Crohn Disease
Contents (8)
Crohn disease is a chronic, transmural inflammatory bowel disease (IBD) characterized by patchy, full-thickness inflammation of the gastrointestinal tract that can affect any segment from mouth to anus but most commonly involves the terminal ileum and colon. With an incidence of 3-20 cases per 100,000 person-years and a prevalence of 26-199 per 100,000 depending on geographic location, it represents one of the most common chronic inflammatory conditions affecting young adults in developed nations. The disease typically presents between ages 15-40 years with a bimodal distribution and affects both sexes equally, though certain populations (Ashkenazi Jewish, Northern European descent) demonstrate higher prevalence. Crohn disease carries significant morbidity due to both intestinal complications (strictures, fistulas, perforation) and extraintestinal manifestations, making accurate diagnosis and early aggressive treatment critical for preventing irreversible complications and optimizing quality of life. For medical trainees, Crohn disease is a high-yield board topic appearing frequently in case questions, with emphasis on distinguishing it from ulcerative colitis and recognizing extraintestinal manifestations.
Crohn disease results from a complex interplay of genetic susceptibility, environmental triggers, and aberrant immune responses to the intestinal microbiota in genetically predisposed individuals. The pathophysiology can be conceptualized through several key mechanisms:
- Genetic susceptibility and NOD2/CARD15 mutations: The disease exhibits familial clustering with concordance rates of 36-50% in monozygotic twins. The first identified susceptibility gene was NOD2/CARD15 (nucleotide-binding oligomerization domain-containing protein 2), located on chromosome 16q12. This intracellular pattern recognition receptor normally detects bacterial peptidoglycan and triggers NF-κB activation for antimicrobial immune responses. Loss-of-function mutations in NOD2 (found in 40-50% of Crohn patients, particularly those with ileal disease) impair bacterial sensing and clearance, leading to dysbiosis and excessive lamina propria immune activation. Genome-wide association studies (GWAS) have identified over 200 genetic loci associated with IBD, including genes involved in autophagy (ATG16L1, IRGM), IL-23 signaling (IL23R, IL12B), and JAK-STAT pathways, indicating multiple dysregulated immune circuits rather than a single primary defect.
- Barrier dysfunction and intestinal epithelial compromise: The intestinal epithelium normally maintains selective permeability through tight junctions formed by claudins, occludin, and zonula occludens-1 (ZO-1). In Crohn disease, increased intestinal permeability ("leaky gut") develops through multiple mechanisms: TNF-α and IL-13 mediated disruption of tight junction proteins, epithelial cell apoptosis, reduced mucin production by goblet cells, and diminished antimicrobial peptide secretion by Paneth cells. Genetic variants affecting barrier proteins (CDH1, JAM2) increase disease susceptibility. This increased permeability allows bacterial translocation and microbial antigen penetration into the lamina propria, perpetuating immune activation in a positive feedback loop.
- Dysbiosis and abnormal microbiota composition: Healthy intestinal microbiota promotes immune tolerance through multiple mechanisms: production of short-chain fatty acids (particularly butyrate) that enhance regulatory T cell (Treg) differentiation via histone deacetylase inhibition, microbial antigen presentation that drives IL-10 and TGF-β secretion, and physical occupation of ecological niches that prevent pathogenic organisms. Crohn disease is characterized by profound dysbiosis: reduced bacterial diversity, decreased Faecalibacterium prausnitzii (a butyrate-producing bacterium with anti-inflammatory properties), expansion of Enterobacteriaceae and Bacteroides fragilis, and altered fungal communities. This dysbiosis precedes clinical disease onset in some at-risk individuals, suggesting it may be both cause and consequence of inflammation. Environmental triggers (infections, antibiotics, diet, smoking) further disrupt normal microbial ecology.
- Th1/Th17-mediated adaptive immune responses with loss of regulatory tolerance: The intestinal lamina propria normally contains a balanced mix of effector T cells and Tregs, maintained through continuous education by dendritic cells (DCs) and microbial antigens. In Crohn disease, DCs become hyperactive and produce excessive IL-12 and IL-23, driving differentiation of naive CD4+ T cells into pathogenic Th1 and Th17 cells rather than tolerogenic Tregs. Th1 cells produce IFN-γ and TNF-α, while Th17 cells produce IL-17 and IL-22; both promote epithelial inflammation and recruitment of additional effector cells. Simultaneously, Tregs (which produce IL-10 and TGF-β) become functionally deficient in number and suppressive capacity. This loss of immune tolerance combined with enhanced pro-inflammatory Th responses perpetuates chronic intestinal inflammation.
