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 are distinct inflammatory bowel diseases that differ fundamentally in their anatomical distribution, depth of inflammation, and microscopic features—with Crohn's showing patchy, transmural inflammation that can affect any part of the GI tract, while ulcerative colitis presents as continuous, mucosal-only inflammation limited to the colon starting from the rectum. 1

Anatomical Distribution

Location and Pattern:

• Ulcerative colitis begins in the rectum and extends proximally in a continuous, uninterrupted fashion with gradually decreasing severity 1, 2
• Rectal involvement is present in >97% of untreated ulcerative colitis cases 1
• Crohn's disease shows patchy, discontinuous distribution with skip lesions throughout the GI tract from mouth to anus 1, 3
• Rectal sparing is a key distinguishing feature of Crohn's disease, occurring commonly, whereas it appears in only up to 3% of ulcerative colitis patients 1

Extent of GI Involvement:

• Ulcerative colitis is limited to the colon only 3, 2
• Crohn's disease can affect any segment of the gastrointestinal tract, with approximately one-third of patients having small intestine involvement not detectable by colonoscopy 1

Depth of Inflammation

Tissue Layer Involvement:

• Ulcerative colitis: Inflammation is confined to the mucosa and occasionally submucosa 1, 2, 4
• Crohn's disease: Transmural inflammation affecting all layers of the intestinal wall 1, 2, 4
• Fibrosis in ulcerative colitis is restricted to mucosa or submucosa, while in Crohn's disease it extends through all layers 1, 2
• Serositis is typically absent in ulcerative colitis (except in fulminant colitis) but commonly present in Crohn's disease 2

Microscopic and Histological Features

Diagnostic Histological Markers:

• Granulomas (non-cryptolytic): Absent in ulcerative colitis but present in Crohn's disease—this is a key distinguishing feature 1, 2, 4
• Crypt abscesses: More common in ulcerative colitis (41%) than Crohn's disease (19%) 1, 2
• Inflammatory pattern: Diffuse without intensity variations in ulcerative colitis; varies in intensity within and between biopsies in Crohn's disease 1, 2
• Mucin depletion: Pronounced in ulcerative colitis but uncommon and mild in Crohn's disease 2

Architectural Changes:

• Crypt architectural irregularity is diffuse and continuous in ulcerative colitis but focal and discontinuous in Crohn's disease 2
• Paneth cell metaplasia is present in ulcerative colitis but uncommon in Crohn's disease 2
• Pyloric gland metaplasia is rare in ulcerative colitis but present in Crohn's disease 2

Clinical Manifestations

Symptom Patterns:

• Ulcerative colitis: Cardinal symptom is bloody diarrhea with colicky abdominal pain, urgency, tenesmo, and a course marked by exacerbations and remissions 3
• Crohn's disease: More heterogeneous symptoms including abdominal pain, diarrhea, weight loss, and complications such as intestinal obstruction from strictures, fistulas, and abscesses 3

Perianal Disease:

• Perianal fistulas and ulcers are rare in ulcerative colitis but common in Crohn's colitis 4
• Fissures are characteristically absent in ulcerative colitis but present in Crohn's disease 2

Growth and Systemic Effects:

• Growth failure is far more severe in Crohn's colitis and may precede intestinal symptoms by months to years 4

Complications and Associated Conditions

Strictures:

• Ulcerative colitis strictures (3.2-11.2% of cases) result from mucosal/submucosal fibrosis 2
• Crohn's disease strictures involve all intestinal wall layers 2

Cancer Risk:

• Both conditions increase colorectal cancer risk, but it is significantly more common in ulcerative colitis 3, 4

Associated Conditions:

• Primary sclerosing cholangitis is more commonly associated with ulcerative colitis than Crohn's disease 1

Environmental Factors:

• Smoking increases the risk of Crohn's disease but decreases the risk of ulcerative colitis 3

Diagnostic Approach

Essential Workup:

• Complete ileocolonoscopy with at least two biopsies from five sites including ileum and rectum is imperative to differentiate the two conditions 1
• Systematic cross-sectional imaging (MRI or CT enterography) is indispensable to evaluate small intestine involvement 1
• Biopsies from unaffected areas are essential to document histologically the spared segments between inflammatory areas 1

Diagnostic Challenges:

• In 5-15% of cases, endoscopic and histological evaluation cannot differentiate between the two diseases, leading to classification as IBD-unclassified 1, 2, 5
• Capsule endoscopy can establish definitive diagnosis by demonstrating small intestine lesions compatible with Crohn's disease in 17-70% of patients with unclassified IBD 1

Treatment Response and Prognosis

Medical Management:

• Ulcerative colitis responds more frequently to medical treatment 3, 4
• Crohn's disease tends to have a more severe clinical course requiring more aggressive medical treatment 5

Surgical Outcomes:

• Total colectomy with ileostomy results in cure for ulcerative colitis 4
• Crohn's disease is likely to recur in the small intestine after surgical resection 3, 4

Common Pitfall: Do not rely on sigmoidoscopy alone even if it suggests ulcerative colitis—complete ileocolonoscopy is imperative to establish the differential diagnosis 1. Approximately one-third of Crohn's disease patients have small intestine involvement that would be missed without cross-sectional imaging 1.