
Waves IV and V occur in the superior olivary complex of the pons with contributions from the cochlear nucleus and nuclei of the lateral lemniscal neurons. Canines don’t typically show a wave IV and we are not really sure why that happens.
Wave V has multiple sites and we are not sure how many sites are involved.
After wave V you may get a 6 and 7, but this is not meaningful for our purposes. Any wave output after that point is considered middle latency.
Normal hearing wave functions chart.
Do you ever get 2 waves that look exactly the same?
Hardly ever. In ABR testing we are looking for waveforms that are relatively repeatable. This repetition indicates to the tester that the ABR test is good.
If you repeat the ABR test 5 times the waveform will deteriorate and become less repeatable. The reason why the waveform deteriorates, the test is prompting the neurons to fire off all the time. The more you exercise the neurons the more the neurons go through depolarization and hyper polarization periods. This constant firing over time makes the waveform less repeatable as the output of the ABR. We know for a fact that the longer you keep a patient on the test table the more the ABR deteriates.
Acting as a filter, the Differential Amplifier removes the high and low extremes. In addition the random firing will get statically averaged out. What's left are the waveforms from the neurons that fire all the time which is the output signal.
Abnormal Canine Hearing:
The wave form that occurs within the first 10ms is an average of all neurons firing. This chart shows no waveform activity in the patient thought to be hearing impaired.
Wave functions chart of a hearing impaired dog.
The chart shows the impulse signal coming back out.
ABR & Wave information courtesy of Gary A. Cimmino, "Effect of Head Size on Canine Auditory Brainstem Response (ABR) Morphology (Normal Hearing)", University of Connecticut, May 2007.