Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is a group of chronic, immune-mediated disorders that inflame the gastrointestinal tract in a relapsing-remitting pattern. The two dominant forms — Crohn disease and ulcerative colitis — share an underlying theme of a dysregulated immune response to gut microbes in a genetically susceptible host, yet they behave differently enough that telling them apart shapes everything from which drug you reach for to whether surgery can cure the patient. For a student, IBD is one of those topics that rewards understanding the pattern rather than memorising lists: once you grasp "transmural, skip lesions, anywhere" versus "mucosal, continuous, colon only," the clinical picture, complications, and management fall into place.

This matters because IBD typically strikes young people (peak onset 15–35 years), lasts a lifetime, and profoundly affects quality of life, fertility decisions, cancer risk, and mental health. It is also a field transformed in the last three decades by biologic therapy, so the exam-relevant knowledge and the real-world practice have both moved fast.

Learning Objectives

• Distinguish Crohn disease from ulcerative colitis by distribution, depth, and histology.
• Explain the pathophysiology: genetics, mucosal immunity, microbiome, and barrier dysfunction.
• Recognise the clinical presentations, extraintestinal manifestations, and complications of each.
• Outline the diagnostic workup, including endoscopy, imaging, and laboratory markers.
• Apply the stepwise ("step-up") medical management and know the indications for surgery.
• Avoid the classic pitfalls in differentiating and treating IBD.

Quick Answer

IBD comprises Crohn disease and ulcerative colitis, chronic relapsing inflammatory conditions driven by an inappropriate immune response to the gut microbiome in genetically predisposed people. Crohn disease can affect any part of the GI tract mouth-to-anus (classically terminal ileum), is transmural and discontinuous (skip lesions), and causes strictures, fistulae, and abscesses. Ulcerative colitis is limited to the colon, always involves the rectum, spreads continuously and proximally, and is confined to the mucosa/submucosa. Diagnosis rests on clinical picture plus ileocolonoscopy with biopsies, supported by imaging (MR/CT enterography) and biomarkers such as faecal calprotectin. Management is stepwise — aminosalicylates, corticosteroids for flares, immunomodulators (azathioprine, methotrexate), and biologics (anti-TNF, anti-integrin, anti-IL-12/23) — with colectomy potentially curative in UC but only palliative/organ-sparing in Crohn.

Where It Came From

Chronic bloody diarrhoea has been recognised for centuries, but ulcerative colitis was first clearly delineated as a distinct entity by Sir Samuel Wilks in 1859, who separated it from infective dysentery on the basis of the sterile, chronically inflamed colon he saw at autopsy. The great puzzle was the small-bowel disease that behaved differently.

The landmark moment came in 1932, when Burrill B. Crohn, along with colleagues Leon Ginzburg and Gordon Oppenheimer at Mount Sinai Hospital in New York, published "Regional Ileitis: A Pathologic and Clinical Entity" in JAMA. They described a series of young patients with a chronic, scarring, granulomatous inflammation of the terminal ileum — a segment that formed strictures and fistulae and did not fit the picture of tuberculosis or colitis. Because Crohn's name came first alphabetically on the paper, the disease bore his name, though earlier reports (notably by the Polish surgeon Antoni Leśniowski in 1904) had touched on it. The real motivation was practical: surgeons kept resecting inflamed, fistulating ileum without a name for what they were treating, and misdiagnosing it as intestinal tuberculosis or appendiceal disease. Naming the entity allowed physicians to study its natural history rather than lumping it with infections.

The next revolution was mechanistic. Through the late 20th century, epidemiology (rising incidence in industrialised nations, the "hygiene hypothesis"), the 2001 discovery that NOD2/CARD15 gene variants predispose to Crohn disease, and the recognition of the gut microbiome's role reframed IBD as a disorder of the interface between host immunity and microbes. Therapeutically, the approval of infliximab (anti-TNF) in 1998 opened the biologic era, shifting goals from merely controlling symptoms toward healing the mucosa itself.

Two Diseases, One Family: Crohn vs Ulcerative Colitis

The single most useful mental model is depth and distribution.

Ulcerative colitis is a disease of the colonic mucosa. It begins in the rectum (proctitis) and extends proximally in a continuous, uninterrupted sheet — left-sided colitis, then extensive colitis or pancolitis. Inflammation is confined to the mucosa and superficial submucosa. Because it is superficial and continuous, it bleeds readily (bloody diarrhoea with mucus is the hallmark) but rarely fistulates or strictures.

