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.

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.

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 an example of osmotic diarrhea. Another example occurs with ingestion of poorly absorbed solutes, including non-digestible sugars such as lactulose or sorbitol, or osmotic laxatives such as polyethylene glycol and milk of magnesia. Fat malabsorption from pancreatic insufficiency or bile acid depletion can also lead to osmotic diarrhea. When poorly absorbed fats and carbohydrates reach the colon, they are hydrolyzed by colonic bacteria, producing short-chain fatty acids that further increase the solute load. Injury to the small intestine mucosa results in decreased absorptive surface area and loss of brush border digestive enzymes, causing generalized malabsorption. Examples include decreased gut length from surgical excision and decreased absorptive area secondary to severe celiac or Crohn’s disease.

Osmotic diarrhea subsides with fasting, which eliminates the osmotic load. This can be a useful diagnostic test for malabsorptive diarrhea in the hospitalized patient receiving intravenous fluids. Stool studies are also helpful, revealing an acidic pH due to fermentation, with increased osmotic gap. The osmotic gap is calculated after measuring stool sodium and potassium using the following formula:

290 – 2(Na+ + K+)

Because of the relatively free movement of water across the intestinal mucosa, all stools can be assumed to have an osmolarity equivalent to serum (290). The sum of sodium and potassium must be multiplied by 2 to account for their accompanying anions. An osmotic gap >50 is consistent with the presence of other unmeasured solutes in the stool, indicating that the diarrhea is osmotic in nature.

Secretory diarrhea (Figure 4–1c; Table 4–1) is caused by derangements in mechanisms regulating fluid and electrolyte movement in the small bowel mucosa, leading to active secretion of electrolytes, with passive loss of water.3 This is characterized by chloride and bicarbonate secretion and inhibition of sodium and chloride absorption. Disorders caused by bacterial toxins, such as cholera (Vibrio cholerae) and traveler’s diarrhea (enterotoxigenic Escherichia coli), are classic examples. Secretory diarrhea can also result from secretagogues produced by tumors; Zollinger–Ellison syndrome (gastrin) and vasoactive intestinal peptide (VIP)-secreting tumors, or VIPomas, are rare examples of this in children. Regardless of cause, secretory diarrhea is characterized by high-volume, frequent watery stools, which persist despite fasting. Stool osmolality is similar to plasma osmolality, with no osmotic gap present.2

Inflammatory diarrhea, typically seen with bacterial infection and inflammatory bowel disease (Figure 4–1e; Table 4–1), is caused by disruptions in the epithelial function of the bowel.3 Bacterial toxins can alter ion transport, leading to increased secretion. In addition, some bacteria can invade the mucosa, directly disrupting function. Injury to the epithelial barrier, including the tight junctions, alters electrochemical gradients. Activated macrophages secrete cytokines such as tumor necrosis factor and interleukins such as IL-2 that increase intestinal permeability. Inflammation-associated diarrhea is aggravated by the release of various secretagogues, including cytokines, prostaglandins, histamine, and leukotrienes. Brush border and enterocyte injury (or death) from inflammation also contributes to diarrhea. In inflammatory diarrhea, the intestinal mucosa is thickened, hyperemic, and edematous. Stools may contain red or white blood cells. There is frequently loss of serum proteins into the stool. A component of osmotic diarrhea may also be present in inflammatory diarrhea secondary to malabsorption.

Motility disorders can cause diarrhea (Figure 4–1d; Table 4–1). Increased transit time decreases the ability of the colon to absorb fluid.4 Increased motility may be secondary to increased secretion of serotonin, histamine, or other mediators. IBS and hyperthyroidism are examples of diarrhea secondary to increased motility. Decreased motility causes stasis, which leads to small bowel bacterial overgrowth and inflammatory diarrhea.

The above categorizations of diarrhea are helpful in narrowing down cause. However, it is important to keep in mind that many causes of diarrhea will have one or more characteristics. For example, patients with Crohn’s disease often have osmotic diarrhea secondary to malabsorption, but will also have a secretory component secondary to inflammation. In this situation, fasting results in diminished stool output without complete resolution of diarrhea.

Children with acute diarrhea may present to their health care provider because of frequent passage of stool, abdominal pain, poor oral intake, fever, irritability, or dehydration. The clinical history should include the onset, duration, nature, and severity of symptoms. Assessment of hydration status is the most important aspect of the physical exam.

Patients with chronic diarrhea will present to their provider for concerns of ongoing frequent stools, abdominal pain, blood in stool, bloating and gassiness, or growth failure. History should include onset and duration of symptoms including if symptoms are intermittent. Stool frequency, consistency, and visual presence of blood should be determined. Any modifying factors such as association with diet or stress should also be elicited. Nocturnal defecation is concerning for colonic inflammation and makes functional diarrhea (IBS or toddler’s diarrhea) less likely. The presence of abdominal pain including timing, severity, and location can be an important clue to diagnosis. Extraintestinal manifestations such as a skin rash or joint pain that can be associated with celiac disease or inflammatory bowel disease should be inquired about. The past medical history may reveal a history of bowel surgery that may raise suspicion for malabsorption due to short bowel syndrome. Surgery may also lead to intestinal strictures, adhesions, or loss of the ileocecal valve, any of which may predispose to small bowel bacterial overgrowth. Medication and supplement history should be obtained. Some medications (orlistat) cause malabsorptive diarrhea, and laxative use must be ruled out. Ingestion of non-absorbable sugars such as “sugar-free” candy or gum or excessive juice intake can precipitate abdominal cramping, gas, and diarrhea. Family history can also be helpful, particularly history of inflammatory bowel disease or celiac disease in a first- or second-degree relative. Social history should include recent travel and exposures such as animal or sick contacts. Attendance at daycare increases the risk for infectious diarrhea.

