Crohn’s disease is a chronic, episodic, inflammatory bowel disease (IBD) that affects the entire wall of the bowel or intestines. Crohn’s disease can affect any part of the gastrointestinal tract from mouth to anus; as a result, the symptoms of Crohn’s disease vary among afflicted individuals. The disease is characterized by areas of inflammation with areas of normal lining between in a symptom known as skip lesions. The cardinal symptoms are chronic diarrhea, abdominal pain, and fatigue; weight loss, fever, anemia, and perianal or extraintestinal findings may also be presenting features. Chronic diarrhea is the most common symptom, though some patients do not experience it. Constipation is not typical and, when present, usually reflects an obstructing stricture.^[1][2] Treatment for Crohn’s disease depends on the location and severity of disease, complications, and the person’s response to previous medical treatments when treated for recurring symptoms. ^[3]

Descriptions consistent with Crohn’s disease appear in the medical literature from the 18th century onward, including reports by Giovanni Battista Morgagni (1769), Samuel Wilks (1859), Antoni Leśniowski (1903), and Thomas Kennedy Dalziel (1913).^[4][5] The disease was established as a distinct clinicopathologic entity in 1932, when Burrill Bernard Crohn, Leon Ginzburg, and Gordon Oppenheimer described 14 patients with inflammation of the terminal ileum under the title “Regional ileitis: a pathologic and clinical entity.”^[4][6] Over the following decades the concept expanded from an ileal disease to a pan-enteric, transmural inflammatory bowel disease (IBD). It was then stratified by location and behavior, which was later formalized in the Montreal classification. Its etiology has been reframed as a dysregulated mucosal immune response to the gut microbiota in a genetically susceptible host.^[7][8]

Crohn’s disease is classified using the Montreal classification (2005), which replaced the 1998 Vienna classification and stratifies patients by three axes: age at diagnosis (A), disease location (L), and disease behavior (B), with modifiers for upper gastrointestinal involvement (L4) and perianal disease (p).^[9][10] Disease location is relatively stable over time, whereas disease behavior commonly progresses from inflammatory to stricturing or penetrating phenotypes.^[11] The Paris classification (2011) adapts the Montreal system for pediatric patients.^[12]

Crohn’s disease (CD) arises when environmental triggers and gut microbial dysbiosis provoke a poorly regulated mucosal immune response across an impaired epithelial barrier in a genetically susceptible host.^[13][14] No single gene, microbe, or environmental factor is sufficient on its own. The result is a self-perpetuating cycle of barrier breakdown, microbial translocation, and Th1/Th17-skewed transmural inflammation, which current therapies interrupt at specific points.^[15][14] More than 200 susceptibility loci have been identified. They converge on four processes: innate microbial sensing (NOD2), autophagy (ATG16L1), epithelial barrier integrity, and IL-23/Th17 signaling.^[16][17] The hallmark pathologic features are:

• Transmural inflammation
• Skip lesions
• Non-caseating granulomas
• A propensity for stricture and fistula formation

Crohn’s disease (CD) has no single identifiable cause. The best-supported model is a dysregulated mucosal immune response to the gut microbiota in a genetically susceptible host, triggered and modified by environmental exposures, resulting in impaired epithelial barrier function and chronic transmural inflammation.^[13][2] Genetic susceptibility, environmental exposure, and the microbiome each appear necessary but not individually sufficient.^[15] More than 200 susceptibility loci have been identified, with NOD2 the strongest single-gene risk factor in populations of European ancestry. Defective innate handling of intestinal bacteria (NOD2 and autophagy pathways) and a Th1/Th17–IL-23–driven effector response are central mechanisms.^[16] Established environmental contributors include cigarette smoking and early-life antibiotic exposure. No single infectious agent has been proven causal; the role of Mycobacterium avium subspecies paratuberculosis (MAP) remains unproven and controversial.

The most common disease that mimics the symptoms of Crohn’s disease is ulcerative colitis, as both are inflammatory bowel diseases that can affect the colon with similar symptoms. It is important to differentiate these diseases, since the course of the diseases and treatments may be different. In some cases, however, it may not be possible to tell the difference, in which case the disease is classified as indeterminate colitis.

CD is no longer confined to high-income Western populations. In early-industrialized regions (North America, Europe, Oceania), adult incidence has plateaued or declined, while prevalence keeps rising (“compounding prevalence”). The steepest incidence increases are now in newly industrialized regions of Asia, Latin America, and Africa.^[18][19] Peak age at diagnosis is roughly 15–35 years, with a smaller later peak. Pediatric and very-early-onset disease continue to rise even where adult incidence has stabilized.^[13] There is no consistent sex predominance in adults. Prevalence in the United States is highest in White Americans, but CD occurs in all racial and ethnic groups.^[20]

Crohn’s disease (CD) develops in genetically susceptible individuals through the interaction of inherited risk, environmental exposures, and gut microbial dysbiosis. No single factor is necessary or sufficient to cause disease.^[13][15][21] The strongest risk factor is a first-degree relative with CD. The most important modifiable risk factor is cigarette smoking.^[13][2] Other established risk factors include childhood antibiotic exposure, appendectomy, urban residence, and an industrialized (“Western”) lifestyle and diet. Breastfeeding and physical activity are associated with lower risk.^[22] Most environmental associations come from observational data, and causality has not been established for most of them.^[22]

No professional society recommends screening asymptomatic members of the general population for Crohn’s disease. In practice, “screening” covers three separate activities. The first is case finding in symptomatic patients, where fecal calprotectin (FC) is the guideline-endorsed first-line, non-invasive test for deciding who needs endoscopy.^[23] The second is risk stratification of asymptomatic first-degree relatives (FDRs), which is still investigational and confined to research and prevention-trial settings.^[24] The third is colorectal cancer (CRC) surveillance in established Crohn’s colitis, a formal, guideline-mandated program whose start time and intervals differ between societies.^[25][26]

Crohn’s disease is a chronic, relapsing–remitting, transmural inflammatory bowel disease that tends to progress over time from purely inflammatory disease to stricturing or penetrating complications.^[16][23] Disease location is usually stable, but up to half of patients develop a stricture, internal fistula, or perianal complication within 20 years of diagnosis, and contemporary population-based cohorts show that roughly 1 in 5 patients with uncomplicated disease progress and about 1 in 4 require resection within 10 years.^[23][27] Major complications include bowel obstruction from strictures, fistulae, intra-abdominal and perianal abscesses, extraintestinal manifestations, nutritional deficiencies, treatment-related toxicity, and an increased risk of small bowel and colorectal adenocarcinoma.^[16][28] There is no cure. Long-term mortality is modestly higher than in the background population, and patients with Crohn’s colitis need risk-stratified colonoscopic surveillance for colorectal cancer.^[29][26]

Many people with Crohn’s disease have symptoms for years prior to the diagnosis. The usual onset is between 15 and 30 years of age but can occur at any age. Abdominal pain may be the initial symptom of Crohn’s disease. Crohn’s disease, like many other chronic, inflammatory diseases, can cause a variety of systemic symptoms. In addition to systemic and gastrointestinal involvement, Crohn’s disease can affect many other organ systems

Physical examination findings in crohn’s disease include fever, fatigue, weakness, bloody diarrhea, abdominal pain, hypotension and tachycardia.

