Chronic Diarrhea and Malabsorption: BAM Microscopic Obscure

0:00Welcome to Board Pearls. This is episode two of two of the Chronic Diarrhea and Malabsorption chapter, in the Stomach and Small Bowel Disorders module. This episode is 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. The patient who arrives after a clean standard workup isn't random, because a predictable handful of diagnoses account for most of what the first pass missed, and the same three keep recurring on the boards because they recur in practice.

0:34Bile acid malabsorption starts with where the bile acids should have gone. Normally, conjugated bile acids secreted into the duodenum solubilize fat in the upper small bowel and are reabsorbed in the terminal ileum through a specific transporter, returning to the liver, with most reabsorbed each cycle and a small fraction lost in stool. When that reabsorption fails, bile acids spill into the colon, and above a threshold concentration the dihydroxy bile acids act on a colonocyte receptor to inhibit electrolyte absorption and drive active chloride and water secretion. The phenotype is urgent watery diarrhea, often nocturnal, often with incontinence, characteristically relieved within a week or two of starting a bile acid sequestrant.

1:18The Camilleri classification organizes it into four types, and matching the scenario to the type is rewarded. Type one is ileal disease or resection, Crohn's ileitis, ileocecal resection, or radiation, all stripping out the reabsorbing enterocytes, and the hundred-centimeter rule lives here and has to be reasoned. Under a hundred centimeters of resected ileum, the residual ileum is overwhelmed by spillover but the liver can still make enough bile acid to keep the small-bowel concentration above the micellar threshold, so fat absorption is preserved, the colon sees the excess, and you get bile acid diarrhea that cholestyramine fixes. Over a hundred centimeters, the liver can't keep up, the small-bowel bile acid concentration falls below the micellar threshold, fat absorption fails, and the picture flips to fatty acid diarrhea with steatorrhea, and here cholestyramine binds the few bile acids the patient still needs and makes it worse, so the right move is medium-chain triglycerides, which don't need micelles, plus a low-fat diet. Type two is idiopathic primary bile acid diarrhea, the one that's been hiding inside the IBS-D label, where the mechanism is impaired ileal feedback through the enterokine FGF19, which normally travels to the liver to suppress the rate-limiting synthesis enzyme, so low FGF19 lets synthesis run unchecked and overwhelms the small bowel. Many IBS-D patients have measurable bile acid malabsorption, which is why an empiric sequestrant trial sits inside the IBS-D approach. Type three is the altered-anatomy and altered-motility group, with post-cholecystectomy diarrhea the prototype, developing within weeks to months of the surgery from continuous low-volume bile flow into the duodenum during fasting that the ileum wasn't built to handle, and type four is drug-induced, mainly metformin.

3:06Diagnostic testing varies by region. The European reference test is a radiolabeled synthetic bile acid retention scan, with retention under a threshold at seven days diagnosing it and lower values grading severity, but it isn't available in the US, which is itself a testable point. The serum surrogates are two: the synthesis intermediate C4, which rises when hepatic synthesis is upregulated, and fasting FGF19, which is low in the idiopathic type because deficient ileal production is the mechanism. In the US, the most practical move in the right phenotype is an empiric therapeutic trial of a sequestrant, where response within a week or two confirms the diagnosis while treating the patient. Cholestyramine starts low and titrates up to a daily maximum, taken before meals, a non-absorbable resin that also binds fat-soluble vitamins and other drugs, so it's given an hour before or four hours after other medications, with bloating and its gritty texture the main tolerability issues; colestipol is an alternative, and colesevelam in tablet form is generally better tolerated. A few traps: the short-bowel patient from massive resection has low colonic bile acid concentration despite massive malabsorption because the high volume dilutes it, so cholestyramine worsens the steatorrhea, and the treatment is medium-chain triglycerides; bacterial overgrowth can deconjugate bile acids and mimic this; and celiac and chronic pancreatitis should be excluded before the label is accepted.

4:36Microscopic colitis is the second commonly missed cause and the place where colonoscopy without random biopsies fails the patient. It's the umbrella for two histologic entities, lymphocytic and collagenous colitis, both producing chronic watery non-bloody diarrhea with a normal-appearing colon, so the diagnosis lives entirely in the biopsy. The diarrhea is secretory in character, with a low osmotic gap, persistence during a fast, and frequent nocturnal stools, and it's statistically the leading cause of chronic watery diarrhea in patients over sixty, with a female predominance and a peak in older age. Severe weight loss or bloody stools should push you toward an alternative diagnosis instead. The autoimmune comorbidity is the rule, with thyroid disease, rheumatoid arthritis, and celiac co-occurring, and it's far more common in celiac patients, which carries the vignette of the celiac with persistent watery diarrhea on a strict gluten-free diet, where the move is colonoscopy with random biopsies, not a repeat duodenal biopsy. Drug associations are tested directly because they change management: NSAIDs, proton pump inhibitors especially lansoprazole, SSRIs, and a few others, plus current smoking as an independent risk factor, and removing the offending drug is the first therapeutic move and is sometimes curative.

