Irritable Bowel Syndrome (IBS) Detailed Notes

Irritable Bowel Syndrome (IBS)

IBS is a functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits in the absence of detectable organic pathology. Diagnostic precision has evolved from subjective terms to the Rome criteria.

Rome IV Criteria

Diagnostic Criteria:

• Recurrent abdominal pain, on average, at least one day per week in the last three months, associated with two or more of the following:

  • Related to defecation

  • Associated with a change in frequency of stool

  • Associated with a change in form (appearance) of stool

• Symptoms must have started at least 6 months prior to diagnosis.

Associated findings:

• Relief of pain on defecation with passage of gas.

• Change in frequency of stool.

• Change in stool appearance.

Subtypes of IBS

• IBS-D: Diarrhea predominant

• IBS-C: Constipation predominant

• IBS-M: Mixed type (alternating between diarrhea and constipation)

Patients can switch between subtypes over time.

Differentiation from Inflammatory Bowel Disease (IBD)

It's crucial to distinguish IBS from IBD (e.g., Crohn's disease, ulcerative colitis). Symptoms of IBD can include rectal bleeding and weight loss. IBD involves structural changes and inflammation in the gut; stricture formation can cause colicky pain, diarrhea, or constipation.

Key Differentiating Features:

Diagnostic Tests to Differentiate:

• Endoscopy: Direct visualization of the GI tract via colonoscopy or sigmoidoscopy to identify inflammation, ulcers, or strictures.

• Biomarkers:

  • Fecal Calprotectin: Elevated levels indicate intestinal inflammation, suggestive of IBD. Normal range is typically < 50 mcg/g, but this can vary by lab.

  • Fecal Lactoferrin: Another marker of intestinal inflammation, less commonly used than fecal calprotectin. Useful in detecting neutrophil activity in the gut.

• Imaging: CT or MR enterography can help visualize bowel wall thickening, inflammation, and strictures. Capsule endoscopy can be used to visualize the small intestine when other imaging is inconclusive.

Etiology and Pathophysiology of IBS

The exact cause of IBS is multifactorial, involving gut-brain interactions, altered motility, visceral hypersensitivity, inflammation, and psychosocial factors. Dysbiosis, genetic predisposition, and environmental factors all play significant roles.

Dysbiosis:

• Imbalance in gut flora (good vs. bad bacteria), affecting the fermentation process and gas production.

• Altered Firmicutes to Bacteroidetes (FB) ratio, impacting metabolic functions in the gut.

• Increased intestinal permeability (“leaky gut”), leading to greater antigen exposure to antigen-presenting cells (APCs).

• Increased cytokine production, resulting in:

  • Altered enteric neuronal function (affecting peristalsis and bowel motility)

  • Altered smooth muscle function in the GI tract

These factors can contribute to diarrhea, constipation, or mixed symptoms.

Clinical Features of IBS

• Constipation:

  • Intractable to laxatives, due to altered colonic motility and reduced responsiveness to stimulant laxatives.

  • Hard, narrow caliber stools due to prolonged colonic transit and increased water reabsorption.

• Diarrhea:

  • Subjective; defined as stool volume > 200 mL/day or stool weight > 200 grams/day. Often associated with urgency and incomplete evacuation.

• Belching, Bloating, and Borborygmi:

  • Excessive gas and abdominal noises, often exacerbated by certain foods or eating habits.

• Absence of Nocturnal Diarrhea or Bleeding:

  • Differentiates IBS from IBD, as these symptoms are more indicative of structural or inflammatory conditions.

• Functional Overlap:

  • Nausea, vomiting, and dyspepsia (often with negative workup for other GI disorders like H. pylori). These symptoms suggest upper GI involvement and altered visceral sensitivity.

• Postprandial Abdominal Pain:

  • Pain after eating due to altered central pain processing. This can be immediate or delayed, depending on individual sensitivity and meal composition.

  • Visceral distension perceived as pain due to faulty central pain processing in the brain. Altered pain thresholds and magnified perception of normal physiological processes.

Workup for IBS

• Complete Blood Count (CBC): To rule out anemia (more common in IBD) and to assess overall health.

• Stool Studies: Microscopic examination for ova and parasites (rule out parasitic infections). Consider testing for Clostridium difficile in diarrhea-predominant cases.

• Imaging Studies: CT or MR enterography to rule out IBD and other structural abnormalities. These help visualize the bowel and identify potential issues such as tumors or strictures.

• Sigmoidoscopy with Biopsy: To exclude IBD, microscopic colitis, and other structural abnormalities. Biopsies are essential to rule out inflammation at the cellular level.

• Breath Hydrogen Test: To assess for lactose malabsorption and small intestinal bacterial overgrowth (SIBO). Elevated hydrogen levels indicate bacterial fermentation of unabsorbed carbohydrates.

• Celiac Serology: Anti-tissue transglutaminase (anti-TTG) and anti-endomysial antibodies to rule out celiac sprue, especially in younger patients with diarrhea and anemia. Consider total IgA levels to rule out IgA deficiency, which can cause false negatives.

Treatment of IBS

The approach is multimodal, focusing on symptom management and addressing underlying factors. Treatment should be individualized based on the predominant symptoms.

• Dietary Modifications:

  • Low-FODMAP diet (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) to reduce gas production and abdominal distension. This involves eliminating high-FODMAP foods like garlic, onions, apples, and dairy products.

• Constipation Management:

  • Tegasirad: 5-HT4 receptor agonist to promote peristalsis. Limited availability due to cardiovascular risks.

  • Lubiprostone: Chloride channel opener to increase water secretion into the gut, softening the stool. Works by electro neutrality concept (loss of negative charges).

  • Linaclotide: Guanylate cyclase-C agonist to increase cyclic GMP levels, improving GI motility and reducing visceral pain perception (nociceptor stimuli).

• Antispasmodics:

  • Dicyclomine to relieve crampy abdominal pain. Should be used cautiously due to anticholinergic side effects.

• Neuromodulators:

  • SSRIs (e.g., paroxetine) to modulate central pain processing. Useful in patients with comorbid anxiety or depression.

• Antibiotics:

  • Rifaximin or neomycin to restore gut flora balance (Firmicutes and Bacteroidetes ratio). Rifaximin is less likely to cause systemic side effects.

• Prebiotics and Probiotics:

  • Prebiotics promote the growth of beneficial bacteria. Inulin, fructooligosaccharides (FOS).

  • Probiotics are live microbes (e.g., Bifidobacterium, Lactobacillus) to restore the FB ratio in the gut. Selection should be based on specific symptoms and strains.

• Diarrhea Management:

  • Loperamide to reduce GI motility. Use with caution to avoid constipation.

  • Cholestyramine for bile acid diarrhea (due to bacterial deconjugation of bile acids). May cause bloating and constipation.

• Fiber Supplementation:

  • Bran and psyllium can be used. Ensure adequate fluid intake to prevent worsening of constipation.

The overall goal is to improve the patient's quality of life by addressing their specific symptoms and underlying contributing factors. A multidisciplinary approach, including dietary, pharmacological, and psychological interventions, is often necessary.