Irritable Bowel Syndrome¶
Chapter 338 | Part 10: Disorders of the Gastrointestinal System
KEY CLINICAL POINTS¶
- Diagnosis based on Rome IV criteria: recurrent abdominal pain ≥ 1 day/week + ≥ 2 of stool frequency/form changes
- IBS-C, IBS-D, IBS-M subtypes with dynamic subtype shifts over time
- Multifactorial pathophysiology including visceral hypersensitivity, gut-brain interaction, and microbiota dysbiosis
- Low FODMAP diet effective for 50-80% of patients with symptom improvement
- Tricyclic antidepressants (TCAs) and serotonin modulators are first-line pharmacologic agents
1. DEFINITION & OVERVIEW¶
IBS is a functional bowel disorder characterized by abdominal pain/discomfort and altered bowel habits in the absence of structural abnormalities. Diagnosis relies on clinical presentation and Rome IV criteria. No definitive biomarkers exist, requiring exclusion of organic diseases.
Table 338-1 Rome IV Diagnostic Criteria for Irritable Bowel Syndrome¶
| Criteria | Description |
|---|---|
| Recurrent abdominal pain | ‡1 day/week in last 3 months |
| Related to defecation | Yes |
| Change in stool frequency | Yes |
| Change in stool form | Yes |
1.1 Rome IV Diagnostic Criteria¶
Recurrent abdominal pain ≥ 1 day/week in last 3 months associated with ≥ 2 of: 1) related to defecation, 2) change in stool frequency, 3) change in stool form. Criteria fulfilled for last 3 months with symptom onset ≥ 6 months prior.
1.2 Subtypes¶
IBS-C (constipation), IBS-D (diarrhea), IBS-M (mixed). Subtypes are unstable, with 75% of patients changing subtypes over 1 year.
2. EPIDEMIOLOGY¶
Affects all ages, with peak onset <45 years. Women diagnosed 2-3x more often than men (80% of severe cases). Global prevalence ~10% in adults. Risk factors include stress, psychological factors, and post-infectious triggers.
2.1 Demographics¶
Female predominance (80% of severe cases). Prevalence 70% for mild, 25% moderate, 5% severe. Higher in younger populations.
2.2 Risk Factors¶
Stress, psychological disturbances, postinfectious triggers (10% of infectious enteritis cases develop IBS), and genetic/environmental interactions.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Multifactorial: gut motility abnormalities, visceral hypersensitivity, central nervous system dysfunction, psychological factors, immune activation, and gut microbiota dysbiosis. TRPV1 channels, mast cell activation, and serotonin pathways play key roles.
Table 338-2 Some Common Food Sources of FODMAPs¶
| FOOD TYPE | FREE FRUCTOSE | LACTOSE | FRUCTANS | GALACTO-OLI GOSACCHARID ES | POLYOLS |
|---|---|---|---|---|---|
| Fruits | Apple, cherry, mango, pear, watermelon | Peach, persimmon, watermelon | Apple, apricot, pear, avocado, blackberries, cherry, nectarine, plum, prune | ||
| Vegetables | Asparagus, artichokes, sugar snap peas | Artichokes, beetroot, Brussels sprout, chicory, fennel, garlic, leek, onion, peas | |||
| Grains and cereals | Wheat, rye, barley | ||||
| Nuts and seeds | Pistachios | ||||
| Milk and milk products | Milk, yogurt, ice cream, custard, soft cheeses | ||||
| Legumes | Legumes, lentils, chickpeas | Legumes, chickpeas, lentils | |||
| Other | Honey, high-fructose corn syrup | Chicory drinks | Inulin, FOS | Sorbitol, mannitol, maltitol, xylitol, isomalt |
3.1 Visceral Hypersensitivity¶
Exaggerated sensory responses to visceral stimuli. Rectal balloon inflation causes prolonged distention-evoked contractions. Brain imaging shows increased mid-cingulate cortex activation.
3.2 Gut-Brain Interaction¶
Central nervous system (CNS) factors modulate pain perception. Functional MRI shows altered brain activation patterns in response to colonic stimulation.
3.3 Microbiota Dysbiosis¶
Altered gut flora with increased intestinal permeability. Bile acid malabsorption and small intestinal bacterial overgrowth (SIBO) are common in IBS-D.
4. CLINICAL FEATURES¶
Abdominal pain (essential criterion), altered bowel habits (constipation, diarrhea, or mixed), bloating, gas, and dyspepsia. Symptoms fluctuate and often overlap with other functional disorders.
4.1 Abdominal Pain¶
Episodic, crampy, or constant. Worsened by eating/emotional stress, relieved by defecation. Not associated with painless diarrhea or constipation.
4.2 Altered Bowel Habits¶
IBS-C: hard stools, infrequent bowel movements, straining. IBS-D: loose stools, urgency, mucus, incontinence. IBS-M: alternating constipation/diarrhea.
