Residency · Residency · Gastroenterology

Irritable Bowel Syndrome

Definition and Epidemiology

Rome IV Criteria

The Rome IV criteria define irritable bowel syndrome as recurrent abdominal pain occurring on average at least 1 day per week in the last 3 months, associated with two or more of the following: a relationship to defecation (either improvement or worsening), an association with a change in stool frequency, or an association with a change in stool form or appearance. Symptom onset must have occurred at least 6 months before the diagnosis, with criteria fulfilled for the last 3 months. The diagnosis should be made in the absence of alarm features, which include weight loss, rectal bleeding, nocturnal symptoms, family history of colorectal cancer, inflammatory bowel disease, or celiac disease, iron deficiency anemia, and new symptom onset after age 50.

Subtypes (Bristol Stool Form Scale)

IBS is subclassified based on the Bristol Stool Form Scale. IBS-D (diarrhea-predominant) is defined by more than 25% of bowel movements being Bristol type 6 to 7 and less than 25% being Bristol type 1 to 2. IBS-C (constipation-predominant) is defined by more than 25% Bristol type 1 to 2 and less than 25% Bristol type 6 to 7. IBS-M (mixed) is defined by more than 25% of each extreme, and IBS-U (unclassified) applies when criteria for the other subtypes are not met. Subtype classification should be based on the distribution of days with abnormal stool form, not on overall predominance.

Epidemiology

The global prevalence of IBS is approximately 4 to 5% using Rome IV criteria, though this rises to 10 to 15% under the less restrictive Rome III criteria. There is a female predominance of 1.5 to 2:1 in Western countries, and peak incidence occurs between 20 and 40 years of age. There is significant overlap between IBS and other functional and somatic symptom disorders, including functional dyspepsia (50%), fibromyalgia (30 to 70%), chronic fatigue, migraine, temporomandibular joint disorder, chronic pelvic pain, and interstitial cystitis.

Pathophysiology — Biopsychosocial Model

Visceral Hypersensitivity

The pathophysiology of IBS is multifactorial and best understood through the biopsychosocial model. Visceral hypersensitivity manifests as lowered pain thresholds to rectal and colonic distension, with enhanced central processing of visceral afferent signals representing central sensitization. This phenomenon can be quantified with rectal barostat testing, though this remains primarily a research tool.

Altered Gut Motility

Altered gut motility patterns differ by subtype. IBS-D is associated with accelerated colonic transit and an exaggerated gastrocolic reflex, while IBS-C demonstrates delayed colonic transit, with pelvic floor dyssynergia overlapping in 25 to 50% of cases. Altered small bowel motility patterns have also been documented.

Gut-Brain Axis Dysfunction

Dysfunction of the gut-brain axis involves bidirectional communication via the vagus nerve, the hypothalamic-pituitary-adrenal axis, and immune signaling. Enhanced stress responsiveness mediated by corticotropin-releasing factor contributes to symptom generation. Functional MRI studies reveal altered brain activation patterns, with hyperactivation of the amygdala, anterior cingulate cortex, and insula. Psychological comorbidity is common, with anxiety present in 30 to 50% and depression in 20 to 40% of patients, and these conditions may either precede or follow the development of IBS.

Intestinal Permeability and Immune Activation

Increased mucosal permeability from tight junction dysfunction contributes to the pathophysiology. Low-grade mucosal inflammation is evidenced by increased mast cell density, particularly in proximity to enteric nerves, and increased intraepithelial lymphocytes. Post-infectious IBS develops in 10 to 15% of patients following acute gastroenteritis, with risk factors including the severity of the initial infection, female sex, and pre-existing anxiety or depression, and symptoms may persist for years.

Microbiome Alterations

Dysbiosis in IBS includes reduced microbial diversity, an altered Firmicutes-to-Bacteroidetes ratio, and reduced short-chain fatty acid production. These findings form the basis for probiotic and antibiotic (rifaximin) therapies. Fecal microbiota transplantation has produced mixed results and is not recommended outside of clinical trials.

