Irritable Bowel Syndrome

• Visceral Hypersensitivity and Altered Nociception: Patients with IBS demonstrate decreased pain thresholds to intestinal distension compared to healthy controls—a phenomenon documented on barostat studies. The underlying mechanism involves increased density and heightened responsiveness of visceral nociceptors (TRPV1 and TRPV4 channels) in the gut mucosa and muscularis. Additionally, altered expression of serotonin receptors (particularly 5-HT3 and 5-HT4) on enteric neurons impairs normal pain signal modulation. Elevated levels of substance P and calcitonin gene-related peptide (CGRP) in the lamina propria enhance nociceptive signaling. These changes result in the cardinal symptom of abdominal pain triggered by normal or low-level colonic contractions that would not ordinarily cause discomfort. The anterior insula and anterior cingulate cortex (areas involved in pain perception) show heightened activation on functional MRI when IBS patients experience visceral stimulation, indicating central sensitization—amplification of pain signals within the spinal cord and brain mediated by NMDA receptors, dynorphins, and dysfunction of descending inhibitory pain pathways involving serotonin and norepinephrine.

• Abnormal Gastrointestinal Motility and Contractility: The enteric nervous system in IBS exhibits dysregulation of smooth muscle contraction patterns. Patients demonstrate postprandial colonic hypermotility (exaggerated mass movements after eating, particularly common in IBS-diarrhea) or postprandial hypomotility (reduced propulsive contractions in IBS-constipation). These abnormalities reflect altered enteric neurotransmission: decreased acetylcholine and enhanced VIP (vasoactive intestinal peptide) and nitric oxide release impair normal coordinated peristalsis. Dysfunction of the myenteric plexus (Auerbach's plexus) leads to dysrhythmic contractions—rapid, uncoordinated bursts of activity that fail to produce effective fecal propulsion. Small intestinal bacterial overgrowth (SIBO), present in 4-78% of IBS patients, produces fermentable substrates that trigger colonic distension and triggering of the gastrocolic reflex, explaining postprandial symptom exacerbation. The relationship between motility dysfunction and symptoms is not absolute: some patients have normal motility studies yet remain symptomatic, underscoring that visceral hypersensitivity contributes more significantly to symptoms than abnormal contractions alone.

• Dysbiosis and Altered Intestinal Microbiota: IBS patients display significant alterations in fecal bacterial composition (dysbiosis), characterized by reduced microbial diversity, decreased Faecalibacterium prausnitzii and Roseburia species (butyrate-producing bacteria), and increased Proteobacteria and pathogenic gram-negative organisms. This dysbiosis disrupts the production of short-chain fatty acids (SCFAs)—particularly butyrate—which normally maintain intestinal barrier integrity by providing energy for colonocytes and regulating tight junction proteins (occludin, claudins, zonula occludens-1). Reduced SCFA production leads to increased intestinal permeability ("leaky gut"), allowing translocation of lipopolysaccharides (LPS) and bacterial antigens across the epithelial barrier. LPS activation of toll-like receptor 4 (TLR4) on dendritic cells and macrophages triggers low-grade mucosal inflammation and increased production of pro-inflammatory cytokines (TNF-α, IL-6, IL-8, IL-17). Dysbiotic microbiota also produce altered levels of tryptophan metabolites (kynurenine pathway activation), reducing the synthesis of aryl hydrocarbon receptor (AhR) ligands, which normally maintain intestinal barrier function and immune tolerance. The dysbiosis is thought to be both a cause and consequence of IBS, as abnormal motility and secretion favor overgrowth of pathogenic species while dysbiosis perpetuates barrier dysfunction and inflammation.

