Irritable Bowel Syndrome¶
Chapter 338 | Disorders of the Gastrointestinal System · Part 10 – Gastrointestinal Disorders · Chapter 338
Key Clinical Points¶
- IBS is a functional gastrointestinal disorder characterized by abdominal pain and altered bowel habits without structural abnormalities.
- Diagnosis is based on Rome IV criteria: recurrent abdominal pain (≥1 day/week for 3 months) associated with ≥2 of: relation to defecation, change in stool frequency, or change in stool form.
- Alarm features (age >40 at onset, weight loss, nocturnal symptoms, rectal bleeding, anemia) require investigation to rule out organic disease.
- Core pathophysiology includes visceral hypersensitivity, central neural dysregulation, and gut-brain interaction.
- Postinfectious IBS occurs in >10% of patients with infectious enteritis (4-fold higher risk than non-infected individuals).
- Low FODMAP diet improves symptoms in 50–80% of patients, primarily addressing pain and bloating.
- Rifaximin (550 mg TID for 2 weeks) is effective for IBS-D with bloating and gas.
- Secretagogues (Linaclotide, Lubiprostone) are used for IBS-C; Eluxadoline is used for IBS-D.
- Gut dysbiosis in IBS involves decreased Bifidobacterium/Faecalibacterium and increased Enterobacteriaceae/Lactobacillaceae/Bacteroides.
- Up to 25% of patients with IBS-D may have underlying bile acid malabsorption.
1. DEFINITION & OVERVIEW¶
• Definition: Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by abdominal pain or discomfort and altered bowel habits in the absence of detectable structural abnormalities. • Clinical Characteristics: ◦ No specific diagnostic markers exist; diagnosis is based on clinical presentation. ◦ Symptoms fluctuate over time and often overlap with other functional disorders (fibromyalgia, headache, backache, and genitourinary symptoms). ◦ Significant impact on quality of life and high direct/indirect healthcare costs. • Rome IV Diagnostic Criteria: ◦ Requirement: Symptoms for ≥3 months, with onset at least 6 months prior to diagnosis. ◦ Core Criteria: Recurrent abdominal pain (average ≥1 day per week in the last 3 months) associated with ≥2 of the following: → Related to defecation → Associated with a change in frequency of stool → Associated with a change in form (appearance) of stool
2. EPIDEMIOLOGY¶
• Prevalence: Approximately 10% of adults and adolescents worldwide. ◦ Most patients present before age 45. • Gender Distribution: ◦ Women are diagnosed 2–3 times more often than men. ◦ Women comprise 80% of the population with severe IBS. • Comorbidities: High prevalence in other functional GI disorders (e.g., noncardiac chest pain, esophageal hypersensitivity).
3. ETIOLOGY & PATHOPHYSIOLOGY¶
• Multifactorial Nature: Involves genetic susceptibility and environmental insults.
• 3.1 GI Motor Abnormalities: ◦ No consistent abnormalities in unstimulated conditions. ◦ Prominent under stimulated conditions (e.g., post-prandial). ◦ IBS-D: Increased motility index and peak amplitude of high-amplitude propagating contractions (HAPCs) → rapid colonic transit → abdominal pain.
• 3.2 Visceral Hypersensitivity: ◦ Exaggerated sensory responses to visceral stimulation. ◦ Food intolerance perception is ≥2x more common than in the general population. ◦ Lower thresholds for first sensation of gas, discomfort, and pain (especially with lipids). ◦ Selection: Afferent pathway disturbances are specific to visceral innervation; somatic pathways are spared.
• 3.3 Central Neural Dysregulation: ◦ Evidence from functional brain imaging (MRI). ◦ Mid-cingulate cortex: Greater activation in response to distal colonic stimulation → increased perception of unpleasantness. ◦ Prefrontal lobe: Shows hypervigilance of visceral pain.
• 3.4 Abnormal Psychological Features: ◦ Present in up to 80% of patients. ◦ Stress alters sensory thresholds. ◦ History of sexual or physical abuse associated with higher distress and worse outcomes (linked to activation of posterior/middle dorsal cingulate cortex).
• 3.5 Postinfectious IBS: ◦ >10% of patients with infectious enteritis develop IBS (4-fold risk compared to non-infected individuals). ◦ Higher risk in females, younger patients, and those with severe/prolonged infections (parasitic/bacterial gt viral).
• 3.6 Immune Activation & Mucosal Inflammation: ◦ Low-grade mucosal inflammation; activated lymphocytes, mast cells, and proinflammatory cytokines (IL-6, IL-1β, TNF). ◦ Mast cell activation mediates barrier dysfunction.
• 3.7 Altered Intestinal Permeability: ◦ "Leaky gut" primarily in IBS-D and postinfectious IBS. ◦ Reduced expression of tight junction proteins → increased passage of macromolecules/bacteria.
• 3.8 Altered Gut Flora: ◦ 4x higher odds of small-intestinal bacterial overgrowth (SIBO) based on breath tests. ◦ Lower diversity in IBS patients. ◦ Decreased: Bifidobacterium, Faecalibacterium. ◦ Increased: Enterobacteriaceae, Lactobacillaceae, Bacteroides.