The laboratory findings in a patient with Crohn’s disease include anemia, low albumin, elevated ESR, elevated serum alkaline phosphatase, deranged LFTs and electrolyte abnormalities.

Xray of the abdomen is not required for the diagnosis of Crohn’s disease. Xray may sometimes be one in case colitis is suspected. Xray is normal in mild to moderate disease and can show dilation and/or “thumb printing sign” in fulminant cases.

CT enterography (CTE) and MR enterography (MRE) are the standard cross-sectional modalities for evaluating the small bowel.^[1] Findings include skip lesions, bowel wall thickening, surrounding inflammation, abscess, and fistulae.

Magnetic resonance imaging (MRI) is another option for imaging the small bowel as well as looking for complications, though it is more expensive and less readily available.

The diagnosis of Crohn’s disease can sometimes be challenging, and a number of tests are often required to assist the physician in making the diagnosis. Ileocolonoscopy with biopsy is the diagnostic gold standard; over 80% of patients with IBD have mucosal involvement within reach of the colonoscope, and the terminal ileum is successfully intubated in 80–97% of cases when the cecum is reached. More proximal small-bowel disease beyond the reach of the colonoscope is evaluated with CT or MR enterography, and small-bowel capsule endoscopy is reserved for high clinical suspicion despite negative ileocolonoscopy and cross-sectional imaging.^[1][16]

Other imaging findings for Crohn’s disease can be seen by the help of barium enema. Barium enema may show ulcerations and skip lesions. Barium enema must be avoided in severe cases as it can lead to the manifestation of toxic megacolon.

Pharmacologic management of luminal Crohn’s disease in adults is stratified by disease severity (mild-to-moderate, moderate-to-severe, severe/fulminant), risk of progression, and prior exposure to advanced therapy. Treatment has two phases: induction of remission and maintenance of remission. Current guidelines have moved away from the historical “step-up” sequence (aminosalicylates → corticosteroids → immunomodulators → biologics) toward early advanced therapy in moderate-to-severe or high-risk disease, with a treat-to-target goal of steroid-free clinical and endoscopic remission rather than symptom control alone.^[1][30][31] Controlled ileal-release budesonide 9 mg once daily is first-line induction therapy for mild-to-moderate ileocecal disease and should not be continued beyond approximately 3 months or used for maintenance; oral mesalamine is not recommended for induction or maintenance.^[1][31] Corticosteroids induce but do not maintain remission. Hospitalized patients with severe disease receive intravenous corticosteroids, with early consideration of infliximab and surgical consultation. Before starting a biologic or JAK inhibitor, screen for latent tuberculosis and hepatitis B, review and update vaccinations (pneumococcus, influenza, COVID-19, herpes zoster, RSV, HPV), and obtain baseline laboratory studies. Anti-TNF agents are cautioned in advanced heart failure and demyelinating disease, and JAK inhibitors carry venous thromboembolic and cardiovascular warnings.^[30]

Surgery is a core but non-curative treatment in Crohn’s disease (CD). Up to approximately 70% of patients require at least one intestinal resection during their disease course. Recurrence at or proximal to the anastomosis is expected rather than exceptional.^[32] Endoscopic recurrence develops in 70–90% of patients within the first postoperative year.^[33] The principal indications for surgery are:

• Medically refractory disease.
• Fibrostenotic obstruction.
• Penetrating disease.
• Free perforation.
• Hemorrhage.
• Neoplasia.^[32]

In selected patients with limited ileocecal disease, early laparoscopic ileocecal resection is now a reasonable first-line alternative to anti-TNF therapy rather than a salvage procedure.^[1][34] Contemporary management emphasizes:

• A minimally invasive, bowel-sparing operation.
• Preoperative optimization to avoid emergency surgery.
• Risk-stratified, endoscopy-guided postoperative prophylaxis.^[32][35]

Smoking cessation is the best-established modifiable intervention affecting disease course and recurrence; former smokers have a flare risk similar to never-smokers, whereas continued smoking is associated with more flares, stricturing and perianal complications, and postoperative recurrence.^[1][36]

Descriptions consistent with Crohn’s disease appear in the medical literature from the 18th century onward, including reports by Giovanni Battista Morgagni (1769), Samuel Wilks (1859), Antoni Leśniowski (1903), and Thomas Kennedy Dalziel (1913).^[1][2] The disease was established as a distinct clinicopathologic entity in 1932, when Burrill Bernard Crohn, Leon Ginzburg, and Gordon Oppenheimer described 14 patients with inflammation of the terminal ileum under the title “Regional ileitis: a pathologic and clinical entity.”^[1][3] Over the following decades the concept expanded from an ileal disease to a pan-enteric, transmural inflammatory bowel disease (IBD). It was then stratified by location and behavior, which was later formalized in the Montreal classification. Its etiology has been reframed as a dysregulated mucosal immune response to the gut microbiota in a genetically susceptible host.^[4][5]

• Morgagni is the most frequently cited early observer. His autopsy observations from the 1760s describe a patient with diarrhea, fever, and ileal ulceration with perforation and enlarged mesenteric nodes, features now recognized as compatible with Crohn’s disease.^[1][6]
• In 1859, Samuel Wilks reported a case of non-infectious intestinal inflammation that is now thought to have represented colonic Crohn’s disease rather than ulcerative colitis.^[2][4]
• In 1903, the Polish surgeon Antoni Leśniowski independently described the condition. In Poland it is still termed Leśniowski–Crohn disease.^[1]
• In 1913, the Scottish surgeon Thomas Kennedy Dalziel reported a surgical series of patients with chronic, non-infectious, transmural intestinal inflammation causing obstruction, some with colonic involvement.^[2][7]
• These reports show that the disease was recognized repeatedly before 1932. None of them, however, consolidated it as a reproducible clinicopathologic entity.^[1]

