There are many different possible causes of loss of bowel control.
1.Anal Sphincter Damage
Probably the most common cause of faecal incontinence is damage to one or both of the anal sphincter muscles. This can be damage to the external anal sphincter, which is responsible for delaying bowel emptying once the rectum fills and the urge to empty the bowel is felt. The muscle may become weak, or may even have a tear or disruption inside where it cannot be seen.
People with a weak or damaged external sphincter muscle typically experience urgency (needing to rush to the toilet as soon as the need is felt) and, if the toilet is not reached in time, urge incontinence (accidental loss of stool on the way to the toilet). This is because the weak or damaged muscle cannot squeeze hard enough to stop the stool from coming straight out (remembering that the internal sphincter will automatically open when contractions start higher up in the bowel causing stool to enter the rectum). Or the muscle cannot squeeze for long enough to provide time for the bowel to relax and the internal sphincter time to close off again. If the stool remains in the upper anal canal, the urge does not wear off, and is felt all the time (and can feel very urgent), until the toilet is reached.
Typically someone with damage to the external anal sphincter has to drop everything and find a toilet with very little warning. This urgent feeling occurs each time that the bowel needs to be emptied. The urge persists until the bowel is emptied and it cannot be forgotten or ignored.
People with a damaged internal anal sphincter usually complain of passive soiling, soft stool or small pellets of stool just leak out without the person realising it is happening and with no control. This may either be after the bowels have been emptied (often lasting for an hour or two after each bowel action), or loss of stool upon physical exertion (stool comes away either as pellets or soft mushy stool, when you walk, bend, or with more vigorous physical exercise such as playing sport). There may be great difficulty in wiping clean after bowel emptying, and many people find that however much toilet paper is used for wiping it still comes away with stool on it.
Sometimes there is damage to both of the sphincter muscles, and this may lead to leakage without being aware of it, as well as urgency and urge incontinence.
Causes of Anal Sphincter Damage
Childbirth is the most common cause of damage to one or both anal sphincter muscles, with one in three women sustaining some sphincter damage during the birth of their first baby if this is born normally through the vagina.
This is even more likely if the baby is very large, or needs the help of forceps to be born, or if there is a tear at the back of the vagina during the birth. Usually this damage is nobodys fault it just happens because the anus and vagina are so close and it is easy for the anal sphincters to get stretched or suffer internal tears.
If the damage is minor, the woman may never know that there has been some damage inside and it may never cause problems. But some women with sphincter damage notice problems immediately after the birth and others seem to develop symptoms at a later date. Often the sphincter muscle damage is hidden, with no obvious damage visible on the skin or in the vagina. Later problems may start at the menopause, especially where there is general weakness of the pelvic floor muscles. It may be that practising pelvic floor exercises, especially after childbirth, may help to prevent later problems, but this has not yet been conclusively proven.
Other possible causes of sphincter damage are any type of injury to the anus. A few people are unlucky enough to have accidental damage during an anal operation such as haemorrhoidectomy (removal of piles) or following an operation for an anal fissure or fistula or from other surgery involving the rectum.
A rectal prolapse can occur when the normal supports to the rectum become weak and the rectal lining or even some of the rectum itself prolapses down through the anus, especially when straining to empty the bowel. This may go back on its own after bowel emptying, or may stay out all the time in severe cases. This prolapse is often associated with bowel leakage and the prolapse itself can stretch and damage the anal sphincter muscles.
Another cause of loss of bowel control is severe diarrhoea. An acute infection in the bowel, such as gastroenteritis or food poisoning, can cause the sudden and explosive onset of diarrhoea. This is usually short-lived, but it may be so severe as to overwhelm even a normal sphincter mechanism if a toilet is not readily at hand. People with diarrhoea often have very high pressure waves in the bowel, and this pressure can be so great and create such extreme urgency that it is impossible to reach the toilet unless it is very close by.
Sometimes diarrhoea can last for a long time. Some people with bowel diseases, such as ulcerative colitis or Crohns disease may experience inability to hold stool reliably during an acute flare-up of the disease. An unpredictable and at times explosive bowel habit can lead to incontinence of stool.
Irritable bowel syndrome (IBS) affects up to one in five of the population. This is a poorly-understood condition which often causes a very variable bowel habit with alternating constipation and diarrhoea, together with a lot of abdominal discomfort and bloating. A few people find that when the stools are very loose their control is poor. For some people IBS seems to be related to diet or stress, for others the cause is a mystery. See Bowel Conditions for more information.
Sometimes people who have had a major operation to remove or reconstruct part of the bowel have problems with diarrhoea and poor bowel control afterwards. Removal of part of the bowel can result in less water being absorbed from the stools and the development of looser bowel motions. This may just last for a few weeks after the operation while everything settles down and you adjust to a new bowel pattern, and you need to be patient. But occasionally diarrhoea and poor control lasts longer. It is always important to discuss this with your surgeon do not just expect problems and put up with them. Sometimes the treatments described below, such as medicines to slow down the bowel and make the stool more solid, or exercises to strengthen the muscles, will help.
It may sound contradictory that severe constipation, or difficulty with bowel emptying, can itself be a cause of faecal incontinence. But it can happen that the bowel becomes so overloaded with stool that small lumps break off and come away, usually without the person feeling that it is happening. Or the wall of the bowel is irritated by the hard stools and so produces more fluid and mucus, which then leaks out through the anus. As this fluid is stained brown by stools this can be mistaken for diarrhoea.
If the bowel becomes very overloaded there can be a very large amount of hard or soft stool in the rectum or colon. This is called impaction. This is a particular problem of physically or mentally frail, dependent elderly or disabled people, particularly those in institutional care. However, impaction is also occasionally found with people with relatively rare bowel disorders such as Hirschsprungs disease, megarectum or megacolon.
Impaction is probably caused by a combination of poor sensation in the rectum, often with immobility, low food and fluid intake, drug side effects, delay in responding to the sensation of needing to empty the rectum, lack of privacy, and other factors which are not as yet fully understood. Some medicines, especially painkillers, have a tendency to constipate. Low food or fluid intake can also make constipation worse. Some people with nerve diseases such as Parkinsons disease or Multiple Sclerosis are prone to constipation.
Repeated straining over many years can eventually lead to muscle weakness around the anus and this may lead to rectal prolapse or anal sphincter weakness (see above).
Constipation itself is a separate subject in its own right and is not covered in detail on this site.
4.Nerve Injury or Disease
As bowel control involves complex co-ordination of nerves and muscles, any damage or disease of the nerves can make control precarious. This includes people who have had a spinal injury, or who have multiple sclerosis. Nerve damage may mean that someone does not get the right messages when the rectum is full. Or the rectum may just empty automatically without feeling or control as soon as it fills. Sometimes this can happen while asleep at night.