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Understanding Crohn's Disease: A Pathological Overview

Crohn's disease is a chronic, transmural inflammatory bowel disease (IBD) that can affect any segment of the gastrointestinal tract from mouth to anus, with predilection for terminal ileum and proximal colon. It is characterized by discontinuous (skip) lesions, granulomatous inflammation, and a high propensity for complications (strictures, fistulas, abscesses).

Pathology Hallmarks

Macroscopic Features

  • Segmental involvement: "Skip lesions" – inflamed segments separated by normal mucosa.
  • Cobblestone appearance: Edematous mucosa intersected by deep longitudinal and transverse ulcers.
  • Creeping fat: Mesenteric fat wrapping around serosa (pathognomonic).
  • Strictures: Fibrotic narrowing from chronic inflammation → obstruction.
  • Fistulas: Transmural penetration → enteroenteric, enterocutaneous, enterovesical, perianal.
  • Abscesses: Intra-abdominal or perianal.

Microscopic Features

  • Transmural inflammation: All layers (mucosa → serosa) involved; distinguishes from ulcerative colitis (mucosal only).
  • Non-caseating granulomas: Present in 30–50% of resections; epithelioid histiocytes ± multinucleated giant cells; not specific (also in TB, sarcoid, foreign body).
  • Architectural distortion: Crypt branching, atrophy, basal plasmacytosis.
  • Neural hyperplasia: Hypertrophied nerve trunks in submucosa/serosa – contributes to pain/dysmotility.

Clinical Phenotypes (Montreal Classification)

CategorySubtypes
Age at diagnosis (A)A1: <17 yr; A2: 17–40 yr; A3: >40 yr
Location (L)L1: Ileal; L2: Colonic; L3: Ileocolonic; L4: Upper GI (modifier)
Behavior (B)B1: Non-stricturing, non-penetrating; B2: Stricturing; B3: Penetrating; p: Perianal modifier

Complications

  • Intestinal: Obstruction (stricture), perforation, fistula, abscess, hemorrhage, malignancy (small bowel adenocarcinoma, colorectal cancer).
  • Extra-intestinal (EIMs): Arthritis (peripheral/axial), erythema nodosum, pyoderma gangrenosum, uveitis, PSC, osteoporosis, nephrolithiasis.
  • Nutritional: Iron/B12/D deficiency, protein-losing enteropathy, growth failure in children.

Treatment Paradigm

  • Induction: Corticosteroids (budesonide for ileocecal), exclusive enteral nutrition (pediatrics), anti-TNF/vedolizumab/ustekinumab for moderate-severe.
  • Maintenance: Immunomodulators (azathioprine/6-MP, methotrexate), biologics, JAK inhibitors (upadacitinib).
  • Surgery: Strictureplasty, resection-anastomosis, fistula repair, abscess drainage; not curative (recurrence at anastomosis common).
  • Monitoring: Fecal calprotectin, CRP, imaging (MRE, CTE), endoscopy.

Etiopathogenesis (Current Model)

Genetic susceptibility (NOD2, ATG16L1, IL23R, >200 loci) + environmental triggers (smoking, antibiotics, diet, microbiome) → dysregulated innate/adaptive immunity → Th1/Th17-mediated chronic inflammation → tissue destruction.

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