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Bowel Problems: The Basic Facts

The bowel: what it is, what it does
The bowel, also known as the colon or large intestine, is the lower portion of the digestive system. This is the internal plumbing that takes the part of our food that can’t be used in the body and makes it ready for disposal. The food we eat begins its journey at the mouth, and proceeds through the throat and esophagus to the stomach.

Major digestive action starts in the stomach, and is continued in the small or upper intestine. The food, which is moved through the digestive system by a propulsive action, has become mainly waste and water by the time it reaches the bowel, a five-foot-long tube.

By the time the stool reaches the final section of the bowel, called the sigmoid colon, it has lost much of the water that was present in the upper part of the digestive system. The stool finally reaches the rectum, and—on command from the brain—is consciously eliminated from the body with a bowel movement through the anal canal.

Normal bowel functioning can range from three bowel movements a day to three a week. Despite the widely recommended “one movement a day,” physicians agree that such frequency is not necessary. The medical definition of “infrequent” bowel movements is “less often than once every three days.” Most physicians agree that a movement less often than once a week is not adequate.

The rectum, the last 4–6 inches of the digestive tract, signals when a bowel movement is needed. It remains empty until just before a bowel movement. The filling of the rectum sends messages to the brain via nerves in the rectal wall that a bowel movement is needed.

From the rectum, the stool passes into the anal canal, guarded by ring-shaped internal and external sphincter muscles. Just prior to being eliminated, the stool is admitted to the anal canal by the internal sphincter muscle, which opens automatically when the rectal wall is stretched by a mass of stool. The external sphincter, on the other hand, is opened by a conscious decision of the brain, so that bowel movements can be performed only at appropriate times.

Constipation and diarrhea
If the contents of the bowel move too fast, not enough water is removed and the stool reaches the rectum in a soft or liquid state known as diarrhea. If movement of the stool is slow, too much water may be absorbed by the body, making the stool hard and difficult to pass. This condition is constipation. Constipation can prevent any of the stool from being eliminated, or it can result in a partial bowel movement, with part of the waste retained in the bowel or rectum.

Common causes. Diarrhea and constipation are frequent companions of travelers, resulting from encounters with unfamiliar or contaminated food or water, or simply because of a change in an accustomed level of activity. Diarrhea can also be triggered by a viral, bacterial, or parasitic infection.

Continued diarrhea may also stem from food allergies or sensitivity to particular kinds of foods, such as highly spiced dishes or dairy products. (Intolerance to dairy products can often be accommodated by drinking lactose-reduced milk or by eating dairy products together with tablets containing lactose-digesting enzymes.)

Non-MS-related constipation may also be caused by common medications such as calcium supplements or antacids containing aluminum or calcium. Other drugs that may lead to constipation include antidepressants, diuretics, opiates, and antipsychotic drugs.

Ironically, one of the most common causes of non-MS-related constipation is a voluntary habit: delaying bowel movements to save time on busy days or to avoid the exertion of a trip to the bathroom. Eventually the rectum adapts to the increased bulk of stool and the urge to eliminate subsides. The constipating effects, however, continue, and elimination becomes increasingly difficult.

For some women, constipation is a premenstrual symptom, and during pregnancy it may be one way that the colon reacts to a change in the level of sex hormones.

Irritable bowel syndrome, also known as spastic colon, is an umbrella term for a number of conditions in which constipation and diarrhea alternate, accompanied by abdominal cramps and gas pains. Your doctor can determine if you have a disease or simply a syndrome associated with stress.

Constipation and MS
Constipation is the most common bowel complaint in MS. It’s easy to slip into poor dietary habits or physical inactivity. These can disrupt the digestive system, as can depression which is also common. As explained above, various medications can also make the situation worse.

But there is more to the problem than poor habits. MS can cause loss of myelin in the brain or spinal cord, a short-circuiting process that may prevent or interfere with the signals from the bowel to the brain indicating the need for a bowel movement, and/or the responding signals from the brain to the bowel that maintain normal functioning.

Common MS symptoms such as difficulty in walking and fatigue can lead to slow movement of waste material through the colon. Weakened abdominal muscles can also make the actual process of having a bowel movement more difficult.

People with MS often have problems with spasticity. If the pelvic floor muscles are spastic and unable to relax, normal bowel functioning will be affected.

Some people with MS also tend not to have the usual increase in activity in the colon following meals that propels waste toward the rectum.

And finally, some people with MS try to solve common bladder problems by reducing their fluid intake. Restricting fluids makes constipation worse. This is so common in MS that the first step to take may be to get medical help for your bladder problems so that adequate fluid intake, which is critical to bowel functions, will be possible.

A long-term delay in dealing with bowel problems is not an option. Besides the obvious discomfort of constipation, complications can develop. Stool that builds up in the rectum can put pressure on parts of the urinary system, increasing some bladder problems. A stretched rectum can send messages to the spinal cord that further interrupt bladder function. Constipation aggravates spasticity. And constipation can be the root cause of the most distressing bowel symptom, incontinence. See Incontinence section.


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Copyright National Multiple Sclerosis Society, 2004