Diarrhea is defined as three or more watery stools per day. There are four basic mechanisms: increased intestinal secretion (e.g., cholera), decreased intestinal absorption (e.g., enterotoxins, inflammation, or ischemia), increased osmotic load (e.g., laxatives, lactose intolerance), and abnormal intestinal motility (e.g., irritable bowel syndrome). Most cases are infectious in etiology.
Determine if the diarrhea is acute (<3 weeks duration) or chronic (>3 weeks duration). Acute diarrhea is more likely to represent a serious problem, such as infection, ischemia, intoxication, or inflammation. Inquire about associated symptoms. Features such as fever, pain, presence of blood, or type of food ingested may help in the diagnosis of infectious gastroenteritis, food poisoning, diverticulitis, or inflammatory bowel disease. Neurological symptoms can be seen in certain diarrheal illnesses, such as seizure with shigellosis or hyponatremia, or paresthesias and reverse temperature sensation with ciguatoxin.
Details about the host can also better define the diagnosis. Malabsorption from pancreatic insufficiency or HIV-related bowel disorders need not be considered in a healthy host. Dietary practices, including frequent restaurant meals, exposure to day care centers, consumption of street vendor food or raw seafood, overseas travel, and camping with the ingestion of lake or stream water, may isolate the vector and narrow the differential diagnosis for infectious diarrhea (e.g., lakes or streams—Giardia; oysters suggest Vibrio; rice suggests Bacillus cereus; eggs suggest Salmonella; and meat suggests Campylobacter, Staphylococcus, Yersinia, Escherichia coli, or Clostridium). Certain medications, particularly antibiotics, colchicine, lithium, and laxatives, can all contribute to diarrhea. Travel may predispose the patient to enterotoxigenic E. coli or Giardia. Social history, such as sexual preference, drug use, and occupation, may suggest diagnosis such as HIV-related illness or organophosphate poisoning.
The physical examination begins with assessment of hydration status. Abdominal examination can narrow the differential diagnosis and reveal the need for surgical intervention. Even appendicitis can present with diarrhea in up to 20% of cases. Rectal examination can rule out impaction or presence of blood, the latter suggesting inflammation, infection, or mesenteric ischemia.
Diagnosis and Differential
The most specific tests in diarrheal illness all involve examination of the stool in the laboratory. Stool culture testing should be limited to severely dehydrated or toxic patients, those with blood or pus in their stool, immunocompromised patients, and those with diarrhea lasting longer than 3 days. Consider testing for Salmonella, Shigella, Campylobacter, Shiga toxin-producing E. coli, or amoebic infection. Make the laboratory aware of which pathogens you suspect. In patients with diarrhea >7 days, those who have traveled abroad, or consumed untreated water, an examination for ova and parasites may be useful to rule out Giardia or Cryptosporidium. Multiple samples may be required. Assay for Clostridium difficile toxin may be useful in ill patients with antibiotic-associated diarrhea or recent hospitalization.