IBS and Functional Bowel: ROME IV Pathophys IBS C

0:00Welcome to Board Pearls. This is episode one of two of the Irritable Bowel Syndrome and Functional Bowel chapter, in the Stomach and Small Bowel Disorders module. This episode is the diagnosis and subtyping, the brain-gut mechanisms that explain why the disease behaves as it does, and the drugs for the constipation-predominant form.

0:18The first move when a patient with chronic abdominal pain and disordered defecation walks in is to decide what kind of diagnosis you're making, because IBS is not a diagnosis of exclusion. It's a positive clinical diagnosis built on a stereotyped symptom pattern with limited targeted testing when there are no alarm features. The classic vignette tests exactly this: a physician orders colonoscopy, enterography, capsule endoscopy, and a full neuroendocrine panel in a young woman with classic IBS-D and no alarm features, and that workup is excessive, because targeted serologic and stool testing is enough. Reflexively excluding everything that could look like IBS is the wrong answer.

1:01The diagnosis rests on the Rome criteria: recurrent abdominal pain, on average at least one day a week over the last three months, associated with two or more of three features, that the pain is related to defecation, associated with a change in stool frequency, or associated with a change in stool form, with symptom onset at least six months before diagnosis. Note that the criterion is pain related to defecation, not necessarily improved by it, because some patients have pain that worsens with defecation. Subtyping is by the predominant Bristol stool form on the days the patient has abnormal bowel movements, not on all days, which matters because a patient with normal stool most days but hard pellets on the bad days is classified by the bad days. IBS-C is hard stool, Bristol one or two, on at least a quarter of abnormal movements with loose stool on fewer than a quarter; IBS-D is the converse; IBS-M reaches at least a quarter at each end; and IBS-U meets the pain criteria but can't be subtyped. Subtyping matters because the prescription drugs split sharply along it, so if you can't subtype the vignette you can't pick the drug.

2:11Alarm features change the workup, and you keep a short list memorized because the vignette will plant one: new symptoms after age fifty, significant unintentional weight loss, nocturnal symptoms that wake the patient, blood in the stool or iron deficiency, fevers, a family history of colorectal cancer or IBD or celiac, abnormal exam findings, and travel to parasite-endemic regions. Any of these push toward directed testing before applying the IBS label, though the presence of one doesn't rule out IBS, it just changes the testing burden first. And targeted testing in suspected IBS is much narrower than the old rule-out-everything approach: in the diarrhea and mixed subtypes, you screen for celiac with the transglutaminase antibody plus a total IgA, for IBD with CRP and calprotectin, and for Giardia when the exposure fits, while routine blood counts, thyroid studies, imaging, and broad parasite panels aren't recommended without a specific driver. Colonoscopy is reserved for alarm features and for age-based cancer screening, which now starts at forty-five, and there are no exclusionary tests recommended for suspected constipation-predominant disease beyond that screening.

3:23That targeting rule has an important inversion that's tested directly: when the screening tests are abnormal, the diagnosis is no longer IBS. A patient with chronic non-bloody diarrhea and a clearly elevated calprotectin, a high CRP, and mild anemia is not an IBS-D patient with non-specific labs, they need colonoscopy with ileal biopsies and small-bowel imaging before a functional label is attached, and a positive transglutaminase antibody with a normal total IgA is celiac until proven otherwise. So the screening tests work both ways: negative results enable the IBS label and positive results suspend it.

4:00The IBS-D differential deserves attention because several mimics are eminently treatable: microscopic colitis in the older patient on a proton pump inhibitor or SSRI or NSAID with watery diarrhea and a normal-looking colon diagnosed on random biopsies; celiac captured by the transglutaminase antibody; giardiasis after travel or daycare captured by stool testing; bile acid malabsorption in the post-cholecystectomy or post-ileal-resection patient who responds to cholestyramine, overlapping a substantial share of IBS-D; bacterial overgrowth with bloating and a low B12 and high folate; lactose and fructose maldigestion confirmed by breath testing or a dietary trial; and IBD for anyone with bloody stool, weight loss, or an elevated calprotectin. Two newer mimics are worth memorizing: alpha-gal syndrome, an IgE reaction to a sugar in mammalian meat appearing hours after eating it, diagnosed by the specific IgE and linked to lone star tick bites, and sucrase-isomaltase deficiency, a meaningful cause of unexplained IBS-D, diagnosed on a disaccharidase assay from duodenal biopsy and treated with sucrose restriction and enzyme replacement.

