Brainstem Auditory Evoked Potentials
The recording of cortical potentials related to auditory stimuli has proven to be difficult. In many patients, the primary auditory cortex is located deep within the Sylvian fissure. This location generates potentials whose dipole is perpendicular to the cortical surface, thereby rendering them undetectable by surface or scalp electrodes. However, detection and analysis of BAEPs have been developed for a number of neurosurgical procedures involving areas that are traversed by the ascending auditory signal, including both extra-axial (nerve) and intraaxial (brainstem) tissues. These procedures include resection of vestibular schwannomas, microvascular decompression of cranial nerves, retrolabyrinthine vestibular neurectomy, clipping of basilar artery aneurysms, treatment of posterior fossa AVMs, and resection of tumors residing in the cerebellopontine angle (CPA) or brainstem.
BAEPs are generated via the presentation of trains of clicks to one or both ears, resulting in an afferent signal that can be detected by scalp electrodes as it passes through the vestibulocochlear nerve, lower brainstem and midbrain. As with SSEP, changes in waveform amplitude or prolongation of signal latency are suggestive of impending or actual damage to the pathway, and persistent loss of the BAEP is more indicative of permanent damage than transient loss. Pre-operative assessment must be performed to obtain the baseline performance of the ascending auditory pathway for each individual prior to surgical manipulation. In most cases, hundreds or even thousands of responses to rapid (10-30 Hz) stimuli are averaged to obtain high quality waveforms. There are five major peaks, numbered I-V, which are particularly relevant in the analysis of BAEPs (Fig. 1.5). These waves are thought to be generated from the proximal eighth cranial nerve (I), the entry zone into the brainstem (II), the cochlear nuclear complex (III), the superior olive (IV), and the contralateral lemniscus or nucleus (V). These associations become important in the operating room, as brainstem ischemia may prolong the latency of peak V but leave peaks I and III essentially unaffected. Although it is possible to record directly from an exposed eighth nerve, the
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