Crohn disease is a transmural disease that can appear anywhere from mouth to anus, most often the terminal ileum and proximal colon. Inflammation penetrates the full thickness of the bowel wall, producing the characteristic complications: fibrotic strictures (obstruction), fistulae (bowel-to-bowel, bowel-to-bladder, bowel-to-skin), abscesses, and perianal disease. Its distribution is patchy — normal mucosa sits between inflamed segments ("skip lesions") — and the endoscopic appearance is often a cobblestone pattern from deep linear ulcers crossing swollen mucosa.

A worked distinction: a 24-year-old with 6 weeks of bloody diarrhoea, urgency, and tenesmus, whose colonoscopy shows continuous friable mucosa from the rectum to the splenic flexure and stops sharply there, has ulcerative colitis. A 24-year-old with crampy right-lower-quadrant pain, weight loss, non-bloody diarrhoea, a palpable RLQ mass, and a perianal fistula, whose colonoscopy shows patchy aphthous ulcers in the caecum and a strictured, cobblestoned terminal ileum with normal intervening mucosa, has Crohn disease.

Pathophysiology: Genes, Microbes, and a Leaky Barrier

IBD arises from the convergence of four factors:

1. Genetic susceptibility. Over 200 risk loci are identified. In Crohn disease, NOD2 mutations impair the intracellular sensing of bacterial muramyl dipeptide by Paneth cells and macrophages, blunting antimicrobial defensin release. Genes affecting autophagy (ATG16L1, IRGM) also cluster in Crohn — pointing to defective clearance of intracellular bacteria.
2. Barrier dysfunction. A compromised mucus layer and leaky epithelial tight junctions allow luminal bacteria to contact the immune system.
3. Dysbiosis. Reduced microbial diversity, fewer protective short-chain-fatty-acid producers (e.g. Faecalibacterium prausnitzii), and expansion of adherent-invasive E. coli shift the balance toward inflammation.
4. Dysregulated immunity. The classic teaching is that Crohn disease is driven by a Th1/Th17 response with high interferon-gamma, TNF, and IL-12/23, producing granulomatous transmural inflammation; ulcerative colitis leans toward an atypical Th2 profile. This is a simplification, but it explains why anti-TNF and anti-IL-12/23 agents work.

Non-caseating granulomas are the histologic signature of Crohn disease when present (found in ~30% of biopsies) but are not required for diagnosis; UC never shows granulomas. Two curious epidemiologic facts anchor the microbe/environment link: smoking worsens Crohn disease but is paradoxically protective in ulcerative colitis, and appendicectomy reduces UC risk. These are favourite exam points.

Clinical Features and Extraintestinal Manifestations

Both diseases can produce systemic effects because inflammation is not confined to the gut. Shared extraintestinal manifestations include:

• Skin: erythema nodosum (correlates with disease activity), pyoderma gangrenosum (does not).
• Eyes: episcleritis (activity-related), uveitis.
• Joints: peripheral arthritis (activity-related) and axial disease — sacroiliitis and ankylosing spondylitis (HLA-B27, independent of gut activity).
• Hepatobiliary: primary sclerosing cholangitis (PSC), strongly linked to ulcerative colitis and carrying a markedly elevated colorectal cancer risk.

Crohn disease adds nutritional consequences from small-bowel involvement: vitamin B12 deficiency (terminal ileal disease or resection), iron and fat-soluble vitamin malabsorption, and gallstones/renal oxalate stones after ileal resection.

Making the Diagnosis

There is no single test; diagnosis integrates clinical, endoscopic, histologic, radiologic, and laboratory data.

• Ileocolonoscopy with multiple biopsies is the cornerstone — it maps distribution, samples the terminal ileum, and provides histology.
• Imaging: MR or CT enterography assesses small-bowel Crohn disease, strictures, and penetrating complications; MRI pelvis is best for perianal fistulae; capsule endoscopy can find proximal small-bowel disease (avoid if stricture suspected — risk of capsule impaction).
• Biomarkers: faecal calprotectin is a sensitive, non-invasive marker of intestinal inflammation, useful to distinguish IBD from irritable bowel syndrome and to monitor activity. CRP and ESR support activity assessment; anaemia and low albumin reflect severity.
• Exclude infection: stool cultures and Clostridioides difficile toxin are mandatory before immunosuppression, since infective colitis mimics a flare.

Serology (pANCA positive in UC, ASCA positive in Crohn) has limited sensitivity and is not used to make the diagnosis. About 10% of colitis confined to the colon cannot be classified and is labelled IBD-unclassified (or indeterminate colitis on pathology).

