Crohn Disease

Crohn disease is a chronic, progressive, transmural inflammatory bowel disease that can affect any segment of the gastrointestinal tract from mouth to anus, characterized by skip lesions, granulomatous inflammation, and a relapsing-remitting course.

Also Called / Synonyms: Crohn disease (CD), regional enteritis, granulomatous enterocolitis, Crohn's syndrome, ileitis terminalis (historical), and Crohn's-like enteritis (when associated with primary immunodeficiency such as X-linked agammaglobulinemia ^[5]C4).

Crohn disease is a lifelong condition with substantial morbidity; it carries an increased risk of colorectal cancer in patients with colonic involvement, necessitating endoscopic surveillance ^[6]A1a.

Nomenclature and Historical Context

First described by Dr. Burrill B. Crohn in 1932 as "regional ileitis," the disease was later recognized to involve the entire GI tract. The term "Crohn disease" now encompasses all phenotypes, though historical terms like "granulomatous colitis" persist in pathology literature. The distinction from ulcerative colitis rests on transmural inflammation, skip lesions, and the presence of non-caseating granulomas.

Disease Phenotypes and Classification Systems

Clinicians classify Crohn disease using the Montreal classification (adopted 2005) and the Paris classification (2011) for pediatric-onset disease. These systems stratify patients by age at diagnosis, disease location, and disease behavior, which predict natural history and guide therapeutic decisions. The Montreal system defines three axes:

The Paris classification refines pediatric CD by splitting L4 into L4a (upper GI proximal to ligament of Treitz) and L4b (distal to ligament of Treitz but proximal to terminal ileum), and adding a B2B3 category for patients with both stricturing and penetrating behavior at diagnosis. It also recognizes growth failure as a distinct modifier (G1: present).

Special Phenotypes: Perianal Disease and Very Early Onset IBD

Perianal Crohn disease (fistulas, abscesses, fissures) is a distinct phenotype that may occur independently of luminal disease activity. The Montreal/Paris systems include a perianal modifier (p), but complicated fissuring perianal disease causing severe pain is not separately captured and may require aggressive therapy ^[10]B3b.

Very early onset inflammatory bowel disease (VEOIBD) is defined as CD diagnosed before age 6 years. These patients often have a distinct genetic basis (monogenic defects) and may present with Crohn's-like enteritis in the setting of primary immunodeficiencies such as X-linked agammaglobulinemia ^[5]C4. Anti-TNF therapy is used in VEOIBD but durability may be lower than in older children ^[9]B3b.

Pearl: The Montreal and Paris classification systems provide the essential framework for subtyping Crohn disease by age, location, and behavior, which directly informs prognosis and treatment strategy; perianal disease and VEOIBD represent special phenotypes that require dedicated management approaches ^[10]B3b[5]C4.

A dysregulated mucosal immune response to commensal gut microbiota in a genetically susceptible host drives the chronic, transmural inflammation that defines Crohn disease. This process involves a cascade of genetic, immunological, and microbial events that converge on a breakdown of intestinal homeostasis.

Genetic Susceptibility and Autophagy

Loss-of-function mutations in NOD2 (nucleotide-binding oligomerization domain 2) impair intracellular bacterial sensing, reducing defensin production and predisposing to ileal disease. The ATG16L1 T300A variant (rs2241880) disrupts autophagy, leading to defective clearance of intracellular pathogens, increased IL-1β secretion, and enhanced susceptibility to perianal Crohn disease ^[13]A1a. The ZIP8 A391T variant, carried by approximately 5% of the population, causes aberrant N-glycosylation via manganese-dependent glycosyltransferases, impairing epithelial barrier function and promoting inflammation ^[23]C4. These genetic hits collectively impair the host's ability to maintain immune tolerance to the gut microbiome.

Mucosal Immune Dysregulation

In inflamed intestinal mucosa, CD4+ T cells exhibit a skewed phenotype: Th17 cells are expanded, producing IL-17 and IL-22, but a subset of FoxP3+ regulatory T cells (Tregs) also co-express IL-17, indicating a plastic, pro-inflammatory state ^[11]C4. TNF-α is a central mediator, driving granuloma formation, fibroblast activation, and fibrosis. Leukocyte trafficking into gut tissue depends on α4 integrin binding to mucosal addressin cell adhesion molecule-1 (MAdCAM-1); blockade with natalizumab reduces inflammation and induces remission ^{[15]A1a[16]A1a}. Anti-TNFα therapies (e.g., infliximab , adalimumab ) target this pathway, but immunogenicity leads to loss of response in a subset of patients, driven by anti-drug antibodies ^[18]B2a.

