[Intraoperative ABR monitoring during cerebello-pontine angle surgery]

Intraoperative auditory brainstem response (ABR) monitoring was performed in 27 patients undergoing C-P angle surgery (12: hemifacial spasm, 10: trigeminal neuralgia, 2: glossopharyngeal neuralgia, 3: C-P angle tumor, 2 epidermoid, 1 meningioma). Because of the introduction of this method, no patient suffered from postoperative hearing disturbance in this series. During C-P angle surgery, the wave V of ABR changes according to the retraction of the cerebellum and the manipulation of the eighth cranial nerve. Many authors have discussed this change, however the timing and the mechanism of disappearance of wave V is unclear. Accordingly, the authors discussed the correlation between the prolongation of wave V latency and its amplitude. The wave V amplitude was measured from the positive peak of wave V to the next negative peak. Then, the correlation between the prolongation of wave V latency and its reduction ratio (%) of amplitude was represented as a parabola. The wave V reduces its amplitude when the prolongation of the latency is from 1.5 ms to 2.0 ms. Once the prolongation of the latency is over 1.5 ms, the amplitude of wave V seems to be reduced suddenly, because it takes over 1'30'' to finish each record. But the authors demonstrated the gradual reduction of the amplitude of wave V in Figure 3. As mentioned above, the prolongation of wave V latency must be less than 1.5 ms, and the neurosurgeon must recognize this turning point during C-P angle surgery.