- Innate lymphoid cell (ILC) dysregulation: Beyond adaptive immunity, ILCs (particularly ILC3s) are expanded in Crohn disease and produce IL-17 and IL-22 upon IL-23 stimulation, contributing substantially to mucosal inflammation independent of T cells. ILC dysregulation appears to be genetically programmed in susceptible individuals (related to IL23R variants) and environmentally triggered by dysbiosis.
- Transmural inflammation as a distinguishing feature: Unlike ulcerative colitis, Crohn disease involves all layers of the bowel wall (transmural inflammation). The inflammatory infiltrate initially affects the mucosa and submucosa but progressively involves the muscularis propria and serosa. This full-thickness involvement explains the high propensity for complications: penetrating disease leads to fistula formation as inflammation erodes through serosal layers and adhesive loops, while muscular inflammation with fibrosis causes stricture formation. The "skip lesions" (patchy distribution with normal mucosa between inflamed segments) reflect heterogeneous antigen presentation or epitope spreading driven by specific bacterial antigens in different segments.
- Fibrostenotic complications and myofibroblast activation: Chronic transmural inflammation stimulates myofibroblasts in the submucosa to excessive collagen deposition and smooth muscle hyperplasia, leading to progressive stricture formation. TGF-β and other fibrogenic cytokines drive this process. Over time, repeated inflammation-healing cycles accumulate fibrosis that may not fully resolve even with anti-inflammatory therapy, explaining why some patients develop strictures despite mucosal healing.
Crohn disease is fundamentally an idiopathic condition arising from gene-environment interactions rather than a single identifiable cause. The following factors contribute to disease susceptibility and initiation:
- Genetic predisposition (not deterministic inheritance): While Crohn disease shows familial clustering (5-15% of patients have affected first-degree relatives) and twin concordance rates of 36-50% in monozygotic twins versus 5-10% in dizygotic twins, it does not follow Mendelian inheritance. Identified susceptibility loci contribute modest individual risk; NOD2 mutations confer 2-4 fold increased risk for heterozygotes and 15-40 fold risk for homozygotes, but most carriers never develop disease. The polygenic nature (>200 identified loci) explains variable penetrance and phenotypic heterogeneity. Ashkenazi Jewish ancestry and Northern European descent carry higher prevalence, suggesting ancestral environmental-genetic interactions.
- Smoking: One of the strongest environmental risk factors with paradoxical effects on IBD phenotype. Current or recent smoking increases Crohn disease risk 1.5-4 fold and predisposes to ileal disease, stricturing phenotype, and earlier need for surgical intervention, though smoking may paradoxically improve ulcerative colitis. Smoking impairs epithelial barrier function, reduces blood flow, and promotes pro-inflammatory Th1 responses. Smoking cessation is one of the few interventions shown to reduce relapse rates.
- Infections: Both acute bacterial/viral infections and chronic atypical infections have been hypothesized to trigger disease. Measles virus has been detected in granulomas of some patients, and mycobacterial species (Mycobacterium avium paratuberculosis) have been proposed as etiologic agents, though causality remains unproven. Viral infections (gastroenteritis, respiratory viruses) may trigger onset through molecular mimicry or by disrupting microbiota. Conversely, some infections (helminths, early childhood infections) may be protective in genetically susceptible populations ("hygiene hypothesis"), explaining higher IBD rates in developed versus developing nations.
- Dietary factors: High fat, refined carbohydrate, and low fiber diets increase risk, while diets rich in fruits, vegetables, and omega-3 fatty acids appear protective. Western dietary patterns promote dysbiosis and barrier dysfunction. Specific food antigens and food additives (emulsifiers, artificial sweeteners) may breach tolerance in susceptible individuals. Food allergy or intolerance coexists in some patients and may perpetuate symptoms.
- Oral contraceptives and hormone replacement therapy: Modest increase in Crohn disease risk (1.5-2 fold), possibly through effects on intestinal permeability, microbiota composition, or immune responses. The risk declines after discontinuation.
