Motor evoked potentials from external anal and urethral sphincter muscles by transcranial cortical stimulation during surgery

Abstract

This chapter addresses stimulation and recording techniques, and validates the use of the ring electrodes in repetitive transcranial electrical stimulation (rTES) studies of the external urethral sphincter (EUS). This technical report is useful, since complex spinal instrumentation can potentially endanger sacral nerve roots leading to bowel and bladder dysfunction after surgery. The chapter discusses that bowel function is maintained through voluntary contraction of anorectal musculature that is under descending control from the cerebral cortex. Cortical mapping studies demonstrate the anal response to be bilaterally represented on the superior motor cortex of both cerebral hemispheres. The chapter reviews that the external anal sphincter (EAS) muscle differs from the muscles in the limbs both physiologically and anatomically. There is a lesser degree of voluntary control over the EAS muscle than over other skeletal muscles. The EAS and the external urethral sphincter (EUS) are driven from lower motor neurons residing in the pudendal nerve.