Key Differences Between Crohn's Disease and Ulcerative Colitis
Anatomical Distribution
- Ulcerative colitis begins in the rectum and extends proximally in a continuous fashion, while Crohn's disease demonstrates patchy, discontinuous inflammation with skip lesions and can affect any part of the GI tract from mouth to anus 1, 2, 3
- Rectal sparing strongly suggests Crohn's disease, as it occurs commonly in CD but is rare in UC 2
- The terminal ileum is the most commonly affected site in Crohn's disease, which can involve small intestine alone, colon alone, or both 3
Depth of Inflammation
- Ulcerative colitis inflammation is limited to the mucosa and occasionally the submucosa only, while Crohn's disease exhibits transmural inflammation extending through all layers of the intestinal wall 1, 3
- This transmural nature in CD drives the development of stricturing and penetrating complications including fistulas 3
- Fibrosis in UC is restricted to mucosa or submucosa, whereas in CD it extends through all layers 1
Microscopic and Histological Features
- Granulomas are absent in UC but present in CD, serving as a key distinguishing feature 1, 2
- Crypt abscesses are more common in UC than in CD 1
- Inflammatory pattern: UC shows diffuse inflammation without variations in intensity, while CD shows focal inflammation that varies in intensity within and between biopsies 1
- Mucin depletion is pronounced in UC but uncommon and mild in CD 1
Complications
- Fistulas represent a hallmark complication of Crohn's disease due to transmural inflammation, with approximately one-quarter of perianal fistulas presenting at or before diagnosis 3
- Strictures in UC result from mucosal/submucosal fibrosis, whereas CD strictures involve all layers 1
- Colorectal cancer risk is increased in both conditions, but the risk appears to be the same in Crohn's colitis as in UC when matched for extent and duration of colonic involvement 4
Diagnostic Approach
- Perform complete ileocolonoscopy with at least two biopsies from five different sites to differentiate CD from UC 2
- Systematically complete with cross-sectional imaging to evaluate small intestine involvement, as about one-third of CD patients have small intestine disease not detectable by colonoscopy 2
- In 5-15% of IBD cases, endoscopic and histological evaluation cannot distinguish between CD and UC; capsule endoscopy can establish a definitive CD diagnosis by demonstrating small intestine lesions in 17-70% of these patients 2