When stool (feces) leaks out from the rectum accidentally, it is known as fecal incontinence. Under normal circumstances, stool enters the end portion of the large intestine, called the rectum, where it is temporarily stored until a bowel movement occurs. As the rectum fills with stool, the anal sphincter muscle (a circular muscle surrounding the anal canal) prevents feces from coming out of the rectum until it is time to have a deliberate (controlled) bowel movement.
Various conditions can cause incontinence. The most common reason for incontinence is that the anal sphincter becomes too weak to hold the stool in the rectum. Alternatively, sometimes the rectum may start to lose its capacity to store the stool, or the person may be unable to feel that the rectum is full. Also, a person must be able to be aware of the need to empty the bowel, and be mobile enough to reach the bathroom in time. Diarrhea from any cause makes incontinence worse (since it is more difficult to control liquid stool than solid stool).
The anal sphincter may become weak either from direct damage to the muscle or from damage to the nerves that cause the muscle to contract normally.
Damage to muscles can be caused by:
Inflammatory bowel disease (especially Crohn's disease)
Damage to nerves can be caused by:
Spinal cord injury
Sometimes the sphincter muscle may be weak simply from aging, since all our body muscles tend to weaken as we grow older.