Which layer of the colon is predominantly affected in ulcerative colitis, and how does this differ from Crohn disease?

## Depth of Inflammation: Ulcerative Colitis vs. Crohn Disease **Key Point:** Ulcerative colitis is a **mucosal and submucosal disease**, whereas Crohn disease is **transmural**, involving all layers from mucosa to serosa. This fundamental difference in depth of inflammation explains many clinical and pathological distinctions. ### Layer-by-Layer Comparison | Layer | Ulcerative Colitis | Crohn Disease | | --- | --- | --- | | **Mucosa** | Involved (primary) | Involved | | **Submucosa** | Involved | Involved | | **Muscularis propria** | Spared (not involved) | Involved | | **Serosa/Visceral peritoneum** | Spared | May be involved | | **Depth pattern** | Superficial | Transmural | ### Clinical Consequences of Transmural vs. Superficial Inflammation **Ulcerative Colitis (Mucosal/Submucosal):** - Bleeding and bloody diarrhea (mucosal ulceration) - Toxic megacolon (acute severe disease) - Lower risk of fistula and stricture formation - No skip lesions (continuous inflammation) **Crohn Disease (Transmural):** - Fistula formation (transmural penetration) - Strictures and fibrosis (full-thickness scarring) - Perforation risk (serositis) - Skip lesions (patchy, discontinuous involvement) - Cobblestone appearance (transmural edema and ulceration) **High-Yield:** On endoscopy and imaging, the **depth of inflammation** is the most important discriminator: - **UC:** Continuous, superficial ulceration starting at rectum, extending proximally - **CD:** Patchy (skip lesions), deep fissuring ulcers, cobblestoning, fistulas **Mnemonic: "TUC"** — **T**ransmural = Crohn, **UC** = Ulcerative Colitis (superficial).