Crohn's Disease vs Ulcerative Colitis — IBD Comparison

Creator: Progress Note
Published: 2026-07-22
Keywords: Crohn's disease vs ulcerative colitis, IBD comparison USMLE, Crohn's disease treatment, ulcerative colitis surgery, skip lesions granulomas Crohn

The 60-second version

Crohn's: any GI location + skip lesions + transmural + granulomas + fistulas
UC: rectum → continuous proximal spread + mucosal only + bloody diarrhea + PSC association
Toxic megacolon and PSC are associated with UC; perianal fistulas are associated with Crohn's
UC surgery (total proctocolectomy) is curative; Crohn's surgery is not curative
Both can cause peripheral arthritis, erythema nodosum, aphthous ulcers — not disease-specific

Distribution and Pathology

Crohn's disease: can involve any part of the GI tract (mouth to anus) — most commonly terminal ileum + right colon (ileocolitis). Skip lesions (normal mucosa between diseased segments). Transmural inflammation (full thickness of bowel wall) → fistulas (enteroenteric, enterovesical, enterocutaneous, perianal), abscesses, strictures. Histology: non-caseating granulomas (pathognomonic but only found in 30–40%), cobblestone appearance, fat wrapping ('creeping fat'), aphthous ulcers progressing to linear ulcers. Ulcerative colitis: starts at rectum and extends proximally in a continuous manner (no skip lesions). Limited to colon. Always involves rectum (rectal sparing = Crohn's). Mucosal/submucosal inflammation only (not transmural). Histology: crypt abscesses, cryptitis, pseudopolyps, continuous mucosal involvement. Bloody diarrhea is the hallmark (rectal involvement disrupts mucosal vasculature).

Clinical Features and Complications

Crohn's: abdominal pain (RLQ — can mimic appendicitis), non-bloody diarrhea (if terminal ileum involved: B12/bile acid malabsorption), weight loss, fistulas, perianal disease (fissures, fistulas, skin tags). Complications: strictures (obstructive symptoms), fistulas (enterovesical → fecaluria; enterocutaneous → skin drainage), abscesses, short bowel syndrome after resections. Ulcerative colitis: bloody mucoid diarrhea, tenesmus, abdominal cramping. Complications: toxic megacolon (colon diameter >6cm + systemic toxicity) → emergency colectomy; colorectal cancer (risk increases with extent and duration — pancolitis + >10 years → surveillance colonoscopy every 1–2 years); primary sclerosing cholangitis (PSC) — strongly associated with UC, not Crohn's.

Extraintestinal Manifestations and Treatment

Both IBD types: aphthous oral ulcers, peripheral arthritis (parallels bowel disease activity), erythema nodosum, episcleritis. Axial arthropathy (AS/sacroiliitis) and pyoderma gangrenosum do NOT parallel bowel activity. PSC is specifically associated with UC. Treatment — Crohn's: mild-moderate: 5-ASA (limited role); budesonide (ileal/right-colon Crohn's); antibiotics (metronidazole + ciprofloxacin for perianal). Moderate-severe: corticosteroids (short-term), immunomodulators (azathioprine, 6-MP), biologics (anti-TNF: infliximab, adalimumab; anti-integrin: vedolizumab; anti-IL-12/23: ustekinumab). Surgery for complications — not curative. Treatment — UC: mild-moderate: oral or rectal 5-ASA (mesalamine). Moderate-severe: corticosteroids, then azathioprine or anti-TNF biologics. Refractory/severe: infliximab (Remicade) or tofacitinib (JAK inhibitor). Total proctocolectomy is CURATIVE (unlike Crohn's).

Test yourself

Quick check — 2 questions

1. A 25-year-old man has RLQ pain, 10 lb weight loss, and non-bloody diarrhea for 6 months. Colonoscopy shows segmental areas of erythema and ulceration with normal mucosa in between, and biopsy reveals non-caseating granulomas. The most likely diagnosis is:

2. A patient with 15-year history of UC pancolitis presents with progressive jaundice, pruritus, and elevated ALP. MRCP shows 'beads on a string' appearance of intra- and extrahepatic bile ducts. Diagnosis?

Frequently asked questions

What is toxic megacolon and what IBD complication does it represent?

Toxic megacolon is a life-threatening complication of severe colitis (most commonly UC, but can occur in Crohn's colitis or infectious colitis from C. difficile, CMV). Definition: transverse colon diameter >6cm (usually ≥8cm) on plain abdominal film + systemic toxicity (fever >38.6°C, tachycardia >120, leukocytosis, anemia, altered mentation). Pathophysiology: transmural inflammation paralyzes the muscular wall. Risk: perforation → peritonitis → septic shock. Treatment: NPO, bowel rest, IV steroids/cyclosporine/infliximab for UC; urgent colectomy if no improvement in 24–72 hours or clinical deterioration.

How does rectal involvement help differentiate Crohn's from UC?

The rectum is ALWAYS involved in UC — it is the defining starting point of disease. Disease extends continuously from the rectum proximally. Rectal sparing (normal rectum with more proximal colitis) is a key feature of Crohn's colitis and should prompt consideration of Crohn's over UC. In Crohn's, the terminal ileum is the most commonly affected site, not the rectum. Perianal disease (fistulas, skin tags, fissures) is a hallmark of Crohn's. On endoscopy: UC shows continuous mucosal erythema/friability; Crohn's shows linear 'bear-claw' ulcers, cobblestoning, and skip areas.

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