• Mucosal Immune Activation and Low-Grade Inflammation: Despite the absence of findings on colonoscopy, IBS tissue demonstrates subtle but consistent immune abnormalities. Increased density of intraepithelial lymphocytes (IELs), particularly CD8+ T cells, and increased numbers of lamina propria lymphocytes (predominantly CD4+ T cells) indicate chronic immune activation. Mast cells are increased 5-10 fold in IBS colonic mucosa; these cells release histamine, tryptase, and proteases (including tryptase and chymase) upon activation, directly stimulating enteric neurons and contributing to visceral hypersensitivity. Epithelial cell integrity is compromised with reduced expression of tight junction proteins and increased zonulin expression (driven by dysbiosis-associated LPS), widening intercellular tight junctions. This leads to increased paracellular permeability measurable by lactulose-mannitol ratios in urine. The intestinal barrier dysfunction is further exacerbated by altered mucus layer composition and thickness; dysbiotic bacteria produce reduced amounts of mucin-degrading enzymes, but the overall mucus barrier is impaired. Increased epithelial apoptosis, driven by bacterial LPS and pro-inflammatory cytokines, impairs the normal epithelial renewal process. Post-infectious IBS (developing after acute gastroenteritis) specifically demonstrates persistent innate immune activation with elevated fecal calprotectin (though typically <150 μg/g), increased IL-6 and IL-8, and altered gut-associated lymphoid tissue (GALT) structure.

• Altered Brain-Gut Axis and Central Nervous System Dysfunction: The bidirectional communication between the brain and gut via the vagus nerve and hypothalamic-pituitary-adrenal (HPA) axis is dysregulated in IBS. Patients demonstrate hyperresponsiveness of the HPA axis to stressors, with exaggerated cortisol secretion and impaired negative feedback. Elevated baseline cortisol and abnormal cortisol patterns alter intestinal permeability, motility, and immune function through glucocorticoid receptors on epithelial cells, immune cells, and enteric neurons. Abnormal corticotropin-releasing hormone (CRH) signaling—CRH directly stimulates mast cells and increases visceral sensitivity—amplifies pain perception. Central dopamine and serotonin dysfunction impairs mood regulation and pain modulation; patients with IBS have higher rates of depression and anxiety, and alterations in 5-HT reuptake transporter (SERT) expression contribute to both mood and GI symptoms. Dysregulation of mu-opioid receptors and endogenous opioid peptide signaling impairs normal pain inhibition. Brain imaging studies reveal altered connectivity between the insula (visceral sensory processing), anterior cingulate cortex (emotional processing), and prefrontal cortex (cognitive control), resulting in reduced inhibitory control over pain signals. Additionally, patients demonstrate altered responses to visceral pain in the amygdala and hippocampus, brain regions involved in emotional memory and conditioned fear responses, potentially perpetuating symptom amplification.

• Genetic and Epigenetic Factors: Family aggregation studies demonstrate that IBS has a heritable component; first-degree relatives have a 2-3 fold increased risk. Candidate genes implicated include those encoding serotonin receptors (HTR1A, HTR1B), the serotonin transporter (SLC6A4), catechol-O-methyltransferase (COMT) (affecting dopamine and norepinephrine degradation), and neuropeptide Y. Genome-wide association studies (GWAS) have identified susceptibility loci at chromosomes 5p15 (near the IL-17 pathway genes) and 5q33 (near immunoglobulin genes), linking genetic predisposition to immune dysregulation. Epigenetic modifications (DNA methylation and histone acetylation) alter expression of genes involved in intestinal permeability and immune function, and these epigenetic patterns can be influenced by environmental factors including diet, stress, and early-life microbiota exposure. The fact that monozygotic twin concordance (~60%) is higher than dizygotic (~40%) but neither approaches 100% demonstrates that genetic predisposition requires environmental triggers for disease manifestation.

• Altered Intestinal Secretion and Absorption: IBS-diarrhea subtype demonstrates enhanced colonic secretion of chloride and water, likely driven by altered expression of chloride channels (particularly CFTR—cystic fibrosis transmembrane conductance regulator—and CaCC channels) and dysbiosis-induced activation of intestinal secretory pathways. Dysbiotic bacteria produce increased quantities of secondary bile acids (from primary bile acid deconjugation), which activate TGR5 receptors and FXR receptors on enterocytes and enteric neurons, increasing fluid secretion and accelerating colonic transit. IBS-constipation is associated with reduced secretion and impaired colonic propulsion, possibly related to downregulation of secretory pathways and altered P2Y2 receptor signaling (involved in purinergic signaling that promotes secretion). Absorptive defects lead to incomplete absorption of dietary carbohydrates and lipids, producing excess substrate for colonic bacterial fermentation, generating bloating and altered gas production.

