IBS and Functional Bowel: IBS D Neuromod Bloating

0:00Welcome to Board Pearls. This is episode two of two of the Irritable Bowel Syndrome and Functional Bowel chapter, in the Stomach and Small Bowel Disorders module. This episode is the diarrhea-predominant subtype, the treatments that work across subtypes, and bloating.

0:16The diarrhea-predominant and constipation-predominant patients share a disease label, but their drug options aren't parallel: the constipation drugs act on intestinal secretion, while the diarrhea drugs act on transit, the microbiome, bile acid spillover, and visceral sensation. The class logic is what to learn, because once the mechanism is in hand the contraindications and safety flags become predictable rather than memorized.

0:40Start with loperamide, because the boards test what it is and isn't. It's a peripheral opioid agonist that slows transit without crossing into the brain, so it controls stool frequency and urgency and that's all, it does not improve abdominal pain because it never reaches the central pain circuits or the visceral nerves. It's dosed as needed up to sixteen milligrams a day with excellent tolerability. The trap is the IBS-D patient whose dominant symptom is crampy pain rather than frequency, because loperamide changes stool form and not what bothers them, so the move is to add something that targets the pain, a low-dose tricyclic or rifaximin, not more loperamide.

1:19Rifaximin is the gut-non-absorbed antibiotic, dosed at five hundred fifty milligrams three times daily for fourteen days, and it has a real place in IBS-D by reducing bacterial fermentation with downstream effects on bile acid signaling and visceral nerves. It gives a modest but real benefit over placebo for global symptoms and bloating, but the number that matters more for the exam is the retreatment rule: responders who relapse can be retreated with the same fourteen-day course and respond again, and the labeling permits up to two retreatments for three courses total when symptoms recur. So the patient who responded four months ago and is now back at baseline isn't someone to lock out of the drug, you retreat. Its systemic absorption is minimal, which is why the safety stays benign and meaningful resistance hasn't emerged with repeated use, and the indication is IBS-D, not IBS-C.

2:14Eluxadoline is the drug whose contraindication list is the teaching. It's a mixed opioid agonist-antagonist designed to slow transit and reduce visceral hypersensitivity while sidestepping the constipation that pure opioid agonism causes, dosed at a hundred milligrams twice daily, reduced to seventy-five in mild-to-moderate hepatic impairment or with certain interacting drugs. The contraindications all flow from one mechanism: it causes spasm of the sphincter of Oddi. In a patient with an intact gallbladder, the reservoir buffers the pressure, but in a patient without a gallbladder the duct has nowhere to send it, and the spasm produces acute pancreatitis and severe biliary pain. So prior cholecystectomy is an absolute contraindication, as are known biliary obstruction or suspected sphincter of Oddi disease, more than three drinks a day or any binge drinking because the pancreatitis risk compounds, a history of pancreatitis or structural pancreatic disease, severe hepatic impairment, and a history of severe constipation. The board vignette is the post-cholecystectomy IBS-D patient started on eluxadoline who returns with severe epigastric pain and a high lipase, where the drug should never have been started.

3:23Bile acid sequestrants treat the bile acid malabsorption hiding inside the IBS-D label, and this links back to the chronic diarrhea chapter, where a substantial share of IBS-D patients have measurable bile acid malabsorption and an empiric sequestrant trial is itself the diagnostic move. Cholestyramine is the favored answer, with colesevelam in tablet form better tolerated because cholestyramine's gritty texture is the main compliance problem, and colestipol a third option. The patient who responds within a week or two probably had bile acid malabsorption as the real driver, so the IBS-D label deserves revisiting, with the caveats that sequestrants bind other drugs so you separate the dosing, and chronic high-dose use can cause fat-soluble vitamin malabsorption.

4:08Alosetron is the drug to know in narrow strokes: a serotonin-3 antagonist that slows colonic transit and reduces visceral hypersensitivity, restricted to women with severe IBS-D refractory to conventional therapy, starting low and titrating. It was pulled from the market after cases of severe constipation and ischemic colitis, then reintroduced under a restricted-prescribing risk-management program. The favored teaching points are the women-only labeling, the severe-disease-only indication, the ischemic colitis and severe constipation boxed warnings, and the contraindication in the mixed subtype because the constipation risk lands worst in patients who already alternate. It's not used in men, and the other testable scenario is the alosetron patient returning with days of obstipation, distension, and pain, where the move is to stop the drug and evaluate for impaction, obstruction, and ischemic colitis, not to titrate up. And probiotics and fecal transplant sit in the negative-finding group: probiotics have modest inconsistent benefit and aren't harmful to continue but aren't evidence-based to start, and fecal transplant has been studied in IBS and doesn't help, so the patient wanting to fix their gut with good bacteria gets redirected to subtype-matched therapy.