• In May 1932, Crohn, a gastroenterologist at Mount Sinai Hospital, presented 14 cases to the American Medical Association as “terminal ileitis.” The term was changed to “regional ileitis” so that the disease would not be read as fatal. The series was published in JAMA later that year as “Regional ileitis: a pathologic and clinical entity.”^[1][3]
• The original description remains closely aligned with current understanding:^[8]
  • A chronic disease of young adults with fever, diarrhea, right-lower-quadrant pain, anemia, and weight loss.
  • Pathologically, segmental transmural inflammation, granulomas, linear ulcers, and a cobblestone mucosa.
  • Clinical behaviors that anticipate modern phenotyping: inflammatory, stenotic, perforating/abscess-forming, and fistulizing disease.
• The surgeon Albert A. Berg operated on the patients but was not listed as an author. Because authors were listed alphabetically, Crohn appeared first, and the eponym “Crohn’s disease” entered general use soon afterward.^[1][6]
• The appropriateness of the eponym has been debated given the earlier descriptions. Recent historical review concludes that the 1932 paper and Crohn’s later contributions were pivotal, so continued use of the eponym is justified.^[1]

• Between 1930 and 1960, the literature expanded and refined the definition of the disease.^[6] Involvement beyond the ileum, including the upper gastrointestinal tract, made the term “regional ileitis” untenable. Crohn’s disease is now recognized as able to affect any segment of the digestive tract from mouth to anus, often discontinuously.^[8][5]
• In 1960, Lockhart-Mummery and Morson described isolated colonic Crohn’s disease and distinguished it from ulcerative colitis. This formally extended the entity beyond the small bowel.^[4]

• Phenotypic classification evolved to stratify disease by location (isolated ileal, colonic, ileocolonic, and isolated upper gastrointestinal) and by behavior. This approach was formalized in the Montreal classification, which is used for prognosis and treatment selection.^[4] Whether Crohn’s disease should be reclassified as ileum-dominant versus isolated colonic disease remains an open question.^[4]
• Etiologic theories progressed from food allergy, psychogenic causes, and single-pathogen hypotheses toward a multifactorial model: a dysregulated mucosal immune response to the intestinal microbiota in a genetically susceptible host.^[5]
• NOD2/CARD15 was the first susceptibility gene identified for Crohn’s disease. It is associated with Crohn’s disease rather than ulcerative colitis, and more than 200 additional risk loci have since been described.^[9]

Crohn’s disease is classified using the Montreal classification (2005), which replaced the 1998 Vienna classification and stratifies patients by three axes: age at diagnosis (A), disease location (L), and disease behavior (B), with modifiers for upper gastrointestinal involvement (L4) and perianal disease (p).^[1][2] Disease location is relatively stable over time, whereas disease behavior commonly progresses from inflammatory to stricturing or penetrating phenotypes.^[3] The Paris classification (2011) adapts the Montreal system for pediatric patients.^[4]

Crohn’s disease is a transmural inflammatory disease that can involve any segment of the gastrointestinal tract from mouth to anus; rectal sparing is typical but not obligatory.^[5] Phenotypic classification is recommended at diagnosis because location, behavior, and age at onset carry prognostic weight.^[6]

The Montreal classification is the most widely used phenotypic classification of Crohn’s disease in clinical practice and research.^[1][2]

• Examples: a patient diagnosed at age 23 with ileal stricturing disease and no perianal involvement is A2L1B2; a patient diagnosed at age 21 with ileocolonic disease, an intra-abdominal abscess, and perianal fistulas is A2L3B3p.

• Location is a relatively stable trait; in a 10-year cohort, location changed in approximately 16% of patients.^[3]
• Approximate distribution at diagnosis:^[5]
  • Ileocolonic (L3): ~35–45%
  • Ileal (L1): ~30%
  • Colonic (L2): ~20%; may be difficult to distinguish from ulcerative colitis
  • Isolated upper GI (L4): <5%
• Location correlates with behavior: ileal disease is more often stricturing, whereas colonic and ileocolonic disease are more often penetrating.^[3]

• B1 (inflammatory): inflammation without stricture or penetrating complications.
• B2 (stricturing): constant luminal narrowing with prestenotic dilation and/or obstructive signs or symptoms, without penetrating disease.
• B3 (penetrating): intra-abdominal fistula, inflammatory mass, and/or abscess not attributable to a postoperative complication; isolated perianal or rectovaginal fistulas do not qualify as B3.^[2]
• A key change from the Vienna classification was removal of perianal fistulizing disease from the penetrating category and its recording as a separate modifier (p), because perianal disease can coexist with any behavior category.^[2][7]

• Unlike location, behavior is dynamic and progresses through repeated cycles of inflammation and tissue repair.^[8]
• In a 10-year cohort, approximately 46% of patients changed behavior category, predominantly from B1 to B2 or B3.^[3]
• In a large referral-center cohort, the cumulative probability of stricturing or penetrating disease reached approximately 88% at 20 years.^[9]
• In the prospective BioCrohn inception cohort (advanced-therapy era), the 5-year cumulative probability of complications was 41.5% (stricturing 15.6%, penetrating 25.9%).^[10]
• In the Swiss IBD Cohort Study, immunosuppressive therapy and anti-TNF therapy for more than 1 year were associated with a lower risk of migration from B1 to complicated behavior.^[11]

The Paris classification modifies the Montreal system for pediatric-onset Crohn’s disease while preserving compatibility with adult classification:^[4]

• Complete phenotyping requires ileocolonoscopy with ileal intubation and biopsies plus cross-sectional small-bowel imaging (e.g., MR enterography or CT enterography) to define location and detect stricturing or penetrating complications.^[6]
• Behavior should be reassessed over time; new obstructive symptoms, abscess, or fistula indicate migration to B2 or B3.^[8]

Crohn’s disease (CD) arises when environmental triggers and gut microbial dysbiosis provoke a poorly regulated mucosal immune response across an impaired epithelial barrier in a genetically susceptible host.^[1][2] No single gene, microbe, or environmental factor is sufficient on its own. The result is a self-perpetuating cycle of barrier breakdown, microbial translocation, and Th1/Th17-skewed transmural inflammation, which current therapies interrupt at specific points.^[3][2] More than 200 susceptibility loci have been identified. They converge on four processes: innate microbial sensing (NOD2), autophagy (ATG16L1), epithelial barrier integrity, and IL-23/Th17 signaling.^[4][5] The hallmark pathologic features are:

• Transmural inflammation
• Skip lesions
• Non-caseating granulomas
• A propensity for stricture and fistula formation

CD reflects the interplay of four interdependent elements rather than a linear sequence:^[1][3][2]

• Genetic susceptibility lowers the threshold for immune activation.
• Environmental triggers act largely through the microbiome and the epithelial barrier. These include smoking, diet, antibiotic exposure, and enteric infection.
• Microbial dysbiosis expands pathobionts and depletes protective commensals.
• Dysregulated innate and adaptive immunity produces transmural, granulomatous inflammation.

