Introduction: A novel intraoperative neuromonitoring (IONM) method based on electric stimulation of pelvic autonomic nerves under electromyography of the internal anal sphincter (IAS) has been recently introduced to rectal cancer surgery. The aim of this experimental study was to investigate the impact of selective surgical pelvic autonomic nerve damaging on the evoked IONM signal of the IAS. Methods: For this purpose 12 pigs underwent low anterior rectal resection. The pelvic splanchnic nerves were identified by means of electric stimulation under continuous electromyography of the IAS. For standardized neurostimulation, currents of 3 mA, frequency of 30 Hz and monophasic rectangular pulses with pulse duration of 200 mu s were chosen. Surgical autonomic nerve damaging was performed on both pelvic sides while recording the evoked IONM signal of the IAS. Nerve damaging was carried out proximal to the stimulation point at the level of the pelvic splanchnic nerves and then distally at the level of the inferior hypogastric plexus. Scissors, monopolar diathermy, ultracision and waterjet were used for nerve damaging, each in 3 animals. Results: Selective instrumental nerve damaging during neurostimulation resulted in a dissection specific superimposed artefact followed by a change in the evoked electromyographic amplitude, which could be observed online on the monitor of the IONM device. Comparison of the recorded IONM signals before and after proximal nerve damaging demonstrated a significant decrease in the evoked amplitude level (median 2.8 mu V (interquartile range (IQR) 1.6; 5.7) vs. median 1.4 mu V (IQR 0.6; 2.9), (p=0.026)). Nerve damaging performed distally to the stimulation point resulted in absence of evoked IONM signals. Conclusion: The present study provided first differentiated insights into the dynamic IONM signal behaviour in response to selective pelvic autonomic nerve damaging. Further studies with neurophysiological and surgical IONM signal analysis are mandatory to reach a more accurate IONM resulting in effective nerve sparing pelvic surgery
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