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Diarrhea is defined as a change in bowel movement pattern resulting in an increase in stool volume and/or frequency, usually with loose to watery stool consistency. Acute and chronic diarrhea are defined based on duration of symptoms. Acute diarrhea lasts <14 days, whereas chronic diarrhea persists for longer. In general, to be considered diarrhea, three or more stools are passed per day. Patients with chronic diarrhea may have periods of loose or frequent stools with normal bowel movements in between episodes. Diarrhea can also be defined based on stool volume; however, measuring this accurately is often difficult. Normal stool volume is about 5–10 g of stool/kg body weight/day for infants and about 100–200 g of stool/day in children and adults. A 24-hour stool volume of >10 g/kg in infants and >200 g in children and adults is considered diarrhea. Acute diarrhea is common. It is most often secondary to viral infections. Other causes of acute diarrhea include toxin-induced diarrhea and antibiotic-associated diarrhea. Infectious diarrhea is most often found in children under age 5 years with a rate of about one to three episodes per year. Causes of chronic diarrhea are more diverse and range from functional disorders such as Toddler’s diarrhea and irritable bowel syndrome (IBS) to disorders that may impact overall health such as inflammatory bowel disease or celiac disease.

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Diarrhea occurs due to a derangement in small bowel, colonic, or pancreatic function. Besides the classification of acute and chronic, diarrhea may be divided further by pathophysiologic mechanism (Figure 4–1; Table 4–1).1 The small bowel both secretes and absorbs water and electrolytes, as well as absorbs nutrients. Imbalance between secretion and absorption can lead to diarrhea. The primary function of the colon is to absorb fluid and electrolytes as well as storage of its contents until it can be expelled. The colon absorbs fluid and electrolytes. Disruption of the colon’s function leads to frequent, loose, and occasionally bloody stools. The motor function of the intestines regulates forward propulsion of its contents. Abnormalities in this motility may also lead to diarrhea.

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FIGURE 4–1
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(A) Normal intestine. Overall effect is absorption of fluid. (B) Osmotic diarrhea—osmotic agents draw fluid into the intestinal lumen. (C) Secretory diarrhea—increased fluid into the intestinal lumen, usually secondary to opening of channels by toxins or hormones. (D) Motility—increased rate of transit reduces absorption. (E) Inflammatory—decreased absorption of water, electrolytes, and nutrients.

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Table Graphic Jump Location
Table 4–1. Categories of Diarrhea
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Osmotic diarrhea (Figure 4–1b; Table 4–1) occurs when increased solute load in the bowel lumen leads to decreased fluid absorption and increased fluid secretion.2 Stools are loose and often foul-smelling. Bloating, abdominal distention, and gas are also characteristic. Carbohydrate malabsorption, such as from lactose intolerance, is ...

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