The genetics point to a “network dyshomeostasis” across interconnected pathways rather than a single defective pathway. These pathways cover microbial sensing, barrier integrity, autophagy, cytokine signaling, and cell-stress responses.^[5]

• Genome-wide association studies have identified more than 200 inflammatory bowel disease (IBD) risk loci, which confirms a polygenic architecture.^[1][4]
• Most loci confer only modest risk (odds ratios ~1.1–1.2) and lie in regulatory regions. Known variants explain only roughly 19–26% of CD heritability, which underscores the contribution of environment and gene–environment interaction.^[6]
• Monozygotic twin concordance is approximately 50% in CD, compared with approximately 15% in ulcerative colitis (UC).^[4]
• Risk alleles differ by ancestry:^[4]
  • Coding variants in NOD2, ATG16L1, and IL23R predominate in populations of European ancestry.
  • These variants are essentially monomorphic in East Asian cohorts, where TNFSF15 is the leading CD locus.

• CD remains a clinical diagnosis. Routine genotyping is not part of standard care.

• An impaired epithelial barrier is now viewed as a central pathogenic factor rather than a bystander.^[1]
• Several defects allow luminal bacteria and metabolites to reach the lamina propria and activate immune cells:^[15][16]
  • Reduced mucus
  • Dysregulated tight junctions
  • Fewer or dysfunctional Paneth cells with impaired antimicrobial peptide secretion
  • Reduced intestinal stem cell function
• Barrier dysfunction and subclinical gut inflammation have been detected before clinically overt CD in at-risk individuals. This supports an upstream, potentially causal role.^[17]
• Environmental exposures, including smoking, are best understood as acting through the barrier and the microbiome. Psychological stress may modulate disease course but is not a core etiologic pillar.^[17][18]

• The microbiota is essential to CD pathogenesis, but no single causative pathogen has been identified.^[3][6]
• CD is characterized by reduced microbial diversity, with two main shifts:^[1][15]
  • Loss of Firmicutes, notably the butyrate producers Faecalibacterium prausnitzii and Roseburia spp.
  • Expansion of Proteobacteria, including adherent-invasive Escherichia coli (AIEC).
• In the pathobiont concept, otherwise commensal organisms become pathogenic under disease-promoting host conditions. Dysbiosis is both a cause and a consequence of inflammation.^[3][2]
• Inflammation itself drives “late dysbiosis.” It increases luminal oxygen, nitrate, and iron, which favors facultative anaerobes.^[7]
• Loss of protective microbial metabolites further weakens barrier and immune homeostasis. These metabolites include short-chain fatty acids, secondary bile acids, and tryptophan/indole derivatives.^[19]

• CD shows deficient innate responses, including reduced macrophage bactericidal activity and impaired neutrophil recruitment. These deficits permit microbial persistence and translocation.^[20]
• NLRP3 inflammasome activation processes IL-1β and IL-18.^[20]
• Lamina propria macrophages that engulf penetrating bacteria secrete TNF, IL-6, IL-12, and IL-23.^[16]
• Dendritic cells present luminal antigens and polarize naive T cells toward inflammatory effector phenotypes.^[4]

• CD is driven by excessive Th1 and Th17 responses to IL-12, IL-18, and IL-23 from antigen-presenting cells, together with impaired regulatory T cell function.^[13][4]
• Th1/Th17 cells secrete IFN-γ, IL-17, and TNF, which perpetuate transmural inflammation.^[4][20]
• The characteristic granuloma reflects this cell-mediated, rather than antibody-mediated, response.^[4]
• The older strict “Th1 = CD, Th2 = UC” paradigm has been revised. Th17 cells and the IL-12/IL-23 axis are now recognized as central drivers. The two cytokines share the p40 subunit.^[13]
• Gut-selective lymphocyte trafficking depends on α4β7 integrin–MAdCAM-1 interactions, a validated therapeutic target.^[4][18]

Sources for table: ^[21][16]

• Chronic transmural inflammation activates fibroblasts and other mesenchymal cells, including through oncostatin M signaling. This produces the strictures and penetrating/fistulizing complications that distinguish CD from UC.^[14][5]
• Fibrosis may progress despite control of inflammation. No therapy is approved to reverse established fibrostenosis.^[14]

• Transmural, patchy inflammation with skip lesions. The terminal ileum and proximal colon are involved most often.^[2]
• Creeping fat (mesenteric fat wrapping) and a thickened, rubbery bowel wall.^[4]
• Strictures, which appear as the string sign on contrast imaging.
• Aphthous ulcers that progress to linear ulcers, cobblestoning, fissures, and fistulas.^[4]

• Transmural inflammation with lymphoid aggregates throughout the bowel wall.^[4]
• Focal crypt injury:
  • Neutrophilic infiltration of crypts, causing cryptitis or crypt abscess
  • Crypt architectural distortion and branching
• Non-caseating granulomas are highly specific for CD but are present in only a subset of cases. Their absence does not exclude the diagnosis.^[4]
• Paneth cell metaplasia and villous blunting.^[22]

Crohn’s disease (CD) has no single identifiable cause. The best-supported model is a dysregulated mucosal immune response to the gut microbiota in a genetically susceptible host, triggered and modified by environmental exposures, resulting in impaired epithelial barrier function and chronic transmural inflammation.^[1][2] Genetic susceptibility, environmental exposure, and the microbiome each appear necessary but not individually sufficient.^[3] More than 200 susceptibility loci have been identified, with NOD2 the strongest single-gene risk factor in populations of European ancestry. Defective innate handling of intestinal bacteria (NOD2 and autophagy pathways) and a Th1/Th17–IL-23–driven effector response are central mechanisms.^[4] Established environmental contributors include cigarette smoking and early-life antibiotic exposure. No single infectious agent has been proven causal; the role of Mycobacterium avium subspecies paratuberculosis (MAP) remains unproven and controversial.

