Bit · GI
Crohn vs Ulcerative Colitis
Two inflammatory bowel diseases that share bloody diarrhea but split sharply on location, depth, and complications. The pivot is what the bowel looks like on colonoscopy and biopsy.
Mechanism
Both are chronic relapsing autoimmune disorders of the GI tract. The clinical pivot is where and how deep the inflammation goes:
- Crohn disease — can affect anywhere from mouth to anus, but the terminal ileum and right colon are the most common sites. Rectum is often spared. Inflammation is transmural (full thickness) and discontinuous ('skip lesions'). Complications: fistulas (entero-enteric, perianal), strictures, abscesses. Biopsy: non-caseating granulomas (~50%). Imaging: 'string sign' on barium study, creeping fat. Associated with cigarette smoking (worsens disease).
- Ulcerative colitis — restricted to the colon. Always involves the rectum and extends proximally in a continuous distribution. Inflammation is limited to the mucosa and submucosa only — no transmural disease, no fistulas, no granulomas. Pseudopolyps and crypt abscesses on histology. Complications: toxic megacolon, primary sclerosing cholangitis (esp. with p-ANCA), colorectal cancer (risk rises with extent and duration). Smoking actually reduces UC risk.
Differentiator Table
| Crohn disease | Ulcerative colitis | |
| Location | Mouth to anus; terminal ileum + right colon most common; rectum often spared | Colon only; ALWAYS rectum, extending proximally |
| Pattern | Skip lesions (discontinuous) | Continuous from rectum |
| Depth of inflammation | TRANSMURAL | Mucosa + submucosa only |
| Granulomas (non-caseating) | Present (~50%) | Absent |
| Fistulas / strictures | Yes — perianal disease, entero-enteric fistulas, strictures | No |
| Bloody diarrhea | Less prominent (often just diarrhea + pain) | Hallmark — bloody, mucousy |
| Tobacco | Worsens disease | Paradoxically protective |
| Pathognomonic findings | Skip lesions, cobblestoning, creeping fat, string sign, non-caseating granulomas | Pseudopolyps, crypt abscesses, lead-pipe colon |
| Extraintestinal | Erythema nodosum, oral aphthous ulcers, episcleritis, kidney stones (oxalate), gallstones (terminal ileum), arthritis | Primary sclerosing cholangitis (esp. p-ANCA+), pyoderma gangrenosum, uveitis, ankylosing spondylitis, arthritis |
| Cancer risk | Increased (colon, small bowel) | Markedly increased colorectal cancer; surveillance colonoscopy required |
| Surgery | Not curative — disease recurs; reserved for complications | CURATIVE colectomy |
| Toxic megacolon | Less common | More common |
The Pivot
Three questions:
- Is the rectum involved AND is the disease continuous? → UC.
- Skip lesions, transmural inflammation, perianal fistulas, granulomas, or terminal ileal disease? → Crohn.
- Surgery curative? UC: yes (colectomy removes the disease). Crohn: never — disease recurs at the surgical anastomosis.
NBME-Style Stem
A 23-year-old man presents with chronic non-bloody diarrhea, right lower quadrant pain, and 6 kg weight loss over 4 months. He has perianal fistulas. Colonoscopy shows patchy inflammation of the terminal ileum and ascending colon with normal-appearing mucosa interspersed (skip lesions). Biopsy shows non-caseating granulomas. Which of the following is the most likely diagnosis?
Concept Anchor
Crohn is transmural, patchy, mouth-to-anus, with fistulas and granulomas — surgery never cures it. UC is mucosal, continuous, colon-only, starting at the rectum — and colectomy removes the disease entirely.