Brainstem auditory evoked potentials (BAEPs) were monitored intraoperatively during 44 posterior fossa operations. Technically satisfactory recordings were obtained from 42 patients (95%) and, in 41 of these cases (98%), the intraoperative changes in BAEPs were correlated with the postoperative neurological findings. The functional integrity of the brainstem was assessed by intraoperative BAEP monitoring in 33 patients. Transient, reversible changes were observed in six (18%), and none had postoperative brainstem dysfunction. The functional integrity of the audi tory pathway was monitored in 11 cases, and nine of these patients (82%) exhibited alterations of BAEP waveforms. Transient, reversible dysfunction occurred in three, and permanent changes in six (obliteration in five). Seven patients experienced postoperative hearing loss, six of whom had in traoperative changes in BAEPs that became permanent. The seventh had only transient derange ment during surgery. BAEP monitoring appeared to be a much more sensitive indicator of auditory than of brainstem function.The study suggests that the maximum delay in wave V latency in reversible dysfunction is about 1.0 msec. Therefore, surgery should be interrupted before the wave V latency is delayed by 1.0 msec, in order to avert postoperative neurological deficits. However, when BAEP changes are rapid, it is extremely difficult to halt the surgical procedure before such alterations become irreversible. The authors conclude that a more timely indicator of reversible trauma must be developed.
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