Constipation can be small, hard stools with normal colonic muscle tone, or alternatively, normal stool with lack of muscle tone and failure to respond to
the stimulus of faecal bulk.
It is important to note that severe constipation may paradoxically present with overflow diarrhoea and faecal incontinence.
PHYSIOLOGY:
Constipation arises when the stool is propelled at a slower than normal rate
through the large intestine, thereby allowing a greater amount of water to
be absorbed from the stool than normal. This results in hard stools that are
less easily propelled down to the rectum.
There are 4 main types of muscular gut movements: pendular and segmental movements that mix the intestinal contents, and peristaltic and vermiform
movements that propel the faecal matter in the large intestine.
Three of four times a day, there is a strong peristaltic wave that propels the
contents about a third of the way along the colon. This tends to come about
as a result of the gastrocolic reflex, which is triggered by the arrival of food in
the stomach.
The sigmoid colon acts as a storage area until defaecation, which is brought about by a mass peristaltic movement of the matter into the rectum.
This generally tends to occur after the morning meal in persons with normal
eating habits. Defaecation is triggered by a spinal reflex and can be voluntarily inhibited by contracting the external anal sphincter or is facilitated by relaxing the sphincter and contracting the abdominal muscles.
In normal evacuation, the stool enters and distends the rectum, causing an
involuntary reflex that relaxes the internal anal sphincter. To avoid
spontaneous defaecation, reflex contraction of the external anal sphincter
and pelvic floor muscles (innervated by the pudendal nerve) occurs.
The brain registers that the rectum is full and the external sphincter is then
voluntarily relaxed and the rectum is evacuated with assistance from abdominal wall muscle contraction.
Prolonged gastrointestinal transit time may be caused by a diet low in fibre, physical inactivity, acute or chronic medical conditions, medications and laxative over-use.
Common medical conditions associated with constipation include IBS,
diverticular disease, and hypothyroidism.
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