• Post-Infectious IBS (PI-IBS): Onset following acute bacterial gastroenteritis occurs in 7-31% of IBS patients (highest incidence in IBS-diarrhea). Risk factors for developing PI-IBS include severity of the initial infection, bacterial virulence factors (especially in Campylobacter and Salmonella infections), antibiotic use during infection, and genetic susceptibility. The pathophysiology involves persistent mucosal inflammation, altered microbiota composition, and increased intestinal permeability lasting months to years after the acute infection. Patients with prior Giardia lamblia infection are at particular risk, likely due to intestinal barrier damage and immunological changes that persist post-cure. This subset has better long-term prognosis compared to non-infectious IBS, with some studies showing partial or complete symptom resolution.

• Psychological Stressors and Early-Life Trauma: Severe psychosocial stressors, major depression, generalized anxiety disorder, and post-traumatic stress disorder are associated with IBS onset and exacerbation. Early-life adversity—including childhood abuse (physical, emotional, or sexual), parental loss, and adverse childhood experiences—significantly increases IBS risk, particularly when combined with genetic predisposition. The mechanism involves HPA axis programming alterations that persist into adulthood, creating vulnerability to stress-induced visceral symptom amplification. Patients with IBS report higher rates of somatization and have significantly more health-related anxiety and catastrophizing behavior patterns. These psychological factors are not merely consequences of chronic illness but are independent risk factors for IBS development.

• Dietary Factors: Fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs)—poorly absorbed short-chain carbohydrates present in wheat, fructose, lactose, sugar alcohols, and legumes—are major dietary triggers. These substrates are rapidly fermented by colonic bacteria, producing gas (methane, hydrogen, CO2), which distends the colon and triggers symptoms in susceptible individuals via the glucose-dependent insulinotropic peptide (GIP) pathway and direct mechanoreceptor stimulation. Dietary fat, particularly saturated fat and fat malabsorption, triggers cholecystokinin (CCK) release and stimulates colonic motility. Capsaicin (in spicy foods) and caffeine activate TRPV1 and adenosine receptors on sensory neurons, enhancing pain perception. Reduced fiber intake is associated with constipation-predominant IBS; however, insoluble fiber can exacerbate symptoms in some patients. Individual food intolerances (not true allergies) are reported in 50-80% of IBS patients, most commonly to wheat, dairy, and cruciferous vegetables.

• Dysbiosis and Altered Microbiota: Antibiotic exposure, particularly broad-spectrum antibiotics during critical developmental windows (childhood), substantially increases IBS risk by disrupting the indigenous microbiota and preventing establishment of protective bacterial species. Proton pump inhibitor (PPI) use alters gastric pH and proximal small intestinal bacterial overgrowth. High salt intake shifts microbiota composition toward inflammatory Th17 cell-promoting species. The hygiene hypothesis suggests that reduced microbial exposure in modern societies, reflected in reduced diversity of commensal bacteria, increases susceptibility to dysbiosis-mediated IBS.

• Female Gender and Hormonal Factors: Women have nearly double the prevalence of IBS compared to men, particularly in reproductive years. Symptoms often correlate with menstrual cycle phase; exacerbation during luteal phase (high progesterone, low estrogen) and improvement during follicular phase suggest estrogen-mediated effects on GABA receptors and pain modulation. Estrogen downregulates expression of mu-opioid receptors in the CNS, potentially reducing endogenous pain inhibition. Oral contraceptive use and hormone replacement therapy have variable effects on IBS symptoms. Reproductive factors (parity, lactation) influence microbiota composition and immune tolerance.

• Genetic Predisposition: As discussed, twin studies and family clustering support heritable components, though no single Mendelian inheritance pattern exists. Patients with IBS frequently have positive family histories. Genetic variations in serotonergic pathways, mucosal immune genes, and genes affecting tight junction function contribute to disease susceptibility.

• Smoking and Alcohol: Active smoking is associated with increased IBS prevalence, possibly through alterations in gut permeability and microbiota. Alcohol consumption correlates with IBS symptoms, particularly in diarrhea-predominant disease, through effects on intestinal motility and bacterial overgrowth.