Management: The Step-Up Ladder and When to Cut

Treatment has two phases — inducing remission in a flare and maintaining remission — and is tailored to disease location, severity, and the presence of complications.

Ulcerative colitis.

• Mild-moderate: 5-aminosalicylates (mesalazine), topical (suppository/enema) for distal disease plus oral for extensive disease. 5-ASAs are the maintenance backbone.
• Moderate-severe flare: corticosteroids to induce remission (never for maintenance), stepping up to thiopurines (azathioprine), biologics (infliximab, adalimumab, vedolizumab, ustekinumab), or JAK inhibitors (tofacitinib).
• Acute severe UC is a medical emergency: admit, IV hydrocortisone, VTE prophylaxis, and assess on day 3 (Travis/Oxford criteria). Failure to respond triggers rescue therapy (IV infliximab or ciclosporin) or urgent colectomy. Watch for toxic megacolon — colonic dilatation over ~6 cm with systemic toxicity — which needs surgery if it does not improve.
• Colectomy is curative in UC because the disease is confined to the colon.

Crohn disease.

• Induce remission with corticosteroids (budesonide for ileocaecal disease has fewer systemic effects); 5-ASAs are largely ineffective in Crohn.
• Maintain with thiopurines, methotrexate, or biologics. Early use of anti-TNF (with an immunomodulator) is favoured in aggressive or fistulating disease — a "top-down" rather than "step-up" approach.
• Surgery is not curative and disease recurs, classically at the anastomosis, so it is reserved for complications: obstructing strictures (resection or stricturoplasty), fistulae, abscesses (drainage plus antibiotics), and perianal disease (setons plus anti-TNF).
• Smoking cessation is one of the most effective interventions in Crohn disease.

Across both diseases the modern goal is treat-to-target: aiming not just for symptom control but for objective mucosal healing (endoscopic and biomarker remission), which reduces hospitalisation, surgery, and cancer.

Real-World Applications

• Cancer surveillance: Long-standing colitis (both UC and Crohn colitis) raises colorectal cancer risk; surveillance colonoscopy with dye-spray (chromoendoscopy) typically begins ~8 years after diagnosis, sooner and more often if PSC coexists.
• Perioperative and pregnancy planning: Most IBD drugs (including anti-TNF) are compatible with pregnancy; methotrexate is teratogenic and must be stopped well before conception. Active disease is more dangerous to pregnancy than most medications.
• Vaccination and infection screening: Before starting immunosuppression, screen for latent TB and hepatitis B, and vaccinate (avoid live vaccines on biologics).
• Nutrition: Exclusive enteral nutrition can induce remission in paediatric Crohn disease without steroids — an elegant real-world example of the microbiome/diet link.

Common Mistakes

• Assuming any bloody diarrhoea in a young person is IBD. Infective colitis (including C. difficile) can look identical and can co-exist with, or trigger, an IBD flare. Correction: always send stool studies before escalating immunosuppression.
• Using corticosteroids for maintenance. Steroids induce remission but do not maintain it and cause serious long-term harm (osteoporosis, diabetes, cataracts). Correction: steroid dependence is an indication to step up to a steroid-sparing agent (thiopurine or biologic).
• Believing 5-ASAs work well in Crohn disease. They are effective in UC but have little role in Crohn. Correction: reserve aminosalicylates for UC and use budesonide/immunomodulators/biologics in Crohn.
• Forgetting that colectomy "cures" only UC. Because Crohn can affect the whole gut, resection is palliative and recurs. Correction: frame surgery in Crohn as complication management, not cure.
• Telling a Crohn patient smoking does not matter. Smoking clearly worsens Crohn disease outcomes. (In UC it is protective, but cessation is still advised for overall health.)

Comparison and Connections

Feature	Ulcerative Colitis	Crohn Disease
Site	Colon only, always rectum, continuous	Mouth to anus, often terminal ileum, skip lesions
Depth	Mucosa/submucosa	Transmural
Bleeding	Common, bloody diarrhoea	Less common; more pain, weight loss
Strictures/fistulae	Rare	Common
Granulomas	Absent	Present in ~30%
Endoscopy	Continuous friable mucosa	Cobblestoning, aphthous/linear ulcers
Smoking	Protective	Harmful
Serology	pANCA more often	ASCA more often
Surgery	Colectomy curative	Not curative; recurs

IBD contrasts with irritable bowel syndrome (IBS), which is a functional disorder with no inflammation, normal calprotectin, and no bleeding or systemic features. It also differs from infective and ischaemic colitis, and from microscopic colitis (watery diarrhoea, normal-looking colon, diagnosis only on biopsy).