• Family history is the strongest single risk factor; an affected first-degree relative confers an incidence rate ratio of approximately 7.8 (95% CI 7.05–8.56).^[1]
• Twin studies: concordance in monozygotic twins is approximately 50% for CD (vs approximately 15% for ulcerative colitis), indicating strong heritability alongside an essential environmental contribution.^[4]
• Polygenic architecture: Genome-wide association studies have identified more than 200 IBD risk loci. Most confer modest individual risk (odds ratios approximately 1.1–1.2) and lie in regulatory regions. Known variants explain only a minority of estimated heritability.^[1][4]
• Population differences: Coding risk variants in NOD2, ATG16L1, and IL23R predominate in populations of European ancestry and are essentially monomorphic in East Asian cohorts. In those cohorts TNFSF15 is the dominant CD locus, with an effect size exceeding that of NOD2 in European-ancestry populations.^[4]

• CD is characterized by a Th1/Th17-skewed mucosal response with impaired regulatory T cell activity. Inflamed mucosa shows elevated TNF-α, IFN-γ, IL-1, IL-6, IL-12, IL-23, and IL-17.^[1]
• IL-12 and IL-23, which share the p40 subunit, are produced by macrophages and dendritic cells and act as upstream drivers. IL-23 is especially important in sustaining pathogenic Th17 responses.^[4]
• Whether immune abnormalities are primary or secondary to barrier breakdown remains unresolved. Many susceptibility genes point to impaired innate handling of commensal bacteria rather than classic autoimmunity.^[2][10]

• CD is associated with dysbiosis:^[1]
  • reduced microbial diversity, with fewer Firmicutes and Bacteroides and more Proteobacteria and Actinobacteria
  • depletion of butyrate producers such as Faecalibacterium prausnitzii and Roseburia spp.
  • enrichment of adherent-invasive Escherichia coli (AIEC)
• No single causative organism has been identified. The implicated microbes are mostly pathobionts: commensals that become pathogenic under disease-promoting host conditions. They may perpetuate inflammation rather than initiate it.^[3][11]
• Gene–microbe interaction is central. Loss-of-function NOD2 and autophagy variants allow pathobionts to accumulate and penetrate the mucosa, driving T-cell–mediated inflammation. Examples include AIEC adhesion via aberrant epithelial CEACAM6 expression, and Mucispirillum schaedleri in Nod2/Cybb-deficient models.^[10][12]

Environmental exposures (the exposome) act across the life course, with particular influence during early-life windows of microbiome and immune maturation.^[13]

Crohn’s disease should be differentiated from other causes of diarrhea. It is very important to differentiate it from ulcerative colitis as the management of both conditions is different though the initial presentation may be confused for any of these disorders.^[1][2]

The most common disease that mimics the symptoms of Crohn’s disease is ulcerative colitis, as both are inflammatory bowel diseases that can affect the colon with similar symptoms. It is important to differentiate these diseases, since the course of the diseases and treatments may be different. In some cases, however, it may not be possible to tell the difference, in which case the disease is classified as indeterminate colitis.^{[1][2][3][4][5][6]}

∞Small bowel diarrhea: watery, voluminous with less than 5 WBC/high power field

Large bowel diarrhea: Mucousy and/or bloody with less volume and more than 10 WBC/high power field
† It could be as high as 1000 based on patient’s immunity system.

Template:WikiDoc Sources

Crohn’s disease must be differentiated from:^{[10][11][12][13][14][15][16][17][18]}

Duodenal ulcer

Oral Crohn’s lesions must be differentiated from other diseases causing oral lesions such as leukoplakia and herpes simplex virus infection.

