10.3 Crohn's Disease, Ulcerative Colitis & Extraintestinal Manifestations
Key Takeaways
- Crohn's disease shows skip lesions, transmural inflammation and non-caseating granulomas; ulcerative colitis is continuous, mucosal and extends proximally from the rectum.
- Pyoderma gangrenosum and primary sclerosing cholangitis run independently of bowel disease activity, whereas erythema nodosum and episcleritis parallel it.
- Toxic megacolon is defined by transverse colonic dilatation above 6 cm with systemic toxicity and requires urgent surgical review.
Inflammatory bowel disease (IBD) and small bowel malabsorptive disorders are frequently tested on MRCP(UK) Part 1. Candidates are expected to differentiate Crohn's disease from ulcerative colitis across clinical, endoscopic, and histological domains, understand the complex pharmacology of induction and maintenance agents, execute emergency protocols for acute severe ulcerative colitis, and master the diagnosis and systemic complications of coeliac disease.
1. Crohn's Disease vs Ulcerative Colitis
Inflammatory bowel disease displays a bimodal age incidence, peaking between 15–30 years with a second smaller peak between 60–80 years. Environmental factors exert divergent influences: cigarette smoking worsens Crohn's disease (increasing flare frequency, stricture formation, and surgical resection rates), whereas smoking exerts a paradoxically protective effect in ulcerative colitis (non-smokers and ex-smokers are at highest risk of developing UC, and smoking cessation frequently precipitates a severe flare). Appendicectomy performed in early life is protective against ulcerative colitis.
Comparative Clinical, Endoscopic, Radiological & Histological Features
| Domain | Crohn's Disease (CD) | Ulcerative Colitis (UC) |
|---|---|---|
| Anatomical Distribution | Any part of GI tract from mouth to anus; terminal ileum and proximal colon most common (~50%); rectal sparing in ~50% | Confined to colon and rectum; begins in rectum (proctitis in 95%) and extends proximally in an uninterrupted, continuous manner |
| Pattern of Involvement | Discontinuous ("skip lesions") with intervening areas of normal mucosa | Continuous and symmetrical; no skip areas (except occasional "backwash ileitis" or caecal patch) |
| Depth of Inflammation | Transmural (involves mucosa, submucosa, muscularis propria, and serosa) | Mucosal and submucosal only; muscularis propria and serosa are spared (except in toxic megacolon) |
| Macroscopic / Endoscopic Findings | Deep, longitudinal, serpiginous "knife-like" or "bear-claw" ulcers; "cobblestone" mucosa; strictures; serosal "creeping fat" | Diffuse mucosal erythema, granular appearance, extreme contact friability, superficial bleeding ulcers, inflammatory pseudopolyps |
| Histopathological Hallmarks | Non-caseating epithelioid granulomas (~50%, pathognomonic); transmural lymphoid aggregates; fissuring ulcers; knife-like clefts | Crypt abscesses (neutrophilic collections within crypt lumens); crypt architectural distortion and branching; goblet cell depletion; no granulomas |
| Radiological Findings | Barium follow-through / MRE: "String sign of Kantor" (terminal ileal narrowing/stricture); asymmetric bowel wall thickening; internal fistulae | Barium enema / CT: Loss of normal colonic haustrations ("lead-pipe" or "hose-pipe" colon); fine mucosal spiculation ("collar-button" ulcers) |
| Cardinal Complications | Fibrostenotic strictures, bowel obstruction, fistulae (enterocutaneous, enterovesical, enterocolic), perianal disease, abscesses, gallstones | Toxic megacolon (transverse diameter >6 cm), colonic perforation, life-threatening lower GI haemorrhage, primary sclerosing cholangitis, colorectal cancer |
| Perianal Disease | Frequent (~30–50%): Complex branching perianal fistulae, deep fissures, indolent perianal skin tags, ischiorectal abscesses | Rare; limited to simple, superficial fissures or excoriations secondary to severe diarrhoea |
| Serological Biomarkers | Anti-Saccharomyces cerevisiae antibodies (ASCA positive in ~60%) | Perinuclear antineutrophil cytoplasmic antibodies (p-ANCA positive in ~65%) |
2. Extraintestinal Manifestations (EIMs) of IBD
Extraintestinal manifestations occur in up to 40% of patients with IBD. For the MRCP examination, these must be categorized by whether their clinical activity mirrors luminal gastrointestinal inflammation:
A. Manifestations that Parallel Luminal Bowel Disease Activity
These complications flare in tandem with active intestinal inflammation and characteristically resolve once luminal bowel disease is brought into remission (or surgically resected in UC):
- Peripheral Arthropathy (Type 1): Pauciarticular (<5 joints involved), asymmetric, affecting large weight-bearing lower limb joints (knees, ankles, hips). Non-deforming, non-erosive, and self-limiting; parallels bowel activity closely.
