Investigation of stimulus position in the production of the strength duration curve of the external anal sphincter muscle in women with anorectal dysfunction

Abstract

The strength duration test (SDT) is a minimally invasive test of the innervation of a muscle and has been successfully adapted for use on the external anal sphincter (EAS). The SDT has previously been performed on the EAS placing a stimulating probe in the 3 o'clock position. The aim of this study was to determine whether there was any variation in the strength duration curves (SDC) produced at different positions around the EAS. Thirty-one patients with anorectal dysfunction attended our Institution for Anorectal Physiology Studies and were recruited to the study. After undergoing anal manometry, pudendal nerve terminal motor latency measurement and endoanal ultrasonography, the SDT was performed in four stimulus positions (3, 6, 9 and 12 o'clock). The sequence of positions was randomly selected. Four SDCs were obtained successfully in 25 patients. There was no significant difference seen between the four positions at the longer current durations (1-100 ms). Significant differences occurred at the shortest current durations (0.3 and 0.1 ms) when comparing adjacent stimulus positions except between the 6 and 9 o'clock positions, which were comparable. When opposing positions were compared (3-9 and 6-12 o'clock) at these short durations no significant difference was found. There was no significant difference between the SDC produced at 3 and 9 o'clock. These positions closely correspond to where the pudendal nerves enter the EAS. It therefore appears acceptable to perform the SDT in either of these positions, though 3 o'clock remains our preferred position.