Irritable bowel syndrome is probably the most common cause of diarrhea in the United States. The disorder is thought to be functional in that no organic abnormality of the GI tract can be detected. Typical symptoms include diarrhea that is often watery, usually alternating with constipation, along with bloating and abdominal discomfort. A reasonable evaluation should include history, physical examination, complete blood count, and Hemoccult testing. In severe or atypical cases, evaluation of the colon, by use of diagnostic modalities such as flexible sigmoidoscopy, colonoscopy, barium enema, and possibly colorectal biopsies, may be indicated.
Laxative abuse is the most common cause of factitious diarrhea.
Melanosis coli, caused by anthraquinone laxative pigment in macrophages, has a distinct appearance on sigmoidoscopy, with speckled black mucosa, indicating past laxative use. X-ray and endoscopic studies are typically normal. These two disorders of unknown etiology are diagnosed only by biopsy, which underscores the importance of including colorectal biopsy in the evaluation of unexplained chronic watery diarrhea. When bile salt absorption is impaired, as in the ileitis of patients with Crohn's disease or those who have undergone ileectomy, unconjugated bile salts in the colon can cause a so-called choleretic diarrhea by stimulating secretion. Other patients are thought to have a primary disorder of bile acid absorption.