- Erythema Nodosum: Tender, hot, erythematous subcutaneous nodules classically located over the anterior pretibial shins. Present in ~15% of CD and ~10% of UC during active flares.
- Episcleritis: Injected, fiery-red vascular engorgement of the superficial episcleral plexus. Painless or mildly aching; visual acuity is completely preserved; vessels blanch upon topical phenylephrine application. Activity mirrors intestinal flares.
- Aphthous Stomatitis: Painful oral mucosal ulcerations that appear during active exacerbations.
B. Manifestations Independent of Luminal Bowel Disease Activity
These complications pursue an autonomous clinical course and do NOT correlate with intestinal disease activity; they may antedate bowel symptoms, persist during deep endoscopic remission, and progress even after total proctocolectomy:
- Axial Spondyloarthropathy & Ankylosing Spondylitis: Strongly associated with the HLA-B27 allele (present in 50–70% of IBD patients with axial disease). Manifests as inflammatory back pain (worse at rest, improving with physical exercise, nocturnal wakening) and progressive bilateral sacroiliitis leading to syndesmophyte formation and "bamboo spine". Runs an entirely autonomous course.
- Peripheral Arthropathy (Type 2): Polyarticular (>=5 joints), symmetric, predilection for small joints of the hands (metacarpophalangeal and proximal interphalangeal joints). Pursues a chronic course lasting years, independent of bowel activity.
- Pyoderma Gangrenosum: Debilitating, painful, non-infectious inflammatory dermatosis. Initiates as a pustule or erythematous nodule that rapidly breaks down into an expanding, necrotic ulcer with violaceous, undermined, purplish borders and a purulent cribriform base. Classically exhibits pathergy—the rapid induction or worsening of ulceration following minor mechanical trauma, venepuncture, or biopsy. Surgical debridement is strictly contraindicated as it exacerbates tissue necrosis. Treatment requires systemic high-dose corticosteroids, oral ciclosporin, or anti-TNF therapy (infliximab). Runs a course completely independent of bowel activity.
- Primary Sclerosing Cholangitis (PSC): Chronic fibrosing inflammatory obliteration of intrahepatic and extrahepatic bile ducts. Approximately 70–80% of patients with PSC have underlying IBD (predominantly ulcerative colitis); conversely, ~5% of patients with UC develop PSC. Characterized by elevated alkaline phosphatase and "beaded" strictures on MRCP. Runs a totally independent course; proctocolectomy does not arrest hepatic progression. Concurrently elevates the lifetime risk of cholangiocarcinoma and accelerates colorectal carcinogenesis, mandating annual screening colonoscopy with chromoendoscopy from the time of PSC diagnosis.
- Anterior Uveitis (Iritis): Severe ocular pain, marked photophobia, blurred vision, ciliary flush, and sluggish pupil. Slit-lamp examination demonstrates inflammatory cells and protein flare in the anterior chamber. Sight-threatening ophthalmic emergency requiring immediate topical corticosteroids and cycloplegic-mydriatic agents to prevent posterior synechiae formation.
- Metabolic & Vascular Sequelae:
- Nephrolithiasis: Calcium oxalate stones in Crohn's ileal disease (unabsorbed fatty acids bind luminal calcium, leaving free oxalate to be hyperabsorbed in the colon: "enteric hyperoxaluria"). Uric acid stones in ileostomies due to chronic dehydration and acidic urine.
- Venous Thromboembolism (VTE): 3- to 4-fold elevated risk of deep vein thrombosis and pulmonary embolism during active flares driven by acute phase reactants, elevated factor VIII, and hyperfibrinogenaemia.
A 45-year-old man with established ileocolonic Crohn's disease presents to the outpatient clinic with an intensely painful, rapidly enlarging ulcer on his left pretibial region that began as a small pustule following a minor graze 2 weeks ago. On examination, there is a 6 x 5 cm purulent ulcer with ragged, violaceous, undermined borders surrounded by an erythematous, indurated halo. A visiting surgical registrar suggests urgent surgical excision and sharp bedside curettage of the ulcer bed to remove devitalized tissue. Which of the following statements concerning this complication is most accurate?