Physiologically, diarrhea is a disorder of intestinal absorption of water and electrolytes. Normally, eight to ten litres of fluid enters the gastrointestinal tract daily; two litres from dietary sources, and six to eight litres from endogenous secretions. Most is absorbed in the small bowel, and only 1.5 litres enters the colon. 150mL is excreted in the stool. A physiologic definition of diarrhea is stool weight > 200gm per day. Diarrhea may be acute (< 4 weeks duration) or chronic (> 4 weeks duration). It should be distinguished from incontinence (the involuntary release of rectal contents) and pseudo-diarrhea (increased stool frequency with no change in weight or consistency). Diarrhea is not always pathologic and may be self-induced by laxation (e.g., factitious diarrhea due to Munchausen's syndrome).
For an effective diagnosis to be made, a good history must first be taken. Key points include assessing whether the diahrrea is acute (<4 weeks) or chronic (>4 weeks), sudden onset or gradual, if there has been recent foreign travel or close contact with individuals who have diarrhea (infectious), recent medication or drug use, previous intestinal surgery, a past history of diarrheal problems or inflammatory bowel disease (IBD), the presence of abdominal, rectal, or anal pain with bowel movements, and nocturnal diarrhea (which often indicates an organic cause). Alarm symptoms are important including gross blood loss with bowel movements, anemia, and weight loss.
Acute diarrhea is usually infective and self-limited, resolving in 2 to 6 weeks. In addition to sigmoidoscopy, stool cultures for bacterial and parasitic pathogens should be performed (culture and sensitivity, O & P??) plus Clostridium difficile toxin assay if there is a history of recent exposure to antibiotics (within 3 months). Viruses are common causes of acute diarrhea, but are not readily culturable. Most infectious diarrheas in otherwise healthy patients don't require antibiotic treatment, but fluid and electrolyte replacement can be critical in some cases.
Chronic diarrhea can be the result of several causes:
This type of diarrhea results from damage to the intestinal epithelium, usually leading to exudation of mucous, serum proteins, and blood into the intestinal lumen and the inability to normally absorb water and electrolytes (a leaky mucosa which functions poorly). Examples include infectious colitis due to enteroinvasive organisms.
The predominant mechanism in this type of diarrhea is that there are changes in the balance between absorption and secretion of water and electrolytes, with increased secretion of bicarbonate and chloride ions and decreased sodium and chloride absorption. Secretory diarrheas are often mediated by secretagogues such as bacterial endotoxins, laxatives, drugs, hormones, etc. Clinical features of this type of diarrhea are:
- Large volume watery diarrhea (>1L / day)
- No osmotic gap (calculated stool osmolality = measured plasma osmolality)
- Fasting for 24-48 hours has little or no effect on the diarrhea unless the secretagogue involves something ingested
3. Osmotic or malabsorptive
This type of diarrhea results from the ingestion of poorly digested or absorbed substances, with poor water and electrolyte resorption resulting from osmotic drag due to unabsorbed solutes in the gut lumen. Clinical features are:
- Diarrhea stops when patients fast or remove poorly absorbed solutes from their diet
- There is an osmotic gap
This is a poorly understood and controversial etiology for diarrhea, but basically states that hypermotility (rapid intestinal transit) reduces contact time with absorptive cells, and leads to poor nutrient, electrolyte, and water absorption. Hypomotility may lead to bacterial overgrowth in the small bowel (normally nearly sterile), which can lead to deconjugation of bile acids and reduction in micelle concentrations and hence fat malabsorption. Bacterial overgrowth may also lead to degradation of carbohydrates by bacteria with the production of short chain organic acids and therefore increased osmolality of small bowel luminal contents. Major intestinal surgery can also lead to motility problems or create blind loops in the intestine which can predispose to bacterial overgrowth syndromes. Examples include irritable bowel syndrome, diabetes, scleroderma, and post vagotomy syndromes.