4.3 Other Symptoms¶
Bloating, flatulence, dyspepsia, fatigue, and anxiety. Symptoms may worsen during premenstrual phases.
5. DIFFERENTIAL DIAGNOSIS¶
Inflammatory bowel disease (IBD), celiac disease, lactose intolerance, infections, microscopic colitis, and other GI disorders. Exclude organic causes with imaging and labs.
5.1 Red Flags for Organic Disease¶
Weight loss, rectal bleeding, nocturnal diarrhea, steatorrhea, or symptoms starting after age 40.
5.2 Associated Conditions¶
Fibromyalgia, chronic fatigue, and mood disorders may coexist due to shared neurobiological pathways.
6. INVESTIGATIONS & DIAGNOSIS¶
Rome IV criteria as primary diagnostic tool. Stool calprotectin (to exclude IBD), lactose breath test, and imaging for alarm features. Exclude celiac disease with serology.
6.1 Diagnostic Tests¶
Stool calprotectin (normal <50 µg/g), lactose breath test, colonoscopy for alarm features, and abdominal imaging.
6.2 Exclusion Criteria¶
Rule out IBD, celiac disease, infections, and structural abnormalities with appropriate testing.
7. MANAGEMENT & TREATMENT¶
Dietary modifications (low FODMAP diet), pharmacologic agents (antispasmodics, antidiarrheals, laxatives), and psychological interventions. Target symptoms with specific therapies.
Table 338-3 Spectrum of Severity in IBS¶
| MILD | MODERATE | SEVERE | |
|---|---|---|---|
| Prevalence | 70% | 25% | 5% |
| Symptoms constant | 0 | + | +++ |
| Health care issues | + | ++ | +++ |
| Practice type | Primary | Specialty | Referral |
Table 338-4 Possible Drugs for a Dominant Symptom in IBS¶
| SYMPTOM | DRUG | DOSE |
|---|---|---|
| Diarrhea | Loperamide | 2–4 mg when necessary/maximum 12 g/d |
| Diarrhea | Diphenoxylate hydrochloride and atropine sulfate (Lomotil) | 1–2 tabs as needed or daily (up to 4 times daily) |
| Diarrhea | Cholestyramine resin | 4 g with meals, increased to tid |
| Diarrhea | Eluxadoline | 100 mg bid |
| Diarrhea | Alosetrona | 0.5–1 mg bid (for severe IBS, women) |
| Constipation | Psyllium husk | 3–4 g bid with meals |
| Constipation | Methylcellulose | 2 g bid with meals |
| Constipation | Calcium polycarbophil | 1 g qd to qid |
| Constipation | Lactulose syrup | 10–20 g bid |
| Constipation | 70% sorbitol | 15 mL bid |
| Constipation | Polyethylene glycol 3350 | 17 g in 250 mL water qd |
| Constipation | Lubiprostone | 8 or 24 µg bid |
| Constipation | Magnesium hydroxide | 15–60 mL qd |
| Constipation | Linaclotide | 72, 145, and 290 mg qd |
| Constipation | Plecanatide | 3 mg qd |
| Constipation | Tenapanor | 50 mg bid |
| Abdominal pain | Smooth-muscle relaxant | qd to qid ac |
| Abdominal pain | Tricyclic antidepressants | Start 25–50 mg hs, then adjust |
7.1 Dietary Interventions¶
Low FODMAP diet (3-step process: restriction, reintroduction, maintenance). Psyllium, soluble fiber, and prebiotics may help.
7.2 Pharmacologic Agents¶
Loperamide for diarrhea, lubiprostone/linaclotide for constipation, TCAs for pain, and rifaximin for postinfectious IBS.
7.3 Psychological Approaches¶
Stress management, cognitive behavioral therapy, and mindfulness for symptom exacerbation.
8. PROGNOSIS & COMPLICATIONS¶
Chronic but not life-threatening. Complications include reduced quality of life and work absenteeism. No serious complications, but symptoms may fluctuate over time.
8.1 Quality of Life¶
Significant impairment in daily activities. Direct/indirect healthcare costs are high due to symptom fluctuation.
8.2 Long-Term Outcomes¶
Symptoms may persist for years but do not progress to IBD or colorectal cancer. Psychological factors influence disease course.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: no specific contraindications. Pediatrics: IBS is rare in children. Elderly: consider drug interactions and comorbidities. Avoid restrictive diets in eating disorders.
9.1 Pregnancy¶
Symptoms may worsen during premenstrual phases. Safe medications include TCAs and low-dose antispasmodics.
9.2 Psychosocial Factors¶
Anxiety/depression are common comorbidities. Cognitive behavioral therapy may improve outcomes.
10. KEY POINTS & CLINICAL PEARLS¶
- Use Rome IV criteria for diagnosis. 2. Low FODMAP diet is effective for 50-80% of patients. 3. Target symptoms with specific pharmacologic agents. 4. Exclude organic causes with appropriate testing. 5. Psychological factors significantly influence disease course.