Inflammatory Bowel Disease: Crohn's Disease and Ulcerative Colitis

Introduction

Inflammatory bowel disease (IBD) describes chronic, relapsing-remitting inflammatory conditions of the gastrointestinal tract, of which the two principal forms are Crohn's disease and ulcerative colitis.¹ Around 1 in 123 people in the UK are affected, and incidence peaks in young adulthood (15-30 years) with a smaller second peak in later life.

The aetiology is multifactorial, involving a dysregulated mucosal immune response to gut microbiota in a genetically susceptible individual, modified by environmental factors. Around 10-15% of colitis cannot initially be classified as either condition and is labelled IBD unclassified.

Comparing Crohn's disease and ulcerative colitis

Clinical features

Crohn's disease

Presentation reflects transmural, patchy inflammation and depends on the site involved. Typical features are crampy abdominal pain (classically right lower quadrant with terminal ileal disease, mimicking appendicitis), non-bloody diarrhoea, weight loss and systemic upset with fever and fatigue. Mouth ulceration is common, and perianal disease - fistulae, fissures, abscesses and skin tags - is a strong pointer to Crohn's rather than UC. Malabsorption from small bowel involvement causes deficiencies of iron, vitamin B12 (terminal ileum) and fat-soluble vitamins.

Ulcerative colitis

The hallmark is bloody diarrhoea with mucus, accompanied by urgency and tenesmus, and lower abdominal cramping that is often relieved by defecation. Disease limited to the rectum (proctitis) may cause rectal bleeding and urgency with normal or even constipated bowel habit, whereas extensive colitis causes frequent bloody stools with systemic upset, fever, tachycardia and weight loss. Symptoms are typically more gradual in onset than infective colitis and follow a relapsing-remitting course.

Extraintestinal manifestations

Both conditions have manifestations outside the gut, which occur in up to a third of patients and may precede bowel symptoms.¹ Some parallel disease activity and some do not:

• Musculoskeletal: enteropathic arthritis (peripheral arthritis tracks disease activity), sacroiliitis and ankylosing spondylitis (independent of activity), osteoporosis
• Skin: erythema nodosum (tender red shin nodules, tracks activity), pyoderma gangrenosum (painful ulcer with a violaceous undermined edge, independent of activity)
• Eyes: episcleritis (tracks activity), uveitis and scleritis (independent) - uveitis is painful with photophobia and needs urgent ophthalmology review
• Hepatobiliary: primary sclerosing cholangitis, strongly associated with ulcerative colitis and conferring a substantial risk of cholangiocarcinoma and colorectal cancer
• Haematological: anaemia (iron deficiency, anaemia of chronic disease, B12 deficiency) and a markedly increased venous thromboembolism risk during flares
• Renal: oxalate stones in Crohn's disease with fat malabsorption

Investigations

Initial tests

• Bloods: full blood count (anaemia, thrombocytosis), CRP and ESR (raised in active disease), urea and electrolytes, liver function tests, albumin (low in severe disease), haematinics (iron studies, B12, folate)
• Faecal calprotectin: a protein released by neutrophils into the stool, raised in intestinal inflammation. Its main use is distinguishing IBD from irritable bowel syndrome, where it is normal, avoiding unnecessary colonoscopy²
• Stool culture and Clostridioides difficile toxin: essential to exclude infective colitis, which can both mimic and precipitate a flare
• Coeliac serology, where the presentation is compatible

Endoscopy and histology

Colonoscopy with ileal intubation and multiple biopsies is the definitive investigation, allowing assessment of disease distribution and histological confirmation. In suspected acute severe colitis, a flexible sigmoidoscopy without bowel preparation is preferred, as full colonoscopy risks perforation.

Imaging

• MR enterography: the investigation of choice for small bowel Crohn's disease, assessing disease extent, strictures and fistulae without radiation
• MRI pelvis: for perianal fistulising Crohn's disease
• Abdominal X-ray: in acute severe colitis, to look for colonic dilatation (toxic megacolon) and to assess disease extent from faecal loading
• CT abdomen: in the acute setting for suspected perforation, obstruction or abscess
• Capsule endoscopy: for suspected small bowel Crohn's disease not seen on other imaging; contraindicated if a stricture is suspected

Management of Crohn's disease

Inducing remission

Corticosteroids (oral prednisolone, or budesonide for ileocaecal disease with fewer systemic effects) are first-line for inducing remission.³ Exclusive enteral nutrition is an effective steroid-sparing alternative and is preferred first-line in children, where growth is a concern. Aminosalicylates are much less effective in Crohn's disease than in UC and have a limited role.

