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Inflammatory Bowel Disease for the USMLE Step 2

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Inflammatory Bowel Disease (IBD) for the USMLE Step 2 Exam
Overview
  • Inflammatory Bowel Disease (IBD) includes Ulcerative Colitis (UC) and Crohn’s Disease (CD), both characterized by chronic inflammation of the gastrointestinal (GI) tract.
  • UC involves the colon, while CD can affect any part of the GI tract from mouth to anus.
Pathophysiology
  • Immune dysregulation: An abnormal immune response to gut microbiota leads to chronic inflammation.
    • UC: Inflammation is confined to the mucosa and submucosa.
    • CD: Transmural inflammation affects all layers of the bowel wall.
  • Genetic predisposition: Mutations such as NOD2 in Crohn’s disease, along with familial clustering.
  • Environmental factors: Smoking worsens Crohn's disease but is protective in UC.
  • Microbiome: Dysbiosis, or altered gut flora, contributes to disease development.
Ulcerative Colitis (UC)
  • Location: Always involves the rectum and can extend proximally in a continuous pattern, affecting only the colon.
  • Histology: Limited to mucosal and submucosal inflammation.
Ulcerative Colitis
  • Clinical presentation:
    • Bloody diarrhea is the hallmark.
    • LLQ pain and tenesmus (feeling of incomplete evacuation).
  • Complications:
    • Toxic megacolon: Acute colonic dilation with risk of perforation.
    • Colon cancer: Increased risk after 8–10 years of extensive disease.
Crohn’s Disease (CD)
  • Location: Can involve any part of the GI tract, most commonly the terminal ileum and proximal colon. Characterized by skip lesions (discontinuous areas of inflammation).
  • Histology: Transmural inflammation, leading to deeper tissue damage.
Crohn’s Disease
  • Clinical presentation:
    • Non-bloody diarrhea and RLQ pain.
    • Perianal disease: Fistulas, abscesses, and skin tags.
  • Complications:
    • Strictures: Fibrotic narrowing of the bowel causing obstruction.
    • Fistulas: Abnormal connections between bowel and adjacent structures (e.g., bladder, skin).
    • Malabsorption: Common with extensive small bowel involvement, leading to deficiencies such as vitamin B12 and fat-soluble vitamins.
Diagnosis
  • Endoscopy with biopsy:
    • UC: Continuous inflammation starting at the rectum.
    • CD: Skip lesions, deep ulcers, and non-caseating granulomas.
  • Imaging:
    • CT/MRI enterography: Preferred for evaluating small bowel involvement in Crohn’s disease.
  • Lab tests:
    • CRP and ESR are elevated during active inflammation.
    • Fecal calprotectin: A sensitive marker of GI inflammation.
Treatment
  • Induction of remission:
    • Corticosteroids: For moderate to severe flares.
    • Budesonide: Preferred for mild Crohn’s disease affecting the ileum and right colon.
  • Maintenance of remission:
    • 5-Aminosalicylates (5-ASA): Effective in mild to moderate UC.
    • Immunomodulators: Azathioprine or methotrexate used for long-term management to reduce reliance on corticosteroids.
    • Biologics: Anti-TNF agents (e.g., infliximab) and newer agents like vedolizumab (anti-integrin) for moderate to severe UC or CD.
Surgery
  • UC: Total colectomy is curative and indicated in refractory disease or cancer prevention.
  • CD: Surgery is reserved for complications (e.g., strictures, fistulas), but recurrence is common postoperatively.
Monitoring
  • Colon cancer screening: Patients with extensive colonic involvement should undergo colonoscopy every 1–2 years starting 8 years after disease onset.
  • Bone density screening: Patients on long-term corticosteroids should be monitored for osteoporosis.
Key Points
  • Ulcerative Colitis (UC) affects only the colon, with continuous inflammation starting in the rectum. Presents with bloody diarrhea.
  • Crohn’s Disease (CD) can involve any part of the GI tract, with skip lesions and transmural inflammation. Presents with non-bloody diarrhea and complications like fistulas and strictures.
  • Diagnosis relies on endoscopy with biopsy for both diseases, supported by imaging and laboratory markers of inflammation.
  • Treatment includes corticosteroids for flares and immunosuppressants or biologics for long-term control.
  • Regular colon cancer screening is important for patients with long-standing colonic involvement.

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