• Abdominal Pain or Discomfort: This is the cardinal symptom, present in >90% of patients. Pain is typically diffuse and cramping in character, though some patients describe sharp, stabbing, or aching sensations. Location varies but frequently involves the left lower quadrant and suprapubic region, corresponding to the sigmoid colon and rectum where visceral sensation is heightened. Importantly, pain severity does not correlate with objective measures of motor or secretory dysfunction, distinguishing IBS from organic disease. Pain characteristically improves or resolves with defecation (a hallmark feature), worsens with eating (particularly within 30 minutes of meals, related to gastrocolic reflex activation), and fluctuates throughout the day with stress and emotional factors. Nocturnal pain is rare and should prompt investigation for organic disease. The Rome IV criteria specify that pain must occur at least 1 day per week on average for the preceding 3 months (with symptom onset ≥6 months prior). Pain intensity ranges from mild (minimally disruptive to daily function) to severe (debilitating, leading to work absenteeism and social withdrawal). Some patients report pain as the predominant symptom with minimal alterations in bowel habit, while others experience predominantly altered bowel patterns with minimal pain.

• Altered Bowel Habits: IBS is classified into four subtypes based on stool pattern: IBS-constipation (IBS-C),

Step 1 — Apply the criteria

• Rome IV criteria (Rome Foundation): recurrent abdominal pain on average ≥1 day per week in the last 3 months, associated with ≥2 of 3: (1) related to defecation, (2) associated with a change in stool frequency, (3) associated with a change in stool form. Symptom onset must be ≥6 months before diagnosis.
• Subtyping: uses the Bristol Stool Form Scale applied only to days with abnormal stools. IBS-C = >25% of stools Bristol types 1–2 and <25% types 6–7; IBS-D = >25% types 6–7 and <25% types 1–2; IBS-M = >25% types 1–2 and >25% types 6–7; IBS-U = meets IBS criteria but stool pattern fits none of these. Subtype drives drug selection, so both halves of each definition matter.

Step 3 — Targeted testing (ACG)

• CBC in all patients; anemia is never attributable to IBS.
• Celiac serology (tissue transglutaminase IgA with total IgA) in patients with diarrhea or mixed symptoms — celiac disease is the classic mimic.
• Fecal calprotectin (or lactoferrin) plus CRP in IBS-D to exclude IBD; normal calprotectin is conventionally <50 µg/g, and values in IBS are normal or only trivially raised, far below the markedly elevated levels of active Crohn disease or ulcerative colitis.
• Consider testing for bile-acid diarrhea in IBS-D, particularly post-cholecystectomy; the AGA guideline on laboratory evaluation of functional diarrhea and IBS-D makes this a conditional suggestion based on very low-quality evidence.
• ACG recommends against routine stool ova-and-parasite testing, routine food-allergy testing, and routine colonoscopy in younger patients without alarm features.

First-line (all subtypes)

• Soluble fiber: psyllium/ispaghula — ACG recommends soluble fiber and recommends against insoluble fiber (wheat bran), which worsens bloating and pain.
• Dietary modification: a limited trial of a low-FODMAP diet with structured reintroduction, ideally dietitian-guided (ACG suggests). Prolonged blanket restriction is discouraged.
• Peppermint oil: suggested by ACG (conditional recommendation); acts via smooth-muscle L-type calcium channel blockade to relieve pain and cramping.
• Antispasmodics (dicyclomine, hyoscyamine): antimuscarinics still widely used in US practice for postprandial cramping, but ACG 2021 issues a conditional recommendation against the antispasmodics currently available in the US for global IBS symptoms, citing low-quality evidence. Do not treat them as guideline-endorsed first-line therapy.

Subtype-directed escalation

• IBS-C: secretagogues — guanylate cyclase-C agonists (linaclotide, plecanatide), chloride channel activator (lubiprostone), or the NHE3 inhibitor tenapanor; the 5-HT4 agonist tegaserod is limited to women <65 with no history of ischemic cardiovascular events (MI, stroke, TIA, angina) and no more than one cardiovascular risk factor, typically after inadequate response to secretagogues. Polyethylene glycol improves stool frequency but not pain — a favorite exam distinction.
• IBS-D: rifaximin (nonabsorbed antibiotic, targets dysbiosis/SIBO; may be re-treated on recurrence); loperamide for stool consistency only; the mixed opioid agent eluxadoline; and the 5-HT3 antagonist alosetron, restricted to women with severe refractory IBS-D.

Refractory / global therapy

• Gut–brain neuromodulators: tricyclic antidepressants (amitriptyline, nortriptyline) are recommended by ACG for global symptoms — central antinociception plus useful anticholinergic slowing in IBS-D. SSRIs are preferred when constipation or comorbid mood disorder predominates.
• Brain–gut behavioral therapy: cognitive behavioral therapy and gut-directed hypnotherapy (ACG recommends).