Disease	Presentation	Risk Factors	Diagnosis	Affected Organ Systems	Important features	Picture
Diseases predominantly affecting the oral cavity
Oral Candidiasis	Dysphagia or odynophagia White patches on the mouth and tongue	Newborn babies Denture users Poorly controlled diabetes As a side effect of medication, most commonly having taken antibiotics. Inhaled corticosteroids for the treatment of lung conditions (e.g, asthma or COPD) may also result in oral candidiasis which may be reduced by regularly rinsing the mouth with water after taking the medication. People with poor nutrition, specifically vitamin A, iron and folate deficiencies. People with an immune deficiency (e.g. as a result of AIDS/HIV or chemotherapy treatment). Women undergoing hormonal changes, like pregnancy or those on birth control pills. Organ transplantation patients	Clinical diagnosis Confirmatory tests rarely needed	Localized candidiasis Oral and esophageal candidasis Candida vulvovaginitis Chronic mucocutaneous candidiasis Invasive candidasis Candidaemia Candida endocarditis Candida osteoarticular disease	Oral candidiaisis is a benign self limiting disease unless accompanied by immunosuppression.
Herpes simplex oral lesions	Fever Sore throat Painful ulcers	Stress Recent URTI Female sex	Physical examination Viral culture Tzanck smear	Orofacial Infection Anogenital Infection Ocular Infection Herpes Encephalitis Neonatal Herpes Herpetic Whitlow Herpes Gladiatorum	The symptoms of primary HSV infection generally resolve within two weeks
Aphthous ulcers	Painful, red spot or bump that develops into an open ulcer	Being a female Between the ages of 10-40 Family history of aphthous ulcers	Physical examination Diagnosis of exclusion	Oral cavity	Self-limiting , Pain decreases in 7 to 10 days, with complete healing in 1 to 3 weeks
Squamous cell carcinoma	Non healing ulcer, nodule, indurated plaque or mass May involve skin, lips, inside the mouth, throat or esophagus	Chronic sun or UV exposure Fair skin Elderly age (>45 yrs) Male sex Smoking	Physical exam Biopsy	Oral Cavity Floor of mouth Lateral tongue Throat Esophagus	Malignant Can spread to TMJ Some times associated with leukoplakia
Leukoplakia	White leathery spots on the mucous membranes of the tongue and inside of the mouth Lateral borders of tongue	Atypical Tobacco use Chronic irritation Immunodeficiency Bloodroot (sanguinaria)	Physical exam Diagnosis of exclusion Biopsy	Vulvar lesions occur independent of oral lesions	Associated with HIV Persistant white spots Benign but can progress to carcinoma after almost 10 years Oral proliferative verrucous leukoplakia is an aggressive sub type with multiple lesions and higher conversion to warts or carcinoma[19]
Melanoma	A lesion with ABCD Asymmetry Border irregularity Color variation Diameter changes Bleeding from the lesion	UV radiations Genetic predisposition Old age Male gender Family or personal history of melanoma Multiple benign or atypical nevi	ABCD characteristics Bleeding or ulceration may show malignancy Serum LDH may be elevated in case of malignancy Biopsy	Can metastasize All UV radiation or sun exposed areas can be effected independently 1-2 to hundreds of granules	Neural crest cell derivative Development begins with disruption of nevus growth control Progression involves MAPK/ERK pathway N-RAS or BRAF oncogene also involved
Fordyce spots	Rice-like granules or spots Small, painless, raised, pale, red or white 1 to 3 mm in diameter	Greasy skin types Some rheumatic disorders Hereditary nonpolyposis colorectal cancer Lower gingiva (gums) Vestibular mucosa	Physical exam Small keratin-filled pseudocysts May be seen on incidental mucosal biopsy Biopsy not done for them primarily	Oral cavity Vermilion border of the lips Oral mucosa of the upper lip Buccal mucosa in the commissural region often bilaterally Genitals	Benign neoplasms with sebaceous features Visible sebaceous glands No surrounding mucosal change Several adjacent glands may coalesce into a larger cauliflower-like cluster
Burning mouth syndrome	Burning or tingling on the lips, tongue, or entire mouth	Nutritional deficiencies Chronic anxiety or depression Diabetes type 2 Menopause Oral thrush or dry mouth, or damaged nerves transmitting taste Female gender Menopause	Presentation Physical exam	Oral cavity	Pain typically is low in the morning and builds up over the day Low dosages of benzodiazepines, tricyclic antidepressants or anticonvulsants may be effective
Torus palatinus	Bony growth on midline of the hard palate Nodular mass covered with normal mucosa	Genetic predisposition Autosomal dominant	Physical exam Types Flat tori Spindle tori Nodular tori Lobular tori	Hard palate	More common in Asian and Inuit populations Twice more common in females Repeated trauma can cause bleeding Surgery may be required in symptomatic
Diseases involving oral cavity and other organ systems
Behcet’s disease	Painful mouth sores Acne like skin lesions Headache, fever, poor balance, disorientation Abdominal pain, diarrhea or bleeding Uveitis Joint swelling and joint pain Genital sores wit pain and scaring Aneurysms	Over active immune system	Physical examination	Mouth Genitals GIT Eye Joints Skin Vascular system Brain	Outbreaks of exaggerated inflammation Affects smaller blood vessels
Crohn’s disease	Chronic, episodic diarrhea or constipation Abdominal pain Vomiting Weight loss or weight gain	Smoking Whites and European Jews Hormonal contraception Diets high in microparticles, sweet, fatty or refined foods Industrialized country	Typical history and symptoms Skip lesions on biopsy Anti-Saccharomyces cerevisiae antibodies (ASCA) Anti-neutrophil cytoplasmic antibodies (ANCA)	Eyes Joints Skin	May lead to Obstructions Abscesses Free perforation Hemorrhage
Agranulocytosis	Fever or chills Frequent infections Unusual redness, pain, or swelling around a wound Mouth ulcers Abdominal pain Burning sensation when urinating Sore throat	Medications[20] Cytotoxic chemotherapy Hematologic malignancies Autoimmune disorders	Neutropenia <100 cells per micro litre Eosinopenia Basopenia	Oral cavity Skin GIT Urinary system Conjunctiva	Immunocompromization Types Drug-induced Malignant Autoimmune
Syphilis[21]	Chancre Regional lymphadenopathy	Multiple sexual partners Illicit drug use Unprotected sex Men who have sex with men Residence in highly prevalent areas HIV infection Presence of other STIs Previous history of STIs Intravenous drug use	Darkfield microscopy Non treponemal tests like VDRL and RPR test) Treponemal testsFTA-ABS tests, (TP-PA) assay, enzyme immunoassays, and chemiluminescence immunoassays)	Oral cavity Penis Cervix Labia Anal canal Rectum CNS CVS	Primary syphilis Chancre Secondary syphilis Condylomata lata Latent syphilis Asymptomatic Tertiary syphilis Gummas Neurosyphilis
Coxsackie virus	Fever Sores in the mouth Rash with blisters Aches	Pregnancy immunodeficiency	History and Physical exam Throat swabs Swabs from the lesion Tzanck test	Oral cavity Skin	Symptomatic treatment
Chicken pox	Conjunctival symptoms Catarrhal symptoms Characteristic spots on the trunk appearing in two or three waves Itching	Pregnancy Premature infants born to susceptible mothers All infants born at less than 28 weeks gestation or who weigh =1000 grams Immunocompromised	History and physical exam PCR to detect VZV in skin lesions (vesicles, scabs, maculopapular lesions)	Oral cavity Skin	Sodium bicarbonate in baths or antihistamines for itching Paracetamol (acetaminophen) for fever Prednisolone is contraindicated
Measles	Fever Rash Cough Coryza (runny nose) Conjunctivitis (pink eye) Malaise Koplick spots in mouth	Unvaccinated individuals[23][24] Crowded and/or unsanitary conditions Traveling to less developed and developing countries Immunocompromized Winter and spring seasons Born after 1956 and never fully vaccinated Health care workers	History and examination PCR for Measles-specific IgM antibody PCR for Measles RNA	Oral cavity Skin Respiratory tract Eyes Throat	Caused by Morbillivirus Primary site of infection is the respiratory epithelium of the nasopharynx Transmitted in respiratory secretions, via aerosol droplets containing virus particles

Crohn’s disease must be differentiated from diseases that cause abdominal pain and chronic diarrhea. The table below summarizes the findings that differentiate watery causes of chronic diarrhea:^{[25][26][27][28][29]}

Crohn’s disease (CD) affects more than 1 million people in the United States and more than 3 million worldwide.^[1] CD is no longer confined to high-income Western populations. In early-industrialized regions (North America, Europe, Oceania), adult incidence has plateaued or declined, while prevalence keeps rising (“compounding prevalence”). The steepest incidence increases are now in newly industrialized regions of Asia, Latin America, and Africa.^[2][3] Peak age at diagnosis is roughly 15–35 years, with a smaller later peak. Pediatric and very-early-onset disease continue to rise even where adult incidence has stabilized.^[4] There is no consistent sex predominance in adults. Prevalence in the United States is highest in White Americans, but CD occurs in all racial and ethnic groups.^[5]

• U.S. administrative-claims data estimate a standardized CD prevalence of 305 per 100,000 population (about 1.01 million Americans).^[5]
• Total inflammatory bowel disease (IBD) prevalence in the same data is 721 per 100,000 (about 2.39 million Americans).^[5]
• Across North America, Oceania, and much of Europe, IBD prevalence exceeds 0.3%.^[2]

• Incidence varies widely by region. Contemporary population-based estimates are summarized below.