Microbiome and Dysbiosis

Gut microbial composition is profoundly altered in Crohn disease, with reduced diversity, depletion of Faecalibacterium prausnitzii, and expansion of adherent-invasive Escherichia coli (AIEC). Fungal dysbiosis also contributes: colonic Engyodontium species trigger neutrophil antimicrobial activity via the CARD9 pathway, suppressing Lactobacillus johnsonii and reducing Treg expansion mediated by L-glutamic acid ^[22]C4. Mendelian randomization analyses support a causal relationship between specific gut microbes (e.g., Roseburia, Lachnospira) and inflammatory proteins (e.g., IL-6, TNF-α) in Crohn disease ^[20]B2c. Fecal microbiota transplantation (FMT) aims to correct dysbiosis but shows variable efficacy, with meta-analyses reporting pooled clinical remission rates of approximately 30-40% ^{[19]A1a[24]D5}.

Epithelial Barrier and Transmural Inflammation

Transient Receptor Potential (TRP) channels, including TRPV1 and TRPA1, are upregulated in colitis, contributing to visceral hypersensitivity and neurogenic inflammation ^[12]D5. Disruption of tight junctions increases intestinal permeability, allowing luminal antigens to activate submucosal immune cells. The resulting inflammation extends transmurally, leading to fibrosis, strictures, and fistula formation. MRI-based motility scores correlate with disease activity and can detect early functional changes before structural damage occurs ^{[17]B2b[21]D5}.

Key Genetic Variants in Crohn Disease Pathogenesis

Pearl: The interplay of genetic susceptibility (NOD2, ATG16L1, ZIP8), Th17/Treg imbalance, and gut dysbiosis creates a self-perpetuating cycle of inflammation that explains the chronic relapsing course of Crohn disease and provides targets for biologic therapy.

Incidence of Crohn disease varies from 1-2 per 100,000 in Asia to 20-30 per 100,000 in North America and Europe, with prevalence reaching 0.5% in Canada by 2030 ^[25]B2c. The disease affects all ages but peaks in the second to fourth decades; a second smaller peak occurs after age 60. Sex distribution is nearly equal, though some cohorts report a slight female predominance. Temporal trends show rising incidence in newly industrialized regions (Asia, South America) while rates have stabilized in high-incidence Western countries ^[25]B2c.

Risk Factors

Crohn disease arises from a complex interplay of genetic susceptibility and environmental exposures. Family history confers the strongest risk: first-degree relatives have a 10- to 20-fold increased risk (OR 10-20). Among modifiable factors, central obesity, vitamin D deficiency, and maternal diet during pregnancy have emerged as significant contributors.

Central obesity independently predicts incident Crohn disease. In the UK Biobank (479,590 participants, median follow-up 12.5 years), each 1-standard-deviation increase in waist-to-hip ratio raised CD risk by 45% (HR 1.45, 95% CI 1.18-1.78) after adjusting for BMI and confounders ^[30]B2b. This effect was stronger in women and persisted across sensitivity analyses.

Vitamin D deficiency (<50 nmol/L) increases long-term risk of elderly-onset IBD. In the same UK Biobank cohort, participants with deficiency had a 1.6-fold higher hazard of incident CD compared to those with sufficiency (≥75 nmol/L) (HR 1.61, 95% CI 1.12-2.31) ^[32]B2b. The association was dose-dependent, with each 25 nmol/L decrease in serum 25(OH)D linked to a 12% increased risk (HR 1.12, 95% CI 1.04-1.21).

Maternal diet in pregnancy influences offspring risk. In the Norwegian Mother, Father and Child Cohort (85,129 children), higher maternal diet quality (assessed by a validated index) was associated with lower risk of CD in the child (HR 0.74 per 1-SD increase, 95% CI 0.56-0.98) ^[36]B2b. Greater intake of vegetables and fish during pregnancy showed protective trends, while high intake of processed meat increased risk (HR 1.42, 95% CI 1.02-1.98).

Pediatric constipation may precede CD diagnosis. In a cross-sectional study of 238 children with new IBD, 19.7% had functional constipation before diagnosis, with a higher prevalence in CD (44% of IBD cases) ^[34]C4. Whether constipation is a risk factor or early symptom remains unclear.

Upper gastrointestinal involvement is common in Asian populations. In a Korean cohort, 48.7% of CD patients had histologic esophagogastroduodenal involvement at diagnosis, though most were asymptomatic ^[29]B2b. This highlights geographic variation in disease phenotype.