Bile Acid Malabsorption

Type 2 or primary idiopathic bile acid malabsorption has been identified in 25 to 50% of patients with IBS-D. A defective FGF19-FXR feedback loop leads to excess colonic bile acid delivery. Diagnosis is achieved with SeHCAT retention below 10% (not available in the US) or serum C4 above 52 ng/mL. This subgroup responds to bile acid sequestrant therapy.

Diet — Fermentable Carbohydrates

FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols) are poorly absorbed, osmotically active carbohydrates that are rapidly fermented by colonic bacteria, generating gas (hydrogen, carbon dioxide, and methane) and increasing luminal water content. A low-FODMAP diet is effective in 50 to 80% of IBS patients for global symptom improvement.

Diagnostic Approach

Positive Diagnosis

IBS should be diagnosed positively based on the Rome IV criteria with an appropriate symptom pattern. In patients under 45 to 50 years without alarm features, the diagnosis can be made without extensive testing. The ACG 2021 guidelines recommend against routine colonoscopy, CT, food allergy testing, and stool ova and parasites testing in typical IBS without alarm features.

Limited Testing Recommended

A limited panel of tests is recommended for all IBS patients, including a complete blood count and CRP, as a normal CRP excludes IBD with greater than 95% negative predictive value. For IBS-D specifically, fecal calprotectin below 50 mcg/g essentially excludes IBD, celiac serologies (tTG-IgA plus total IgA) should be obtained, and either a C4 level or an empiric bile acid sequestrant trial should be considered. For IBS-C that is refractory to initial therapy, anorectal manometry with a balloon expulsion test should be considered to evaluate for pelvic floor dyssynergia. Colonoscopy is indicated when alarm features are present, when age-appropriate colorectal cancer screening is due, or when there is diagnostic uncertainty.

<image>A diagnostic algorithm for IBS presented as a flowchart. Start with "Chronic abdominal pain + altered bowel habits >= 6 months." First decision: "Meets Rome IV criteria?" If yes: "Alarm features present?" (list alarm features in a box: age >50, rectal bleeding, unintentional weight loss, nocturnal symptoms, family history CRC/IBD, iron deficiency anemia, new symptom onset >50). If alarm features: "Full evaluation (colonoscopy, labs, imaging as appropriate)." If no alarm features: "Limited workup" with three parallel boxes: "All: CBC, CRP" — "IBS-D: tTG-IgA, fecal calprotectin, consider bile acid testing" — "IBS-C: if refractory, anorectal manometry + balloon expulsion test." Arrow from limited workup to "Diagnose IBS (positive diagnosis)" then "Classify subtype: IBS-D, IBS-C, IBS-M, IBS-U" based on Bristol Stool Form Scale. Include a note box: "ACG recommends AGAINST: routine colonoscopy, CT, food allergy testing, stool O&P in typical IBS without alarm features." Use green for diagnostic confidence, yellow for additional testing, red for alarm features requiring investigation.</image>

Treatment

Dietary Interventions

Low-FODMAP Diet

The low-FODMAP diet follows a three-phase approach: strict elimination for 4 to 6 weeks, systematic reintroduction over 6 to 8 weeks, and a personalized long-term maintenance diet. This approach is effective in 50 to 80% of patients for global symptom improvement, as demonstrated in the Halmos randomized controlled trial. It requires guidance from a trained dietitian, and there is a risk of unnecessary nutritional restriction and disordered eating if the reintroduction phase is not completed. The major FODMAP groups include fructans (wheat, onion, and garlic), galacto-oligosaccharides (legumes), lactose (dairy), excess fructose (honey, mango, and high-fructose corn syrup), and polyols (sorbitol and mannitol, found in stone fruits and artificial sweeteners).

Fiber Supplementation

Soluble fiber, particularly psyllium or ispaghula husk, is effective for global IBS symptoms with a number needed to treat of 7. It should be started at 3 to 4 g per day and titrated slowly. In contrast, insoluble fiber such as wheat bran may worsen bloating and pain and is not recommended for IBS.

Other Dietary Approaches

A gluten-free diet may benefit a subset of IBS patients independent of celiac disease, reflecting non-celiac gluten or wheat sensitivity, though the mechanism is debated and may overlap with FODMAP reduction. Traditional dietary advice, including regular meals, adequate fluid intake, and limiting caffeine, alcohol, and spicy food, represents a reasonable and simple first-line intervention.

