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more than one structure, and in addition each structure contributes to more than one peak. The two earliest peaks (I and II) in the human BSAER are generated by the distal and proximal portions of the vestibulocochlear nerve, which together have a length of about 2.5 cm. The BSAER is little affected by general anesthesia, but hypothermia results in prolongation of the latencies by an amount that becomes noticeable when the patients temperature is lowered by 2.50 to 3.00 C.
When BSAERs are used to monitor function of the vestibulocochlear nerve, the results must be interpreted with as little delay as possible so that changes in the recorded potentials can be detected as soon as possible after they occur. Early detection is important because if too much time elapses between the occurrence of a change in the evoked potentials and the detection of the change, it may be difficult to determine what surgical manipulation caused the change and, therefore, what to do to reverse the change. Thus to prevent permanent injury, changes in recorded potentials should be communicated to the surgeon as soon as possible after the change has occurred. Signal averaging has been the prevailing method for improving the quality of recorded potentials so they can be interpreted, but this examined preoperatively. Cerumen, if present, must be removed from the ear canal. Also, a complete audiogram and preferably BSAER recordings should be obtained preoperatively. If the patient is deaf before the operation it is obviously not meaningful to attempt to monitor BSAERs intraoperatively. It is also of In operations where the vestibulocochlear nerve becomes exposed, recording CAPs directly from the exposed nerve can solve some of these problems. CAPs recorded from the vestibulocochlear nerve can be viewed directly on an oscilloscope, and therefore these potentials do not need to be averaged; or only a few responses need to be averaged
The intracranial portion of the vestibulocochlear nerve is more fragile than other cranial nerves of the posterior fossa because the transition zone between peripheral and central myelin (Obersteiner-Redlich zone) is located inside the internal auditory meatus. It is covered by myelin that originates from oligodendrocytes; both perineurium and epineurium are absent and the endoneurium is finer than it is in a peripheral nerve. Since this intracranial portion of the vestibulocochlear nerve is very fragile, when recording CAPs directly from the vestibulocochlear nerve, great care must be taken not to injure the nerve with the recording electrode. We use a fine, malleable, multistrand Teflon-coated silver wire with a small cotton wick sutured to the uninsulated tip with 5-0 silk. This electrode can be placed on the exposed vestibulocochlear nerve and it will stay in place when the wire is fixed in the edge of the wound, for example, by tucking it under a dural suture.
When the vestibulocochlear nerve is stretched moderately, for example by retraction, the latency of the negative peak will increase with little change in the waveform of the recorded CAP. Heating of (Continued on page 125)
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