Expansion
Crohn is transmural and patchy anywhere; colitis is mucosal and continuous from the rectum
Expansion
| Feature | Crohn disease | Ulcerative colitis |
|---|---|---|
| Site | Mouth to anus, often terminal ileum | Colon only, from the rectum |
| Distribution | Patchy, skip lesions | Continuous |
| Depth | Transmural | Mucosal |
| Complications | Fistulae, strictures, abscess | Toxic megacolon, bleeding |
| Histology | Non-caseating granulomas | Crypt abscesses, goblet cell depletion |
| Smoking | Worsens | Protective |
| Surgery | Not curative | Curative (colectomy) |
The physiological consequences follow the anatomy. Terminal ileal Crohn disease causes B12 deficiency, bile salt diarrhoea, gallstones and oxalate renal stones, all traceable to loss of ileal function.
Ulcerative colitis, being confined to the colon, impairs water and electrolyte absorption, causing bloody diarrhoea with urgency and tenesmus, and carries a substantial long-term colorectal cancer risk requiring surveillance.
Extraintestinal manifestations occur in both: arthritis, erythema nodosum, pyoderma gangrenosum, uveitis and primary sclerosing cholangitis, which is particularly associated with ulcerative colitis.