5:23That's the subtype-specific layer. The next layer is the therapies that work across subtypes because they target mechanisms that aren't subtype-specific, the shared visceral hypersensitivity, central pain processing, and brain-gut signaling, so they're the answer for the mixed and unclassified subtypes and for the patient who's already cycled through subtype-specific drugs. Antispasmodics come first because they're the most prescribed and least evidence-supported: dicyclomine and hyoscyamine are anticholinergics used in short bursts for cramping, on the rationale that smooth-muscle overcontraction drives the pain, but pooled data show no significant benefit over placebo, so the continued use rests on plausibility and familiarity rather than outcomes, with the trade-off of anticholinergic side effects that matter especially in older patients. So the patient on dicyclomine for six months with no benefit and three side effects is one to stop the drug in, switching to evidence-based therapy, a low-dose tricyclic, hypnotherapy, behavioral therapy, or peppermint oil. Peppermint oil is the exception in that group: enteric-coated peppermint relaxes intestinal smooth muscle through calcium channel blockade, with real trial evidence for reducing pain and bloating, and the enteric coating is mechanistically required because uncoated oil dissolves in the stomach and causes reflux, so it's worth offering before chronic anticholinergics for episodic cramping.

6:45Neuromodulators are the central layer and the one to spend the most time on. They work by reducing visceral hypersensitivity and modulating central pain processing, not through an antidepressant effect, so the analgesic dose is lower than the antidepressant dose and the response takes weeks. The key teaching is that you select the agent by its side-effect profile to match the subtype, a side-effect-matched choice rather than a head-to-head efficacy one. Low-dose tricyclics are the favored neuromodulator for IBS-D, because the anticholinergic effect that slows transit is therapeutic in diarrhea and unhelpful in constipation, and the analgesic benefit is independent of any effect on mood. So the patient on amitriptyline with intolerable anticholinergic effects who asks about an SSRI is the teaching pivot: SSRIs haven't been shown to help IBS-D pain, and the move is to switch from a tertiary-amine tricyclic like amitriptyline to a secondary-amine one like nortriptyline or desipramine, which have fewer of those side effects. SSRIs are favored in IBS-C because they accelerate transit, chosen pragmatically when depression or anxiety coexists, but they aren't the answer for IBS-D pain. The serotonin-norepinephrine drugs like duloxetine are useful across subtypes and are the answer when fibromyalgia, chronic pelvic pain, or migraine accompanies the IBS, since all three respond through the central pain pathway, and mirtazapine fits the IBS-D patient with nausea or insomnia.

8:16Behavioral and mind-body therapies are the chronically undervalued layer the boards test more aggressively now, and the evidence is stronger than most appreciate. Cognitive behavioral therapy targets catastrophizing, hypervigilance to gut sensations, and avoidance, with durable benefit, and the favored detail is that phone- and web-based versions are as effective as in-person, which matters for the rural patient who can't drive to a clinic, who is a candidate for remote therapy, not someone to write off. Gut-directed hypnotherapy is the favored mind-body answer because it has the most consistent data, with durable response and a real mechanism, since imaging after a course shows altered brain activation, so the refractory patient who's cycled through subtype drugs and a neuromodulator gets sent to hypnotherapy, not a fifth drug. And integrated multidisciplinary care, a gastroenterologist, dietitian, behavioral health, and physical therapy when pelvic floor dysfunction overlaps, has the strongest outcomes for refractory disease.

