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Constipation
06 March,13
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The pattern of normal defaecation is extremely variable, which makes constipation hard to define, and to some extent subjective. However, expert consensus considers that it occurs if 2 or more of the following features are present for at least 12 weeks in the previous year: less than 3 defaecations in a week, or hard or lumpy stool, straining, feeling of incomplete evacuation or anorectal blockage, or need for manual assistance, on at least a quarter of occasions. New onset constipation, especially in patients over 50 years of age, or accompanying symptoms such as anaemia, abdominal pain, weight loss, or overt or occult blood in the stool should provoke urgent investigation because of the risk of malignancy or other serious bowel disorder.

Constipation can be a symptom of a range of disorders or of drug toxicity and appropriate management of any underlying cause is required. It tends to be more common in the elderly, in women, and in late pregnancy.

Certain subtypes of constipation have been identified: patients may be subdivided according to whether intestinal transit time is slow or normal, or whether there is pelvic floor dysfunction (dyssynergia; outlet obstruction; anismus).

There is relatively little evidence to guide management. In general, stepped or graded management is favoured, beginning with advice on lifestyle (including exercise and adequate hydration), and dietary modification if appropriate. An increase in fibre intake, preferably through a high-fibre diet, is useful in patients with normal transit times although it is less beneficial in slow-transit constipation. In patients with pelvic floor dysfunction, biofeedback training may be instituted.

Where lifestyle changes and dietary modification are insufficient, a laxative may be considered. Because of the lack of clear evidence to guide the choice of laxative, practice varies considerably. Many favour the use of either a bulk laxative or an osmotic laxative as a first choice option, with stimulant laxatives tending to be reserved as a second-line option, and diphenylmethane derivatives being preferred to anthraquinones.

Bulk laxatives include bran, ispaghula, methylcellulose and related compounds, psyllium, or sterculia. Bulk laxatives are of particular value in those with small hard stools. However, they have a delayed onset of action (up to 72 hours) and they may exacerbate symptoms of flatulence, bloating, and cramping; use should be avoided where these are severe. A gradual increase in dose may help reduce such effects. They may not be the first choice for elderly patients who are frail or immobile since the resulting soft faeces may result in faecal incontinence, and should only be given where fluid intake is adequate, because of the risk of obstruction.

Osmotic laxatives include saline laxatives such as magnesium hydroxide and magnesium sulfate, poorly absorbed sugars such as lactulose or sorbitol, and macrogols (often formulated with electrolytes). Some favour the use of macrogol 3350 or 4000, for which there is reasonable evidence of benefit in chronic constipation, although in practice lactulose, which is less effective in the elderly than a combination of bulking and stimulant laxatives, is widely used. In the USA, a saline laxative such as magnesium hydroxide has been recommended, although saline laxatives may not be acceptable to some patients because they can cause watery stool and urgency.

Stimulant laxatives in current use include diphenylmethane derivatives such as bisacodyl or sodium picosulfate, and anthraquinone-containing drugs such as senna. They have a more rapid onset of action than bulk laxatives or lactulose, and are usually given at night to help produce a bowel motion the following morning.

Many traditional stimulant laxatives have fallen from use because of adverse effects, and it has been widely thought that their prolonged use or abuse may irreversibly damage colonic nerves and muscles; however, the evidence for this is poor, and appropriate stimulant laxatives used at licensed doses are unlikely to cause significant harm. Tolerance can occur but seems to be uncommon in the majority of users. Combined preparations are also available, and a combination of senna and fibre has been found to be more effective than the osmotic laxative lactulose in elderly patients with chronic constipation.

Other laxatives include the docusates, surfactants that are used as stool softeners, but their efficacy used alone for constipation is not well established; perhaps for this reason, they are often formulated with a stimulant or osmotic laxative. Stool softeners would have potential value for patients with haemorrhoids or anal fissures, or those in whom straining is potentially hazardous (such as the elderly or those with existing cardiovascular disease). Liquid paraffin has also been used as a lubricant for the passage of stool, but should be used with caution because of its adverse effects, which include anal seepage and the risks of granulomatous disease of the gastrointestinal tract or of lipoid pneumonia on aspiration. Although it has been recommended in some countries for constipation in children, the UK CSM considers it should not be used in those under 3 years of age.

Prokinetic drugs have theoretical advantages in patients with slow-transit constipation, but few suitable drugs are available, although selective 5-HT4 agonists such as tegaserod are used in constipation-predominant irritable bowel syndrome and are under investigation in chronic constipation.

Rectal administration of laxatives using enemas or suppositories is appropriate in patients requiring rapid relief from constipation. Phosphate enemas should be used with caution because of the risks of absorption of significant amounts of phosphate.

Glycerol and bisacodyl may be given as suppositories to promote faecal evacuation; they should not be used regularly, as changes in the rectal epithelium have been seen with bisacodyl, and the effect of glycerol on rectal mucosa is unclear
During pregnancy the preferred options for management are increased fluid intake, exercise, and dietary fibre; bulking agents may be given as fibre supplements.

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