Physical exam should begin with evaluation of growth parameters. Patients with lactose intolerance and IBS usually do not have weight loss or decrease in growth percentiles. However, patients with celiac disease, Crohn’s disease, or pancreatic insufficiency may experience growth failure. Decreased subcutaneous tissue and loose skin are also signs of weight loss and possible malabsorptive disorders. Thyroid mass or fullness with or without proptosis, along with brisk reflexes and tachycardia, is suggestive of hyperthyroidism. Evaluate for abdominal distention, location of tenderness, and fullness. Tenderness and fullness in the right lower quadrant is concerning for Crohn’s disease. Digital clubbing may be a sign of chronic disease such as Crohn’s disease, celiac disease, or cystic fibrosis. Edema is suggestive of a protein losing enteropathy that may be secondary to inflammatory bowel disease or lymphangiectasia. Perianal exam may reveal skin tags, fissures, or fistulas that are suggestive of Crohn’s disease. Skin exam may reveal dermatitis herpetiformis (celiac disease), erythema nodosum (IBD), or pyoderma gangrenosum (IBD). Perianal rash may be present secondary to prolonged diarrhea, but also may be suggestive of carbohydrate malabsorption as stool often is acidic. A combination of perianal and perioral rash may be a sign of acrodermatitis enteropathica. Additionally, signs of malnutrition should be noted such as sparse, brittle hair, cheilosis, or smooth tongue.

Acute Diarrhea

The most common causes of acute diarrhea are infectious agents (Table 4–2). Rotavirus is the most common cause of severe acute diarrhea and accounts for about 40% of hospitalizations due to diarrhea in children under age 5 years.5 It is spread from person to person by the fecal–oral route and is most common during the winter months. The illness usually begins with fever and vomiting followed by frequent, loose, foul-smelling diarrhea that can last as long as 10 days. Malnourished patients or those with immunodeficiency can develop chronic diarrhea. Dehydration is common. Rotavirus can be detected by antigen or PCR testing of the stool.6 An oral vaccine for the most common serotypes is now available and has been found to decrease hospitalizations and emergency room visits for acute diarrhea by 95%.7

Calciviruses (Norwalk virus and norovirus) are responsible for outbreaks of gastrointestinal illnesses, spread by the fecal–oral route, often via contaminated food or water. Outbreaks occur in confined environments such as cruise ships or daycare centers. The illness usually lasts 2–3 days. Symptoms include fever, abdominal cramping, diarrhea, and vomiting.6 Astrovirus is another common cause of viral gastroenteritis that is transmitted via the fecal–oral route. Astroviral diarrhea tends to be less severe than rotavirus or norovirus.6 Hepatitis A can present with acute diarrhea, with or without jaundice. It is often a foodborne illness, but can also be transmitted via close contacts.8

Bacterial causes of diarrhea may present similarly to viral-induced diarrhea, but invasive or toxigenic bacteria may cause bloody stools, and more prominent fever and abdominal pain. Abdominal pain can be quite severe and may be mistaken for appendicitis. Salmonella and Campylobacter are the most frequently isolated bacterial causes of diarrhea.

Campylobacter species can be found in contaminated poultry or other farm animals. It can precipitate immunoreactive complications such as Guillain–Barre’ syndrome, reactive arthritis, or Reiter’s syndrome.9

Salmonella species can be contracted from animals such as poultry and reptiles who are carriers. It is also associated with the ingestion of poultry, eggs, and dairy; however, outbreaks from other contaminated foods such as peanuts and produce have occurred. It is most commonly seen during the summer and fall.9 Prolonged excretion of Salmonella in the stool can occur. Invasive infections such as osteomyelitis and meningitis can occur in young infants or immunocompromised patients.8

Patients infected with Shigella often have high fevers in addition to abdominal pain and diarrhea that may or may not be bloody. Only a small number of organisms are required to initiate infection that is most often spread from person to person but may also be contracted from contaminated food or water.8 Infection with Shigella can be complicated by hemolytic uremic syndrome, Reiter’s syndrome, or toxic megacolon.9

Yersinia enterocolitica infectious diarrhea is less common than other bacterial causes of diarrhea. It is transmitted by contaminated food, especially pork products such as chitterlings (pork intestines).8 It mimics appendicitis in about 40% of patients. Yersinia can lead to a migratory arthritis, Reiter’s syndrome, and erythema nodosum.9

Contaminated food or water is the source of E. coli. There are five subtypes of E. coli. Shiga toxin-producing E. coli is the most common cause of bloody diarrhea. Fever is either not present or minimal, and patients often present with severe abdominal pain and high white blood cell count. It has been associated with hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. It can occur in outbreaks usually associated with contaminated food (undercooked beef or unpasteurized apple cider) or animal exposures (such as petting zoos). Enterotoxigenic E. coli is a cause of “traveler’s diarrhea” and causes 1–5 days of loose stools and abdominal cramping. Enteropathogenic, enteroinvasive E. coli and enteroaggregative E. coli are seen mostly in undeveloped countries. Enteropathogenic E. coli causes watery diarrhea. Enteroinvasive E. coli causes dysentery and enteroaggregative E. coli is usually watery but may contain blood.9

Foodborne bacterial toxins are another cause of acute diarrhea. Staphyloccocus species cause a foodborne illness that is characterized by an abrupt onset of vomiting, abdominal pain, and diarrhea. It lasts 1–2 days and is usually caused by foods that are not kept warm or cold enough to inhibit staphylococcal growth. Illness usually occurs within 2–4 hours after ingestion. Bacillus cereus produces several toxins. Emetogenic toxins produce vomiting and abdominal pain; enterotoxins result in diarrhea in about one-third of patients. Symptoms depend on the toxin types being produced and usually last <24 hours.8