11. Diarrhea / W.G. Thompson 

Lax bowels, the flux. 

Diarrhea is best described as too frequent passage of too loose (unformed) stools. Diarrhea is frequently accompanied by urgency, and occasionally incontinence. When considering a patient with diarrhea the following must be considered: frequency (>3 movements/day), consistency (loose/watery), urgency, volume (>200 g/day) and whether the condition is continuous. Persistent, frequent, loose, urgent, large-volume stools are most likely to have a pathology. Lesser and intermittent symptoms are more likely to be functional. 

Diarrhea is due to one or more of four mechanisms: osmotic attraction of water into the lumen of the gut, secretion of excess fluid into the gut (or decreased absorption), exudation of fluid from the inflamed surface of the gut, and rapid gastrointestinal transit. 

Osmotic diarrhea results if the osmotic pressure of intestinal contents is higher than that of the serum. This may result from malabsorption of fat (e.g., in celiac disease) or of lactose (e.g., in intestinal lactase deficiency). Certain laxatives, such as lactulose and magnesium hydroxide, exert their cathartic effect largely through osmosis. Certain artificial sweeteners, such as sorbitol and mannitol, have a similar effect. Characteristically, osmotic diarrhea ceases when the patient fasts. 

Secretory diarrhea occurs when there is a net secretion of water into the lumen. This may occur with bacterial toxins, such as those produced by E. coli or Vibrio cholerae, or with hormones, such as vasoactive intestinal polypeptide (VIP), which is produced by rare islet cell tumors (pancreatic cholera). These provoke adenylate cyclase activity in the enterocyte (intestinal epithelial cell), increase cyclic AMP and turn on intestinal secretion. A similar effect may occur as a result of excess bile salts in the colon (choleretic enteropathy) and from the cathartic affect of hydroxylated fatty acids resulting from the bacterial action on malabsorbed fat. Such a diarrhea does not diminish with fasting. Osmotic and secretory diarrhea result from abnormalities in the small intestine such that the flow of water through the ileocecal area overcomes the absorptive capacity of the colon. 

Exudative diarrhea results from direct damage to the small or large intestinal mucosa. This interferes with the absorption of sodium salts and water and is complicated by exudation of serum proteins, blood and pus. Infectious or inflammatory disorders of the gut cause this kind of diarrhea. 

Acceleration of intestinal transit may result in diarrhea (e.g., as a result of hyperthyroidism). The rapid flow-through impairs the ability of the gut to absorb water, resulting in diarrhea. 

In most instances of diarrhea two or more of these four mechanisms are at work, so these pathogenetic concepts are seldom of great help in diagnosis. 

It is important to establish the frequency of defecation, the duration of the diarrhea, the nature of the stool and its volume. If diarrhea has been present for less than two weeks, it is most likely a result of an infection or toxin. A history of many previous attacks, on the other hand, may indicate a recurrence of inflammatory bowel disease. The frequency of the stool gives some idea of severity; one should establish whether incontinence is also present. To elicit the latter history may require direct or leading questions. Stool from malabsorption is often foul-smelling and contains oil droplets. A history of nutrient deficiency, anemia or weight loss also suggests malabsorption. Watery diarrhea, particularly when large in volume, supports a diagnosis of small bowel disease. However, a large villous adenoma of the distal colon may produce a watery diarrhea. The presence of blood or pus in the stool suggests an exudative diarrhea, a type of diarrhea that is often relatively small in volume and indicative of colitis. Loose bowel movements interspersed with normal or even constipated ones are evidence of the irritable bowel syndrome. 

There are many causes of diarrhea, some of which are summarized in Table 2. The presence of profound weight loss and malnutrition in a young person points to a malabsorption syndrome due to small bowel or pancreatic disease or to inflammatory bowel disease. Metabolic conditions such as hyperthyroidism or the overuse of (magnesium-containing) antacids or laxatives might also be responsible for chronic diarrhea. 

TABLE 2. Anatomic approach to the causes of chronic diarrhea

Travel to tropical countries can be marred by an attack of so-called traveler’s diarrhea. The most common cause is toxigenic E. coli (it is known as toxigenic because a toxin is produced). However, a large variety of intestinal infestations can occur with travel. Pseudomembranous colitis may occur within weeks of the use of antibiotics. Campylobacter or cryptococcosis may be acquired from pets. Contaminated water may result in giardiasis, amebiasis or cryptococcosis. Chronic use of alcohol may damage the small intestinal mucosa. Diabetics frequently have diarrhea because of autonomic neuropathy, perhaps with bacterial overgrowth. 

Finally, it is essential to establish if the patient is homosexual. Almost any of the usual gastrointestinal pathogens can be spread by homosexual activity, including lymphogranuloma venereum and gonococcus. Homosexuals are liable to the gastrointestinal complications of AIDS. 

The differential diagnosis is very complex. A careful history is often the most important diagnostic tool. Patients examined for the first time deserve at least a sigmoidoscopy to rule out local colon disease. If a small intestinal diarrhea is suspected, a three-day collection of stool to determine daily weight and fat content is necessary. If there is steatorrhea, or if stool weight exceeds 500 g per day, there is likely to be small intestinal or pancreatic disease. Smaller volumes, particularly if accompanied by blood, point to inflammation of the colon. 

The recent onset of acute diarrhea requires careful examination of the stool for pus cells and culture for bacterial pathogens, or a study for ova and parasites in the case of suspected protozoa. Viral studies are important in infants, and special studies are required in AIDS.