Pathophysiology of adult fecal incontinence.

Fecal incontinence occurs when the normal anatomy or physiology that maintains the structure and function of the anorectal unit is disrupted. Incontinence usually results from the interplay of multiple pathogenic mechanisms and is rarely attributable to a single factor. The internal anal sphincter (IAS) provides most of the resting anal pressure and is reinforced during voluntary squeeze by the external anal sphincter (EAS), the anal mucosal folds, and the anal endovascular cushions. Disruption or weakness of the EAS can cause urge-related or diarrhea-associated fecal incontinence. Damage to the endovascular cushions may produce a poor anal "seal" and an impaired anorectal sampling reflex. The ability of the rectum to perceive the presence of stool leads to the rectoanal contractile reflex response, an essential mechanism for maintaining continence. Pudendal neuropathy can diminish rectal sensation and lead to excessive accumulation of stool, causing fecal impaction, mega-rectum, and fecal overflow. The puborectalis muscle plays an integral role in maintaining the anorectal angle. Its nerve supply is independent of the sphincter, and its precise role in maintaining continence needs to be defined. Obstetric trauma, the most common cause of anal sphincter disruption, may involve the EAS, the IAS, and the pudendal nerves, singly or in combination. It remains unclear why most women who sustain obstetric injury in their 20s or 30s typically do not present with fecal incontinence until their 50s. There is a strong need for prospective, long-term studies of sphincter function in nulliparous and multiparous women.