Controversies and Guideline Disagreement

Vitamin D supplementation for primary prevention is not recommended by any major guideline, despite observational evidence of deficiency as a risk factor. The association may be confounded by reverse causation (subclinical disease reducing sun exposure) or shared lifestyle factors. No randomized trial has tested vitamin D for CD prevention. Similarly, central obesity as a modifiable target lacks interventional data; weight loss trials have not assessed CD incidence as an endpoint.

Pearl: Central obesity and vitamin D deficiency are independently associated with incident Crohn disease, but causality remains unproven; family history remains the strongest identifiable risk factor, with a 10- to 20-fold increase in first-degree relatives ^{[25]B2c[30]B2b[32]B2b}.

The disease process often announces itself insidiously long before structural damage is evident. Patients typically describe a story spanning weeks to months, rarely an acute catastrophe, of progressive abdominal pain, chronic diarrhea, and unintentional weight loss . The pain, usually right lower quadrant or periumbilical, is crampy and postprandial, reflecting the segmental transmural inflammation that narrows the bowel lumen ^[37]D5. Diarrhea is non-bloody in most cases, driven by malabsorption, bile salt wasting, and bacterial overgrowth in the setting of a diseased terminal ileum; the presence of gross blood should raise suspicion for colonic involvement or an alternate diagnosis ^[4]B2a. Weight loss results from a combination of cytokine-driven anorexia, nutrient malabsorption, and increased intestinal protein loss.

Neurological & Systemic Examination Findings

The physical examination is a critical bedside tool for localizing disease and assessing severity. The key findings are organized below by system.

Phenotypic Variants

Crohn disease presents with distinct clinical phenotypes that determine management and prognosis. The Montreal classification is the standard system for categorizing these variants.

Red Flags

Certain symptoms mandate urgent evaluation and often hospital-level care. FVC < 15 mL/kg (though rarely measured at bedside) suggests neuromuscular weakness in the context of associated myopathy; more practically, inability to tolerate oral intake for >24 hours, high-volume diarrhea (>10 stools/day), and fever >38.5°C are red flags for severe or complicated disease ^[4]B2a. Peritonitis-equivalent signs, involuntary guarding, rebound tenderness, abdominal rigidity, indicate perforation or abscess. Unexplained tachycardia (>100 bpm) and hypotension (SBP <90 mmHg) suggest intra-abdominal sepsis or an adrenal crisis from chronic corticosteroid use.

Atypical Presentations

Several presentations challenge the clinician because they deviate from the classic terminal ileal picture. Isolated perianal disease may present solely as a non-healing fissure or complex fistula before any intestinal symptoms appear; such patients require colonoscopy to exclude occult CD ^[13]A1a. Gastroduodenal Crohn (Montreal L4) manifests as epigastric pain, early satiety, and vomiting without diarrhea, easily mistaken for peptic ulcer disease until endoscopic biopsies reveal granulomas ^[38]C4. Systemic onset with fever, arthralgias, erythema nodosum , or anterior uveitis can precede abdominal symptoms by months, mimicking seronegative spondyloarthritis or Behçet disease ^[41]D5. Severe weight loss and growth failure in children may be the sole presenting feature, with gastrointestinal symptoms dismissed as functional until linear growth deceleration appears on the growth chart ^[38]C4. Finally, patients with Niemann-Pick type C may develop granulomatous enteritis that mimics CD but resists anti-TNF therapy, requiring a high index of suspicion when atypical features like vertical supranuclear gaze palsy or hepatosplenomegaly are present ^[42]C4.

Pearl: When a patient presents with right lower quadrant pain and chronic diarrhea, the presence of perianal disease, a tender abdominal mass, or oral ulcerations increases the pre-test probability of Crohn disease over ulcerative colitis to >90% in appropriate epidemiological contexts ^{[37]D5[13]A1a}.

The diagnosis of Crohn disease rests on a combination of endoscopic, histologic, and cross-sectional imaging findings, with no single test sufficient in isolation. The gold-standard diagnostic test is ileocolonoscopy with multiple biopsies from the terminal ileum and each colonic segment, even when the mucosa appears normal ^[43]C4. This approach provides both macroscopic and microscopic evidence of chronic inflammation and allows exclusion of alternative diagnoses such as ulcerative colitis, infection, or ischemia.

Gold-Standard Test: Ileocolonoscopy with Biopsies

Ileocolonoscopy achieves a sensitivity of approximately 90% for detecting active colonic and terminal ileal disease when performed by an experienced endoscopist ^[43]C4. The procedure should include careful inspection of the terminal ileum, which is involved in 70-80% of patients. Key endoscopic features include aphthous ulcers, deep serpiginous ulcers, cobblestoning (a network of fissures and edematous mucosa), and strictures. The presence of skip lesions, areas of inflammation alternating with normal mucosa, is highly characteristic and helps distinguish Crohn disease from ulcerative colitis, which typically shows continuous inflammation from the rectum proximally.

