Pathophysiology

A total of 8 to 9 L of fluid enters the healthy intestines on a daily basis. Only 1 to 2 L are derived from food and liquid intake; the rest is from salivary, gastric, pancreatic, biliary, and intestinal secretions. Each day, about 90% of this fluid is absorbed in the small intestine, 1 L enters the colon, and about 100 mL is excreted in stool. Normal stool output is approximately 100 to 200 g/d. Diarrhea is defined as stool output greater than 200 g/d in children older than 2 years of age and greater than 10 mL/kg/d in children younger than 2 years of age. It is also described more practically as an increase in liquidity and frequency of bowel movements. Diarrhea can be categorized by duration, as either acute (≤2 weeks) or chronic (>2 weeks), or by mechanism, as osmotic or secretory. It can also be categorized by the presence or absence of malabsorption (Figure 111-1).

Figure 111-1 Diarrhea.

Both secretory and osmotic diarrhea are caused by defective or impaired mucosal absorption. In osmotic diarrhea, excess amounts of nonabsorbed substances, such as lactose, lactulose, fructose, or sorbitol, remain in the intestinal lumen, causing luminal water retention. After these luminal substances enter the colon, they are processed by colonic flora, producing large amounts of organic acids, increased flatulence, and faster transit. The fecal osmolar gap [290 mOsm/L − {2 × (measured stool sodium + measured stool potassium)}] is usually greater than 50 mOsm/L in the setting of osmotic diarrhea. When an abnormal gap is found, reducing substances, stool pH, and fecal fat should be measured. Osmotic diarrhea improves with fasting. Examples of osmotic diarrhea include lactase deficiency, celiac disease, and short bowel syndrome. Secretory diarrhea is the result of abnormal ion transport in epithelial cells, leading to decreased absorption of electrolytes and increased secretion of fluid. The fecal osmolar gap is less than 50 mOsm/L, and the diarrhea persists despite fasting. Examples include congenital chloride and sodium diarrhea, cholera, and neuroendocrine tumors.

Another important underlying mechanism of diarrhea is dysmotility. For example, pseudo-obstruction may result in bacterial stasis, overgrowth and resultant diarrhea, while hyperthyroidism may be associated with diarrhea because of rapid intestinal transit.

The character of the stool can help to determine the origin of diarrhea. Watery, voluminous, nonbloody stool with few or no white blood cells (WBCs) and low pH (<5.5) is likely to emanate from disease of the small intestine. Low-volume, mucusy, often bloody diarrhea with a large number of WBCs and higher pH often originates from the colon. The most common electrolyte abnormalities related to diarrhea include hypokalemic metabolic acidosis caused by bicarbonate and potassium losses in stool.

Bloody diarrhea is a concerning symptom. The most common cause is infection, especially in a setting of fever and acute onset. If bloody diarrhea is progressive and persistent, chronic inflammatory causes should be considered. The age of the patient is also important. In infants, milk protein–induced enterocolitis is a common cause of bloody stools.

Etiology And Pathogenesis

The most common cause of acute diarrhea is infection (see Chapter 96). In young children, this is most often viral, with the most common agents being rotavirus, adenovirus, astrovirus, and norovirus. Norovirus causes 60% to 90% of nonbacterial gastroenteritis in the United States, affecting 23 million Americans each year. Rotavirus is a leading cause of death in children younger than 5 years of age worldwide. In immunocompromised hosts, viruses, including cytomegalovirus, Epstein-Barr virus, and BK virus, should be considered. It is estimated that 70% of infectious diarrhea is foodborne, and thus a detailed history of exposures is very important (Table 111-1). Exposure to untreated water may cause giardiasis. Use of public swimming pools poses a risk of Shigella, Giardia, Cryptosporidium, and Entamoeba infection, with the last three being chlorine resistant. Home pets can transmit infections. For example, turtles carry Salmonella spp. History of foreign travel may narrow exposures based on the specific destination. The most common etiology of traveler’s diarrhea remains enterotoxigenic Escherichia coli. Cryptosporidium and Giardia spp. are responsible for most parasitic infections in developed countries. Cyclospora outbreaks have occurred in the United States. Clostridium difficile infection, previously thought to affect only hospitalized patients or those taking antibiotics, is now responsible for 40% of community-acquired diarrhea. A recent increase in C. difficile infections has been observed, some attributable to the resistant strain, BI/NAP1. An overgrowth of toxin-producing Clostridium organisms causes pseudomembranous colitis, which may be a potentially life-threatening condition. Vibrio cholerae remains a cause of illness and death in war zones and developing countries. The mechanism of infectious diarrhea is primarily secretory. It can quickly lead to electrolyte abnormalities and acidosis. Infection may result in villous atrophy, which can add an osmotic component. Mucosal healing after infection may lead to transient postinfectious diarrhea.

Table 111-1 Foodborne Infectious Agents

Several other causes of acute diarrhea, particularly in afebrile children, may be particularly concerning. Intussusception, a telescoping of two segments of bowel that occurs mostly in children between 6 months and 2 years of age, may present with bloody diarrhea (see Chapter 109). The typical presentation is colicky abdominal pain, vomiting, and an abdominal mass. “Currant jelly” stools do not occur in all patients with intussusception but are pathognomonic for the condition. Hemolytic-uremic syndrome (HUS) is an uncommon but potentially fatal illness that may present with acute bloody diarrhea. HUS begins as a mild gastroenteritis that evolves into hematochezia, microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure (see Chapter 64). Less commonly, appendicitis may present with abdominal pain and diarrhea as a result of colonic irritation from the inflamed appendix (see Chapter 5).

Other acute causes of diarrhea include inflammatory bowel disease (IBD; see Chapter 110), overfeeding (caused by increased osmotic loads), antibiotic-associated diarrhea (likely caused by changes in bowel flora), extraintestinal infections (otitis media, urinary tract infection, pneumonia), and toxic ingestions.

Clinical Presentation

In any patient presenting with acute diarrhea, a thorough history and physical examination should guide the immediate and subsequent evaluation and therapy. It is important to quantify the duration and frequency of stooling in addition to emesis, liquid intake, and urine output to assess for hydration status. A travel history should be obtained. Recent antibiotic use may suggest pseudomembranous colitis with C. difficile