The Auditory Brainstem Response: Latencies Obtained in Children While under General Anesthesia

The auditory brainstem response (ABR) is detected by placing electrodes on the scalp and separating the response from background EEG (electroencephalography) measures using averaging and amplification techniques. It consists of a series of peaks that occur within 10 msec after an abrupt stimulus such as a click. Waves I and II originate from the VIII nerve while waves III–V depend on neural functioning of the cochlear nucleus through the lateral lemniscus (Moller and Jannetta, 1982; Moller et al, 1981). The ABR test serves two main purposes in clinical audiology. It can be used as a measure of hearing sensitivity and to examine functional integrity of the VIIIth nerve and auditory brainstem neurons. As a measure of hearing sensitivity, the ABR test is usually performed when more traditional behavioral methods (e.g., hand-raising techniques, conditioned play audiometry, visual reinforcement audiometry) yield inconclusive results or cannot be performed because of the child’s age or developmental status. At a high stimulus level, waves I through V can be observed in normal hearing individuals. As the stimulus level is reduced, early ABR waves disappear (Selters and Brackmann, 1977). However, wave V, which is typically the most robust wave of the ABR, can be detected for stimuli that are close to behavioral thresholds. The ABR can be used to evaluate neurologic function of the auditory pathways. In this procedure, a high level click stimulus (e.g., 70 or 80 dB nHL) is presented to each ear, and the absolute and interpeak latencies are measured. Although the absolute latencies can be influenced by peripheral hearing loss, the interpeak latencies (e.g., I–III, III–V, I–V) are generally accepted as measures of central neural conduction time (Eggermont and Don, 1986) and are influenced by myelination of the nerve fibers. The ABR interpeak latencies are not adultlike until approximately 2 yr of age (Hecox and Galambos, 1974; Salamy, 1984); therefore, age-specific normative data are employed in evaluation of ABR latencies (Gorga et al, 1989; Issa and Ross, 1995). Alternatively the latencies may be compared to one’s own clinical norms. However, these normative databases are typically obtained by recording ABRs under natural sleep and/or oral sedation. In our clinical experience the latencies obtained when ABRs are recorded in a child under general anesthesia are prolonged and often outside of clinical norms. Several factors such as anesthesia (e.g., sevoflurane, isoflurane, enflurane) and temperature changes associated with undergoing anesthesia are known to increase neural conduction time (Dubois et al, 1982; Thornton et al, 1983; Manninen et al, 1985; Markand et al, 1987; Schwender et al, 1995). With the exception of wave I, which is generated by the distal portion of the VIII nerve (Moller and Jannetta, 1982), an increase in central conduction time would result in delayed ABR absolute and interpeak latencies (e.g., III, V, I–III, III–V, I–V). This paper is a retrospective analysis of the latency measures of ABRs recorded to high level click stimuli from children while under general anesthesia. ABRs from a control group, not under general anesthesia, were examined for comparison purposes. Several factors that can potentially influence latency characteristics are reviewed.