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Inflammatory Bowel Disease for the USMLE Step 3

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Inflammatory Bowel Disease (IBD) for the USMLE Step 3 Exam
Overview
  • Inflammatory Bowel Disease (IBD) encompasses Ulcerative Colitis (UC) and Crohn’s Disease (CD), both marked by chronic, relapsing inflammation of the gastrointestinal (GI) tract.
  • UC affects only the colon, while CD can involve any part of the GI tract, most commonly the terminal ileum and colon.
Pathophysiology
  • Immune dysregulation: Abnormal immune response to intestinal microbiota in genetically predisposed individuals.
    • UC: Inflammation is confined to the mucosa and submucosa.
    • CD: Inflammation is transmural, affecting all layers of the bowel wall.
  • Genetics: Mutations such as NOD2 in Crohn’s disease, with familial clustering.
  • Environmental triggers: Smoking worsens CD but is protective in UC.
  • Microbiome: Dysbiosis (altered gut microbiota) plays a role in disease development.
Ulcerative Colitis (UC)
  • Location: Confined to the colon, always involves the rectum, and extends proximally in a continuous manner.
  • Histology: Inflammation is limited to the mucosa and submucosa.
Ulcerative Colitis
  • Clinical presentation:
    • Bloody diarrhea, often with LLQ pain and tenesmus.
  • Complications:
    • Toxic megacolon: Acute colonic dilation with risk of perforation.
    • Colon cancer: Increased risk after 8–10 years of extensive colitis.
Crohn’s Disease (CD)
  • Location: Can affect any part of the GI tract, commonly the terminal ileum and proximal colon, with skip lesions (discontinuous inflammation).
  • Histology: Transmural inflammation leading to deeper tissue damage.
Crohn’s Disease
  • Clinical presentation:
    • Non-bloody diarrhea, RLQ pain.
    • Perianal disease: Fistulas, abscesses, and skin tags.
  • Complications:
    • Strictures: Bowel narrowing due to fibrosis and inflammation, leading to obstruction.
    • Fistulas: Abnormal connections between the bowel and adjacent organs.
    • Malabsorption: Common in patients with small bowel involvement, leading to vitamin B12 deficiency and other nutritional deficits.
Diagnosis
  • Endoscopy with biopsy:
    • UC: Continuous inflammation starting from the rectum.
    • CD: Skip lesions, non-caseating granulomas, and deep ulcers.
  • Imaging:
    • CT/MRI enterography: Preferred for small bowel evaluation and detection of complications such as strictures and fistulas in CD.
  • Lab markers:
    • Elevated CRP and ESR indicate active inflammation.
    • Fecal calprotectin: Elevated in IBD, distinguishing it from irritable bowel syndrome (IBS).
Treatment
Acute Flares
  • Corticosteroids: Used to induce remission during moderate to severe flares.
    • Oral prednisone or IV steroids for severe flares.
    • Budesonide for mild ileocolonic Crohn's disease.
  • Biologic agents: Anti-TNF agents (e.g., infliximab) are often used in steroid-refractory disease.
Maintenance Therapy
  • Aminosalicylates (5-ASA): First-line for mild to moderate UC, but limited role in CD.
  • Immunomodulators: Azathioprine, 6-mercaptopurine (6-MP), or methotrexate for long-term disease control.
  • Biologics: Anti-TNF agents (e.g., infliximab, adalimumab) and newer agents like vedolizumab (anti-integrin) or ustekinumab (IL-12/23 inhibitor) for moderate to severe disease.
Surgery
  • Ulcerative Colitis: Total colectomy is curative and is indicated in cases of refractory disease, high-grade dysplasia, or cancer.
  • Crohn’s Disease: Surgery is not curative and is reserved for complications (e.g., strictures, fistulas). Recurrence after surgery is common.
Monitoring and Surveillance
  • Colon cancer screening: Patients with extensive colitis should undergo regular colonoscopy every 1–2 years starting 8 years after diagnosis.
  • Bone density monitoring: Long-term corticosteroid use increases the risk of osteoporosis, warranting periodic screening.
Key Points
  • Ulcerative Colitis involves continuous colonic inflammation starting from the rectum and presents with bloody diarrhea. Inflammation is limited to the mucosa.
  • Crohn’s Disease affects any part of the GI tract with skip lesions and transmural inflammation, leading to complications such as strictures, fistulas, and malabsorption.
  • Endoscopy with biopsy is critical for diagnosis, supported by imaging and lab markers (CRP, fecal calprotectin).
  • Corticosteroids are used for flares, and immunosuppressants or biologic agents are used for maintenance.
  • Regular colonoscopy is necessary for cancer surveillance in long-standing colonic disease.

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