Auditory Brainstem Response

Auditory evoked responses are electrophysiologic recordings of responses to sounds. With proper test protocols, the responses can be recorded clinically from activation of all levels of the auditory system, from cochlea to cortex (6). Among these responses, the ABR, which neurologists often call the brainstem auditory evoked response, is used most often clinically. An ABR recording is shown schematically in Fig. 132.3. The ABR is generated with transient acoustic stimuli (clicks or tone bursts) and detected with surface electrodes (disks) placed on the forehead and near the ears (earlobe or within the external ear canal). The ABR represents minimal electrophysiologic activity (less than 1 mV) within electroencephalographic activity that is 100 times larger in amplitude. With a commercially available, computer-based device, it is possible to present rapidly (20 to 30 per second) thousands of sound stimuli and, by means of signal averaging, to detect reliable ABR waveforms in a matter of minutes.

FIGURE 132.3. Schematic shows the instrumentation for recording the auditory brainstem response (ABR) and important relations between auditory anatomic features and waveform components. A simple strategy for analysis of ABR waveform in neurodiagnosis also is shown.

Extensive research has shown that the ABR wave components arise from the eighth cranial nerve and auditory regions in the caudal and rostral brainstem (Fig. 132.3). Wave I unquestionably represents the synchronously stimulated compound action potentials from the distal (cochlear end) of the eighth cranial nerve. Wave II may arise from the eighth nerve but near the brainstem (the proximal end). Waves I and II are generated by structures ipsilateral to the ear stimulated. All later ABR waves have multiple generators within the auditory brainstem. Wave III, which usually is prominent, is generated within the caudal pons with likely contributions from the cochlear nuclei, trapezoid body, and superior olivary complex (6). The most prominent and rostral component of the ABR—wave V—is thought to arise in the region of the lateral lemniscus as it approaches the inferior colliculus, probably contralateral to the ear stimulated.

The ABR is not a test of hearing. It reflects synchronous firing of a subset of onset neurons within the auditory system. In ABR waveform analysis, the first objective is to assure that the response is reliably recorded. At the minimum, two replicated waveforms are averaged. If the response cannot be replicated, the test protocol is modified, and technical problems are considered and systematically ruled out. When the existence of a response that can be replicated is confirmed, reproducible absolute latencies for each wave component and relative (interwave) latencies between components are calculated in milliseconds. These latency data for each ear are assessed for symmetry (wave V within 0.4 ms between ears) and compared to appropriate normative data (6).

Common ABR waveform patterns are illustrated in Fig. 132.4. A well-formed and clear wave I at a delayed latency value for the maximum stimulus intensity level is characteristic of conductive or mixed hearing loss. When wave I is small and poorly formed but interwave latency values are within normal limits (the wave I through V latency value less than 4.60 ms), high-frequency sensory (cochlear) hearing loss is suspected. Delayed interwave latency values are the signature of retrocochlear auditory dysfunction. Abnormal delays between the early wave components (I through III) are consistent with posterior fossa lesions that involve the eighth cranial nerve or lower brainstem, whereas prolonged latency of waves III through V suggests intraaxial auditory brainstem dysfunction.

FIGURE 132.4. Auditory brainstem response patterns associated with various types of auditory dysfunction. Information about dysfunction can be inferred from the latency of specific waves and overall structure of the waveforms.

A primary goal in any neurodiagnostic evaluation of ABR is to record a clear and reliable wave I component. Wave I is the benchmark for peripheral auditory function. Subsequent interwave latencies offer indices of retrocochlear (eighth cranial nerve and brainstem) function that are relatively unaffected by conductive or sensory hearing loss. The likelihood that wave I is recorded is enhanced through use of ear canal or tympanic membrane electrode designs and through alterations in the test protocol, such as a slower stimulus rate, rarefaction stimulus polarity, and maximum stimulus intensity level (6).

Reports on ABR dating back to the late 1970s have confirmed that waveforms evoked by high intensity signals yield neurodiagnostic information about cochlear and retrocochlear auditory function. The data can be used in the identification of retrocochlear disorders, such as acoustic neuroma, with an accuracy that exceeds 95%. With the development of sophisticated neuroradiologic techniques, such as magnetic resonance imaging (MRI) with enhancement, reports of normal ABR findings among patients with very small tumors of the posterior fossa have appeared in the literature (7). Inexpensive fast spin echo MRI is technically feasible and available clinically.

False-negative ABR outcomes can occur among patients at risk of retrocochlear auditory dysfunction, usually owing to the presence of small intracanicular vestibular schwannomas. The poor results are evidence of the relatively poorer sensitivity of ABR than that of MRI to mass lesions. However, false-positive outcomes of MRI also have been reported among patients with normal ABR results and no surgical evidence of tumor (8). It must be kept in mind that ABR is a measure of function, whereas computed tomography and conventional MRI are measures of structure (see Chapter 147). Assessment of ABR continues to be a readily available, relatively inexpensive, and reasonably sensitive procedure for initial diagnostic evaluation of eighth-nerve and auditory brainstem status in the care of patients with retrocochlear signs and symptoms. As described in Chapter 131, assessment of ABR is also valuable in electrophysiologic monitoring of the eighth cranial nerve and auditory brainstem function during neurotologic operations such as vestibular nerve section and posterior tumor removal.

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