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Inflammatory Bowel Disease for the ABIM Exam

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Inflammatory Bowel Disease (IBD) for the American Board of Internal Medicine Exam
Overview
  • Inflammatory Bowel Disease (IBD) includes two primary chronic disorders: Ulcerative Colitis (UC) and Crohn’s Disease (CD).
  • Both are immune-mediated conditions characterized by relapsing inflammation of the gastrointestinal (GI) tract but differ in their distribution and depth of tissue involvement.
Epidemiology
  • Prevalence is highest in developed countries, with increasing incidence globally.
  • Peak onset occurs between 15–35 years, with a second smaller peak in those aged 55–70.
  • There is a slight male predominance in UC and a female predominance in CD.
Pathophysiology
  • Genetics: Familial clustering and association with specific genes, such as NOD2 in Crohn's disease, suggest a genetic predisposition.
  • Immune dysregulation: An aberrant immune response to gut flora contributes to the chronic inflammation seen in IBD.
  • Environmental factors: Diet, smoking, and certain infections may trigger or exacerbate IBD. Notably, smoking worsens CD but appears protective in UC.
  • Gut microbiome: An altered microbiome composition (dysbiosis) is linked to IBD development.
Ulcerative Colitis (UC)
Distribution
  • Confined to the colon: UC always involves the rectum and can extend proximally in a continuous manner.
  • Superficial inflammation: Inflammation is restricted to the mucosa and submucosa.
Ulcerative Colitis
Clinical Features
  • Diarrhea with blood: The hallmark symptom, often accompanied by rectal urgency and tenesmus.
  • Abdominal pain: Typically left lower quadrant (LLQ).
  • Systemic symptoms: Fever, weight loss, and fatigue may occur during flare-ups.
Complications
  • Toxic megacolon: Acute severe colonic distension with risk of perforation.
  • Colon cancer: Increased risk after 8–10 years of disease, particularly in patients with extensive colitis.
  • Primary sclerosing cholangitis (PSC): A progressive liver disease strongly associated with UC.
Crohn’s Disease (CD)
Distribution
  • Can involve any part of the GI tract: From mouth to anus, with a predilection for the terminal ileum and proximal colon.
  • Skip lesions: Discontinuous areas of inflammation, a hallmark of Crohn’s disease.
  • Transmural inflammation: Involves all layers of the bowel wall, leading to complications like strictures, fistulas, and abscesses.
Crohn’s Disease
Clinical Features
  • Non-bloody diarrhea: Often the main symptom, though blood may be present if the colon is involved.
  • Abdominal pain: Typically right lower quadrant (RLQ), resembling appendicitis.
  • Perianal disease: Fissures, fistulas, and abscesses are common.
  • Systemic symptoms: Weight loss, fever, and fatigue are common in active disease.
Complications
  • Strictures: Narrowing of the bowel lumen due to chronic inflammation and fibrosis, leading to obstruction.
  • Fistulas: Abnormal connections between the bowel and other organs (e.g., bladder, skin).
  • Malabsorption: Particularly in patients with extensive small bowel involvement, leading to deficiencies in vitamins (e.g., B12, fat-soluble vitamins).
Extraintestinal Manifestations
  • Musculoskeletal: Arthritis is the most common, affecting both large joints and the axial skeleton (ankylosing spondylitis).
  • Dermatologic: Erythema nodosum and pyoderma gangrenosum.
  • Ocular: Uveitis, episcleritis.
  • Hepatobiliary: Primary sclerosing cholangitis (PSC) is more common in UC than CD.
  • Renal: Nephrolithiasis, particularly in Crohn’s patients with ileal disease, due to increased oxalate absorption.
Diagnosis
Labs
  • Inflammatory markers: Elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) indicate active inflammation.
  • Fecal calprotectin: A sensitive marker of GI inflammation, often used to distinguish IBD from irritable bowel syndrome (IBS).
  • Anemia: Microcytic from iron deficiency or macrocytic from B12 deficiency (in Crohn's disease).
Endoscopy
  • Colonoscopy with biopsy: The gold standard for diagnosing and differentiating UC and CD.
    • UC shows continuous inflammation, beginning in the rectum.
    • CD shows skip lesions and deeper ulcers.
  • Capsule endoscopy: Useful for assessing small bowel involvement in Crohn’s disease.
Imaging
  • CT/MRI enterography: Preferred for assessing small bowel disease and complications in Crohn’s disease (e.g., strictures, fistulas).
  • Barium studies: Rarely used today but may show the classic "string sign" of terminal ileal narrowing in Crohn’s disease.
Management
Induction Therapy
  • Corticosteroids: Used to induce remission in moderate to severe flares.
    • Systemic steroids (e.g., prednisone) for more severe disease.
    • Topical steroids (e.g., budesonide) for mild ileocolonic Crohn’s disease.
Maintenance Therapy
  • Aminosalicylates (5-ASA): First-line for mild to moderate UC.
    • Mesalamine is commonly used, though its role in Crohn’s disease is less clear.
  • Immunomodulators: Azathioprine or methotrexate for steroid-sparing effects in both UC and CD.
  • Biologic agents:
    • Anti-TNF agents (e.g., infliximab, adalimumab) for moderate to severe disease unresponsive to other therapies.
    • Anti-integrin therapy (e.g., vedolizumab) and IL-12/23 inhibitors (e.g., ustekinumab) for refractory cases.
Surgical Management
  • Ulcerative Colitis: Total colectomy is curative but reserved for refractory disease or malignancy prevention.
  • Crohn’s Disease: Surgery is not curative and is reserved for complications (e.g., strictures, fistulas). Recurrence is common after surgery.
Monitoring and Surveillance
  • Colon cancer screening: Patients with extensive UC or Crohn’s colitis should undergo colonoscopy every 1–2 years starting 8 years after disease onset.
  • Bone density screening: Chronic steroid use increases the risk of osteoporosis, warranting periodic bone density monitoring.
Key Points
  • IBD includes Ulcerative Colitis (UC) and Crohn’s Disease (CD), with distinct patterns of inflammation and clinical presentation.
  • UC is limited to the colon and presents with bloody diarrhea, while CD can affect any part of the GI tract and often leads to non-bloody diarrhea, fistulas, and strictures.
  • Complications include toxic megacolon and colon cancer in UC and fistulas and strictures in CD.
  • Diagnosis relies on endoscopy with biopsy, supported by imaging and lab tests.
  • Treatment is tailored to disease severity, with corticosteroids for flares, and biologics or immunosuppressants for maintenance.
  • Regular colon cancer screening is critical for patients with long-standing colonic involvement.

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