DIARRHEA - Increased frequency, fluid content, or volume of fecal discharge.
Osmotic diarrhea occurs when unabsorbable, water-soluble solutes remain in the bowel, where they retain water. Osmotic diarrhea occurs with sugar intolerance, including lactose intolerance caused by lactase deficiency, and with the use of poorly absorbed salts (Mg sulfate, Na phosphates) as laxatives or antacids.
Ingestion of large amounts of the hexitols (eg, sorbitol, mannitol), which are used as sugar substitutes, causes osmotic diarrhea as a result of their slow absorption and stimulation of rapid small-bowel motility. Even eating too much of some foods, such as certain fruits, can produce osmotic diarrhea. (See also Osmotic Diarrhea in The Merck Manual of Geriatrics.)
Secretory diarrhea occurs when the small and large bowel secrete more electrolytes and water than they absorb. Secretagogues include bacterial toxins (eg, in cholera), enteropathogenic viruses, bile acids (eg, after ileal resection), unabsorbed dietary fat (eg, in steatorrhea), some drugs (eg, anthraquinone cathartics, castor oil, prostaglandins), and peptide hormones (eg, vasoactive intestinal peptide produced by pancreatic tumors).
Microscopic colitis (collagenous or lymphocytic colitis) causes 5% of secretory diarrhea. It is 10 times more common in women, generally affecting persons > 60. Nausea, vomiting, abdominal pain, flatulence, and weight loss may occur, although the diarrhea is often without other symptoms. Symptoms are often prolonged. Loperamide can be used to control symptoms, and histologic changes may resolve with prednisone or sulfasalazine.
Exudative diarrhea occurs with several mucosal diseases (eg, regional enteritis, ulcerative colitis, TB, lymphoma, cancer) that cause mucosal inflammation, ulceration, or tumefaction. The resultant outpouring of plasma, serum proteins, blood, and mucus increases fecal bulk and fluid content. Involvement of the rectal mucosa may cause urgency and increased stool frequency because the inflamed rectum is more sensitive to distention.
Decreased absorption time occurs when chyme is not in contact with an adequate absorptive surface of the GI tract for a long enough time so that too much water remains in the feces. Factors that decrease contact time include small- or large-bowel resection, gastric resection, pyloroplasty, vagotomy, surgical bypass of intestinal segments, and drugs (eg, Mg-containing antacids, laxatives) or humoral agents (eg, prostaglandins, serotonin) that speed transit by stimulating intestinal smooth muscle.
Diagnosis - The history should note the circumstances of onset, including recent travel, food ingested, source of water, and medication use; duration and severity; associated abdominal pain or vomiting; blood in the stool or change in color; frequency and timing of bowel movements; consistency of stool; evidence of steatorrhea (fatty, greasy, or oily stools with a foul odor); associated changes in weight or appetite; and rectal urgency or tenesmus.
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