Pharmacotherapy — IBS-D

Loperamide at 2 to 4 mg as needed slows transit and increases water absorption, improving stool form, but does not improve pain; the maximum dose is 16 mg per day. Rifaximin at 550 mg three times daily for 14 days demonstrated sustained benefit in the TARGET 3 trial, with re-treatment being effective upon relapse, and has a number needed to treat of 9 for IBS-D without constipation. Eluxadoline is a mixed mu-opioid agonist, delta antagonist, and kappa agonist dosed at 100 mg twice daily (75 mg if the gallbladder is absent or there is mild hepatic impairment). It is contraindicated in patients without a gallbladder due to pancreatitis risk, as well as in biliary obstruction, alcoholism, heavy alcohol use, and sphincter of Oddi dysfunction. Alosetron, a 5-HT3 antagonist at 0.5 to 1 mg twice daily, is FDA-approved only for women with severe IBS-D refractory to conventional therapy, given the risk of ischemic colitis (1 in 1,000) and severe constipation, and requires prescriber certification. Bile acid sequestrants, including cholestyramine 4 g daily to four times daily and colesevelam 625 mg two to three tablets twice daily, are first-line for bile acid malabsorption-associated IBS-D, and an empiric trial is reasonable when testing is unavailable. Ondansetron at 4 to 8 mg three times daily, titrated to response, is an off-label option supported by randomized controlled trial data that slows transit and improves urgency.

Pharmacotherapy — IBS-C

Linaclotide, a guanylate cyclase-C agonist at 290 mcg daily (the IBS-C dose), reduces pain and improves bowel function and should be taken 30 minutes before breakfast on an empty stomach, with diarrhea occurring in approximately 20% as the most common side effect. Plecanatide, another guanylate cyclase-C agonist at 3 mg daily, has similar efficacy and possibly less diarrhea. Lubiprostone, a CLC-2 chloride channel activator at 8 mcg twice daily, is approved for IBS-C in women, with nausea in 8% mitigated by taking it with food. Tenapanor, an NHE3 inhibitor at 50 mg twice daily, reduces sodium and water absorption while also addressing visceral pain and is approved for IBS-C. Tegaserod, a 5-HT4 agonist at 6 mg twice daily, was re-approved in 2019 for women under 65 without cardiovascular risk factors but has limited use due to safety restrictions. PEG 3350 (MiraLAX), an osmotic laxative, improves stool frequency but not pain and serves an adjunctive role.

Pelvic Floor Dyssynergia in IBS-C

Pelvic floor dyssynergia is present in 25 to 50% of patients with refractory IBS-C. Biofeedback therapy achieves a response rate of 70 to 80% and is superior to laxatives, as demonstrated in the Rao trial. Diagnosis requires anorectal manometry combined with a balloon expulsion test, and referral to pelvic floor physical therapy is appropriate.

Centrally Acting Therapies (Neuromodulators)

Tricyclic antidepressants, including amitriptyline, nortriptyline, and desipramine, are started at 10 to 25 mg nightly and titrated to 50 to 75 mg. They are the most effective pharmacologic class for IBS-related pain, with a number needed to treat of 4.5, as confirmed by the ATLANTIS trial. TCAs are preferred for IBS-D due to their anticholinergic slowing effect. SSRIs including citalopram, fluoxetine, and paroxetine have less evidence for visceral pain but are useful when anxiety or depression is the predominant comorbidity. SNRIs such as duloxetine at 30 to 60 mg daily and venlafaxine offer dual pain and mood benefits and may be preferred for IBS-C due to their prokinetic effect. Gabapentinoids (pregabalin and gabapentin) may be considered for refractory visceral pain, though IBS-specific data are limited. Mirtazapine is useful for IBS-D accompanied by weight loss, nausea, or insomnia, leveraging its antihistaminic and anti-5HT3 properties.