Disease-related

• Impaired quality of life and disability: visceral hypersensitivity plus central sensitization produce work absenteeism and social withdrawal; signaled by escalating healthcare utilization despite normal testing.
• Psychiatric comorbidity: anxiety, depression, and somatization share HPA-axis and serotonergic mechanisms with IBS; screen when symptoms are refractory.
• Nutritional deficiency and disordered eating: prolonged unsupervised low-FODMAP or elimination diets restrict fiber, calcium, and prebiotic substrate; signaled by weight loss, which must never simply be attributed to IBS.
• Diagnostic overshadowing: attributing new alarm features to known IBS delays diagnosis of celiac disease, microscopic colitis, IBD, or malignancy. New rectal bleeding, anemia, or nocturnal symptoms demand re-evaluation.
• Unnecessary surgery: IBS patients undergo excess cholecystectomy, appendectomy, and hysterectomy, adding adhesive obstruction risk.

Treatment-related

• Alosetron — ischemic colitis and complications of severe constipation: the mechanism is incompletely understood and is generally attributed to markedly slowed colonic transit and constipation rather than a defined vascular effect. Bloody diarrhea with new abdominal pain, or obstipation with distension, is an emergency: stop the drug immediately.
• Eluxadoline — sphincter of Oddi spasm and acute pancreatitis, especially post-cholecystectomy. New epigastric pain radiating to the back with elevated lipase is an emergency.
• Tegaserod — cardiovascular/ischemic events, the reason for its age and risk-factor restriction.
• High-dose loperamide — QT prolongation and torsades de pointes, an emergency; also toxic megacolon if given during unrecognized colitis.
• TCAs — anticholinergic burden (urinary retention, delirium in the elderly), orthostasis, QT prolongation, and lethality in overdose.
• Secretagogues — severe diarrhea with dehydration and hypokalemia; linaclotide carries a boxed warning against use in patients <2 years of age because of fatal dehydration in juvenile animal studies.
• Opioids — narcotic bowel syndrome: paradoxical worsening pain requiring escalating doses; recognized by improvement only after opioid withdrawal.
• **Rifaximin — rare Clostridioides difficile infection**; suspect with new fever and profuse diarrhea.

• Rome IV is the answer when asked how IBS is diagnosed: recurrent abdominal pain ≥1 day/week for 3 months, onset ≥6 months prior, plus ≥2 of 3 features (related to defecation, change in stool frequency, change in stool form). IBS is a positive diagnosis, not a diagnosis of exclusion.
• Pain relieved by defecation, worse after meals, absent at night, with a normal exam is the classic stem. Nocturnal diarrhea, weight loss, hematochezia, anemia, or fever are NOT IBS — the single best next step there is colonoscopy, not symptomatic therapy.
• The one association examiners love: IBS-D and celiac disease. Order tTG-IgA with total IgA before labeling a diarrhea-predominant patient. Fecal calprotectin is the discriminator against IBD.
• Post-infectious IBS after Campylobacter, Salmonella, Shigella, or Giardia — IBS-D phenotype, better long-term prognosis.
• Soluble fiber (psyllium) helps; insoluble fiber (bran) makes it worse — a frequently tested reversal. Likewise, PEG improves constipation but not pain in IBS-C.
• Alosetron (5-HT3 antagonist) → ischemic colitis and severe constipation; eluxadoline → pancreatitis/sphincter of Oddi spasm, contraindicated without a gallbladder. Both are restricted-use drugs.
• TCAs (amitriptyline) for global IBS symptoms, especially IBS-D; SSRIs are preferred when constipation or comorbid depression dominates. Add CBT or gut-directed hypnotherapy for refractory disease (ACG 2021).
• Common distractors: ordering routine colonoscopy, stool ova and parasites, or food-allergy panels in a young patient meeting Rome IV without alarm features; attributing new bleeding to "known IBS"; prescribing opioids (narcotic bowel syndrome); and expecting IBS to progress to IBD or colon cancer — it does not.
• In an older woman with chronic watery diarrhea and a normal-appearing colon, think microscopic colitis — the diagnosis requires random mucosal biopsies.