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Mr. C is a 35-year-old man who comes to your outpatient office complaining of 3 days of diarrhea.

image What is the differential diagnosis of diarrhea? How would you frame the differential?


Figure 13-1.

Diagnostic approach to diarrhea.

A first step in assessing diarrhea is to categorize the frequency and duration of bowel movements. Diarrhea is defined as at least 3 loose bowel movements per day. Often patients and clinicians will mislabel loose bowel movements as diarrhea solely because the stool is loose. If a patient does fit the clinical criteria for diarrhea, the diarrhea can be further characterized as acute, persistent, or chronic. Acute diarrhea has a duration of less than 14 days, persistent diarrhea 14–29 days, and chronic diarrhea at least 30 days. The differential diagnosis varies significantly based on the duration of symptoms. This chapter focuses specifically on diagnosis and treatment of acute diarrhea.

Acute diarrhea can be further subcategorized into infectious and noninfectious causes, with infectious causes accounting for the majority of cases of acute, self-limited diarrheal illnesses.

Several factors are important in reasoning through a case of acute diarrhea including (1) associated signs and symptoms, (2) duration of symptoms, (3) exposure history, (4) history of immunosuppressive conditions, and (5) epidemiology of specific infectious pathogens.

A detailed history of symptoms and exposures helps guide the evaluation and treatment of acute diarrhea. Noninfectious diarrhea is typically characterized by lack of constitutional symptoms and presence of a culprit medication, supplement, food, or other condition (eg, pancreatic insufficiency). Therefore, taking a thorough history of medications, supplements, diet, and underlying medical conditions is essential to prioritize an initial differential diagnosis for acute diarrhea.

Infectious diarrhea can be associated with symptoms such as fever, myalgias, severe abdominal cramping, nausea, vomiting, and stool with blood or mucus. However, lack of any of these symptoms does not rule out a diagnosis of infectious diarrhea. Symptoms of infectious diarrhea may also suggest whether the diarrhea originates in the large or small bowel, which can narrow the likelihood of specific pathogens. Patients with large volume, watery bowel movements, nausea, vomiting, abdominal cramping, bloating, and gas are more likely to have gastroenteritis and the pathogens most commonly associated with this condition. Mucus or blood in the stool is rarely present in gastroenteritis. In contrast, patients with tenesmus and passage of many small-volume stools with blood and/or mucus are more likely to have colitis and the pathogens more commonly associated with this condition.

Figure 13-1 presents a clinical algorithm and a broad differential diagnosis using this framework.

  1. Noninfectious diarrhea

    1. Medications and other ingestible substances (some with osmotic effect)

      1. Sorbitol (gum, mints, pill fillers)

      2. Mannitol

      3. Fructose (fruits, soft drinks)

      4. Fiber (bran, fruits, vegetables)

      5. Lactulose

    2. Magnesium-containing ...

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