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Small Bowel· Chapter 12

Chronic Diarrhea and Malabsorption

Sort osmotic versus secretory versus fatty versus inflammatory by fecal osmotic gap, then route to bile-acid malabsorption types 1-2-3, microscopic colitis, dumping after Billroth, carcinoid (5-HIAA), and VIPoma (WDHA syndrome). The PERT trial when pancreatic insufficiency is on the differential.

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What this chapter covers

  • Section 12.1: Pathophysiologic classification

    Chronic diarrhea is a problem of too much stool water.

  • Section 12.2: Initial evaluation and stool studies

    Chronic diarrhea evaluation begins with a layered workup that does cheap noninvasive things first and reserves invasive testing for the patients who need it.

  • Section 12.3: Secretory diarrhea workup

    Secretory diarrhea is the bucket where the calculated osmotic gap is under 50 mOsm/kg, the volume runs high (often over 1 liter per day), the diarrhea persists with fasting, and nocturnal stools are common.

  • Section 12.4: Fatty and inflammatory diarrhea

    Fatty diarrhea is the steatorrhea phenotype: greasy, foul-smelling, oil-floating stools with weight loss, fat-soluble vitamin (ADEK) deficiency, and the laboratory findings of hypocalcemia, low 25-hydroxyvitamin D, prolonged INR from vitamin K malabsorption, and night blindness from vitamin A deficiency.

  • Section 12.5: Bile acid malabsorption framework

    Bile acid malabsorption is the syndrome in which excess bile acids enter the colon and stimulate water and electrolyte secretion through the bile-acid-sensitive G-protein-coupled receptor TGR5 (also called GPBAR1) on the colonic enterocyte.

  • Section 12.6: Microscopic colitis

    Microscopic colitis is the umbrella for lymphocytic colitis and collagenous colitis, two histologic entities that produce chronic watery non-bloody diarrhea, normal-appearing colonic mucosa at colonoscopy, and a diagnosis that lives entirely in the biopsy.

  • Section 12.7: Algorithm for obscure chronic diarrhea

    Obscure chronic diarrhea is the patient who has gone through the standard workup (history, medications, surgical history, basic labs with celiac serology and total IgA, calprotectin and elastase, infectious panel, stool osmolality and electrolytes, quantitative fecal fat, EGD with duodenal biopsies, colonoscopy with random biopsies including microscopic colitis sampling, and CT or MR enterography) without a diagnosis.

Podcast episodes

  1. 01

    Framework and Mechanism Workup

    This episode covers the framework and the mechanism-driven workup: sorting chronic diarrhea into osmotic, secretory, fatty, and inflammatory, and letting that mechanism drive the tests.

  2. 02

    BAM Microscopic Obscure

    This episode covers three of the most commonly missed causes and what to do when the standard workup runs out: bile acid malabsorption, microscopic colitis, and the algorithm for obscure chronic diarrhea.