9:14That leaves bloating, which the boards split into two ideas you keep separate: bloating is the subjective sensation of fullness and pressure, while distension is the objective increase in girth, and the two correlate imperfectly, with only some bloating patients having measurable distension. The mechanistic split runs between luminal contents, gas, liquid, retained stool, and abdominal-wall mechanics. The workup begins with constipation, because retained stool is a common reversible driver and treating it often resolves the bloating. Pelvic floor dyssynergia is the next layer, the inability to coordinate abdominal push with anal relaxation during defecation, producing incomplete evacuation and a loaded colon, diagnosed with anorectal manometry and balloon expulsion and treated with biofeedback, so the patient with bloating, incomplete evacuation, prolonged toileting, and a need for digital maneuvers has dyssynergia as the treatable mechanism. Abdomino-phrenic dyssynergia is the more recently recognized one and the favored answer for the patient whose distension is visible but out of proportion to luminal content: paradoxically, during perceived bloating the diaphragm descends instead of relaxing and the abdominal wall relaxes instead of contracting, producing visible protrusion without an actual volume increase, with progressive distension through the day that resolves overnight and no excess gas on CT, treated with physical therapy and diaphragmatic breathing retraining. Bacterial overgrowth contributes in some patients and is the rationale for the rifaximin response, diagnosed by breath testing, with the methane-predominant version associated with constipation rather than diarrhea. And a couple of refractory vignettes to flag: adult disaccharidase deficiencies like sucrase-isomaltase, diagnosed on duodenal biopsy with enzyme activity analysis rather than a serum level, in the patient whose bloating and diarrhea persist soon after sugar-containing meals despite a low-FODMAP trial, and alpha-gal syndrome from tick bites producing delayed symptoms hours after red meat, missed by standard allergy panels and requiring the specific IgE.

11:14The low-FODMAP diet is the dominant dietary intervention for bloating and pain, validated across subtypes, with the acronym naming the short-chain carbohydrates that resist absorption, distend the lumen, and reach the colon to ferment into gas and pain, living in wheat, onions, and garlic, in legumes, in dairy, in certain fruits and honey, and in the sugar alcohols. The teaching the boards expect is the three-phase structure, not the food list: eliminate all the groups for a few weeks, then reintroduce each group separately to find personal triggers, then personalize with chronic restriction limited to those triggers, because indefinite blanket restriction is nutritionally harmful and alters the microbiome, which is why dietitian-supervised three-phase elimination is the right answer rather than open-ended self-management from an internet guide. And fiber sits next to it: soluble fiber, mainly psyllium, has the strongest evidence in IBS-C, while insoluble fiber like wheat bran doesn't help and often worsens bloating because its fermentation lands hard on a hypersensitive gut, so the patient who added wheat bran and got worse is one to switch to psyllium.

12:24A summary across subtypes as one mechanism-mapped picture. For IBS-C, the drugs sort by channel: linaclotide and plecanatide are the cyclic-GMP agonists, lubiprostone the chloride-channel activator dosed at eight micrograms twice daily for IBS-C in women, tenapanor the sodium-exchanger inhibitor, and prucalopride the serotonin-4 prokinetic approved for chronic constipation. For IBS-D, they sort by mechanism: loperamide the peripheral opioid agonist, rifaximin the non-absorbed antibiotic with up to two retreatments, eluxadoline the mixed opioid agent with the contraindication list running through cholecystectomy, alcohol, and pancreatic disease, sequestrants for the bile acid overlap, and alosetron under restricted prescribing for women with severe refractory disease. Across subtypes, neuromodulators select by side-effect match, tricyclics for IBS-D, SSRIs for IBS-C, and serotonin-norepinephrine drugs for either with comorbid pain, with antispasmodics including peppermint oil and behavioral therapies including CBT and hypnotherapy covering the cross-subtype layer. Three dimensions organize the whole map: the mechanism, the subtype indication, and the contraindication or safety flag.

13:43So the way to think about it is that drug selection in IBS is mechanism-matched to the dominant symptom and driver, not subtype-matched in the abstract. The IBS-D patient with crampy pain dominating gets a tricyclic, not more loperamide. The one with bloating and prior antibiotic response gets retreated with rifaximin, not locked out for a single-course rule that doesn't exist. The post-cholecystectomy patient gets a sequestrant, not eluxadoline. The mixed patient who's cycled through subtype drugs gets hypnotherapy or duloxetine, not a fifth switch. And the bloating patient gets evaluated for constipation, pelvic floor dyssynergia, and abdomino-phrenic dyssynergia before more drug trials. Carry mechanism, subtype indication, and contraindication together and you'll pick the right answer when the stem describes a patient whose features push toward one option and away from another.

14:35The next chapter turns to inflammatory bowel disease, where the pathophysiology is structural rather than functional and the drug decisions get richer: ulcerative colitis severity and acute severe UC, Crohn's phenotype by the Montreal classification, biologic positioning across IBD, and dysplasia surveillance in long-standing colitis, with the organizing question being how phenotype and disease activity drive induction and maintenance, and how surveillance in long-standing colitis differs from average-risk cancer screening.

15:03For 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 thirteen, and I'll see you in the next one.