Anatomy and Physiology

See Chapter 28 for anatomy and physiology of the bowel.

Pathophysiology

Large-volume diarrhea is caused by excessive quantities of water or secretions in the intestines. Small-volume diarrhea is caused by excessive intestinal motility.

Large-volume diarrhea can be caused by osmosis. A nonabsorbable substance (such as lactose or nonabsorbable sugar) in the gut causes fluid to be drawn into the lumen by osmosis. This condition also may be caused by excessive mucosal secretions. Bacterial toxins and neoplasms that produce hormones also stimulate secretions. Large-volume diarrhea also can be caused by excessive motility of the intestine. Conditions that affect autonomic nervous system control of digestion (e.g., diabetic neuropathy) increase transit time, thereby preventing adequate absorption of water and electrolytes from the feces.

Small-volume diarrhea usually is caused by an inflammatory condition that affects the gut mucosa.

Disease Process

Worldwide, diarrhea is a major cause of morbidity and mortality, especially in children. Diarrhea is the second most common illness among families in the United States, with an annual incidence of up to 63% per year; 4% to 20% of chronic diarrhea results from laxative abuse.

Medically, stools are classified as diarrhea if the patient has increased frequency, which usually is defined as more than two or three bowel movements per day, and the stools are liquid, not just “soft.” The emphasis in diagnosis is on consistency, not frequency.

Diarrhea is categorized as acute or chronic. By definition, acute diarrhea persists for less than 2 weeks—usually, a few days to 1 week. It can be subdivided into noninflammatory or inflammatory diarrhea. Noninflammatory diarrhea is watery and nonbloody and usually is caused by a bacterium or a virus that is self-limiting. Inflammatory diarrhea consists of WBCs in the stool; these reflect invasion of the organism or toxin into the wall of the intestine (Table 29-1).

Chronic diarrhea has an extensive number of etiologies (Table 29-2). Evaluate the patient carefully for the cause of the diarrhea, and treat the disease, not the symptom. Medications that can cause diarrhea include laxatives, antacids, magnesium-containing products, and antibiotics.

Acute diarrhea can be of infectious (i.e., bacteria, protozoa, or virus) or noninfectious origin (i.e., toxins, an inflammatory process, an ischemic process, or a mechanical process). Chronic diarrhea can be classified according to stool type as follows: (1) watery (secretory and osmotic)—no pus, blood, or fat; (2) fatty—fat; or (3) inflammatory—blood and/or pus. Causes of chronic diarrhea include medications, IBS, enteral feedings, malabsorption syndromes (e.g., celiac disease, fat malabsorption), and malnutrition.

Assessment

The guidelines put forth by the American Gastroenterological Association emphasize comprehensive evaluation of a patient before treatment is provided. Patient history is important for the diagnosis. Similar illness in contacts points to an infection. Ingestion of improperly prepared or stored food suggests infection or bacterial toxins. Exposure to impure water suggests parasites. Travel abroad exposes patients to infections that are characteristic of the local area. Antibiotic use points to Clostridium difficile. The critical laboratory test involves sending the stool for fecal leukocytes, routine stool culture, and C. difficile. Obtain ova and parasites if the patient has had diarrhea for longer than 10 days, has traveled to an endemic region, is experiencing a community water-borne outbreak, has human immunodeficiency virus (HIV), or is a homosexual male. If suspicion involves a specific infectious cause, focused stool testing can be ordered (e.g., Giardia antigen, Norwalk virus, cryptosporidium).

Opioids

Opioid antidiarrheal agents act on the smooth muscle of the intestinal tract to slow GI motility and propulsion. Slowed transit time of intestinal contents allows more fluid to be absorbed from the stool, thereby decreasing fecal volume, as well as fluid and electrolyte loss. Little or no analgesic activity is noted.

The opioids (except loperamide hydrochloride) may be habit forming. Atropine sulfate is added to some formulations to discourage abuse and deliberate overdose.

Adsorbents

Adsorbents act by reducing intestinal motility and by adsorbing fluid. In infectious diarrhea, they bind bacteria and toxins in the GI tract.