Biopsies must be taken from both ulcerated and endoscopically normal mucosa because histologic abnormalities often extend beyond visible lesions ^[43]C4. At least two biopsies from each of five segments (terminal ileum, cecum, ascending, transverse, descending, and sigmoid/rectum) are recommended. Histologic hallmarks include transmural inflammation, non-caseating granulomas (found in 30-50% of biopsies), fissuring ulcers, and architectural distortion of crypts. Granulomas are pathognomonic when present but their absence does not exclude the diagnosis.

Endoscopic Findings by Location

Ileocolonic disease is the most common phenotype (approximately 50% of patients). Typical findings include ulcers, strictures, and fistulous openings. Upper gastrointestinal involvement (esophagus, stomach, duodenum) occurs in up to 48.7% of patients in prospective series, often with subtle endoscopic findings such as erosions, erythema, or nodularity ^[29]B2b. Esophagogastroduodenoscopy (EGD) with biopsies is recommended at diagnosis, especially in children and young adults, because upper tract involvement may be asymptomatic yet histologically active ^[29]B2b. Perianal disease (fissures, fistulae, abscesses) is a separate diagnostic domain; examination under anesthesia may be required when external inspection is inadequate.

Cross-Sectional Imaging

Magnetic resonance enterography (MRE) is the imaging modality of choice for evaluating small bowel disease beyond the reach of the colonoscope, assessing transmural inflammation, and detecting complications such as abscesses, fistulae, and strictures. MRE has a sensitivity of 85-90% and specificity of 90-95% for active small bowel inflammation ^[45]A1a. Key findings include bowel wall thickening (>3 mm), mural hyperenhancement, mesenteric fat stranding, and the comb sign (engorged vasa recta). MRE is preferred over CT enterography (CTE) in young patients and those requiring serial imaging because it avoids ionizing radiation; CTE is reserved for acute settings where speed is critical or when MRE is contraindicated.

Capsule endoscopy is used when ileocolonoscopy and MRE are nondiagnostic but clinical suspicion remains high. It visualizes the entire small bowel mucosa and can detect subtle erosions not seen on cross-sectional imaging. However, capsule retention occurs in 1-5% of patients with known strictures; a patency capsule should be administered first to confirm luminal patency.

Positron emission tomography (PET) with FDG or FAPI tracers is an emerging tool. A meta-analysis of 20 studies (547 patients) reported pooled sensitivity of 76%, specificity of 81%, and accuracy of 85% for FDG-PET in detecting active inflammation ^[45]A1a. FAPI-PET may offer higher specificity for fibrotic disease, but neither tracer is currently part of routine diagnostic algorithms.

Laboratory Studies

No single laboratory test confirms Crohn disease, but several markers support the diagnosis and assess disease activity.

Fecal calprotectin is the most useful noninvasive test for distinguishing inflammatory from functional bowel disease. A level >150 μg/g has a sensitivity of 90% and specificity of 80% for active Crohn disease ^[44]A1b. It is also used to monitor response to therapy and predict relapse.

Histology

Histologic confirmation is essential. The pathognomonic finding is a non-caseating granuloma composed of epithelioid histiocytes and multinucleated giant cells, often located in the submucosa or serosa. Granulomas are present in only 30-50% of biopsies, so their absence does not rule out Crohn disease. Other histologic features include transmural lymphoid aggregates, fissuring ulcers, crypt distortion, and pyloric gland metaplasia in the terminal ileum (a marker of chronic injury). Electron microscopy may reveal abnormal microvilli in uninflamed epithelium, suggesting a primary epithelial defect ^[43]C4.

Diagnostic Algorithm

Step 1: Clinical Suspicion, Symptoms (chronic diarrhea, abdominal pain, weight loss, perianal disease) lasting >4 weeks with no infectious cause.

Step 2: Noninvasive Screening, Obtain fecal calprotectin and CRP. If both are normal and clinical suspicion is low, consider alternative diagnoses. If elevated, proceed to endoscopy.

Step 3: Ileocolonoscopy with Biopsies, The definitive diagnostic step. Perform EGD with biopsies in children and young adults or if upper GI symptoms are present ^[29]B2b.

Step 4: Cross-Sectional Imaging, Obtain MRE (or CTE if MRE unavailable) to assess small bowel involvement, detect complications, and guide therapy. Consider capsule endoscopy if MRE is negative but suspicion remains high.

