Crohn disease
- Lesion
Crohn disease can inflame any part of the gut, in separate patches and through the whole wall, causing narrowings, fistulas and abscesses.
What it is
Crohn disease can involve any part of the gastrointestinal tract, most often the terminal ileum, ileocaecal valve and caecum. About two in five patients have small-bowel disease only, about a third have both small bowel and colon, and the rest have colonic disease alone. The inflammation is patchy (skip lesions) and transmural, which explains its typical complications: strictures, fissures, fistulas and abscesses. Non-caseating granulomas are a hallmark but are found in only about a third of cases.
Structure
- Gross: segmental thick wall with narrow lumen, linear ulcers, cobblestoning, creeping fat.
- Micro: neutrophils attack crypts (cryptitis, crypt abscesses); abrupt transition from ulcer to normal mucosa.
- Chronic injury: crypt distortion and branching, pseudopyloric metaplasia, Paneth cell metaplasia in the left colon, mucosal atrophy.
- Transmural lymphoid aggregates and submucosal fibrosis with muscle hypertrophy.
- Non-caseating granulomas, even in uninvolved segments.
Step by step
- Barrier loss and microbial sensing defects start inflammation at lymphoid-rich sites such as the terminal ileum.
- Aphthous ulcers form over lymphoid follicles.
- Ulcers coalesce into long linear ulcers separated by swollen mucosa (cobblestones).
- Inflammation crosses the whole wall; fissures deepen into fistulas or abscesses.
- Repeated healing lays down fibrosis and muscle hypertrophy: strictures and obstruction.
Clinical notes
- Intermittent diarrhoea, abdominal pain and fever; about one in five begins acutely and mimics appendicitis.
- Extra-intestinal disease: uveitis, migratory polyarthritis, sacroiliitis and ankylosing spondylitis, erythema nodosum, clubbing, primary sclerosing cholangitis (less often than in ulcerative colitis).
- Anti-Saccharomyces cerevisiae antibodies (ASCA) favour Crohn disease.
- Disease often recurs at the anastomosis after resection; smoking worsens it.
Test yourself
Think of the answer, then tap the question to see it.
Which part of the gut is most often involved in Crohn disease?
The terminal ileum, ileocaecal valve and caecum.
Why do fistulas form in Crohn disease but rarely in ulcerative colitis?
Crohn inflammation is transmural, so fissures can tunnel through the entire wall into neighbouring organs or skin.
What proportion of Crohn cases show granulomas?
About a third; their absence does not exclude the diagnosis.
What is creeping fat?
Mesenteric fat that wraps over the serosal surface of the bowel in transmural Crohn disease.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Crohn Disease (StatPearls (NCBI Bookshelf), 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.