1-A-D-21. Changes of brainstem auditory evoked potentials after craniotomy and CSF drainage: Analysis of 344 consecutive patients during microvascular decompression

During microvascular decompression (MVD) surgery for hemifacial spasm and trigeminal neuralgia, slight changes of brainstem auditory evoked potentials (BAEP) can be observed without manipulation around the cochlear nerve. This study is aimed to examine influence of general anesthesia and cerebral spinal fluid (CSF) drainage on the BAEP. We included 344 patients with normal auditory function who underwent MVD. The BAEP data were compared at three points: (1) within a week before MVD, (2) just after induction of anesthesia, (3) after CSF drainage from cerebellar medullary cistern. The effects of anesthetic agents, age and previous surgery were also analyzed. After induction of anesthesia and CSF drainage, the latencies of wave V delayed for 0.318 ms and 0.206 ms on the average. The amplitudes of wave V increased approximately for 10% after CSF drainage. Sevoflurane prolonged latencies longer than propofol. In the elderly patients (>59 year-old), the latency of wave I delayed significantly longer than in younger patients. Previous surgery did not affect the BAEP changes. These findings indicate that the latencies of BEAP can be prolonged slightly without nerve injury, presumably due to local hypothermia, dryness and stretch of the nerves caused by anesthesia, craniotomy and CSF drainage during neurosurgery.