Chronic Diarrhea and Malabsorption

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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 has a 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

   Chronic Diarrhea and Malabsorption: First Workup

   Chronic diarrhea is loose stools for more than four weeks, and small absorptive failures cause big diarrhea. The episode starts with the history, especially the medication list including over-the-counter products and supplements, then the surgical history, the exam, and a rectal exam. It then moves through first blood tests, stool studies, and targeted imaging and endoscopy. The stool osmotic gap sorts diarrhea into osmotic, secretory, inflammatory, or fatty types. Secretory diarrhea is proven with a supervised fast and worked up from common to rare causes: microscopic colitis, bile acid diarrhea, drugs, infection, neuroendocrine tumors such as VIPoma, carcinoid, and gastrinoma, then laxative abuse and idiopathic disease.

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2. 02

   Chronic Diarrhea and Malabsorption: Bile and Fat

   The organizing idea is that chronic diarrhea needs its mechanism identified before it is labeled irritable bowel. The episode starts with bile acids in the colon, how a sequestrant trial confirms the diagnosis, and why a long ileal resection reverses the treatment. It then covers microscopic colitis: culprit drugs, biopsies from both sides of the colon, and staged therapy with budesonide. Next comes fatty stool, proven by fecal fat and split into pancreatic and mucosal causes by fecal elastase. Bloody diarrhea is named by the colonoscopy pattern. Last is the empty workup, with a repeat history, empiric trials in order of likelihood, targeted tests, referral, and only then the irritable bowel label.

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Key topics

• ·Definition and mechanisms of chronic diarrhea
• ·History and medication culprits
• ·Surgical history and exam clues
• ·First labs and stool studies
• ·Imaging and endoscopy
• ·Stool osmotic gap and osmotic diarrhea
• ·Proving secretory diarrhea with a fast
• ·Common causes of secretory diarrhea
• ·Neuroendocrine tumors and VIPoma
• ·Factitious and idiopathic secretory diarrhea
• ·Bile acid physiology and bile acid diarrhea
• ·Diagnosing without the SeHCAT scan
• ·Sequestrant dosing and long ileal resection
• ·Microscopic colitis risk factors and biopsy
• ·Staged treatment with budesonide
• ·Celiac disease and microscopic colitis overlap
• ·Steatorrhea testing and fecal elastase
• ·Pancreatic enzyme dosing
• ·Mucosal causes of fat malabsorption
• ·Colonoscopy patterns in bloody diarrhea
• ·Obscure diarrhea and empiric trials
• ·When irritable bowel applies