Step 5: Histologic Confirmation, Review biopsies for granulomas, transmural inflammation, and other features. If histology is inconclusive but clinical and imaging features are typical, a diagnosis of Crohn disease can still be made.

Step 6: Staging, Once diagnosis is established, assess disease extent (Montreal classification), severity (Crohn's Disease Activity Index, endoscopic scores), and complications (strictures, fistulae, abscesses).

Pearl: Ileocolonoscopy with biopsies remains the gold standard for diagnosis, but MRE is essential for evaluating small bowel disease and detecting complications that alter management ; fecal calprotectin is the most sensitive noninvasive screening tool and should be used to guide the decision to proceed to endoscopy ^{[44]A1b[45]A1a}.

Severity assessment in Crohn disease relies on a triad of clinical activity indices, endoscopic scoring systems, and biomarker panels that together guide treatment intensity and predict disease course. No single tool captures the full spectrum of disease burden; the choice of instrument depends on the clinical setting, trial enrollment, routine monitoring, or preoperative planning.

Clinical Activity Indices

The Crohn's Disease Activity Index (CDAI) remains the gold standard for clinical trials, incorporating eight items (stool frequency, abdominal pain, general well-being, extraintestinal manifestations, abdominal mass, hematocrit, body weight) over a 7-day diary. A CDAI <150 defines remission, 150-220 mild, 221-450 moderate, and >450 severe disease. However, its complexity limits routine use. The Harvey-Bradshaw Index (HBI) simplifies assessment to five clinical parameters (general well-being, abdominal pain, number of liquid stools, abdominal mass, complications) and correlates well with CDAI (r=0.80) while requiring no laboratory data. The HBI is preferred in outpatient clinics for rapid triage.

Endoscopic Scoring Systems

The Simple Endoscopic Score for Crohn's Disease (SES-CD) evaluates four segments (ileum, right colon, transverse colon, left colon, rectum) for ulcer size, ulcerated surface, affected surface, and stenosis. Scores range from 0 to 60; SES-CD ≥7 indicates moderate-to-severe endoscopic activity. Mucosal healing, defined as SES-CD ≤2, is a key therapeutic target associated with sustained remission and reduced hospitalization. The Rutgeerts score specifically assesses postoperative recurrence at the neoterminal ileum after ileocolic resection, with scores i0 (no lesions) to i4 (diffuse inflammation with large ulcers) predicting clinical recurrence.

Biomarker-Based Risk Stratification

C-reactive protein (CRP) and fecal calprotectin are the most widely used biomarkers. CRP >5 mg/L indicates active inflammation but lacks specificity. The CRP-to-albumin ratio (CAR) has emerged as a robust perioperative risk tool. In a propensity-matched cohort of 112 patients with penetrating CD, a preoperative CAR threshold of 0.102 maximized negative predictive value for 30-day complications; high CAR (>0.102) was associated with significantly higher early morbidity and worse 5-year re-operation-free survival ^[54]C4. This simple, inexpensive ratio can guide surgical timing.

Emerging Tools: Radiomics and Machine Learning

Computed tomography enterography (CTE)-based radiomics models outperform conventional scoring for predicting mucosal healing. In a study of 92 CD patients, a LASSO-selected radiomics signature achieved an area under the curve of 0.85 for identifying mucosal healing on follow-up endoscopy, offering a noninvasive alternative to repeat colonoscopy ^[50]B3b. Machine learning (ML) models integrating clinical, endoscopic, and imaging data have shown promise for stratifying disease course. A systematic review of 42 studies found that ML algorithms (random forests, support vector machines) predicted disease flares, need for surgery, and response to biologics with accuracies exceeding 80% ^[14]B2a. However, external validation and standardization remain barriers to clinical adoption.

Postoperative Risk Stratification

Postoperative recurrence is common after ileocolic resection. A multicenter case-control study developed a nomogram for predicting intestinal fistula after CD surgery, incorporating preoperative CRP, albumin, disease duration, and penetrating phenotype. The nomogram showed good discrimination (C-index 0.78) in the validation cohort ^[53]B3b. Additionally, preoperative CAR >0.102 identifies patients at high risk for early complications ^[54]C4. These tools enable tailored postoperative surveillance and early intervention.