- Appendectomy: Paradoxically appears protective for ulcerative colitis but not Crohn disease, suggesting different underlying pathophysiology.
- Stress and psychological factors: Psychological stress may exacerbate disease but is not proven to initiate it. The bidirectional gut-brain axis involves vagal afferent signaling and stress hormone effects on barrier function and immune regulation.
Crohn disease presents with enormous clinical heterogeneity based on disease location, extent, behavior (inflammatory vs. fibrostenotic vs. penetrating), and activity level. The classic presentation often develops insidiously over weeks to months:
- Chronic diarrhea with blood: The hallmark symptom, present in 80-90% of patients. The pathophysiology involves mucosal inflammation reducing absorption and promoting active secretion of electrolytes (through inflammatory cytokine effects on crypt secretory cells), increased intestinal motility from inflamed segments, and bacterial overgrowth in stagnant areas proximal to strictures. Gross or occult blood results from ulceration of inflamed mucosa. Disease location affects diarrhea character: ileal disease causes less frequent but voluminous stools, while colonic disease causes frequent small-volume stools with urgency and tenesmus.
- Abdominal pain and cramping: Present in 75-85% of patients, typically colicky and postprandial. Early disease pain is related to inflammation and edema of the bowel wall; chronic pain may reflect fibrosis, strictures, adhesions, and bacterial overgrowth proximal to obstructive lesions. Right lower quadrant pain is common with ileitis. Crohn disease differs from ulcerative colitis in that pain severity often correlates poorly with inflammation degree due to the muscular and fibrotic component.
- Weight loss and failure to thrive: Occurs in 60-70% of patients due to multiple mechanisms: reduced oral intake from pain and diarrheal urgency, increased caloric losses in diarrhea (particularly with fat malabsorption from extensive small bowel disease), increased metabolic demands from chronic inflammation, and micronutrient malabsorption (vitamin B12, folate, iron, vitamin D). Pediatric Crohn disease presenting with growth failure and delayed puberty represents a particularly high-yield board presentation.
- Nausea and postprandial symptoms: Patients often report that eating triggers abdominal pain and diarrhea, leading to self-imposed dietary restriction and further nutritional compromise. Pyloric or gastroduodenal involvement causes vomiting and early satiety.
- Rectal bleeding and hematochezia: Occurs in 50% of patients with colonic involvement; bright red blood per rectum suggests distal disease while melena may occur with proximal involvement. Bleeding severity is usually mild to moderate; massive hemorrhage is uncommon in Crohn disease (more common in ulcerative colitis) but can occur with deep ulceration eroding blood vessels.
- Fever and systemic symptoms: Low-grade fever occurs in 30% of patients during active disease, reflecting TNF-α and IL-6 production. Higher fevers suggest complications (perforation, fistula with abscess, toxic megacolon).
- Physical examination findings in active disease
- Abdominal tenderness and guarding: Focal right lower quadrant tenderness with ileal disease; may mimic appendicitis. Distension, visible peristalsis, or rebound/guarding suggests severe disease or perforation.
- Perianal findings: Anal fissures, skin tags, perirectal abscesses, or fistulous drainage occur in 30% of patients and may be the presenting symptom, particularly in colonic/rectal disease. These findings are highly specific for Crohn disease and virtually absent in ulcerative colitis.
- Oral ulcers: Aphthous ulcers occur in 5-10% as an extraintestinal manifestation.
- Hepatosplenomegaly: Suggests severe disease or extraintestinal manifestations.
- Clubbing, joint swelling, or erythema nodosum: Indicate extraintestinal manifestations.
- Disease phenotypes based on clinical behavior (Montreal classification)
- Inflammatory (non-stricturing, non-penetrating): Most common phenotype (55%), presents with diarrhea and abdominal pain from mucosal inflammation. Responds well to anti-inflammatory therapy.
- Fibrostenotic (stricturing): Develops in 20% at diagnosis, 40% within 20 years. Presents with abdominal pain (often colicky and postprandial) with less diarrhea due to obstruction; signs of partial small bowel obstruction (abdominal distension, high-pitched bowel sounds, visible peristalsis). Strictures form from fibrosis of chronically inflamed segments. May require surgical intervention.
