Crohn's Disease
A transmural, skip-lesion inflammation that can strike anywhere from mouth to anus, so fistulae, strictures and patchy malabsorption are the predictable consequences of depth and distribution rather than separate diseases.
In a nutshell
Crohn's disease is transmural, skip-lesion inflammation anywhere from mouth to anus. Full-thickness involvement explains fistulae, abscesses and strictures; terminal ileal disease explains B12 deficiency and bile-salt diarrhoea; systemic immune dysregulation explains the extra-intestinal features.
Classic presentation
A young adult with chronic right iliac fossa pain, non-bloody diarrhoea, weight loss and perianal skin tags or a fistula, with raised inflammatory markers.
Key points
- Transmural (full-thickness) inflammation is the defining feature: it explains fistulae, abscesses and fibrotic strictures, which are rare in ulcerative colitis.
- Skip lesions with intervening normal mucosa distinguish Crohn's from the continuous inflammation of ulcerative colitis.
- Terminal ileal disease causes B12 deficiency (site of absorption) and bile-salt malabsorption diarrhoea.
- Perianal disease (fistulae, abscesses, skin tags) is a hallmark of Crohn's and essentially never seen in ulcerative colitis.
- Smoking worsens Crohn's disease (the opposite relationship to ulcerative colitis, where smoking is protective).
First-line investigation
Faecal calprotectin to screen for inflammatory bowel disease, followed by colonoscopy with biopsies and terminal ileal intubation for diagnosis.
First-line management
Corticosteroids (or biologics in more severe/fistulating disease) to induce remission, then thiopurines or biologics to maintain it.
Exam traps
- Perianal fistulae or skin tags in a young patient with diarrhoea point to Crohn's, not ulcerative colitis.
- Smoking is a risk factor for relapse in Crohn's disease: the opposite of its protective effect in ulcerative colitis.
- Rectal sparing and non-bloody diarrhoea favour Crohn's; continuous rectal involvement with bloody diarrhoea favours ulcerative colitis.
Educational content pending clinical review. Not medical advice.