5:55Diagnosis is at colonoscopy with random biopsies, since the mucosa almost always looks normal, and the biopsies must come from at least the right and left colon because both the lymphocyte density and the collagen band are heavier in the right colon, so a left-only or rectum-only protocol misses a fraction of cases. The two forms read differently but treat the same: lymphocytic colitis is defined by more than twenty intraepithelial lymphocytes per hundred surface cells with preserved architecture, and collagenous colitis by a thickened subepithelial collagen band over ten microns, where the normal band is around three, confirmed on a trichrome stain, and in both, crypt architecture is preserved, which is what separates it from inflammatory bowel disease. Treatment goes by severity: mild disease starts with removing the offending drug and smoking cessation, sometimes with loperamide or bismuth, while budesonide for six to eight weeks is the favored first-line for moderate to severe disease, working locally in the colon because of high first-pass metabolism so it avoids the systemic burden of prednisone, with a maintenance dose reserved for the relapsers, since it commonly relapses after stopping. Cholestyramine is added when bile acid malabsorption overlaps, which is common. So the anchor vignette is an older woman with chronic watery non-bloody diarrhea, no weight loss, normal basic labs, a normal-appearing colon, and a biopsy showing the lymphocytosis or the thickened collagen band, where the answer is microscopic colitis, managed by removing the offending drug and then budesonide if symptoms persist.

7:21That leaves the obscure chronic diarrhea patient, the one who's been through the whole standard workup, history and medications reread, basic labs clean, celiac serology with total IgA negative, calprotectin and elastase unremarkable, infectious panel and osmotic gap normal, fecal fat normal, endoscopy top and bottom with the right biopsies done, enterography done, and the diarrhea persists. The next move has to be structured, in three parts: targeted small-bowel investigation, sequential empiric trials, and referral when local capability runs out. Targeted small-bowel investigation extends beyond the standard workup: capsule endoscopy when subtle Crohn's, NSAID small-bowel injury, or lymphoma is suspected and hasn't been seen, or the more distal sprue patterns the duodenal biopsy missed, with patency verified first in anyone with obstructive symptoms to avoid a retained capsule, and balloon enteroscopy for distal biopsy and therapy.

8:21The sequential empiric trials are diagnostic and therapeutic in the same step, ordered by prevalence: a bile acid sequestrant first because it's the statistical winner in the IBS-D-overlap and post-cholecystectomy patient, then rifaximin for the overgrowth and dysbiosis differential, then a lactose-free trial, then a low-FODMAP elimination with structured reintroduction, then a pancreatic enzyme trial when elastase is borderline, and a gluten-free trial is sometimes considered, though the cleaner move is HLA testing where a negative result essentially excludes celiac before committing someone to lifelong avoidance. Targeted labs go beyond the standard panel when the phenotype gives a reason: the neuroendocrine panel when secretory features or red flags are present, somatostatin-receptor imaging to localize a suspected tumor, anorectal manometry for the incontinence that masquerades as diarrhea, quantitative immunoglobulins for common variable immunodeficiency, and a laxative screen when factitious disease is plausible. Referral to a chronic diarrhea center is the move when local tools are exhausted, and the recovered diagnoses are consistent, bile acid diarrhea, carbohydrate malabsorption, idiopathic secretory diarrhea, microscopic colitis missed for lack of random biopsies, surreptitious laxatives, peptide-secreting tumors, incontinence misclassified as diarrhea, and a drug nobody asked about. The unromantic lesson underneath all of it is that a careful repeat history catches a meaningful share before any new test is sent, so the medication and over-the-counter list get reread, the surgical history rechecked, and a diet diary collected.

9:51The practical endpoint of many of these workups is the IBS-D overlap: once organic disease is excluded and the empiric trials are run, the patient with diarrhea, defecation-related pain, and no alarm features carries the IBS-D label and moves into its treatment approach, which is the next chapter, running through loperamide, rifaximin, and eluxadoline with attention to the absent-gallbladder contraindication, sequestrants for the bile acid overlap, and neuromodulators for the central component.

10:20So the way to think about it: when a chronic diarrhea workup runs out, three entities account for most of what's left. Bile acid malabsorption is the urgent watery diarrhea, often nocturnal, often post-cholecystectomy or after ileal disease, that responds to a sequestrant within a week or two, with the hundred-centimeter rule deciding whether cholestyramine helps or harms. Microscopic colitis is the older patient with chronic watery non-bloody diarrhea and a normal-looking colon, diagnosed only on random biopsies, with budesonide first-line after removing the offending drug. And the obscure-diarrhea algorithm sorts the rest by targeted small-bowel imaging, prevalence-driven empiric trials, targeted labs when the phenotype demands them, and referral when local tools are done.

11:06The next chapter takes the patient at the end of that algorithm, the one with no alarm features, and works through the irritable bowel and functional bowel framework: the diagnosis by predominant bowel habit, the brain-gut and visceral hypersensitivity biology that explains why a normal workup doesn't mean a normal patient, and the subtype-targeted pharmacology from neuromodulators through behavioral therapy.

11:29For the full chapter, the practice vignettes, and the topic-tagged question bank, head to board pearls dot com. You'll find the rest of the series on Apple Podcasts, Spotify, or wherever you listen to podcasts. That brings us to the end of episode two of two of chapter twelve, and I'll see you in the next one.