Activation of the external anal and urethral sphincter muscles by repetitive transcranial cortical stimulation during spine surgery

OBJECTIVE: Clinical utility of high intensity repetitive transcranial electrical stimulation (rTES) to elicit a response in external anal and urethral sphincter muscles was investigated in 23 patients undergoing spine surgery. METHODS: During surgery, motor evoked potentials (MEPs) were recorded from external anal sphincter (EAS), external urethral sphincter (EUS), and tibialis anterior (TA) muscles following high voltage rTES under total intravenous anesthesia. RESULTS: No neurologic sequelae occurred during or after the rTES of the motor cortex. Onset latency for the EAS muscle was 20.2 +/- 3.5 msec which was not significant compared to the EUS muscle latency at 19.9 +/- 1.8 msec. The average electrical intensity to evoke EAS response was 789 +/- 78 volts compared to the 831 +/- 11 volts of the EUS muscle. Waveform latency for the TA muscle was robust in all cases while the latencies for EAS and EUS were not always clear. CONCLUSIONS: This preliminary study shows that intraoperative MEP monitoring of the external anal and urethral muscles is feasible method in particular in circumstances where bowel and bladder function are at risk of an inadvertent injury due to surgical manipulation.