Causes and mechanisms

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Irregular bowel habit can exacerbate constipation, as the colon and rectum continue to remove water from stool, hardening it and making passage more difficult. Thus, constipation can be self-perpetuating. In severe chronic constipation, particularly in the elderly, faeces may become so hard, dry and immovable (faecal impaction) that they cannot be passed without medical or surgical assistance, leading to intestinal obstruction.

Reduced motility

Reduced colonic motility may be congenital as in Hirschsprung's disease, where myenteric nerves are absent from the distal colon, causing chronic obstruction and a massively dilated, faeces-filled proximal colon (megacolon).

Paralytic ileus occurs after abdominal surgery, or with electrolyte abnormalities, such as hypokalaemia. Intestinal motility may be reduced acutely by stress, due to sympathetic autonomic nerve activity, and people who are severely injured or otherwise unwell may become constipated for several days.

Neuromuscular dysfunction caused by hypercalcaemia directly reduces intestinal motility.

Reduced colonic motility may also be constitutive, i.e. normal for that person (slow transit constipation).


Drugs such as opiates, antidepressants and others with anticholinergic effects reduce intestinal motility. Similar effects are seen with oral iron supplements and aluminium-containing antacids.

Excessive, chronic use of stimulant laxatives, such as senna, can reduce motility, presumably by damaging or depleting enteric neurons, causing colonic atonia.

5HT3 receptor antagonists that have been used to treat diarrhoea in irritable bowel syndrome (IBS) can also cause severe constipation.

Stool bulk

Stool volume and the frequency of defecation vary with diet, fluid intake and intestinal secretion. Dietary fibre, which mainly comprises non-digestible plant polysaccharides, draws water around itself, increasing stool volume. Thus, chronic constipation is often caused by lack of dietary fibre and/or inadequate fluid intake, which is required to hydrate dietary fibre and to soften the stool.

With fasting, the frequency of defecation declines, partly because of reduced reflex colonic activity and also because of reduced stool volume, although a large proportion of the solid material in stool actually comprises enteric bacteria rather than food residue.

Neuro-psychological dysfunction

Defecation is imbued with social and psychosexual constraints that influence bowel habit, and it can be inhibited voluntarily via the external anal sphincter and by cortical signals acting on autonomic nerves.

Neurological damage to the brain and spinal cord, for example, in multiple sclerosis and peripheral neuropathy can lead to chronic constipation as well as incontinence.

Local causes and obstruction

Local obstruction, for example, by a tumour, may cause pain and difficulty in defecation. Painful local lesions, such as prolapsed haemorrhoids and anal fissure, inhibit the urge to defecate. Constipation and straining at stool contributes to the development of haemorrhoids and fissure.

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Constipation Prescription

Constipation Prescription

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