Chronic diarrhea is a common but vexing problem for physicians and patients. Defined as diarrhea that continues for more than four weeks, chronic diarrhea occurs in 1 to 5 percent of the population. Patients often present late, after other symptoms such as weight loss, rectal bleeding, and abdominal pain have developed. Schiller reviewed the management of chronic diarrhea.

Diarrhea results from incomplete absorption of water from the bowel lumen because of a reduced rate of water absorption or osmotically induced luminal retention of water. Even mild changes in absorption can cause loose stools. The three available management strategies are test and treat; categorize, test, and treat; and empiric therapy. After a complete history and physical examination, a diagnosis could be made which could then be confirmed by appropriate testing. This “test and treat” plan is useful when the history and physical examination yield a high probability of a specific diagnosis, When the evaluation is less clear, it is impractical to test for every possible etiology. A “categorize, test, and treat” plan is useful because the presentation often is nonspecific. Diarrhea can be categorized as watery, fatty, or inflammatory based on gross stool examination or microscopic analysis (see accompanying table). Once the diarrhea is categorized, further testing becomes more specific. An “empiric therapy” plan avoids determining a diagnosis and simply treats the symptoms. This is a reasonable approach, assuming serious causes for the diarrhea have been excluded. Patients must be monitored closely when this plan is followed.

Calculation of the fecal osmotic gap may be useful to differentiate chronic osmotic diarrhea and chronic secretory diarrhea when fatty and inflammatory bowel problems are excluded. In secretory diarrhea, water is held within the bowel lumen by incompletely absorbed electrolytes, whereas in osmotic diarrhea, electrolyte absorption is normal but water is held in the bowel lumen by some other osmotically active substance. This means that secretory diarrhea will have a high electrolyte content while osmotic diarrhea will have a low electrolyte content. Stool electrolyte quantitation is determined by doubling the total of the stool sodium and potassium concentrations and subtracting that amount from 290 mOsm per kg, which is the normal osmolality of bowel stool. This fecal osmotic gap clarifies the diagnosis: values of less than 50 mOsm per kg indicate a secretory diarrhea, and values of greater than 50 mOsm per kg indicate an osmotic diarrhea.

The author concludes that methods for managing diarrhea should depend on the specific presentation of each patient.