Establishing baseline values for brainstem auditory evoked potentials (BAEP) during microvascular-decompression (MVD) for hemifacial spasm (HFS). Santhosh Kumar Mohanraj, MD 1, Parthasarathy D Thirumala, MD 1, MS, Miguel Habeych, MD 1, Donald Crammond, PhD 1 & Jeffrey Balzer, PhD 1. 1 Centre of Clinical Neurophysiology, Department of Neurological Surgery, University of Pittsburgh Medical Centre. Introduction HFS is caused by vascular compression of the cranial nerve VII (CN VII) at root exit zone. MVD is a surgical procedure which alleviating HFS by removing the pressure on CN VII 1. During MVD, BAEP monitoring is done to decrease the incidence of post operative hearing loss (HLS) 2. In BAEP monitoring changes in the waveforms are used to alert the surgeon about impending hearing-loss. These changes are compared to the baseline BAEP values set at the beginning of the procedure. Our study evaluates the appropriate time to establish the baseline values for BAEP during MVD and its implication on the alarm criteria Methods We retrospectively identified 61 patients who had intraoperative monitoring with BAEP during MVD. Compared the BAEP values obtained at the beginning of the procedure and before any major manipulation (dura opening). We analyzed the data by comparing the number of alerts given to the surgeons and the post operative hearing outcomes between baselines set at the beginning of the surgery and before major manipulation. Latency of wave V (LwV) and amplitude of wave V (AwV) are the only values considered since it is the only wave used in the alarm criteria. Results The latencies of wave I,III and V at dura opening were significantly increased when compared to their latency obtained at the beginning of the procedure. But there were no significant decrease in amplitudes were seen. We analyzed the physiological and pharmacological parameters but they were not significant. Higher percentages of alerts were communicated to the surgeon when baseline values were set before the beginning of the procedure when compared to baseline values set before major manipulation. There was one patient with HLS when baseline is set before major manipulation, but on further analysis of this patient’s data we found this as an outlier. Conclusions Significant variation in BAEP responses exist before any major manipulations during MVD. Baselines can be set before any major manipulations without any loss of sensitivity in value of BAEPs during MVD. References 1.Barker FG, 2nd, Jannetta PJ, Bissonette DJ, Shields PT, Larkins MV, Jho HD. Microvascular decompression for hemifacial spasm. J Neurosurg 1995;82: Radtke RA, Erwin CW, Wilkins RH. Intraoperative brainstem auditory evoked potentials: significant decrease in postoperative morbidity. Neurology 1989;39: Total number of patients Patients with significant alerts in LwV at change start Patients with significant alerts in AwV at change start HLS after surgery Baseline set at beginning of the procedure Baseline set at dura opening Comparison of HLS in patients with significant alerts.