• The highest reported prevalence is in Europe (up to 322 per 100,000, Germany) and Canada (~319 per 100,000).^[2]
• Prevalence remains low but is increasing in newly industrialized countries.^[2][3]

• IBD evolves through four epidemiologic stages:^[3]
  1. Emergence: low incidence and low prevalence.
  2. Acceleration in incidence: rapidly rising incidence with low prevalence. This is the current stage for much of Asia, Latin America, and Africa.
  3. Compounding prevalence: incidence decelerates, plateaus, or declines while prevalence climbs steadily. This is the current stage for North America, Western Europe, and Oceania.
  4. Prevalence equilibrium: prevalence plateaus as an aging patient population reaches a demographic balance. This stage is modeled but not yet observed.
• Compounding prevalence reflects the accumulation of chronic cases, low disease-specific mortality, and aging of the IBD population. It does not necessarily mean more new disease.^[3]
• The rise in newly industrialized regions is attributed to urbanization, Westernized diet, and industrialization.^[3][8]

• A north–south gradient and an urban–rural gradient persist within and between countries.^[8][9]
• In sub-Saharan Africa, true incidence and prevalence are uncertain. Contributing factors include limited endoscopic, histopathologic, and imaging capacity, and misattribution to infections that mimic IBD (e.g., intestinal tuberculosis).^[10] Low reported rates should not be read as a true low burden.

• Peak age at diagnosis is ~15–35 years, with a smaller second peak at ~50–60 years.^[6]
• Whether the distribution is truly bimodal is debated. U.S. claims data show IBD incidence peaking in the third decade of life, then holding relatively stable from the fourth to eighth decades rather than forming a distinct second peak.^[5]
• Pediatric and very-early-onset CD incidence continues to rise even where adult incidence has plateaued.^[4]

• There is no consistent sex predominance in adult CD.^[4]
• Regional patterns differ. A slight female predominance is reported in parts of North America and New Zealand, and a male predominance in parts of East Asia (Japan, Korea).^[4][11] The reasons are not fully explained.

• U.S. IBD prevalence per 100,000 population is:^[5]
  • White: 812
  • Black: 504
  • Hispanic: 458
  • Asian: 403
• A historically higher risk in Ashkenazi Jewish populations persists but varies geographically, which implicates both genetic and environmental factors.^[8]
• CD occurs across all racial and ethnic groups. The legacy framing of CD as predominantly a disease of White and European Jewish populations is outdated.^[5][2]

• Having an affected first-degree relative is the strongest known risk factor (incidence rate ratio ~7.77; 95% CI 7.05–8.56).^[4]
• Monozygotic twin concordance is incomplete, which indicates a substantial environmental contribution.^[4]
• Immigrants who move from low- to high-incidence regions acquire increased risk, especially when they migrate at a younger age. This supports early-life environmental programming.^[12]
• Modifiable environmental associations are covered in risk factors. These include smoking, which increases CD risk in contrast to its protective association in ulcerative colitis, as well as antibiotic exposure, ultra-processed food intake, and physical activity.^[6][4]

Crohn’s disease (CD) develops in genetically susceptible individuals through the interaction of inherited risk, environmental exposures, and gut microbial dysbiosis. No single factor is necessary or sufficient to cause disease.^[1][2][3] The strongest risk factor is a first-degree relative with CD. The most important modifiable risk factor is cigarette smoking.^[1][4] Other established risk factors include childhood antibiotic exposure, appendectomy, urban residence, and an industrialized (“Western”) lifestyle and diet. Breastfeeding and physical activity are associated with lower risk.^[5] Most environmental associations come from observational data, and causality has not been established for most of them.^[5]

• Family history is the single strongest risk factor. Having a first-degree relative with CD carries an incidence rate ratio of about 7.8. The risk is higher in relatives of patients with childhood-onset disease (OR ~10.6). About 8–12% of patients report a positive family history.^[1][6][7]
• Twin concordance for CD is about 30–50% in monozygotic twins, compared with about 4% in dizygotic twins and about 15% in monozygotic twins for ulcerative colitis (UC). This reflects a substantial but incomplete genetic contribution.^[8][6]
• Polygenic susceptibility:
  • Genome-wide association studies have identified more than 200–240 susceptibility loci. Together these explain only about 13.6% of CD variance, and most loci carry modest effects (OR 1.1–1.2).^[8][6]
  • Higher-impact genes include NOD2 (CARD15) and ATG16L1, which are CD-specific and impair bacterial sensing and autophagy. IL23R is shared with UC.^[8][2]
  • Risk variants are ancestry-specific. NOD2, ATG16L1, and IL23R coding variants predominate in White populations and are essentially absent in East Asian cohorts, where TNFSF15 is the dominant CD locus.^[8]
• Age: incidence peaks at 15–25 years, although CD occurs at all ages.^[4]
• Geography and industrialization: CD is most common in industrialized countries. North American incidence is 6.3–23.8 per 100,000 person-years.^[4] Incidence is rising rapidly in newly industrialized regions such as Brazil, Taiwan, and East and South Asia. Among immigrants to high-incidence countries, younger age at immigration is associated with higher risk.^[3][7]

• The following exposures are associated with lower CD risk:^[1][5][9]
  • Breastfeeding
  • Physical activity (RR ~0.63)
  • Higher folate and vitamin D levels
  • Greenspace exposure
  • Bed-sharing in childhood

• Dysbiosis is a consistent correlate of CD. It includes reduced microbial diversity, depletion of Firmicutes such as Faecalibacterium prausnitzii, and expansion of Proteobacteria, including adherent-invasive Escherichia coli. No single causative organism has been identified.^[2][10][11]

No professional society recommends screening asymptomatic members of the general population for Crohn’s disease. In practice, “screening” covers three separate activities. The first is case finding in symptomatic patients, where fecal calprotectin (FC) is the guideline-endorsed first-line, non-invasive test for deciding who needs endoscopy.^[1] The second is risk stratification of asymptomatic first-degree relatives (FDRs), which is still investigational and confined to research and prevention-trial settings.^[2] The third is colorectal cancer (CRC) surveillance in established Crohn’s colitis, a formal, guideline-mandated program whose start time and intervals differ between societies.^[3][4]

• Not recommended. Major society guidelines (ACG, AGA, ECCO, BSG, NICE) contain no recommendation to screen the asymptomatic general population for Crohn’s disease. Current research aims to build validated, multidimensional predictive tools for enriched high-risk groups, not for the general population.^[2]
• Genetic testing has no screening role. Genetic testing is reserved for suspected monogenic disease, mainly very-early-onset inflammatory bowel disease (IBD) presenting before age 6, or IBD with immunodeficiency or syndromic features.^[5] It should not be used to screen asymptomatic relatives.