Integrating Scores into Clinical Decision-Making

The choice of severity instrument depends on the context. For clinical trials, CDAI remains mandatory. For routine monitoring, HBI combined with fecal calprotectin provides rapid assessment. For endoscopic targets, SES-CD guides biologic escalation. For preoperative planning, CAR and nomograms refine risk. The SERENE CD exposure-response analysis further suggests that adalimumab average concentration impacts endoscopic response, supporting therapeutic drug monitoring as a stratification tool ^[51]B2b. Ozanimod-induced reductions in circulating T-cell subsets (Th, cytotoxic T cells) correlate with clinical response, hinting at lymphocyte phenotyping as a future biomarker ^[52]B2b. Causal inference from Mendelian randomization identifies specific inflammatory factors (e.g., IL-6, TNF-α) as drivers of disease activity, potentially informing personalized anti-cytokine selection ^[57]B2b. Even sleep quality and mood, as measured by PSQI and PHQ-9, correlate with disease activity, suggesting that patient-reported outcomes should complement objective scores ^[55]C4.

Limitations and Future Directions

Current scores have limitations: CDAI is cumbersome, HBI is subjective, SES-CD requires endoscopy, and biomarkers lack specificity. Machine learning and radiomics offer the potential for noninvasive, integrated risk prediction, but require prospective validation. The field is moving toward composite scores that combine clinical, endoscopic, and molecular data to achieve precision stratification.

Pearl: Severity stratification in Crohn disease requires integration of clinical indices (CDAI, HBI), endoscopic scores (SES-CD), and biomarkers (CRP, fecal calprotectin, CAR); emerging radiomics and ML models promise to refine risk prediction, but no single tool suffices ^{[50]B3b[54]C4}.

Acute exacerbations of Crohn disease require rapid severity classification to determine the appropriate level of care and intervention. The initial assessment integrates clinical activity indices, objective markers of inflammation, and endoscopic or cross-sectional imaging findings from the preceding diagnostic workup.

Step 1: Initial Assessment and Severity Classification

Severity is stratified using the Harvey-Bradshaw Index (HBI) or Crohn's Disease Activity Index (CDAI). Mild disease corresponds to HBI <8 or CDAI <220, moderate disease to HBI 8-16 or CDAI 220-450, and severe disease to HBI >16 or CDAI >450 ^{[62]A1b[64]B2b}. Concurrent features such as fever, tachycardia, hypotension, peritoneal signs, or evidence of obstruction, abscess, or fulminant colitis (≥6 bloody stools daily with systemic toxicity) define a severe or complicated flare that warrants hospitalization. For patients with severe disease, immediate consultation with a gastroenterologist and colorectal surgeon is indicated.

Step 2: First-Line Intervention

Mild-to-moderate active disease (pediatric): Exclusive enteral nutrition (EEN) or the Crohn's disease exclusion diet (CDED) with 50% partial enteral nutrition induces rapid clinical remission within 3 to 6 weeks. In a multicenter randomized trial of 73 children, 78% of those on CDED and 60% on EEN achieved clinical remission at week 6 ^[58]A1b (1b). Dietary therapy is preferred over corticosteroids in children due to its favorable safety profile and positive effects on growth and mucosal healing.

Moderate-to-severe active disease (adults and children): Systemic corticosteroids remain first-line for induction of remission. Prednisone 40-60 mg orally once daily or intravenous methylprednisolone 40-60 mg daily is initiated, followed by a taper over 8-12 weeks ^{[61]A1b[63]C4}. Corticosteroids are not recommended for maintenance therapy due to toxicity and lack of efficacy.

AIEC-colonized ileal disease: In patients with endoscopically active ileal CD colonized with adherent-invasive Escherichia coli (AIEC), a 12-week course of oral ciprofloxacin 500 mg twice daily plus rifaximin 800 mg twice daily significantly improves endoscopic response compared with placebo (TEOREM trial) ^[44]A1b (1b). This antibiotic combination should be considered only when AIEC colonization is confirmed on ileal biopsy.

Step 3: Second-Line and Rescue Therapy

Steroid-refractory or steroid-dependent disease: Infliximab induction therapy is indicated. Administer 5 mg/kg intravenously at weeks 0, 2, and 6 ^[64]B2b. Early infliximab trough levels predict long-term outcomes: a week-2 trough concentration ≥3.5 μg/mL is associated with higher rates of clinical remission and mucosal healing at week 30 ^[64]B2b (2b). Proactive therapeutic drug monitoring (TDM) during maintenance therapy reduces the risk of disease worsening compared with standard dosing without TDM ^[2]A1b (1b).

Fulminant colitis or toxic megacolon: Hospitalize, initiate intravenous corticosteroids (methylprednisolone 60 mg/day), and obtain surgical consultation. If no improvement within 3-5 days, rescue therapy with infliximab (5 mg/kg single dose) or cyclosporine (2 mg/kg/day continuous IV) is considered, though infliximab is preferred due to superior long-term outcomes. Emergency colectomy is indicated for perforation, uncontrolled hemorrhage, or failure of medical therapy.

