Pathogenesis of slow transit and pelvic floor dysfunction: from bench to bedside.

The colon and anorectum function together to provide intraluminal mixing, absorption of water, electrolytes, and short chain fatty acids, dehydration of fecal material, storage, and ultimately, elimination in a socially appropriate manner. Normal function and continence require accommodation of the colon and rectum to the entry of fecal materials, which includes receptive relaxation, perception, and discrimination of rectal contents, and voluntary and reflex motor function of the anorectum. Defecation, on the other hand, requires the reflex relaxation of the internal anal sphincter, voluntary and reflexive relaxation of the external anal sphincters and pelvic floor structures, and adequate rectosigmoid tone to allow funneling of contents through the anal canal. The sensation of urgency with rectal filling, and the motivation and prior learning of the appropriate responses are also required. Continence and defecation, therefore, involve complex sensory, structural, and motor mechanisms that involve both the colon and pelvic floor. These mechanisms and their relative importance to the pathogenesis of slow-transit constipation and pelvic floor dysfunction will be reviewed.