Spinal Accessory Nerve Monitoring in Posterior Fossa Surgery

The role of intraoperative monitoring of lower cranial nerves is not well established. In this study, the authors retrospectively analyzed the intraoperative monitoring data and the clinical outcome of the spinal accessory nerve (SAN) in patients who have undergone posterior fossa surgery. SAN was monitored in 19 patients. Subcutaneous needle electrodes were placed in the trapezius, and any abnormal spontaneous activity was documented for duration and amplitude. A bipolar stimulation probe was used for triggered electromyogram, with a maximum stimulation intensity of 3 mA. All patients were clinically evaluated for SAN dysfunction postoperatively 24 hours and 7 days. Neurotonic discharges, lasting between 3 and 90 minutes were seen in 84% of the patients. Continuous discharges lasting longer than 5 minutes were seen in 36% of the patients. The SAN was stimulated in 57.8% of the patients. With clinical examination, none of our patients had postoperative SAN dysfunction. Long lasting dense neurotonic discharges and high stimulation thresholds have been correlated with postoperative facial nerve injury; however, this does seem be true in the case of SAN monitoring. Neurotonic discharges are commonly false-positive and stimulation intensity up to 3 mA can be related with a good outcome. The intraoperative monitoring of SAN may not be valuable as much as facial nerve monitoring.