Human ear is made up of three parts – the outer, the middle, and the inner ear. The Auditory Brainstem Response (ABR) test tells us how the inner ear, called the cochlea, and the brain pathways for hearing are working. You may also hearing it called an Auditory Evoked Potential (AEP).
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What Is Auditory Brainstem Response (ABR)?
This Auditory Brainstem Response test can be conducted when the baby is sleeping, the three electrodes are placed on the baby’s head, these electrodes are stuck to their skin and connected to a computer.
The measurement of recording a series of six to seven vertex positive waves of which I through V are evaluated by experts. These waves, labeled with Roman numerals in Jewett and Willistonconvention, occur in the first 10 milliseconds after the onset of an auditory stimulus.
Brainstem evoked response audiometry is a useful assessment technique of hearing. A major advantage of this procedure is its ability to test even infants in whom conventional audiometry may not be useful.
The investigation can be used as a screening test for deafness or people with hearing loss in high-risk infants. Early diagnosis and rehabilitation will reduce disability in these children. The ABR is considered to be an exogenous response because it is fully dependent upon the external factors.
What Does Click-ABR Represent?
The sound of clicks alert the whole cochlear, it has been found that the ABR thresholds obtained by clicks match the behavioral thresholds only at high frequencies (2000-4000).
The explanation is derived from cochlear physiology. At the base of the cochlea it was tuned for high frequencies, neurons are very crowded. This crowdedness of neurons results in no time lag between the excitation of neighboring neurons. As a result of the treatment process, the synchronization of responses is only true at the base of the cochlea.
The apex of the cochlea, the neurons are scattered everywhere which results in significant time lags between excitation of neighboring neurons. As a result of this time lag, synchronization is lacking.
The result concludes, and despite the fact that the click excites the entire cochlear partition, the ABR as recorded from the scalp represents activity from the cochlear region between 2800-4000 Hz with less contribution from 2000 Hz region.
Auditory Brainstem Response For Children
Auditory Brainstem Response can also be used as a screening test in newborn hearing screening tests. A screening is a program where you either pass or fail. When the ABR is used for screening, only one loudness level is checked. The baby passes the test if his brain displays that it is hearing the sound. And if the baby fails in the test, more specific testing will be completed.
Use of Electrodes
The electrodes are used to record BERA and always help to give proper output, It should be placed over the scalp, the scalp hair should he oil-free. A patient should be instructed to give a shampoo bath to the hair on the day of the investigation.
The rest of the non-inverting electrode is placed over the vertex of the head, and the inverting electrode, the inverting electrode is placed over the ear lobe or mastoid prominence. There is one more electrode known as the earthing electrode it can be placed over the forehead.
This earthing electrode is very important for the proper functioning of a preamplifier. Electrodes that are placed over the mastoid process or ear lobe should be symmetrical. All the electrodes should be placed and run over the top of the patient’s head.
This helps in the active separation of electrodes from transducer cable. It also minimizes the risk of the cable getting dislodged if the sleeping baby wakes up.
Uses of BERA
Auditory brainstem response is a valuable objective measure of hearing. With the rapid decrease of stimulus intensity, wave latencies increase systematically until the hearing threshold is reached. Below this hearing threshold, the response is absent.
Thus it is possible to estimate the hearing threshold even in individuals who cannot be tested by behavioral methods.
It should be borne in mind that adult-like is RA responses are acquired only after the age of 2. It is possible to test a newborn’s hearing using BERA using age-appropriate norms. BERA cannot be affected by sleep or sedation hence infants can be sedated before performing this test.
It also helps the neurosurgeon in intraoperative monitoring. It can be used to detect lesions and tumors involving the auditory pathway. BERA also used to detect demyelinating lesions involving auditory pathways.
Conclusion
Auditory Brainstem Response is a useful tool in identifying deafness in neonates. It can also be used as a screening tool for prior evaluation of deafness in high-risk infants. The ABR also gives a reasonable and accurate proper assessment of hearing threshold levels objectively when used in adults.
When used with speech stimuli it helps in the identification of complex auditory disorders involving auditory processing areas of the brain.
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