Anatomy and physiology: Neurologic basis for the function of sacral nerve stimulation

Sacral neuromodulation (SNM) was originally evaluated in the 1970's for urinary dysfunction. SNS was approved by the Food and Drug Administration (FDA) for urinary urge incontinence in 1997. Observing that patients treated with SNM for urinary incontinence experienced increased anal pressures and symptomatic improvement of fecal incontinence (FI), investigators explored the role of SNM for the treatment of FI. In 2011, FDA approval was received for the indication of FI. Currently, while SNM is an accepted surgical modality for the treatment of FI, the mechanism by which SNM modulates anorectal physiology to improve bowel function is poorly understood. Early studies focused on targeting anal sphincteric motor response. However, the current hypothesis is that SNM has a more complex mechanism of action and modulates somatic and visceral afferent fibers to the central nervous system. This review highlights the anatomy and physiology for the function of SNM.