Overview
Brainstem auditory evoked responses (ABR), also known as auditory brainstem evoked response (ABR), test both the ear and the brain. They measure the timing of electrical waves from the brainstem in response to clicks or tone bursts in the ear. Computer averaging over time to filters background noise to generate an averaged response of the auditory pathway to an auditory stimulus Three waves (1, 3 and 5) are plotted for each ear. The waveform represents specific anatomical points along the auditory neural pathway: the cochlear nerve and nuclei (waves I and II), superior olivary nucleus (wave III), lateral lemniscus (wave IV), and inferior colliculi (wave V). Delays of one side relative to the other suggests a lesion in the 8th cranial nerve between the ear and brainstem or the brainstem itself.
The main indication for ABR is when an acoustic neuroma is suspected. This generally comes about when there is an asymmetrical sensorineural hearing loss. The most reliable indicator for acoustic neuromas from the ABR is the interaural latency differences in wave V: The latency in the abnormal ear is prolonged. According to a meta-analysis of ABR, they are 93.4% sensitive to acoustic neuroma.
ABR testing may also be useful in situations where an auditory neuropathy is suspected. In this case, it may be combined with otoacoustic emission testing.
ABR testing may show some subtle abnormalities in persons with tinnitus.
ABR’s are commonly abnormal in brainstem disorders such as multiple sclerosis, brainstem stroke, or brainstem degenerative disorders. These are much less common than inner ear disorders, but also are intrinsically much more dangerous.
ABR testing requires reasonable high-frequency hearing. This means that it is often not worth doing in persons over the age of 70. We recommend that either an audiogram or at least a screening test for high frequency hearing be done prior to ABR testing.
Who does ABR testing and who should interpret ABR tests ?
ABR tests are commonly performed by an audiologist or an electrophysiology technician. Audiologists are often associated with otolaryngology practices (ENT doctors), while electrophysiology technicians are often associated with Neurology practices. It is not a difficult test and does not require much training for one to perform. Thus technically, either setting is quite reasonable. Because hearing testing is absolutely required to interpret the test (see above), often the most convenient process is to have an audiologist do the ABR test and audiogram in a single sitting.
Reading of the ABR test is more problematic because ABR’s may be obtained in settings relatively unfamiliar with CNS disorders (i.e. most otology practices) or settings relatively unfamiliar with ear disorders (i.e. most neurology practices).
How Is a BAER Test Performed?
BAER tests are quick and easy, and have virtually no risks or complications. You do not need to prepare for the test in advance, though you may be asked to wash your hair the night before to remove oils that might keep the testing equipment from sticking to your scalp.
You will simply lie back in a reclining chair or on a bed and keep still while the doctor places small electrodes (sticky patches with wires attached) on your scalp and earlobes. The electrodes are connected to a machine that records your brain activity. If your infant or child is being tested and cannot remain still, the doctor may give them a sedative medication.
The doctor will then give you a set of earphones. You should hear a series of clicks or tones played through the earphones, but you do not need to do anything in response to the sounds. The electrodes placed on your scalp and earlobes will record how your brain reacts to the noises you hear. It will show if you are hearing the sounds properly and if they are being conducted from your ears to your brain.
RESULTS
A printout of your test results should show spikes in your brain activity each time you heard one of the clicking sounds or other tones. If your results show flat lines when one of the tones or clicking sounds was played, it may indicate that you have hearing loss.
Abnormal test results can also indicate that you have sustained damage to your brain or nervous system. This could be caused by:
- multiple sclerosis (an autoimmune disease that causes damage to the protective coverings of your nerve cells)
- central pontine myelinolysis (another condition that damages the myelin sheath covering your nerve cells)
- acoustic neuroma (a tumor growing on the nerve that connects your ear to your brain)
- a stroke
- a brain injury
- a brain tumor
- a speech disorder
Additional tests will probably be required to determine the cause if your test results are abnormal,. Once the underlying cause has been identified, your doctor will discuss your treatment options with you.
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