Your Audiology Tutorial: Auditory Brainstem Response (ABR)
Auditory Brainstem Response (ABR) tests evaluate the function of the auditory nerve and the integrity of auditory pathways in the lower brainstem. Also known as brainstem auditory evoked responses (BAERs) and brainstem auditory evoked potentials (BAEPs), these special tests examine electrical responses of short latency manifest as waveforms produced by an auditory stimulus of one frequency (toneburst) or the complete spectrum (click). The transducer (vehicle through which the sounds are transmitted) is usually a moldable foam insert that is placed in the external ear canal. For some set-ups, that insert is covered in gold and acts not only as a transducer but also as a recording electrode. On a computer monitor, these waves are graphed as a function of time (milliseconds) and amplitude (microvolts).
The waveforms are evoked following a series of sweeps (collections/averages of electrical responses), and are comprised of the differences in electrical potentials among electrodes placed on various landmarks on the forehead, scalp, and outer ear. The electode montage: as with any battery, a ground is utilized (electrode usually on scalp or above bridge of nose, but may be placed anywhere on the body) along with the recording electrodes placed on the "vertex" (high forehead or scalp) and the earlobes or mastoid bone (protrusion behind the auricle [outer cartilaginous portion of ear]). The waveforms are analyzed for their morphology and repeatability. Clinicians examine the amplitude of the waves, how long certain waves take to peak,the latency difference between wave peaks, and the differences of such between ears.
ABRs are mostly used clinically for a)neurological or b)audiological purposes. Neurologically, the evoked potentials test investigates the possible presence of tumors (acoustic neuromas, among other names) along the auditory pathway. If such a space occupying lesion is present, ABR waves will likely peak later than at what is considered a normal latency, not appear with definition, or not be recognizable at all. Acoustic neuromas usually appear in one ear (unilaterally) and if present, will cause degradation of that ear's waveforms as described above. The ABR is very sensitive (positive) for the detection of tumors, second only to the MRI (Magnetic Resonance Imaging). ABR waves may also be affected by the severity of the patient's hearing loss, usually in the higher frequency range (above 2000 Hz).
For patients who are unable to participate in behavioral audiometry ("raise your hand when you hear the beep"), the ABR can also be used to predict hearing thresholds (quitest level at which a patient responds 50% of the time). A certain wave (V, generated in the lower midbrain) is measured at various decibel levels for its presence. Wave V will drop out when the level is too quiet; the tester will then increase the toneburst stimulus until the wave begins to re-appear. Threshold ABRs are commonly done on children. As this test requires the patient to be as calm as possible, you might imagine the challenge this can present at times. When medical staff are present, sedation may be administered for the patient if necessary.
ABR testing is also administered via portable devices for newborn hearing screenings (age-specific norms to be used). High risk newborns with hyperbilirubinemia or other pathologies benefit greatly from ABR screens, as the potential for hearing loss is high. The evoked potential tests are also used in the operating room during what is known as intraoperative monitoring (IOM). The ABR monitors the hearing nerve during invasive procedures where blood flow may be compromised to the inner ear. Cochlear function is also monitored via ABR/BAER for hearing preservation during resection and decompression surgeries. We will go more in depth with IOM at a later date.
As you may have guessed, ABR testing, while objective, still involves a fair bit of subjectivity. Marking and identifying waves can yield different results among interpreters, though the amount of variance should be minimal. The presence of waveforms is often subtle, so running multiple trials is essential. Being aware of electrical artifact, which can "muddy up" the waveforms, is also very important; what looks like a peak may just be "noise." Crossed wires and other electronics in the room can contribute to this artifact.
In all, the electrophysiological ABR test is a highly effective method primarily for detecting retrocochlear (beyond the inner ear) dysfunction. It is commonly performed by audiologists and EEG and IOM techs in a variety of clinical settings. It is especially useful in combination with an MRI, but can be used in lieu of it when conditions preclude (such as in the presence of implantable devices).
The waveforms are evoked following a series of sweeps (collections/averages of electrical responses), and are comprised of the differences in electrical potentials among electrodes placed on various landmarks on the forehead, scalp, and outer ear. The electode montage: as with any battery, a ground is utilized (electrode usually on scalp or above bridge of nose, but may be placed anywhere on the body) along with the recording electrodes placed on the "vertex" (high forehead or scalp) and the earlobes or mastoid bone (protrusion behind the auricle [outer cartilaginous portion of ear]). The waveforms are analyzed for their morphology and repeatability. Clinicians examine the amplitude of the waves, how long certain waves take to peak,the latency difference between wave peaks, and the differences of such between ears.
ABRs are mostly used clinically for a)neurological or b)audiological purposes. Neurologically, the evoked potentials test investigates the possible presence of tumors (acoustic neuromas, among other names) along the auditory pathway. If such a space occupying lesion is present, ABR waves will likely peak later than at what is considered a normal latency, not appear with definition, or not be recognizable at all. Acoustic neuromas usually appear in one ear (unilaterally) and if present, will cause degradation of that ear's waveforms as described above. The ABR is very sensitive (positive) for the detection of tumors, second only to the MRI (Magnetic Resonance Imaging). ABR waves may also be affected by the severity of the patient's hearing loss, usually in the higher frequency range (above 2000 Hz).
For patients who are unable to participate in behavioral audiometry ("raise your hand when you hear the beep"), the ABR can also be used to predict hearing thresholds (quitest level at which a patient responds 50% of the time). A certain wave (V, generated in the lower midbrain) is measured at various decibel levels for its presence. Wave V will drop out when the level is too quiet; the tester will then increase the toneburst stimulus until the wave begins to re-appear. Threshold ABRs are commonly done on children. As this test requires the patient to be as calm as possible, you might imagine the challenge this can present at times. When medical staff are present, sedation may be administered for the patient if necessary.
ABR testing is also administered via portable devices for newborn hearing screenings (age-specific norms to be used). High risk newborns with hyperbilirubinemia or other pathologies benefit greatly from ABR screens, as the potential for hearing loss is high. The evoked potential tests are also used in the operating room during what is known as intraoperative monitoring (IOM). The ABR monitors the hearing nerve during invasive procedures where blood flow may be compromised to the inner ear. Cochlear function is also monitored via ABR/BAER for hearing preservation during resection and decompression surgeries. We will go more in depth with IOM at a later date.
As you may have guessed, ABR testing, while objective, still involves a fair bit of subjectivity. Marking and identifying waves can yield different results among interpreters, though the amount of variance should be minimal. The presence of waveforms is often subtle, so running multiple trials is essential. Being aware of electrical artifact, which can "muddy up" the waveforms, is also very important; what looks like a peak may just be "noise." Crossed wires and other electronics in the room can contribute to this artifact.
In all, the electrophysiological ABR test is a highly effective method primarily for detecting retrocochlear (beyond the inner ear) dysfunction. It is commonly performed by audiologists and EEG and IOM techs in a variety of clinical settings. It is especially useful in combination with an MRI, but can be used in lieu of it when conditions preclude (such as in the presence of implantable devices).
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Brian
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