Maintaining remission

Azathioprine or mercaptopurine are first-line maintenance agents; thiopurine methyltransferase (TPMT) activity must be checked before starting, as deficiency predisposes to severe myelosuppression. Methotrexate is an alternative. Biologics - anti-TNF agents (infliximab, adalimumab), ustekinumab or vedolizumab - are used for severe or refractory disease and for fistulising disease. Screening for latent tuberculosis and hepatitis B is mandatory before starting anti-TNF therapy.

Surgery

Surgery is not curative in Crohn's disease, as inflammation typically recurs at the anastomosis, so it is reserved for complications: strictures (strictureplasty or limited resection), fistulae, abscesses, perforation, or disease refractory to medical therapy. Around half of patients require surgery within 10 years. Repeated small bowel resections risk short bowel syndrome.

Management of ulcerative colitis

Inducing remission in mild to moderate disease

Treatment is stepwise and the route depends on disease extent.⁴ Aminosalicylates (mesalazine) are first-line: topical (suppository or enema) for proctitis and left-sided disease, oral for more extensive disease, and often both combined. If there is no response, add an oral corticosteroid.

Acute severe ulcerative colitis

This is a medical emergency requiring admission. Severity is assessed with the Truelove and Witts criteria.

Management involves IV corticosteroids (hydrocortisone), fluid and electrolyte correction, VTE prophylaxis (the thrombotic risk is high despite the bleeding), stool cultures to exclude infection, and stopping opioids and antimotility agents, which can precipitate toxic megacolon. Response is assessed at day 3 using stool frequency and CRP; failure to respond triggers rescue therapy with infliximab or ciclosporin, with early surgical involvement throughout.

Maintaining remission and surgery

Maintenance uses aminosalicylates first-line, with azathioprine or mercaptopurine for those with frequent relapses or who needed rescue therapy, and biologics for refractory disease. Colectomy is curative for the colonic disease, and is performed for acute severe colitis failing medical therapy, chronic refractory symptoms, or dysplasia. Options include subtotal colectomy with end ileostomy acutely, and later completion proctectomy with ileo-anal pouch (restorative proctocolectomy).

Colorectal cancer surveillance

Patients with extensive colitis have an increased colorectal cancer risk that rises with disease duration, extent and severity, and is markedly higher with coexisting primary sclerosing cholangitis. Colonoscopic surveillance begins around 10 years after symptom onset, with intervals of 1-5 years based on risk.

Complications

Crohn's disease

• Strictures causing small bowel obstruction
• Fistulae (enterocutaneous, enterovesical, enterovaginal, perianal) and abscesses
• Malabsorption with B12, iron and fat-soluble vitamin deficiency; gallstones and oxalate renal stones
• Short bowel syndrome after repeated resections
• Increased risk of small bowel and colorectal cancer

Ulcerative colitis

• Toxic megacolon: colonic dilatation above 6 cm with systemic toxicity, risking perforation
• Perforation and massive lower GI haemorrhage
• Colorectal cancer, hence surveillance colonoscopy
• Primary sclerosing cholangitis
• Venous thromboembolism during flares

Red flags

Prognosis

Both conditions follow a relapsing-remitting course over a lifetime, and with modern therapy most patients achieve good disease control and a near-normal life expectancy.¹ In ulcerative colitis, around half of patients relapse in any given year, and roughly 20-30% eventually require colectomy, which cures the colonic disease. In Crohn's disease, around half require surgery within 10 years and recurrence after resection is common, so the emphasis is on sustained medical maintenance to avoid repeated operations.

Adverse prognostic features in Crohn's disease include young age at diagnosis, extensive small bowel involvement, perianal disease, stricturing or penetrating behaviour, and smoking. Early use of immunomodulators and biologics in high-risk patients has improved long-term outcomes and reduced hospitalisation and surgery rates.