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(Continued from page 121)
usually seen on the contralateral side. Wave II is generated bilaterally at or in the proximity of the cochlear nucleus. The latency between Waves I and II is approximately 0.8-1.0 msec. The amplitude of Wave II on the contralateral side may be greater than on the ipsilateral side. Wave III is generated bilaterally from the lower pons near the superior olive and trapezoid body. The latency between Waves I and III is approximately 2.0-2.3 msec in a normal adult. Wave III may be smaller on the contralateral side than on the ipsilateral side. Waves IV and V are probably generated in the upper pons or lower midbrain, near the lateral lemniscus or possibly near the inferior colliculus. In ipsilateral recordings, Waves IV and V may fuse into a complex that can vary between two identifiable components with a common base to a single wave with a tall wide peak. On the contralateral side the peaks tend to be more easily identified. Wave V tends to be the most robust peak and is typically the last to disappear when stimulus intensity is reduced. In addition, there tends to be a large negative-going wave following Wave V, which aids the neurophysiologist in identifying Wave V. Wave V is most closely followed during these cases. The BSAER is stimulated using one of several techniques, depending on the surgical procedure involved-and thus whether or not the auricle is retracted-and other considerations. Most often, we use miniature open-air high-fidelity earphones (i.e., commonly used with personal tape players or radios), which rest in the concha of the ear. The earphones, along with the recording electrodes, are applied following the patient's intubation, prior to final positioning of the patient. After verifying that the earphones are working, they are securely taped in the ears with transparent tape, so that they may remain visible. At the same time, the taping must be adequate to prevent fluids from getting to the earphones and into the ear canal, which might cause device failure or a conductive hearing loss, respectively. Following final patient positioning, the vertex (C2) and ear recording electrodes are placed. The contralateral ear electrode is placed over the mastoid (Mn) and the electrode for the operative side is placed in the pinna of the earlobe (Am). In some cases, it is not possible to place an ipsilateral stimulating earphone and recording electrode because of the planned surgical incision. In those cases, we monitor only the contralateral responses, which still provide valuable information concerning the status of the brain stem.
For a wide variety of cranial base tumors monitored, the intensity level of the click is set to approximately 70 dB nHL. However, when the patient is known to have a hearing loss and/or a given patient's responses are not well defined, higher intensity levels may be required. In such cases an intensity level of 85 dB nHL is typical. Rarefaction and compression clicks are applied in an alternating fashion to minimize the apparent stimulus artifact. The stimulus rate is usually set between 9.3 and 19.3 Hz, because of the well known effects of higher stimulus rates on response latencies. (Continued on page 123)
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