Diarrhea is described as three or more loose or watery stools a day. Infection commonly causes acute diarrhea. Noninfectious etiologies are more common as the duration of diarrhea becomes chronic. Treatment and management are based on the duration and specific etiology. Rehydration therapy is an important aspect of the management of any patient with diarrhea. Prevention of infectious diarrhea includes proper handwashing to prevent the spread of infection.
Diarrhea is categorized into acute or chronic and infectious or non-infectious based on the duration and type of symptoms. Acute diarrhea is defined as an episode lasting less than 2 weeks. An infection most commonly causes acute diarrhea. Most cases are the result of a viral infection, and the course is self-limited. Chronic diarrhea is defined as a duration lasting longer than 4 weeks and tends to be non-infectious. Common causes include malabsorption, inflammatory bowel disease, and medication side effects.
Norovirus is associated with approximately one-fifth of all infectious diarrhea cases, with similar prevalence in both children and adults, and is estimated to cause over 200,000 deaths annually in developing countries.  Historically, rotavirus was the most common cause of severe disease in young children globally. Rotavirus vaccination programs have decreased the prevalence of diarrhea cases associated with rotavirus.
A common cause of chronic diarrhea includes inflammatory bowel disease, Crohn disease, and ulcerative colitis. In Europe, the incidence of ulcerative colitis and Crohn disease has increased overall from 6.0 per 100,000 person-years in ulcerative colitis and 1.0 per 100,000 person-years in Crohn disease in 1962 to 9.8 per 100,000 person-years and 6.3 per 100,000 person-years in 2010, respectively. 
Diarrhea is the result of reduced water absorption by the bowel or increased water secretion. A majority of acute diarrheal cases are due to infectious etiology. Chronic diarrhea is commonly categorized into three groups; watery, fatty (malabsorption), or infectious.
Lactose intolerance is a type of watery diarrhea that causes increased water secretion into the intestinal lumen.  Patients typically have symptoms of bloating and flatulence along with watery diarrhea. Lactose is broken down in the intestine by the enzyme lactase. The byproducts are readily absorbed by the epithelial cells. When lactase is decreased or absent, lactose cannot be absorbed, and it remains in the gut lumen. Lactose is osmotically-active, and it retains and attracts water leading to watery diarrhea.
Common causes of fatty diarrhea include celiac disease and chronic pancreatitis. The pancreas releases enzymes that are necessary for the breakdown of food. Enzymes are released from the pancreas and aid in the digestion of fats, carbohydrates, and proteins. Once broken down, the products are available for uptake in the gut. Patients with chronic pancreatitis have insufficient enzyme release leading to malabsorption. Symptoms often include upper abdominal pain, flatulence, and foul-smelling, bulky pale stools due to malabsorption of fats.
In bacterial and viral diarrhea, the watery stool is the result of injury to the gut epithelium. Epithelial cells line the intestinal tract and facilitate the absorption of water, electrolytes and other solutes. Infectious etiologies cause damage to the epithelial cells which leads to increased intestinal permeability. The damaged epithelial cells are unable to absorb water from the intestinal lumen leading to loose stool.
History should include the duration of symptoms, accompanying symptoms, travel history, and exposures to medications and food. It is important to ask about the stool frequency, type, volume, and presence of blood or mucus. Patients with diarrhea may also complain of abdominal pain or cramping, vomit, bloating, flatulence, fever, and bloody or mucoid stools. Important aspects of the physical exam include the patient’s vital signs, volume status, and abdominal exam. Dry mucous membranes, poor skin turgor, and delayed capillary refill are signs of dehydration. A thorough history and physical exam are important to determine the proper diagnostic workup.
Typically, a patient with acute diarrhea will have a self-limited course and will not require labs or imaging. A stool culture is warranted in a patient with bloody diarrhea or severe illness to rule out bacterial causes. Bloody stools require additional testing for Shiga toxin and lactoferrin. A patient with recent antibiotic use or hospitalization will require testing for Clostridium difficile infection. Imaging is not ordered routinely in a patient with acute diarrhea. However, an abdominal CT may be required when a patient presents with significant peritoneal signs.
A thorough history is important to determine what labs and imaging need to be ordered to distinguish the cause of chronic diarrhea.  Basic lab work for a patient with chronic diarrhea includes a complete blood count, basic metabolic panel, stimulating thyroid hormone, erythrocyte sedimentation rate, liver panel, and a stool analysis. The physician should categorize the type of chronic diarrhea as either watery, fatty, or inflammatory based on the patient’s history and physical exam. Once a probable diagnosis is determined, additional labs and testing specific to the suspected etiology should be ordered.
An important aspect of diarrhea management is replenishing fluid and electrolyte loss.  Patients should be encouraged to drink diluted fruit juice, Pedialyte or Gatorade. In more severe cases of diarrhea, IV fluid rehydration may become necessary. Eating foods that are lower in fiber may aid in making stool firmer. A bland 'BRAT' diet including bananas, toast, oatmeal, white rice, applesauce and soup/broth is well tolerated and may improve symptoms.  Anti-diarrheal therapy with anti-secretory or anti-motility agents may be started to reduce the frequency of stools. However, they should be avoided in adults with bloody diarrhea or high fever because they can worsen severe intestinal infections. Empiric antibiotic therapy with an oral fluoroquinolone can be considered in patients with more severe symptoms. Probiotic supplementation has been shown to reduce the severity and duration of symptoms and should be encouraged in patients with acute diarrhea.
The treatment of chronic diarrhea is specific to the etiology.  The first step is to categorize diarrhea into watery, fatty or inflammatory. Once categorized, an algorithm can be used to determine the next step in management. Most cases require additional fecal studies, lab work or imaging. More invasive procedures like colonoscopy or upper endoscopy may be required.
Proper handwashing can prevent the spread of infectious diarrhea. Patients with infectious diarrhea should not return to work, school, or daycare until their symptoms have resolved. Professionals should encourage parents to vaccinate their children against rotavirus, a common etiology of viral diarrhea. Probiotic therapy can be considered in patients taking antibiotics to prevent C. difficile colitis. 
To decrease the chance of traveler’s diarrhea, encourage patients to drink bottled water, avoid raw fruits and vegetables, and only eat hot, well-cooked foods when they are traveling to developing countries. Bottled water should be used even when brushing teeth. Prophylactic antibiotics for traveler’s diarrhea are usually not recommended. Antibiotics can be considered in individuals with underlying medical diseases who may be affected more significantly by diarrhea. 
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