- Penetrating (fistulizing): Occurs in 15-25%, characterized by full-thickness inflammation eroding through serosal layers, creating communications between bowel loops (internal fistulas), bowel and other organs (bladder, uterus, skin), or bowel and peritoneal cavity (perforation with abscess). Presents with chronic drainage, recurrent abscess formation, and complex systemic symptoms. These patients require aggressive immunosuppression to prevent surgery.
- Important clinical variants
- Crohn colitis: Disease isolated to colon; presents indistinguishably from ulcerative colitis clinically but distinguished by transmural involvement and potential for extraintestinal complications. Approximately 20% of Crohn disease.
- Ileocolonic disease: Most common location (40% of cases); combines features of small bowel obstruction with colitis.
- Isolated small bowel disease: 30% of cases; more insidious onset with weight loss and malabsorption predominating; stricture formation common.
- Gastroduodenal involvement: Rare (5%); presents with vomiting, early satiety, and gastric outlet obstruction symptoms.
- Pediatric Crohn disease: Often presents with growth failure and delayed puberty as primary complaint; disease may be extensive at diagnosis.
- Fulminant presentation: Acute onset with severe diarrhea, fever, abdominal pain, and systemic toxicity; risk for toxic megacolon and perforation; requires hospitalization and aggressive therapy.
Diagnosis of Crohn disease requires synthesis of clinical history, laboratory findings, endoscopic features, and imaging rather than relying on any single test:
- Diagnostic approach and clinical assessment
The diagnostic process begins with detailed history: onset and progression of diarrhea (frequency, consistency, presence of blood, timing relative to meals), abdominal pain pattern and severity, weight loss timeline, systemic symptoms, medication use (NSAIDs, antibiotics), family history of IBD, and extraintestinal symptoms (joint pain, eye symptoms, skin lesions). Perianal symptoms or fistulous drainage are particularly important red flags. Risk factor assessment includes smoking status, recent infections, dietary pattern, and stress triggers. Red flags suggesting complicated disease include high fever (abscess/perforation), severe abdominal pain with guarding (perforation), weight loss >10% (malnutrition), or symptoms of obstruction (distension, absence of flatus, vomiting).
- Inflammatory biomarkers
- C-reactive protein (CRP): Elevated in 50-70% of active Crohn disease patients; normal in 30-50% despite active inflammation, limiting sensitivity. CRP >10 mg/L suggests moderate-severe disease. CRP is elevated in both Crohn disease and ulcerative colitis without distinction.
- Erythrocyte sedimentation rate (ESR): Less specific; elevated in 40-60% with active disease.
- Fecal calprotectin or lactoferrin: More specific than serum markers; elevated in Crohn disease with high negative predictive value (>
Stabilize and exclude complications first
- Severe flare: admit, IV fluids/electrolytes, and pharmacologic VTE prophylaxis — active IBD is a prothrombotic state and prophylactic heparin is recommended even with rectal bleeding.
- Rule out infection and abscess before immunosuppression: stool C. difficile PCR/toxin and cross-sectional imaging (CT/MR enterography). An undrained abscess must be drained (percutaneous or surgical) with antibiotics before steroids or anti-TNF, or sepsis will worsen.
Induction (ACG 2018 Crohn's disease guideline)
- Corticosteroids: prednisone for moderate–severe disease; budesonide (high first-pass metabolism, fewer systemic effects) for mild–moderate ileocecal disease. Steroids induce but never maintain remission.
- Mesalamine/5-ASA: ACG states it is not effective for Crohn disease — a favorite distractor imported from ulcerative colitis.
Maintenance and escalation (AGA guideline on moderate-to-severe Crohn disease)
- Thiopurines: azathioprine/6-mercaptopurine as steroid-sparing maintenance; check TPMT/NUDT15 activity first.
- Methotrexate: alternative maintenance; absolutely contraindicated in pregnancy.
- Anti-TNF biologics: infliximab, adalimumab — first-line for moderate–severe and for perianal fistulizing disease; combination with a thiopurine gives higher remission rates than monotherapy (SONIC).
- Other biologics: anti-integrin vedolizumab (gut-selective), anti-IL-12/23 ustekinumab, anti-IL-23 agents; JAK inhibitor upadacitinib for refractory disease.
- Pre-biologic screening: latent TB (IGRA/PPD), hepatitis B serologies, and age-appropriate vaccination — live vaccines are contraindicated once immunosuppressed (ACIP).