Psychological Therapies

Cognitive behavioral therapy has the strongest evidence base among psychological interventions, achieving effective global IBS symptom improvement with a number needed to treat of 3 to 4. Gut-directed CBT specifically designed for IBS is available. Gut-directed hypnotherapy, following the Manchester protocol, demonstrates comparable efficacy to CBT over 7 to 12 sessions with durable benefit and a number needed to treat of 4. Mindfulness-based stress reduction has moderate supporting evidence. The ACG 2021 guidelines provide a strong recommendation for gut-brain behavioral therapies in IBS.

Probiotics

The effects of probiotics are strain-specific, and not all probiotics are equivalent. Bifidobacterium infantis 35624 has the best evidence for global IBS symptom improvement. VSL#3 has some evidence for bloating, and Lactobacillus plantarum 299v provides modest benefit. The ACG 2021 guidelines issue a conditional recommendation against probiotics due to heterogeneous evidence and the difficulty of recommending specific strains.

Emerging Therapies

Fecal microbiota transplantation has produced mixed results and is not recommended outside of clinical trials. A vibrating capsule (Vibrant) has been FDA-cleared for IBS-C and provides mechanical stimulation of colonic motility. Anti-CRF agents, mast cell stabilizers, and cannabinoid modulators remain investigational.

<image>A comprehensive treatment overview table for IBS organized by subtype. Create a visual grid with three columns: "IBS-D," "IBS-C," and "All Subtypes." Under IBS-D, list in descending order: dietary (low-FODMAP, bile acid sequestrant trial), loperamide, rifaximin 550 mg TID x 14 days, eluxadoline 100 mg BID, alosetron 0.5-1 mg BID (women only, severe), ondansetron 4-8 mg TID, TCA neuromodulators. Under IBS-C: dietary (soluble fiber, low-FODMAP), linaclotide 290 mcg daily, plecanatide 3 mg daily, lubiprostone 8 mcg BID, tenapanor 50 mg BID, biofeedback for dyssynergia, SNRI neuromodulators. Under All Subtypes: low-FODMAP diet, gut-brain behavioral therapy (CBT, hypnotherapy), TCA/SSRI/SNRI neuromodulators, peppermint oil. Include NNT values where available. Use color coding: green for strong evidence/recommendation, yellow for moderate, gray for weak/conditional. Add safety warning icons next to alosetron (ischemic colitis) and eluxadoline (pancreatitis without gallbladder). Format as a clean infographic-style visual reference.</image>

Key Clinical Pearls

• IBS is a positive diagnosis based on Rome IV criteria — avoid excessive testing in patients without alarm features
• Fecal calprotectin <50 mcg/g effectively excludes IBD in IBS-D patients and reduces unnecessary colonoscopy
• Low-FODMAP diet is effective in 50-80% but requires dietitian guidance and should include reintroduction phase to avoid unnecessary restriction
• Soluble fiber (psyllium) is beneficial in IBS; insoluble fiber (wheat bran) may worsen symptoms
• TCAs are the most effective pharmacologic class for IBS-related pain (NNT 4.5) — the ATLANTIS trial confirmed benefit of low-dose amitriptyline
• Bile acid malabsorption affects 25-50% of IBS-D patients — empiric cholestyramine trial is reasonable
• Pelvic floor dyssynergia is present in 25-50% of refractory IBS-C — biofeedback therapy is highly effective (70-80% response)
• Gut-directed CBT and hypnotherapy have NNTs of 3-4, making them among the most effective IBS interventions

References

1. Lacy BE, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021;116(1):17-44.
2. Ford AC, et al. Irritable bowel syndrome. Lancet. 2020;396(10263):1675-1688.
3. Halmos EP, et al. A diet low in FODMAPs reduces symptoms of irritable bowel syndrome. Gastroenterology. 2014;146(1):67-75.
4. Ford AC, et al. Effect of amitriptyline vs placebo on symptoms in patients with irritable bowel syndrome: the ATLANTIS randomized clinical trial. JAMA. 2023;330(12):1125-1135.
5. Pimentel M, et al. Rifaximin therapy for patients with irritable bowel syndrome without constipation (TARGET 3). N Engl J Med. 2011;364(1):22-32.

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