Step 4: Monitoring and Titration

• Clinical response: Reassess HBI or CDAI at weeks 2, 6, and 12. A decrease of ≥70 points in CDAI or ≥3 points in HBI defines response.
• Infliximab trough levels: Measure at week 2 (target ≥3.5 μg/mL) and before the fourth infusion (target 3-7 μg/mL). If trough is subtherapeutic, consider dose escalation to 10 mg/kg or shortening the interval to every 6 weeks ^[64]B2b.
• Corticosteroid taper: Begin taper after 1-2 weeks of clinical response; reduce by 5-10 mg/week of prednisone equivalent. Prolonged use beyond 12 weeks is discouraged.
• Objective assessment: Endoscopic evaluation or CT enterography (CTE) with radiomics may confirm mucosal healing, which is associated with better long-term outcomes ^[50]B3b (3b).

Step 5: Resolution and Transition to Maintenance

Once clinical remission is achieved, transition to a maintenance strategy. For patients who responded to infliximab, continue 5 mg/kg every 8 weeks with proactive TDM ^[2]A1b. For those who achieved remission with dietary therapy, gradual reintroduction of foods under CDED guidance is recommended ^[58]A1b. Corticosteroids should be discontinued. If surgery was required for complications (obstruction, abscess, perforation), the Kono-S antimesenteric functional end-to-end anastomosis reduces endoscopic recurrence at 6 months compared with conventional stapled anastomosis (22.2% vs 51.2%; NNT = 4) ^[59]A1b (1b). Postoperative parenteral n-3 polyunsaturated fatty acid supplementation (e.g., 10 g/day for 7 days) may improve recovery and reduce inflammatory markers ^[60]A1b (1b).

Drug Dosing Table

Treatment Failure Protocol

• No response to corticosteroids by day 7: Initiate infliximab induction (5 mg/kg) ^[64]B2b.
• No response to infliximab by week 6: Check trough level and anti-drug antibodies. If trough <3.5 μg/mL without antibodies, escalate dose to 10 mg/kg or shorten interval to every 6 weeks. If antibodies present, switch to another anti-TNF ( adalimumab ) or different class (vedolizumab, ustekinumab).
• No response to antibiotics by week 6: Discontinue and reassess for alternative therapy (corticosteroids, biologics).
• Clinical deterioration or development of complications (abscess, obstruction, perforation): Urgent surgical consultation.

What NOT to Do

• Do not use antibiotics empirically without evidence of AIEC colonization; they are ineffective in unselected patients ^[44]A1b.
• Do not use growth hormone as routine therapy; a randomized trial showed no benefit over corticosteroids alone for clinical or endoscopic outcomes ^[61]A1b (1b).
• Do not rely solely on symptom-based assessment; objective evaluation with endoscopy or CTE is necessary to confirm mucosal healing and guide therapy ^[50]B3b.

Controversies and Guideline Disagreement

No major guideline disagreements identified for this topic in the reviewed evidence. The role of dietary therapy is well-established in pediatrics but less studied in adults; corticosteroids and anti-TNF agents remain the standard for moderate-to-severe flares across age groups. The use of antibiotics is restricted to the specific AIEC-colonized phenotype.

Pearl: For acute severe Crohn disease, initiate corticosteroids promptly and assess response within 7 days; early infliximab induction with target trough ≥3.5 μg/mL at week 2 improves long-term outcomes, and dietary therapy is a safe first-line option in children ^{[58]A1b[64]B2b}.

Maintenance therapy in Crohn disease aims to sustain remission, prevent relapse, and avoid long-term complications. The evidence ladder prioritizes biologic agents with proven efficacy in randomized controlled trials, supported by therapeutic drug monitoring and dietary interventions in select populations.

Anti-TNF Maintenance Therapy

Anti-tumor necrosis factor (anti-TNF) agents remain the cornerstone of maintenance therapy for moderate-to-severe luminal Crohn disease. Infliximab 5 mg/kg IV every 8 weeks after a three-dose induction regimen achieves sustained clinical remission and mucosal healing. A systematic review of anti-TNF therapy demonstrated that 60-80% of patients successfully withdraw corticosteroids during maintenance, with a pooled steroid-free remission rate of approximately 40% at 1 year ^[1]A1a (1a). Subcutaneous infliximab (120 mg SC every 2 weeks) offers an alternative for patients preferring home administration; real-world pediatric data show 89% persistence at 12 months after switching from intravenous formulation ^[28]C4 (4). Accelerated 1-hour infliximab infusions are safe and do not increase infusion reactions compared with standard 2-hour infusions ^[75]C4 (4).

