Rapid rate magnetic stimulation of human sacral nerve roots alters excitability within the cortico-anal pathway

Sacral nerve root stimulation (SNS) can produce dramatic symptomatic improvement in faecal incontinence (FI). However, the physiological mechanism behind this improvement remains unknown. One hypothesis is that SNS may modulate cortico-anal pathways and drive compensatory changes within the spinal cord or cerebral cortex that beneficially alter sphincter function. Our aim was to assess whether short-term experimental SNS can induce changes in the human cortico-anal pathway. Eight healthy volunteers (mean age 30 years) were studied. Subjects were investigated on three separate occasions and randomized to either active (5 and 15 Hz) or sham rapid-rate lumbosacral magnetic stimulation (rLSMS). Anal sphincter electromyograms (EMG) were recorded from an anal probe following single-pulse transcranial magnetic stimulation, at baseline, immediately, 30 and 60 min following rLSMS at either (i) 5 Hz for 15 min, (ii) 15 Hz for 15 min or (iii) sham stimulation for 15 min. In addition, manometry and anal sphincter sensation was measured in a subset of subjects. Interventions were compared to sham using anova. Fifteen hertz rLSMS increased cortico-anal EMG response amplitude in the 1 h postintervention (F(4, 28) = 3.2, P = 0.027), without a shift in response latency. This effect was not demonstrated with either 5 Hz or sham stimulation. rLSMS had no short-term effect on sensation or physiology. Short-term magnetic stimulation of the sacral nerve roots induces changes in cortico-anal excitability which is frequency specific. These data support the hypothesis that SNS produces some of its beneficial effect in patients with FI by altering the excitability of the cortico-anal pathway.