• 3.9 Abnormal Serotonin Pathways: ◦ Increased enterochromaffin cells in IBS-D. ◦ TPH1 polymorphism associated with IBS subtypes. ◦ SERT downregulation due to gram--negative gut dysbiosis → abnormal mucosal serotonin levels.
• 3.10 Bile Acids: ◦ Up to 25% of patients with IBS-D have idiopathic bile acid diarrhea. ◦ Mechanism: Reduced synthesis of FGF-19 by ileal mucosa or genetic variation → increased colonic bile acid → accelerated transit, increased permeability, and visceral hypersensitivity.
4. CLINICAL FEATURES¶
• Abdominal Pain: ◦ Essential for diagnosis. ◦ Variable intensity/location; often episodic/crampy or constant ache. ◦ Exacerbated by eating/stress; improved by flatus/stool passage. ◦ Females: Worsening during premenstrual/menstrual phases.
• Altered Bowel Habits: ◦ Most consistent clinical feature. ◦ Subtypes: IBS-C (constipation), IBS-D (diarrhea), IBS-M (mixed). ◦ IBS-C: Hard stools, infrequent (<3/week), incomplete evacuation, straining. ◦ IBS-D: Small volumes of loose stools (<200 mL), urgency, mucus, incontinence.
• Gas and Flatulence: ◦ Patients report bloating/distention; however, gas volume is often normal. ◦ Mechanism: Impaired transit → intolerance of gas loads → reflux of gas from distal to proximal intestine (belching).
• Upper GI Symptoms: ◦ 25–50% experience dyspepsia, heartburn, nausea, vomiting. ◦ High overlap between dyspepsia and IBS.
5. DIFFERENTIAL DIAGNOSIS¶
• Epigastric/Periumbilical Pain: → Differentiate from: Biliary tract disease, peptic ulcer, intestinal ischemia, carcinoma (stomach/pancreas). • Lower Abdominal Pain: → Differentiate from: Diverticular disease, IBD (UC/Crohn's), colon cancer. • Postprandial Symptoms (Nausea/Vomiting): → Differentiate from: Gastroparesis, partial intestinal obstruction. • Diarrhea-specific: → Rule out: Lactase deficiency, malabsorption, celiac sprue, hyperthyroidism, IBD, infectious diarrhea. • Constipation-specific: → Rule out: Drug effects (anticholinergic, antihypertensive), hypothyroidism/hypoparathyroidism, acute intermittent porphyria, lead poisoning. • Infectious: → Giardia lamblia or other parasites.
6. INVESTIGATIONS & DIAGNOSIS¶
- Clinical Evaluation: Perform thorough history and physical exam to identify typical features (recurring pain, no progression, absence of systemic symptoms) and rule out organic disease.
- Routine Laboratory Testing: • CBC, fecal calprotectin (or lactoferrin), and/or CRP in IBS-D patients even without alarm features → rule out IBD.
- Serologic Testing: Perform celiac serology if prevalence of celiac disease is ≥1% in the population.
- Stool Examination: Test for ova and parasites if in endemic regions or history suggests exposure to untreated water.
- Specific Symptom-Driven Tests: • For diarrhea/gas: Hydrogen breath test (lactase deficiency) or 3-week lactose-free diet. • For excessive gas/bloating: Glucose hydrogen breath test (rule out SIBO).
- Imaging/Endoscopy: • If dyspepsia present: Upper GI radiographs or esophagogastroduodenoscopy. • If postprandial RUQ pain: Ultrasound of the gallbladder.
- Alarm Features (Require Colonoscopy): • Age >40 at onset. • Weight loss. • Nocturnal symptoms. • Rectal bleeding. • Anemia. • Persistent diarrhea after 48-h fast. • Steatorrheal stools.
- Laboratory Findings Arguing Against IBS: → Evidence of anemia, elevated sedimentation rate, leukocytes/blood in stool, or stool volume >200–300 mL/d.
7. MANAGEMENT & TREATMENT¶
- Education and Counseling: • Reassurance regarding functional nature. • Identification of personal triggers.
- Lifestyle Modifications: • Stress management. • Exercise (recommended for all patients).
- Dietary Management: • Elimination of identified food precipitants. • Low FODMAP diet: Reduces global symptoms in 50–80% of patients (improves pain and bloating). Common sources include apples, wheat, lactose, etc.
- Pharmacotherapy: • Stool-Bulking Agents: → Psyllium husk, Methylcellulose, Calcium polycarbophil. • Antidiarrheal Agents: → Loperamide: 2–4 mg when necessary (max 12 g/d). → Diphenoxylate and atropine sulfate (Lomotil): 1–2 tabs as needed or daily. → Eluxadoline: 100 mg bid (approved for IBS-D). → Alosetrona: 0.5–1 mg bid (severe IBS, women). → Linaclotide: (for IBS-C). → Lubiprostone: (for IBS-C). → Other options: Cholestyramine, Lactulose, 70% sorbitol, Polyethylene glycol 3350, Magnesium hydroxide, Plecanatide, Tenapanor. • Antispasmodics: → Smooth-muscle relaxants: qd to qid ac. • Neuromodulators: → Tricyclic antidepressants (TCAs): Start 25–50 mg hs, then adjust (effective for IBS-D by slowing transit and modulating visceral afferent neural function). • Antimicrobials: → Rifaximin: 550 mg TID for 2 weeks (for IBS-D with bloating/gas). • Probiotics: → Used to modulate gut flora.
