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(Continued from page 122)
The interstimulus intervals may be randomized to minimize contamination with phase-locked or quasi-periodic noise.
Baseline responses, for each ear, are acquired prior to the beginning of surgery. Usually 1024 stimulus presentations are used for the baseline data; however, the number of stimuli may be adjusted to as few as 256 depending on the quality of the responses. These data are compared with the preoperative evaluation and used as baselines throughout the case. If auditory responses are being obtained concurrently with either somatosensory or visual responses, the auditory stimulus is always presented first in the stimulus sequence. Usually, the second stimulus in the sequence is presented 10 msec after the auditory stimulus is presented, allowing the BSAER to fully develop prior to the activation of the second system being monitored.
In the operating room, we usually record and display the BSAER responses on Channel 1, using vertex to mastoid or earlobe electrodes. This permits these farfield auditory responses to be observed sequentially with the farfield somatosensory responses on the same channel.
Waves I to V are relatively resistant to sedative medication and general anesthetics. Thus these responses place no constraints on the anesthesiologist. However, they are sensitive to temperature changes, with absolute and interpeak latencies increasing by approximately 0.20 msec 1 C.
The latency of Wave V is the primary concern in intraoperative monitoring of the BSAERs, since this is the most robust and easily identifiable of the waves in this response. In general, any repeatable or systematic change in the latency of Wave V that exceeds 0.3 msec is reported to the surgeon. However, clear changes in the waveshape, even with latency shifts less than 0.3 msec, are noted. The next sample is taken as soon as possible in order to confirm the stability of the change. In the case where the potentials are completely lost, the neurophysiologist reports the loss and then immediately checks to ensure that both the stimulating system and the recording electrodes are intact. BSAERs and SSEPs were among the first modalities routinely monitored during neurosurgical operations. The BSAER is characterized by a series of 5-7 vertex-positive peaks that occur between 2 ms and 10 ms after the presentation of a high-intensity transient sound such as a click or a short, high-intensity tone burst. These peaks, usually given Roman numerals, are generally assumed to reflect successive activations of the vestibulocochlear nerve and the nuclei and fiber tracts of the ascending auditory pathway (cochlear nucleus, superior olivary complex, lateral lemniscus, and inferior colliculus). This pathway crosses the midline at the level of the superior olivary complex, so that the responses from the lateral lemniscus and inferior colliculus are from the side opposite the stimulated ear Because of the great complexity of the ascending auditory pathway, each peak, except peaks I and II, in the human BSAER receives contributions from (Continued on page 124)
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