Prevention and prediction of postoperative bowel bladder disorder using an anal plug electrode with Tc-MsEP monitoring during spine surgery

A retrospective study was performed to examine the efficacy of intraoperative monitoring of the external anal sphincter (EAS) muscle-evoked potential after a transcranial muscle-action potential (Tc-MsEP) in spinal cord surgery, and to evaluate alarm points for EAS waveform deterioration related to postoperative bowel bladder disorder (BBD). BBD is caused by damage to the hypogastric, pelvic, and pudendal nerves and leads to a significant reduction in quality of life. Intraoperative Tc-MsEP monitoring using the EAS is common to prevent neurological deficit, but the relationship of BBD with intraoperative monitoring of the EAS has not been examined. Waveform derivation from the EAS using a plug-type surface electrode was investigated in 123 spine surgeries in which Tc-MsEP was recorded intraoperatively outside the anal sphincter. An acceptable baseline waveform from the EAS was detected in 105 of the 123 cases (85.3%). Preoperative BBD was present in 16 cases, postoperative BBD occurred in 8 cases, and intraoperative waveform deterioration from the EAS was detected in 25 cases. The derivation rate was significantly lower in cases with preoperative BBD compared to those without BBD (62.5% vs. 88.9%, p<0.01). Waveform deterioration from the EAS had a sensitivity of 100%, specificity of 93.0%, positive predictive value of 50%, and negative predictive value of 100% for detection of postoperative BBD. All postoperative BBD was detected with an EAS amplitude decrease to <30% of baseline. Therefore, in spine surgery, a Tc-MsEP intraoperative EAS amplitude decrease to <30% of the control waveform may be useful for prediction of postoperative BBD.