Auditory Brainstem Response
Auditory brainstem response (ABR) testing objectively assesses the neural synchrony of the auditory system from the level of the eighth nerve to the midbrain. The obtained results can be extrapolated to provide information regarding hearing sensitivity and also can be used for neurodiagnostic purposes. To administer the ABR test, electrodes are placed on the patient's head, and a series of sounds are presented. The patient must remain still and, with children, it may be necessary to sedate to obtain valid results. The diagnostic (not screening) test procedure typically requires 30–60 minutes. The ABR consists of a series of 5–7 waves that occur within the first 10–15 ms following the stimulus. These potentials are shown in Figure 45–5. Waves I and II are thought to be generated in the eighth nerve of the stimulus ear; Waves III–V are thought to be generated in the brainstem and midbrain. For neurologic purposes, the latencies (absolute, interwave, and interaural) and amplitudes (absolute and relative) of Waves I, III, and V are analyzed. Clicks are the most commonly used stimuli because their abrupt rise time and broad spectrum enhance neural synchrony; however, the results are dominated by the high-frequency region.
An example of a normal auditory brainstem response.
The other primary use of electrophysiologic measures is to estimate the auditory thresholds for AC and BC in patients who are unwilling or unable to provide accurate behavioral audiometric thresholds. For these patients, the lowest intensity level at which Wave V can be visualized and repeated is considered the threshold. ABR is frequently used for newborn hearing screening, because it provides accurate information in a relatively short amount of time. For better definition across the frequency range, frequency-specific stimuli, such as tone pips, are used. Because tonal signals have a slower rise–fall time than clicks, the wave morphology may be degraded, making threshold identification more difficult. Normative values from research centers for gender and age are available; however, to be accurate, each clinic should establish its own equipment-specific normative values. More recent additions to the electrophysiologic battery include ASSR (Auditory Steady State Response), which allows for a potentially more rapid means of establishing frequency-specific thresholds, stacked ABR (a more sensitive and specific method for the detection of small tumors), and CHAMP (cochlear hydrops analysis masking procedure) for detection of Meniere disease.
Interpretation of the ABR depends on knowledge of the relevant recording and subject variables. Age and gender, as well as the type, degree, and configuration of the hearing loss, may substantially affect the latencies and amplitudes of the ABR. The following statements summarize the expected outcomes:
The latencies for Waves I, III, and V are within the normal range, and the interaural (between ears) latencies are equal (within 0.2–0.3 ms).
The absolute latencies of all waves are prolonged, but the interwave latencies are not substantially affected. This pattern occurs regardless of the stimulus intensity. The configuration of the hearing loss may also contribute to variability in the latencies.
The degree and configuration of the hearing loss may affect the latencies of Waves I, III, and V. A relatively flat hearing loss of less than 60 dB HL should not impact the ABR latencies, but high-frequency hearing loss may reduce the amplitudes and prolong the absolute latencies of the waves, without increasing the interwave I–V latency difference when stimuli are presented at high intensities.
Retrocochlear Hearing Loss
A variety of effects on the latencies and morphology of the ABR may occur. These may include the absence of waves, prolonged absolute or relative latencies (2 or 3 standard deviations beyond the mean), or prolonged interaural latencies. Wave I may be within normal limits, but the absolute latency for Wave V, and consequently the I–V interwave latency, is prolonged beyond the normal limits.
Electrocochleography (ECOG or ECochG) evaluates the electrical activity generated by the cochlea and the eighth cranial nerve, occurring during the first 2–3 ms subsequent to a stimulus. The active electrode is placed in the ear canal, on the tympanic membrane, or through the tympanic membrane on the promontory; the reference electrode is placed on the vertex or the contralateral earlobe or mastoid. The closer the active electrode is to the cochlea, the larger the response. The principal potentials that are evoked through ECOG are the cochlear microphonic, the summating potential, and the compound action potential. Most commonly, only the summating potential and the compound action potential are of interest. The main application of the ECOG is to help determine if a patient has Meniere disease. The amplitude of the summating potential (reflecting activity of the hair cells) is compared with that of the compound action potential (reflecting whole nerve activity). If the ratio is larger than normal (0.3–0.5), it is considered indicative of Meniere disease. The procedure is considered valid only the patient is symptomatic.
Don M, Kwong B, Tanaka C, Brackmann D, Nelson R. The stacked ABR: a sensitive and specific screening tool for detecting small acoustic tumors. Audiol Neurootol
. (Introduction and basic explanation of the stacked auditory brainstem response test.)
Picton TW, John MS, Dimitrijevic A, Purcell D. Human auditory steady-state responses. Int J Audiol
. (Review of the Auditory Steady State Response test protocol.)