Motor evoked potentials recorded from external anal sphincter by cortical and lumbo-sacral magnetic stimulation: normative data.

Electrophysiological activation of the motor pathways can be obtained by electrical or magnetic stimulation. The latter has the great advantage of being painful and able to stimulate deeply situated nervous structures. Only a few reports describe responses obtained from pelvic floor muscles and external anal sphincter (EAS) by transcranial and lumbo-sacral magnetic stimulation. Our purpose is to present normative data of motor evoked responses from EAS in a group of healthy subjects (age range 19-80 years) using a standardized protocol of magnetic stimulation. Nine females and 7 males, with a mean age of 52.63 years, were included in this study. They had no known neurological and gastrointestinal disease. Magnetic shocks were delivered by a Magstim 200 (Novametrix) and a circular coil, centered on the vertex and on the lumbo-sacral region. Electromyographic recordings were taken from EAS using needle electrodes. The cortical magnetic stimulation was performed in two conditions: at rest and during a mild contraction of pelvic floor muscles. The mean values of motor evoked potentials (MEPs) latencies after cortical stimulation were 26.92+/-3.01 ms at rest and 23.31+/-2.70 ms during facilitation. Motor latency after lumbo-sacral root stimulation was 6.09+/-1.43 ms. The MEPs from EAS are easily obtained and stably reproducible in normal subjects. It can be suggested also as a useful adjunct in the assessment of faecal incontinence.