• Guideline status: The ACG recommends that FC be considered to help distinguish IBD from irritable bowel syndrome (IBS).^[1]
• Rule-out value: FC is most useful for its negative predictive value. In primary care, an FC ≤40 μg/g corresponds to a <1% probability of IBD.^[6][7]

• Combined triage: Pairing the Red Flags Index (≥8) with FC >250 μg/g raised the positive predictive value from 4% to 21% and the negative predictive value from 97% to 100%.^[6]
• Small-bowel limitation: FC is less sensitive for small-bowel inflammation. In a pooled analysis of 214 patients who underwent capsule endoscopy, median FC for comparable inflammation rose stepwise:^[8]
  • proximal small bowel: 63 μg/g
  • distal small bowel: 190 μg/g
  • colon: 542 μg/g
• A normal FC therefore does not exclude isolated proximal or small-bowel CD. When clinical suspicion is high, proceed to capsule endoscopy or cross-sectional enterography.^[9]

• C-reactive protein, erythrocyte sedimentation rate, CBC, and albumin can raise suspicion of CD but are neither sensitive nor specific. Many patients with mild active inflammation have normal CRP and ESR, so a normal panel does not exclude CD.^[1]
• CRP ≥2.7 mg/dL has a positive predictive value of about 90% but poor sensitivity.^[7]
• Serologic markers such as ASCA and pANCA are not sensitive or specific enough for diagnostic screening.^[10]

This approach applies to patients with chronic diarrhea and/or abdominal pain:^[1][11][6]

1. Obtain FC, CBC, CRP, a metabolic panel, and stool studies, including Clostridioides difficile testing, to exclude infection.
2. If FC is ≤40–50 μg/g and there are no alarm features, IBD is unlikely; evaluate for functional or alternative diagnoses.
3. If FC is ≥250 μg/g, alarm features are present (rectal bleeding, weight loss, perianal disease, anemia), or inflammatory markers are abnormal, refer for ileocolonoscopy with biopsies, with or without cross-sectional imaging.
4. If FC is ~50–250 μg/g, repeat FC and/or proceed to endoscopy based on pretest probability.
5. If small-bowel CD is suspected despite a low FC, pursue small-bowel imaging or capsule endoscopy.

• Familial risk: In a Swedish nationwide study, people with IBD had about 3.5-fold higher odds of having an affected parent or full sibling. The odds were highest for pediatric-onset CD (OR 10.6). Risk rose with the number of affected FDRs. The 10-year cumulative incidence of IBD in full siblings of incident cases was 1.7%, compared with 0.4% in siblings of reference individuals.^[12]
• Subclinical inflammation: In the CCC-GEM cohort, healthy FDRs from multiplex families (≥2 affected relatives) had higher FC, a distinct fecal microbial signature, and a higher risk of developing CD than FDRs from simplex families.^[13]
• Practical counseling: Counsel FDRs to seek prompt evaluation if symptoms develop. Advise smoking cessation, since smoking is a strong environmental risk factor for CD within IBD families.^[14][9]

Unlike population screening, colonoscopic surveillance is a formal mandate once colonic CD is established. All societies agree that surveillance is indicated; they differ on when to start and how to set intervals. Detailed dysplasia management belongs in the complications/cancer microchapter.

• Primary sclerosing cholangitis (PSC) is a high-risk modifier. Surveillance colonoscopy should begin at the time of PSC diagnosis, regardless of colitis duration, and continue annually.^[4][16]

Crohn’s disease is a chronic, relapsing–remitting, transmural inflammatory bowel disease that tends to progress over time from purely inflammatory disease to stricturing or penetrating complications.^[1][2] Disease location is usually stable, but up to half of patients develop a stricture, internal fistula, or perianal complication within 20 years of diagnosis, and contemporary population-based cohorts show that roughly 1 in 5 patients with uncomplicated disease progress and about 1 in 4 require resection within 10 years.^[2][3] Major complications include bowel obstruction from strictures, fistulae, intra-abdominal and perianal abscesses, extraintestinal manifestations, nutritional deficiencies, treatment-related toxicity, and an increased risk of small bowel and colorectal adenocarcinoma.^[1][4] There is no cure. Long-term mortality is modestly higher than in the background population, and patients with Crohn’s colitis need risk-stratified colonoscopic surveillance for colorectal cancer.^[5][6]

• Most patients follow a chronic intermittent course. In North American population-based cohorts, about 13% had continuously active (unremitting) disease and about 10% had prolonged remission.^[7]
• Symptoms and bowel damage are often discordant. Structural damage can accumulate silently despite symptomatic remission.^[1]

• At diagnosis, ileal, colonic, and ileocolonic disease each account for roughly one-third of cases. Location stays stable in most patients, and only about 6–14% extend anatomically.^[2]
• Most patients present with non-stricturing, non-penetrating (inflammatory, Montreal B1) disease. Behavior then progresses to stricturing (B2) or penetrating (B3) disease in up to half of patients within 20 years.^[2]
• Contemporary inception cohorts show lower early progression rates than historical series:
  • In the European Epi-IBD cohort, 14% of patients with B1 disease progressed to stricturing and/or penetrating disease within 5 years, 22% underwent surgery, and 36% were hospitalized.^[8]
  • At 10 years, rates were about 20% for progression to complicated disease, 24% for surgery, and 41% for hospitalization. Outcomes were similar despite earlier use of advanced therapy in Western Europe.^[3]
• In US claims data, more than one-quarter of newly diagnosed patients progressed within 3 years. Intestinal obstruction/stenosis was the most common complication, and much of the risk accrued within the first 6 months, reflecting diagnostic delay or complications present at diagnosis.^[9]

• Surgical rates have fallen over time:
  • In the Copenhagen County cohort (diagnosed 1962–1987), the cumulative probability of surgery was 62% at 10 years and 71% at 20 years, plateauing near 74% by 40–50 years. Over half of operated patients needed two or more operations.^[5]
  • In contemporary cohorts, about 24% undergo resection by 10 years.^[3]
• Stricturing or penetrating disease at diagnosis, progression of disease behavior, and former smoking were associated with intestinal resection in the 10-year Epi-IBD cohort.^[3]