diarrhoea

diarrhoea is a world-wide health problem. It ranges from a trivial nuisance for travellers to a fatal illness, particularly among children in underdeveloped countries. Infection is the commonest but not the only cause of diarrhoea. The several causes will be reviewed, but first it is necessary to be clear about the meaning of the term itself.

Diarrhoea is commonly defined as a change in bowel habit resulting in an increase in stool frequency or fluidity or both. Since stool frequency and consistency varies widely in the normal population it is the change from what is usual that is important. Stool frequency alone is not a good indicator of ‘true diarrhoea’ since stool frequency can increase in ‘irritable bowel syndrome’, but the total amount of stool passed each day may be unchanged and within normal limits. It is for this reason that diarrhoea is best defined by stool weight, which for an individual in the Western world is usually less than 200 g/day. In the developing world, stool weight is higher in the normal population (250–400 g/day). Diarrhoea may be acute — an illness of abrupt onset lasting 3–7 days — or chronic (persistent), which is usually defined as lasting more than 14 days.

Mechanisms and causes of diarrhoea

Normally, during a 24-hour period, about 9 litres of fluid enter the small intestine. About 2 litres come from the diet, and the remaining 7 litres from secretions into the alimentary tract which carry the enzymes and other substances necessary for the digestion and absorption of food: saliva, gastric juice, biliary, and pancreatic secretions, and additional fluid secreted by the small intestine itself. Of these 9 litres only 1–2 litres pass on into the colon, so under normal circumstances the small intestine reabsorbs 7 litres of fluid into the bloodstream. The colon also retrieves the major part of the fluid with which it is presented, so that only about 100 ml finally leave the body in faeces. Diarrhoea results when these absorptive mechanisms fail to retrieve the large volume of fluid entering the small intestine, or when there is excessive secretion of fluid into either the small or large intestine. In some circumstances both decreased absorption and increased secretion are combined.

Acute or chronic diarrhoea may be due to a variety of pathophysiological mechanisms and is often categorized into four major groups as follows:

Osmotic diarrhoea

Failure of the intestine to absorb certain solutes results in an increase in the osmolality of intestinal contents which draws an excess of water into the gut from the body fluids down the osmotic gradient. This occurs in conditions that involve carbohydrate malabsorption (monosaccharide intolerance, lactase and sucrase deficiency) or ingestion of non-absorbed saccharides, such as sorbitol, which is used as a sweetener in sugar-free confectionery, and the laxative, lactulose. Magnesium sulphate (Epsom salts) is effective as a laxative because of its osmotic effects. Other conditions which produce malabsorption syndromes, such as coeliac disease, also produce diarrhoea because of the presence of non-absorbed solute in the intestine.

Secretory diarrhoea

Cholera toxin is an example of a bacterial product which produces high-volume watery diarrhoea by promoting massive secretion from the lining of the small intestine. The non-osmotic laxatives, phenolphthalein and senna, partly act by stimulating secretory mechanisms in the intestine. Bile salts, which aid in the process of digestion and absorption of nutrients, are normally absorbed in the last part of the small intestine (terminal ileum). Surgical removal or disease of this part of the bowel causes bile acids to move on into the colon where they interfere with water absorption, causing diarrhoea.

Exudative diarrhoea

occurs usually as a result of damage to the lining of the gut, which leads to impaired absorption or increased losses of body fluids into the lumen. Common causes include intestinal infections, inflammatory bowel diseases (Crohn's disease and ulcerative colitis), other forms of colitis including those due to irradiation and ischaemia, and coeliac disease.

Increased motility

Rapid transit through the intestine can result in true diarrhoea due to the reduced time available for intestinal absorption. This mechanism is thought to be important in some forms of laxative abuse, diabetic diarrhoea, and thyrotoxicosis. Certain functional disorders, such as irritable bowel syndrome, are also associated with more rapid intestinal transit, although in these conditions the change in bowel habit is often regarded as ‘pseudodiarrhoea’ because the increase in bowel frequency is not accompanied by an increase in stool weight.

Acute diarrhoea

Patients with acute watery diarrhoea generally have an intestinal infection. In infants and young children this is most commonly due to a virus infection (rotavirus or adenovirus). There are reported to be no less than 130 million cases annually worldwide, of which about 1 in 150 are fatal.

In adult travellers a frequent cause is a type of E. coli bacterium, which thrives in hot countries and forms toxins that disturb the function of the gut lining. In the indigenous population of industrialized countries, the commonest organisms are bacteria of the Salmonella, Campylobacter, or Clostridium families. The last is particularly common in the elderly and in immunocompromised patients undergoing anti-cancer chemotherapy. Acute diarrhoea accompanied by blood (dysentery) usually indicates infection with an invasive enteropathogen (an organism that attacks the cells of the lining of the gut) such as a member of the genus Shigella (causing bacillary dysentery), an invasive strain of E. coli, or amoebiasis (amoebic dysentery).

Organisms cause acute diarrhoea either by multiplying in a food source, and surviving to be ingested if it is not fully cooked (e.g. Salmonella in chickens), or through living in the gut of people or animals, whose faeces contaminate food or water by reason of defective personal or social hygiene (e.g. E. coli and Shigella from humans; Campylobacter from farm animals, birds, or pets).

In many instances the presence of infection can be determined by microscopic examination of the stool for evidence of parasites and microbiological culture for bacteria.

When acute diarrhoea is due to infection it is usually self-limiting and will remit without specific therapy. When the illness is more severe, oral rehydration therapy with glucose–electrolyte solutions is the mainstay of therapy to correct dehydration and acidosis. There is no need to modify food intake during episodes of diarrhoea, although secondary deficiency of lactase absorption can occur and it sometimes wise to restrict milk and milk products. Breast feeding of infants should continue despite diarrhoea.

Anti-diarrhoeal agents, such as loperamide, are sometimes helpful to reduce bowel frequency but should be avoided in children. Antibiotics are not routinely used for the treatment of acute infective diarrhoea although it is well established that a short course can reduce the severity and duration of symptoms. Some infections, however, do require antibiotic therapy, particularly the invasive organisms that cause dysentery such as severe Shigella and Salmonella infections and amoebiasis.

Persistent diarrhoea

When diarrhoea is persistent, measurement of stool weight over a 3-day period may be needed to confirm whether this is true diarrhoea or ‘pseudo-diarrhoea’. Laxative abuse may be revealed by urine testing. Chronic parasitic infection can be sought and excluded by microscopic examination of the stools. Inflammatory bowel disease usually requires an examination of the colon by endoscopy and possibly a small bowel radiological examination using barium by mouth — a barium ‘follow-through’ examination. If a small bowel disorder such as coeliac disease is considered likely then a small intestinal mucosal biopsy may be required to confirm the diagnosis.

The treatment of chronic diarrhoea depends entirely on the causes. Chronic infections due to certain parasites such as Giardia lamblia and Entamoeba histolytica require anti-microbial chemotherapy. Chronic diarrhoea due to inflammatory bowel disease usually requires treatment with anti-inflammatory drugs such as corticosteroids or 5-aminosalicylic acid. Dietary therapy is sometimes required when the problem is a food sensitivity, such as coeliac disease when a gluten-free diet is appropriate.

Michael Farthing, and Anne Ballinger

See also bacteria; infectious disease; parasites; vomiting.