5:06That's the recognition layer. The mechanistic premise behind it is that IBS is a disorder of brain-gut interaction with five contributors to name: visceral hypersensitivity, altered motility, brain-gut dysregulation, microbiome alterations, and post-infectious or inflammatory triggers, all combining in varying proportions into a final phenotype of pain plus disordered defecation. Recognizing which mechanism a vignette is testing tells you which therapy to choose, because the drug classes map onto the mechanisms.

5:37Visceral hypersensitivity is a lowered threshold for perceiving normal gut distension as painful, demonstrated reproducibly with rectal balloon studies where many IBS patients feel pain at smaller volumes than controls, though the ones with normal thresholds tell you this is one of several contributors rather than a unified theory, and some patients show lowered esophageal and gastric thresholds too, so the sensitization isn't specific to the colon. The mechanism is peripheral and central: peripherally, mast cells, enterochromaffin cells, and afferent nerve endings become primed by inflammation, bile acids, microbial signals, and food, and the spinal neurons then fire at lower input; centrally, imaging shows IBS patients activating pain-processing networks more strongly to the same gut input. This two-way sensitization is why neuromodulators targeting central pain processing and behavioral therapies that reshape pain perception work across subtypes regardless of bowel pattern, which is an episode-two thread.

6:37Altered motility differs by subtype: IBS-D shows accelerated colonic transit and an exaggerated after-meal colonic response, while IBS-C shows delayed transit and reduced propagated contractions. The serotonin-4 receptor is the prokinetic target in the colon, since stimulating it releases acetylcholine and drives the high-amplitude contractions that move stool, which is the rationale for prucalopride later. And the exaggerated after-meal response is why IBS-D patients feel urgency within minutes of eating. Brain-gut dysregulation captures the two-way signaling between the central, autonomic, and enteric nervous systems: IBS patients seen in referral clinics show more anxiety, depression, and prior trauma, but community IBS patients who never seek care are psychologically indistinguishable from controls, which means psychiatric comorbidity modulates the experience and the help-seeking rather than causing the syndrome. So when a vignette asks whether treating anxiety will cure IBS, the answer is that distress modulates symptom severity both ways but doesn't cause IBS, and integrated treatment improves both.

7:42Microbiome alterations show reduced diversity and shifts in specific organisms but no single defining signature, and the clinical relevance is that gut-targeted antibiotics like rifaximin reduce IBS-D symptoms and bloating while probiotics show modest inconsistent benefit, and fecal transplant has not been shown to work for IBS and isn't recommended, so the vignette where a patient wants a transplant because her friend's C. diff was cured by one is testing exactly that: it works for C. diff, not IBS. Post-infectious IBS is chronic IBS after a discrete gastroenteritis, developing in a fraction of patients after bacterial enteritis and more often after C. diff, with the mechanism involving persistent low-grade inflammation and microbiome shifts that fail to reset. The favored vignette is a previously healthy patient with new IBS-D after confirmed enteritis, and the trap is reflexively retesting stool for C. diff in someone with post-infectious symptoms but no active infection, because the false-positive rate is high, so careful history and standard IBS-D therapy is the move.

8:43Bile acid malabsorption hides inside a substantial share of IBS-D vignettes, where excess colonic bile acids drive secretion and motility, with the classic anchors being post-cholecystectomy diarrhea, prior ileal disease, and idiopathic primary disease, and the practical consequence is that a patient labeled IBS-D who responds dramatically to a sequestrant probably had bile acid malabsorption as the real driver. And a few other contributors round it out: altered permeability, low-grade mucosal immune activation, food-triggered symptoms through distension and fermentation rather than IgE allergy, and the overlap with fibromyalgia, interstitial cystitis, and migraine, the central sensitivity syndromes, where the recognition pattern is a young patient with longstanding fibromyalgia and interstitial cystitis who develops crampy pain with alternating stools, whose shared central sensitization calls for integrated neuromodulation alongside IBS-specific therapy rather than forcing a single subtype label.

