Diarrhea is defined as an increase in the liquidity and/or frequency of the stools and can be a primary feature of both acute and chronic conditions. It reflects an increase in stool water content due to impaired water absorption and/or active water secretion by the intestine. Although the advent of oral rehydration therapy (ORT) has dramatically reduced the mortality rates in the past 50 years, diarrhea remains a major cause of morbidity and mortality in children, particularly among those in developing countries.
PATHOGENESIS AND EPIDEMIOLOGY
Most episodes of diarrhea occur secondary to 1 of 5 types of mechanisms: malabsorptive, secretory, osmotic, dysmotility, and inflammatory. Malabsorption is due to a decrease in absorptive surface area, as occurs after intestinal resection (short bowel syndrome) or with intestinal villous atrophy, as seen in celiac disease. Secretory diarrhea is caused by secretagogues such as bacterial toxins (eg, cholera), gut regulatory peptides (eg, vasoactive intestinal polypeptide), short-chain fatty acids, and bile salts, which can induce intestinal water secretion while inhibiting absorption. Secretory diarrhea characteristically persists even when the patient is in a fasting state. Osmotic diarrhea results from the intraluminal presence of malabsorbed solutes, such as lactose, which exert significant osmotic pressure that results in secretion of water into the intestines. In contrast to secretory diarrhea, osmotic diarrhea characteristically decreases or stops completely during fasting. Dysmotility can lead to increased peristalsis, causing diarrhea due to rapid transit, or to decreased peristalsis, causing diarrhea due to bacterial overgrowth. Inflammatory disorders cause diarrhea by several mechanisms including release of prosecretory eicosanoids and cytokines; altered tight junction function, decreasing the mucosal absorptive capacity or the capacity to reabsorb bile acids; and/or disturbances in motility. One or more of these mechanisms may be operative in an individual during an episode of diarrhea.
Acute diarrhea (lasting < 1 week in duration) accounts for 2 to 3 million deaths per year, with most deaths occurring in young children in developing countries. In the United States, it is estimated that 220,000 children under 5 years of age are hospitalized each year for acute diarrheal illnesses, which account for 9% of all hospitalizations in this age group. The common causes of acute diarrhea are listed in Table 380-1. Enteric infections account for most cases of acute diarrhea, with viruses being responsible for the majority (60–80%) in children, especially in those under the age of 2 years. Of these, rotaviruses were previously the most prevalent cause in the United States, but noroviruses have supplanted rotavirus as the most common cause of medically attended diarrhea in children in the era of rotaviral vaccination. Worldwide rotavirus remains the most common cause of moderate to severe diarrhea. This is due to incomplete penetration of the vaccine and diminished vaccine efficacy in children in developing countries. In the Northern hemisphere, rotavirus occurs with a peak incidence in the winter. Other winter-predominant viral agents, ...