MS ExplainedBowel problems
In the healthy bowel, once food has passed through the digestive system, waste material is collected in the colon where it forms into stools. When the colon is stretched to a certain size, nerve responses trigger a reflex process that causes a bowel movement. The stool is passed into the rectum - the area occupying the last six inches of the digestive system. As the rectum fills, it triggers another reflex action that causes the stool to pass to the anal canal.
The anal canal is about an inch long with a valve, or sphincter, at each end. The internal sphincter, which separates the anal canal from the rectum, is opened by an involuntary reflex action under the control of the spinal cord. The external sphincter can be controlled to prevent inappropriate defecation. When someone wishes to defecate, the external sphincter is relaxed and they 'bear down' - increasing the pressure in their abdomen to help push the stool out.
Although the causes of bowel disorders in MS are not fully known, it is thought that damage to nerves controlling different parts of this process can lead to different bowel problems.
Constipation can be caused by interruptions to the sensory messages from the colon or rectum that signal fullness. The body does not realise that there is a need to move the stool on to the next stage in the process.
Constipation can also be due to the slowing down of the digestive process that can occur in people who do not undertake much physical activity or who can not bear down with sufficient pressure due to weakness.
Not drinking enough can also cause constipation. As food passes through the body, water is extracted. If there is not enough fluid in the diet, by the time food reaches the end of the process, it has become a dry, hard stool which does not move as easily through the system.
Faecal incontinence is the inability to control defecation. This happens when damage to nerves occurs between the reflex area in the spinal cord and the areas of the brain that give voluntary control to bowel movements. This does not necessarily mean that the individual experiences diarrhoea, but if this lack of coordination affects the parts of the process where stools are formed, it can result in ill formed and runny stools.
Faecal incontinence can also be caused by constipation. A build up of constipated matter can sometimes mean that the sphincters in the anus and the rectum do not close properly, allowing runny stools to escape around the blockage.
- More information from the MS Trust
- Bowel problems factsheet
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