9:38That's the mechanism map, and the treatment of IBS-C maps onto it, escalating from general measures to mechanism-targeted drugs. First-line is education and a positive therapeutic relationship, which by itself reduces visits and improves symptoms, plus soluble fiber like psyllium introduced gradually, preferred over insoluble fiber like wheat bran, which commonly worsens bloating without proportional benefit, along with fluid, activity, scheduled toileting, and pulling out the constipating medications, opioids, anticholinergics, calcium channel blockers, iron, and calcium. Over-the-counter polyethylene glycol improves stool frequency but has a less consistent effect on the whole IBS-C picture including pain, and that asymmetry is exactly why the prescription drugs target secretion and motility rather than osmotic bulk alone, so polyethylene glycol is useful when the patient wants an over-the-counter option, and the prescription drugs are the answer when the stem specifies pain plus constipation.

10:40The prescription drugs split into three mechanisms, each ending with water in the lumen but reaching it differently. Lubiprostone activates a chloride channel on the enterocyte, pulling water in to soften stool, dosed at eight micrograms twice daily for IBS-C, with the approval narrowly written for women, versus twenty-four micrograms twice daily for chronic idiopathic constipation, three times the IBS-C dose, and that dose split is frequently tested. Its common side effect is nausea, mitigated by taking it with food, so the patient who calls back with severe nausea on an empty stomach hasn't failed the drug, she missed the food instruction. Linaclotide and plecanatide share a mechanism, stimulating the enterocyte to raise cyclic GMP, which opens the chloride channel to drive secretion and also modestly reduces visceral pain through submucosal nerves, which is why they move both the pain and the constipation, not just the constipation. Linaclotide for IBS-C is two hundred ninety micrograms once daily on an empty stomach at least thirty minutes before the first meal, and that empty-stomach timing gets tested, while plecanatide is three milligrams once daily with or without food, with diarrhea the main side effect for both, both contraindicated in young children with a boxed warning for dehydration, and the two are essentially interchangeable, chosen by tolerability and cost. Tenapanor inhibits a sodium exchanger in the small intestine and proximal colon so trapped luminal sodium pulls water in, and it also seems to reduce visceral hypersensitivity, dosed at fifty milligrams twice daily before meals, with diarrhea the main side effect, and it's the answer when the vignette names the sodium-exchanger mechanism.

12:16Prucalopride sits apart from the others because it's a prokinetic rather than a secretagogue, a selective serotonin-4 agonist that boosts acetylcholine release and stimulates the high-amplitude contractions that move stool, dosed at two milligrams daily, one in renal impairment, and it's approved for chronic idiopathic constipation rather than IBS-C specifically, a distinction the boards may test, with off-label use reasonable in refractory IBS-C when slow transit dominates. Its cardiovascular safety is the tested part: the older agents in this class had off-target activity at other receptors and a cardiac channel that caused arrhythmia and ischemic events and led to withdrawal, while prucalopride was designed for selectivity and has a better cardiovascular record, though prescribers still review risk. So the refractory IBS-C patient with no cardiac history being considered for a prokinetic should point you to prucalopride, with the recognition that its label is chronic constipation, not IBS-C.

13:16Selection across the IBS-C class is mechanism-aware but driven mostly by tolerability and cost: linaclotide and plecanatide interchangeable, lubiprostone the answer when the chloride-channel mechanism or the dose split is the teaching point, tenapanor the sodium-exchanger inhibitor, and prucalopride the prokinetic for failures of the secretagogues or slow-transit phenotypes, with SSRIs sometimes used because they accelerate transit and polyethylene glycol as an over-the-counter adjunct. There's no drug specific to the mixed subtype, so you target the predominant symptom on a given day and layer in low-dose tricyclics or behavioral therapy across both patterns, which is an episode-two thread.

13:56So carry three things from this episode. IBS is a positive clinical diagnosis applied after a brief targeted screen for celiac, IBD, and infection. The brain-gut model with its five mechanisms maps onto the drug classes. And IBS-C treatment layers fiber and polyethylene glycol onto first-line care before reaching for the secretagogues by mechanism and the prokinetic when they fail, with the dose splits the boards test directly.

14:25Episode two picks up IBS-D pharmacology, eluxadoline with its post-cholecystectomy and alcohol contraindications, rifaximin with its retreatment rule, and sequestrants for the bile acid overlap, then the all-subtype neuromodulators with behavioral therapy, and the selective approach to bloating with the low-FODMAP framework and the biofeedback overlap when pelvic floor dyssynergia is doing the work.

14:50For 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 one of two of chapter thirteen, and I'll see you in the next one.