Crohn Disease

Overview
  • Crohn Disease (CD) is a subtype of Inflammatory Bowel Disease (IBD) and readers should refer to the IBD page for a discussion of this disease's etiopathogenesis, extra-intestinal clinical consequences, and epidemiology.
Morphology
  • Overview
    • Pathology can manifest at any location within the alimentary tract although disease is predominantly seen in the small intestine and less often in the large intestine or both. The terminal ileum is involved in nearly 90% of patients but disease typically spares the rectum whereas Ulcerative Colitis (UC) almost always affects the rectum and rarely the ileum.
  • Gross Appearance:
    • The pathology of CD is typically sharply demarcated and often affects multiple, scattered segments with intervening areas of normal mucosa, a manifestation described as "Skip Lesions". However, mucosal ulcerations often coalesce longitudinally and transversely, leaving islands of normal mucosa in between, somewhat akin to cobblestones in a road, and thus termed a "Cobblestone Appearance". CD lesions frequently span the entire alimentary wall and thus fissures can form along with fistulas between alimentary segments. Characteristically, mesenteric fat can encase involved segments and is known as "Creeping Fat". Finally, inflammation, edema, and fibrosis of involved segments can cause narrowing of the intestinal lumen, visible on barium studies as a "String Sign".
  • Histologically
    • Inflammation observed in CD is dominated by neutrophils which typically populate the bowel epithelium as well as the Crypts of Lieberkuhn, forming characteristic "Crypt Abscesses". Active inflammatory disease leads to ulcerations of the intestinal mucosa which can coalesce as described above to form "Cobblestones". Unlike UC, inflammation in CD is typically transmural and may contain non-caseating granulomas that may even be observed in non-involved segments. Ultimately, inflammation associated with CD leads to distorted architecture of the intestinal mucosa and destruction of Crypts of Lieberkuhn. Over time, thickening and fibrosis of the intestinal wall can result in luminal narrowing as described above. In long-standing cases of CD dysplasia can be observed within the intestinal epithelium, heralding the increased risk of intestinal carcinomas associated with CD.
Clinical Consequences
  • Overview
    • The clinical manifestation of CD is diverse and depends on which segment or segments of the alimentary tract are pathologically involved. Symptomology, as described below, is usually episodic and relapses and remits over years. Episodes may be separated by years although they become more frequent with time and can lead to a variety of important complications.
  • Episode Symptomology
    • Flares of Crohn Disease are characterized by fever and symptomology associated with the alimentary segment or segments involved. Because pathology most frequently involves the small intestine, especially the terminal ileum, episodes are often characterized by a diarrhea as well as abdominal pain, especially in the right-lower quadrant. Diarrhea is usually due to malabsorption arising from loss of intestinal absorptive surface area. During episodes patients often display significant anorexia which combined with malabsorption can lead to substantial weight loss.
  • Complications
    • Fissuring of involved intestinal segments can lead to bowel perforation and leakage of luminal contents into the abdominal cavity and thus result in abdominal abscesses. Alternatively, different segments of the alimentary tract may fistulize, leading to unique consequences depending on the segments involved. Narrowing of the intestinal lumen due to inflammatory, edematous, and fibrotic mechanisms can lead to bowel obstruction. With greater duration and severity of disease, patients with CD become more at risk for developing colorectal carcinoma.