STEPTRACKERGASTROINTESTINAL | PATHOLOGY + IMMUNOLOGY + PHARMACOLOGY | HIGH-YIELD REVIEW

Inflammatory Bowel Disease

Crohn disease and ulcerative colitis side by side — where they strike, how deep they go, and what they lead to.

FOCUS ON
Crohn: anywhere mouth to anus (terminal ileum most), skip lesions, transmural, noncaseating granulomas → fistulas, strictures, perianal disease. UC: starts in the rectum and spreads continuously, mucosa only, crypt abscesses → bloody diarrhea, toxic megacolon, PSC. Both raise colorectal cancer risk.
FEATURE
CROHN DISEASE
ULCERATIVE COLITIS
CLUE
Location
Any part of the GI tract; terminal ileum most common; rectum often spared
Rectum always, extends proximally; colon only (backwash ileitis possible)
Crohn: "skips" · UC: continuous
Depth
Transmural
Mucosa and submucosa
Depth explains fistulas
Gross · imaging
Skip lesions, linear ulcers, cobblestone mucosa, creeping fat, bowel-wall thickening; string sign (narrowed ileum)
Diffuse friable mucosa, pseudopolyps; loss of haustra → lead pipe colon
Barium findings are classic
Histology
Noncaseating granulomas, lymphoid aggregates (Th1, Th17)
Crypt abscesses, distorted crypts, neutrophils; no granulomas (Th2)
Granulomas = Crohn
Symptoms
Diarrhea (often non-bloody), RLQ pain, weight loss, fever
Bloody diarrhea, tenesmus, urgency, LLQ cramps
Blood is the UC tell
Complications
Fistulas (enterovesical, enterocutaneous, perianal), strictures → obstruction, abscesses; malabsorption (B12, bile acids) → calcium oxalate stones, gallstones
Toxic megacolon, perforation, hemorrhage
Fistula = Crohn
Extraintestinal (both)
Erythema nodosum, pyoderma gangrenosum, uveitis/episcleritis, aphthous ulcers, migratory arthritis, sacroiliitis/ankylosing spondylitis (HLA-B27)
Same list plus primary sclerosing cholangitis (stronger link)
PSC → cholangiocarcinoma risk
Serology
ASCA (anti-Saccharomyces) ⁺
p-ANCA ⁺ (MPO-ANCA)
Supportive only
Cancer risk
↑ colorectal cancer when colon involved
↑↑ colorectal cancer; depends on extent and duration
Surveillance colonoscopy ~8 years after onset
Smoking · surgery
Smoking worsens; surgery not curative (recurs)
Smoking is protective; colectomy is curative
Classic contrast
Genetics
NOD2 (defective sensing of bacterial peptidoglycan)
HLA and barrier genes; strong link with PSC
NOD2 = Crohn

Picture it — distribution and depth

ileumrectumcontinuous from rectum
Ulcerative colitis — whereStarts in the rectum and extends continuously upward.
ileumrectumskip lesions
Crohn disease — whereTerminal ileum most often; skip lesions anywhere; perianal disease.
mucosasubmucosamuscularisserosa
Ulcerative colitis — depthMucosa and submucosa only; crypt abscesses.
mucosasubmucosamuscularisserosatransmural · fissure · granuloma
Crohn disease — depthTransmural: fissures, fistulas, noncaseating granulomas.

Schematic diagrams, not to scale. Red = abnormal.

Mechanism — an overreaction to gut flora

Genetic riskNOD2, IL-23R
+
Barrier defectBacteria cross
→
Abnormal immune responseTh1/Th17 (Crohn) · Th2 (UC)
→
Chronic inflammationRelapses and remissions
Years of colitis
→
Flat dysplasiaEarly p53
→
Colorectal carcinomaYounger, multifocal

Drugs for IBD

DRUG
ACTION
WATCH FOR
5-ASA (mesalamine, sulfasalazine)
Topical anti-inflammatory; mainly UC
Sulfasalazine: oligospermia, sulfa reactions
Glucocorticoids
Induction of remission
Not for maintenance
Azathioprine / 6-MP
Purine antagonist
Check TPMT; marrow suppression
Infliximab, adalimumab
Anti-TNF-α
Screen for latent TB
Vedolizumab
Anti-α4β7 integrin (gut-selective)
Natalizumab (α4) → PML (JC virus)
Ustekinumab
Anti-IL-12/23 (p40)
Infection

How it's tested

COMMON CONFUSIONCrypt abscesses favor UC but can appear in Crohn; granulomas and transmural inflammation are what make it Crohn.
THE TRAPA UC patient with fever, tachycardia and a dilated colon on X-ray has toxic megacolon. Colonoscopy and barium enema can cause perforation — avoid them.
YOUR APPROACHRectum involved? Continuous or skipping? Transmural (fistula, stricture) or mucosal (bleeding)? Then name the extraintestinal and cancer risk.

If you see… think…

Young adult, RLQ pain, perianal fistula, string sign → Crohn disease
Noncaseating granulomas in a colon biopsy → Crohn disease
Bloody diarrhea, tenesmus, continuous disease from the rectum → ulcerative colitis
Lead pipe colon on barium enema → chronic UC
UC + ↑ alkaline phosphatase + beaded bile ducts → PSC
Air bubbles in urine in a patient with Crohn → enterovesical fistula
Crohn with ileal resection + oxalate stones → fat binds Ca²⁺, oxalate absorbed
Painful ulcer with violaceous undermined border on the shin → pyoderma gangrenosum
Fever, tachycardia, distended colon > 6 cm in UC → toxic megacolon
Start infliximab → screen for latent TB first