Practice Questions

Recall

Q: Which layers of the bowel wall does ulcerative colitis involve, and how far does it extend? A: The mucosa and superficial submucosa only; it involves the rectum and extends continuously and proximally through the colon, stopping at (or before) the ileocaecal valve.

Understanding

Q: Why does Crohn disease cause fistulae and strictures while ulcerative colitis usually does not? A: Crohn inflammation is transmural, penetrating the full wall thickness — deep ulcers erode into adjacent structures (fistulae) and fibrotic healing narrows the lumen (strictures). UC inflammation is superficial, so it bleeds but does not typically perforate the wall or fibrose it.

Application

Q: A 28-year-old with known UC presents with 10 bloody stools per day, fever, tachycardia, and albumin 24 g/L. What is the diagnosis and immediate management? A: Acute severe ulcerative colitis. Admit; give IV corticosteroids (hydrocortisone), IV fluids, VTE prophylaxis, and exclude C. difficile; involve surgery early. Assess response at day 3 — if failing, offer rescue infliximab or ciclosporin, or urgent colectomy. Monitor for toxic megacolon with serial exams and abdominal imaging.

Analysis

Q: A patient labelled with UC undergoes total colectomy but later develops a perianal fistula and terminal ileal inflammation. What does this suggest and why does it matter? A: The original diagnosis was likely Crohn disease (or IBD-unclassified). It matters because it changes expectations — the colectomy was not curative, an ileal pouch may be contraindicated or at risk of Crohn-related complications, and the patient needs Crohn-directed therapy such as anti-TNF for the fistula.

FAQ

Is IBD the same as IBS? No. IBD (inflammatory bowel disease) is genuine, visible inflammation with tissue damage, bleeding, and systemic effects. IBS (irritable bowel syndrome) is a functional disorder — real and distressing, but with no inflammation, normal calprotectin, and no bleeding or weight loss.

Can IBD be cured? Ulcerative colitis can be effectively cured by removing the colon, since disease is confined there. Crohn disease has no cure because it can affect any part of the gut; the aim is durable remission and preventing complications.

Will I need surgery? Many people never do, especially with modern biologics. Up to half of Crohn patients eventually need surgery for a complication; in UC, surgery is for disease that fails medical therapy or for cancer risk.

Does diet cause IBD? Diet does not directly cause IBD, though Western diets and the microbiome are implicated in susceptibility. Specific foods may trigger symptoms individually, and exclusive enteral nutrition can induce remission in children with Crohn — but there is no single "IBD diet" that cures the disease.

Is it safe to have children with IBD? Yes, for most people. Well-controlled disease and appropriate medications (most, including anti-TNF, are pregnancy-compatible) give good outcomes. Methotrexate must be avoided. Active disease poses more risk than most drugs, so planning conception during remission is ideal — a decision to make with your specialist.

Quick Revision

• IBD = Crohn disease + ulcerative colitis; chronic, immune-mediated, relapsing-remitting, peak onset 15–35 years.
• Crohn: mouth-to-anus, terminal ileum, transmural, skip lesions, cobblestoning, granulomas, strictures/fistulae; smoking harmful; surgery not curative.
• UC: colon only, rectum always, continuous, mucosal, bloody diarrhoea; smoking protective; colectomy curative.
• Pathophysiology: genes (NOD2 in Crohn) + barrier defect + dysbiosis + dysregulated immunity (Th1/Th17 in Crohn).
• Diagnosis: ileocolonoscopy + biopsies, enterography, faecal calprotectin; exclude C. difficile.
• Management: 5-ASA (UC), steroids to induce (never maintain), thiopurines/methotrexate, biologics (anti-TNF, vedolizumab, ustekinumab), JAK inhibitors; treat-to-target mucosal healing.
• Acute severe UC: IV steroids, day-3 review, rescue therapy or colectomy; beware toxic megacolon.
• Long-standing colitis raises colorectal cancer risk — surveillance colonoscopy; PSC links to UC.

Related Topics

Prerequisites

• Gastroenterology overview
• Basic mucosal immunology — see ../../34._Immunology/index.md

Related Topics

• Colorectal cancer and surveillance — see ../../32._Oncology/index.md
• Primary sclerosing cholangitis and hepatobiliary disease
• Irritable bowel syndrome (functional contrast)

Next Topics

• Biologic and immunomodulator pharmacology — see ../../5._Pharmacology/index.md
• Surgical management of the colon and small bowel — see ../../10._General_Surgery/index.md