Surgery — not curative
- Reserved for obstructing fibrotic strictures (stricturoplasty or limited resection), perforation, abscess, refractory bleeding, or dysplasia. Bowel-sparing technique is essential; endoscopic recurrence at the anastomosis is the rule, so post-operative prophylaxis (often anti-TNF) is started early in high-risk patients.
Avoid
- NSAIDs (flares), opioids/antimotility agents in severe colitis (toxic megacolon), anti-TNF in untreated latent TB or NYHA III–IV heart failure. Smoking cessation is the single most effective non-drug intervention.
Disease complications
- Fibrostenotic stricture with small bowel obstruction: TGF-β–driven collagen deposition narrows the lumen; colicky pain, distension, vomiting, air-fluid levels, string sign on enterography. Complete obstruction is a surgical emergency.
- Fistula and abscess: transmural ulceration erodes into adjacent structures. Enterovesical fistula → pneumaturia/fecaluria and recurrent polymicrobial UTI; enterocutaneous → feculent skin drainage; perianal → drainage, seton required. Intra-abdominal abscess is an emergency — fever plus a tender mass; drain before immunosuppressing.
- Free perforation and toxic megacolon: peritonitis, rigid abdomen, or colonic dilation with systemic toxicity — both emergencies requiring surgery. Toxic megacolon is more classic for UC but occurs in Crohn colitis.
- Terminal ileal disease/resection: loss of the sole site of B12–intrinsic factor absorption → megaloblastic anemia and subacute combined degeneration. Bile-salt malabsorption → cholesterol gallstones; unabsorbed fat binds calcium so free oxalate is absorbed → calcium oxalate nephrolithiasis. Short bowel syndrome after repeated resections.
- Malignancy: colonic Crohn carries colorectal cancer risk comparable to UC — surveillance colonoscopy after roughly 8 years of colitis; small bowel adenocarcinoma is rare but disproportionately increased.
- Systemic: iron/anemia of chronic disease, osteoporosis, growth failure in children, VTE, and secondary AA amyloidosis (proteinuria).
Treatment complications
- Corticosteroids: hyperglycemia, osteoporosis, adrenal suppression, masked perforation.
- Thiopurines: dose-independent pancreatitis, myelosuppression (low TPMT/NUDT15), hepatotoxicity, non-melanoma skin cancer, lymphoma.
- Anti-TNF: reactivation of latent TB (often disseminated/extrapulmonary) and hepatitis B, histoplasmosis, drug-induced lupus, demyelination; combination with a thiopurine in young males raises hepatosplenic T-cell lymphoma risk.
- Natalizumab: PML from JC virus reactivation — check JCV antibody.
- JAK inhibitors: herpes zoster, and boxed warnings for VTE and major cardiovascular events.
- Buzzwords that mean Crohn: skip lesions, cobblestoning, creeping fat, string sign of the terminal ileum, transmural inflammation, and non-caseating granulomas (present in only a minority of biopsies — their absence never excludes the diagnosis).
- Perianal fistula, fissure, or abscess is the single association examiners test most: it is essentially diagnostic of Crohn and virtually absent in ulcerative colitis, which stays mucosal and continuous from the rectum.
- Smoking worsens Crohn and "protects" against UC — the classic paradox. Cessation reduces relapse and reoperation.
- Best next step with fever + tender abdominal mass: cross-sectional imaging and drainage of the abscess. Do not start or continue anti-TNF therapy until it is drained.
- Before any anti-TNF: latent TB screening (IGRA/PPD, chest x-ray) and hepatitis B serologies. A stem describing new cough and weight loss months after starting infliximab is reactivation TB.
- Terminal ileum = B12: ileal disease or resection gives B12 deficiency, bile-salt diarrhea (responds to cholestyramine when <100 cm resected), cholesterol gallstones, and calcium oxalate kidney stones — the distractor is uric acid stones.
- Mesalamine is the trap: 5-ASA is UC therapy; ACG does not support it for Crohn. Likewise, corticosteroids induce but never maintain remission — a stem with a patient on prednisone for months needs a steroid-sparing agent (thiopurine or biologic), not a higher steroid dose.
- Surgery is not curative: recurrence at the anastomosis is expected, so resection is limited to obstruction, perforation, abscess, refractory bleeding, or dysplasia — unlike colectomy in UC.
- **Check a stool C. difficile assay** in any IBD patient with a flare before escalating immunosuppression.