What is the difference between Crohn's disease and ulcerative colitis?

Key Differences Between Crohn's Disease and Ulcerative Colitis

Crohn's disease and ulcerative colitis differ fundamentally in their anatomical distribution, depth of inflammation, and microscopic features—with Crohn's showing transmural, patchy inflammation that can affect any part of the GI tract from mouth to anus, while ulcerative colitis remains confined to the colonic mucosa with continuous inflammation starting from the rectum. 1

Anatomical Distribution

Location and Pattern:

• Ulcerative colitis begins in the rectum (present in >97% of untreated cases) and extends proximally in a continuous, uninterrupted fashion with gradually decreasing severity 1, 2
• Crohn's disease demonstrates patchy, discontinuous inflammation with skip lesions (normal bowel between inflamed segments) and can affect any part of the GI tract from mouth to anus 1, 3
• Rectal sparing strongly suggests Crohn's disease, as it occurs commonly in CD but is rare in UC (only up to 3% of cases) 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

Tissue Layer Involvement:

• Ulcerative colitis inflammation is limited to the mucosa and occasionally the submucosa only 1, 4
• Crohn's disease exhibits transmural inflammation extending through all layers of the intestinal wall (mucosa, submucosa, muscularis propria, and serosa) 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

Key Distinguishing Pathology:

• Granulomas (non-cryptolytic) are absent in UC but present in CD, serving as a key distinguishing feature 1, 2
• Crypt abscesses are more common in UC (41%) than in CD (19%) 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
• Crypt architectural irregularity is diffuse and continuous in UC but focal and discontinuous in CD 1

Additional Microscopic Differences:

• Paneth cell metaplasia is present in UC but uncommon in CD 1
• Pyloric gland metaplasia is rare in UC but present in CD 1
• Neuronal hyperplasia is rare in UC but common in CD 1
• Muscular hypertrophy is absent in UC but present in CD 1
• Serositis is typically absent in UC except in fulminant colitis, but commonly present in CD 1

Complications

Disease-Specific 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
• Fistulas are rare in ulcerative colitis but common in Crohn's colitis 4
• Strictures in UC (3.2% to 11.2% of cases) result from mucosal/submucosal fibrosis, whereas CD strictures involve all layers 1
• Fissures are characteristically absent in UC but present in CD 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 5

Diagnostic Approach

Essential Workup:

• Perform complete ileocolonoscopy with at least two biopsies from five different sites (including ileum and rectum, even from normal-appearing areas) to differentiate CD from UC 2
• Systematically complete with cross-sectional imaging (MRI or CT enterography) 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 (termed IBD-unclassified); capsule endoscopy can establish a definitive CD diagnosis by demonstrating small intestine lesions in 17-70% of these patients 2, 6

Common Pitfall: Never rely on sigmoidoscopy alone even if it suggests UC—complete ileocolonoscopy is imperative to establish the differential diagnosis and assess for skip lesions or ileal involvement that would indicate Crohn's disease 2