## Site of Involvement in Crohn Disease **Key Point:** The terminal ileum and right colon (ileocolonic region) represent the most common site of involvement in Crohn disease, occurring in approximately 40–50% of cases. ### Distribution Pattern in Crohn Disease The characteristic distribution of Crohn disease follows a segmental pattern: | Site | Frequency | Features | |------|-----------|----------| | Terminal ileum + right colon (ileocolonic) | 40–50% | Most common; skip lesions typical | | Small bowel alone | 20–30% | Jejunum and ileum | | Colon alone | 15–20% | Discontinuous involvement | | Rectum alone | <5% | Rare as isolated site | | Perianal region | 20% | Fistulas, abscesses, skin tags | **High-Yield:** The **ileocecal valve** region is the classic site where Crohn disease preferentially affects the terminal ileum, with the inflammation often sparing the cecum initially — this creates a characteristic pattern on imaging. ### Pathological Hallmarks Supporting Ileocolonic Involvement 1. **Transmural inflammation** — extends through all bowel layers, causing strictures and fistulas 2. **Skip lesions** — discontinuous areas of inflammation separated by normal mucosa 3. **Non-caseating granulomas** — present in ~30–50% of cases, more common in ileocolonic disease 4. **Cobblestone appearance** — due to deep fissuring ulcers and edema **Clinical Pearl:** The terminal ileum's specialized lymphoid tissue (Peyer's patches) and its role in antigen sampling may explain the predilection for Crohn disease at this site. ### Why Ileocolonic Site Matters Clinically - **Stricture formation** — common, may require surgical intervention - **Fistula development** — enterocutaneous, enterovesical, rectovaginal - **Nutritional deficiency** — particularly vitamin B₁₂ and iron malabsorption - **Surgical considerations** — ileocecal resection is the most common surgical procedure [cite:Robbins 10e Ch 17]
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