8. PROGNOSIS & COMPLICATIONS¶
• Prognosis: ◦ Generally good; no progression of disease. ◦ Malnutrition is extremely rare. ◦ Sleep deprivation is uncommon as pain is typically only during waking hours. • Complications: ◦ Impact on quality of life and productivity. ◦ Potential for overlap with other functional disorders (fibromyalgia, etc.).
9. SPECIAL CONSIDERATIONS¶
• Pediatric/Adolescent: → Symptoms often begin before age 45; similar management principles apply. • Postinfective Patients: → Higher risk in females and younger patients; may require more aggressive investigation of underlying triggers.
10. KEY PEARLS & CLINICAL TRAPS¶
• Rome IV is the gold standard for diagnosis based on pain + 2 features (defecation, frequency, form). • Alarm features are the primary trigger to escalate to invasive testing (colonoscopy/endoscopy). • Visceral hypersensitivity is a core mechanism; patients have lower thresholds for pain than healthy controls. • Rifaximin is specifically indicated for IBS-D with gas/bloating. • Low FODMAP diet is highly effective but should not lead to nutritionally deficient diets. • Bile acid malabsorption is a common underlying cause in a subset of IBS-D patients.
Reference Tables¶
TABLE 338-1 Rome IV Diagnostic Criteria for Irritable Bowel Syndrome a Recurrent abdominal pain, on average, at least 1…¶
Harrison's 22e, p.2571
- Recurrent abdominal pain, on average, at least 1 day per week in the last
3 months, associated with ≥2 of the following criteria:
1. Related to defecation
2. Associated with a change in frequency of stool
3. Associated with a change in form (appearance) of stool
TABLE 338-2 Some Common Food Sources of FODMAPs¶
Harrison's 22e, p.2576
| FOOD TYPE | FREE FRUCTOSE | LACTOSE | FRUCTANS | GALACTO- OLIGOSACCHARIDES |
POLYOLS |
|---|---|---|---|---|---|
| Fruits | Apple, cherry, mango, pear, watermelon |
Peach, persimmon, watermelon | Apple, apricot, pear, avocado, blackberries, cherry, nectarine, plum, prune |
||
| Asparagus, artichokes, sugar snap peas |
Artichokes, beetroot, Brussels sprout, chicory, fennel, garlic, leek, onion, peas |
||||
| Grains and cereals | Wheat, rye, barley | ||||
| Pistachios | |||||
| Milk and milk products | Milk, yogurt, ice cream, custard, soft cheeses |
||||
| Legumes, lentils, chickpeas | Legumes, chickpeas, lentils |
||||
| Other | Honey, high-fructose corn syrup |
Chicory drinks | |||
| Inulin, FOS |
TABLE 338-4 Possible Drugs for a Dominant Symptom in¶
Harrison's 22e, p.2577
| SYMPTOM | DRUG | DOSE |
|---|---|---|
| Diarrhea | Loperamide | 2–4 mg when necessary/maximum 12 g/d |
| Diphenoxylate hydrochloride and atropine sulfate (Lomotil) |
1–2 tabs as needed or daily (can be taken up to 4 times daily) |
|
| Cholestyramine resin | 4 g with meals, can be started qd and increased to tid |
|
| Eluxadoline | 100 mg bid | |
| Alosetrona | 0.5–1 mg bid (for severe IBS, women) |
|
| Psyllium husk | ||
| Methylcellulose | ||
| Calcium polycarbophil | ||
| Lactulose syrup | ||
| 70% sorbitol | ||
| Polyethylene glycol 3350 | ||
| Lubiprostone | ||
| Magnesium hydroxide | ||
| Linaclotide | ||
| Plecanatide | ||
| Tenapanor | ||
| Abdominal pain | Smooth-muscle relaxant | qd to qid ac |
| Tricyclic antidepressants | Start 25–50 mg hs, then adjust | |
| Low FODMAP diet | ||
| Low FODMAP diet | ||
| Probiotics | ||
| Rifaximin |
TABLE 338-3 Spectrum of Severity in IBS Clinical Features Prevalence Correlations with gut physiology Symptoms constant…¶
Harrison's 22e, p.2577
| MILD | MODERATE | SEVERE | |
|---|---|---|---|
| Clinical Features | |||
| Prevalence | 70% | 25% | 5% |
| +++ | ++ | ||
| Symptoms constant | 0 | + | +++ |
| 0 | + | ||
| Health care issues | + | ++ | +++ |
| Primary | Specialty |