What is an ABR?
The auditory brainstem response (ABR) is an electrical recording of brain activity that specifically tests the hearing nerve and the areas in the central nervous system that process hearing. The test was originally derived from an EEG, an electrical recording of generalized brain activity.
What Is the ABR Test?
The Auditory Brainstem Response (ABR) test is a helpful objective tool that measures the ability to hear and the ability of the hearing to be properly processed in the brain. The test uses a special computer to measure the way the hearing nerve responds to different sounds.
The electrical activity is termed auditory evoked potential and is extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The resulting recording is a series of waves of which I through V are evaluated. These waves, labeled with roman numerals occur in the first 10 milliseconds after playing a clicking sound in the ear. The ABR is considered a type of reflexive response because it is dependent upon an external sound stimulation.
The auditory structures that generate the auditory brainstem response are believed to be as follows:
- Wave I – generated by the portion of the auditory nerve (the nerve responsible for hearing) that is outside of the brain (central nervous system or CNS)
- Wave II – generated by the nerve activity in the portion of the auditory nerve that is considered part of the CNS
- Wave III – generated by the nerve activity in the brainstem that receives sound input from the auditory nerve
- Wave IV – generated by the activity of nerves in the higher brainstem centers that are involve in processing multiple aspects of hearing
- Wave V – generated by the activity of nerves in the midbrain and brainstem centers that process hearing information
How the test is performed ?
This is a painless, non-invasive test that takes between 30 minutes and 1.5 ours in adults, in younger children anesthesia may be necessary which will require ambulatory testing in the hospital.
The test begins with the attachment of skin recording electrodes to your ear lobes, forehead, and possibly scalp. It also involves the placement of small foam-tipped earphones into each ear. This test will be conducted in a semi-dark, soundproof room by a certified clinical audiologist. During the test, you will hear moderately loud clicking and, at times, hissing sounds delivered to your ears. The audiologist will be recording and watching the electrical responses generated by your hearing nerves and brain in response to these clicking sounds. These responses appear like traces, first on the screen of a computer, later on a printout. Based on the shape and timing of the various portions of these traces, the audiologist will be able to determine the condition of your hearing nerves and other nervous system components that are responsible for hearing. If the results of the test are considered to be abnormal, you may be referred by your physician for additional tests.