defecation

defecation is the process by which faeces (stools) are ejected from the rectum. This is a co-ordinated neuromuscular process involving relaxation of the muscles that normally maintain continence at other times.

When sufficient faecal material has entered the rectum, the ‘call to stool’ is evoked. Distension of the rectum by inflating a balloon can reproduce this sensation and invoke the relaxation of the sphincters which allow defecation to proceed. Providing the continence mechanisms are intact the relaxation can be voluntarily overridden—the ‘call to stool’ can be delayed until it is socially convenient to defecate.

The most common physiological stimulus to defecation is eating. This initiates the ‘gastro–colonic reflex’ which results in increased motor activity in the colon and the passage of faeces from the colon into the rectum. Sitting or squatting straightens the angle between the rectum and the short anal canal, and contractions in the colon force additional stool into the rectum to initiate a defecation reflex and sphincter relaxation. Although many individuals ‘strain’ (perform a Valsalva manoeuvre: see blood pressure), which increases intra-abdominal pressure and facilitates movement of faeces into the rectum, this is not strictly necessary since the process will proceed automatically.

There are two major — and opposite — disorders of defecation, namely faecal incontinence and obstructed defecation.

Faecal incontinence

results when the anal sphincter is no longer competent to prevent the unscheduled evacuation of faeces. This may occur as a result of injury to the local nerves serving the sphincters or as a result of disease of the central nervous system, notably dementia, mental retardation, stroke, brain tumours, and spinal cord lesions. Local nerves may be affected as part of widespread nerve damage (polyneuropathy) in conditions such as as diabetes mellitus, although perhaps the most common situation is that of sacral nerve damage associated with pregnancy and delivery. Incontinence may also occur as a result of primary muscle disorders or as a result of direct sphincter damage following surgery, radiation, or inflammatory disorders. Incontinence can also occur in the irritable bowel syndrome and in situations of extreme anxiety.

Obstructed defecation.

Recently it has become apparent that some individuals with constipation have a problem with co-ordination of the process of defecation, and a failure to relax pelvic and sphincter muscles to allow the evacuation of faeces. This may be part of the spectrum of sacral nerve damage, although there is also evidence that there may be a psychological component in addition.

Management of defecation disorders

Degenerative neuromuscular disorders which affect the defecatory process are extremely difficult to treat and in some instances a colostomy is the only socially acceptable intervention. However, when there is evidence of traumatic damage to the anal sphincter, surgical repair is a possibility. In individuals with only partially impaired sphincter function, continence can be maintained for some time with the use of simple anti-diarrhoeal drugs and possibly bulking agents. There is no universally accepted treatment for obstructed defecation, although psychotherapy and behaviour therapy, particularly using biofeedback techniques, have been successful.

Michael Farthing, and Anne Ballinger

See also alimentary system; autonomic nervous system; constipation; faeces; toilet practices.