How sacral nerve stimulation works in patients with faecal incontinence

Aim Sacral nerve stimulation (SNS) reduces incontinence episodes and improves the quality of life of patients treated for faecal incontinence. However, the exact mechanism of action of this technique remains unclear. The present article reviews the pertinent neuroanatomy and neurophysiology related to SNS and provides explanations for potential mechanisms of action. Method A systematic review of the literature was performed for studies of the potential mechanisms of action of SNS, using MEDLINE, PubMed, Embase and the Cochrane Library. Articles dealing with the technique, adverse events and economic evaluations of SNS, as well as literature reviews, were excluded, except for reviews dealing with the mechanisms of action of SNS. The following inclusion criteria were used to select articles: (i) articles in English, (ii) randomized, double-blinded, sham-controlled studies, and (iii) cohort studies. Case-control studies or retrospective studies were cited only when randomized or cohort studies could not be found. Results We propose three hypotheses to explain the mechanism of action of SNS: (i) a somato-visceral reflex, (ii) a modulation of the perception of afferent information, and (iii) an increase in external anal sphincter activity. Conclusion The mechanism of action of SNS in patients with faecal incontinence almost certainly depends on the modulation of spinal and/or supraspinal afferent inputs. Further research on humans and animals will be required to gain a better understanding of the mechanisms of action of SNS.