Adalimumab 40 mg SC every other week after a 160/80 mg induction is another first-line option. The SERENE CD exposure-response analysis found that higher adalimumab trough concentrations were associated with increased rates of clinical remission at week 56 (OR 1.34 per 1 μg/mL increase, 95% CI 1.05-1.71) ^[51]B2b (2b). Transition from reference adalimumab to biosimilar SB5 does not compromise disease control; a randomized crossover trial reported that 88% of patients maintained clinical status on reference vs 86% on biosimilar (difference -2%, 95% CI -10% to 6%) ^[70]A1b (1b).

Vedolizumab Maintenance

Vedolizumab, a gut-selective α4β7 integrin antagonist, is recommended for patients with prior anti-TNF failure or contraindications. Vedolizumab 300 mg IV every 8 weeks after standard induction maintains remission in approximately 40% of patients at 1 year. In the pediatric VedoKids cohort, baseline drug clearance predicted deep biochemical remission at week 30, supporting therapeutic drug monitoring to optimize exposure ^[27]B2b (2b).

Natalizumab

Natalizumab (300 mg IV every 4 weeks) is effective for induction and maintenance of remission in anti-TNF-refractory Crohn disease, as shown in Cochrane systematic reviews ^{[15]A1a[16]A1a} (1a). However, its use is limited by the risk of progressive multifocal leukoencephalopathy (PML); mandatory JC virus antibody testing and MRI surveillance are required. It is reserved for patients who have failed all other biologics and are JC virus antibody-negative.

Dietary Therapy for Maintenance

Exclusive enteral nutrition (EEN) and the Crohn disease exclusion diet (CDED) with partial enteral nutrition are effective induction therapies, particularly in pediatric patients. A short 3-week trial of dietary therapy can predict long-term success: children who achieve clinical remission by week 3 are more likely to maintain remission at week 6 ^[58]A1b (1b). Nutritional therapy also improves growth and nutritional status, making it a valuable adjunct or alternative to pharmacologic maintenance in selected patients ^[65]D5 (5).

What NOT to Do

• Do not use 5-aminosalicylates (5-ASA) for maintenance in Crohn disease. A meta-analysis of 5-ASA discontinuation found no increased risk of relapse compared with continued use (RR 1.02, 95% CI 0.83-1.25), confirming lack of efficacy ^[72]A1a (1a).
• Do not use antibiotics for long-term maintenance. The TEOREM trial showed that ciprofloxacin plus rifaximin for 12 weeks improved endoscopic response in AIEC-colonized patients (45% vs 22%; NNT = 4.3), but this was an induction strategy with no maintenance data ^[44]A1b (1b).
• Do not use fecal microbiota transplantation (FMT) for maintenance. A Cochrane review found insufficient evidence to support FMT for induction or maintenance of remission in Crohn disease ^[24]D5 (5).
• Do not use tesnatilimab (anti-NKG2D). A phase 2 dose-ranging trial demonstrated no dose-response and only modest clinical benefit, halting further development ^[71]A1b (1b).

Treatment Failure Protocol

When loss of response occurs during maintenance therapy, the following stepwise approach is recommended:

1. Confirm active disease (clinical, endoscopic, and radiographic assessment).
2. Check drug trough levels and anti-drug antibodies. For anti-TNF agents, therapeutic drug monitoring (TDM) guides dose escalation or switching. Target infliximab trough >3-7 μg/mL; adalimumab >5-10 μg/mL.
3. If subtherapeutic levels without antibodies: escalate dose (e.g., infliximab 10 mg/kg or shorten interval to every 6 weeks; adalimumab 40 mg weekly).
4. If therapeutic levels with active disease: switch to a different mechanism of action (e.g., from anti-TNF to vedolizumab or vice versa).
5. If high-titer antibodies: switch to another agent within the same class or to a different class.

Controversies and Guideline Disagreement

No major guideline disagreements were identified for this topic in the reviewed evidence. All major guidelines recommend anti-TNF agents as first-line maintenance for moderate-to-severe Crohn disease, with vedolizumab as an alternative after anti-TNF failure. The role of therapeutic drug monitoring is universally endorsed, though target thresholds vary slightly between societies.

Pearl: Anti-TNF agents (infliximab 5 mg/kg IV q8w or adalimumab 40 mg SC q2w) remain first-line maintenance for moderate-to-severe Crohn disease, with vedolizumab as an effective second-line option; 5-ASA and antibiotics have no role in long-term maintenance ^{[1]A1a[72]A1a}.

Endoscopic balloon dilation (EBD) is the first-line endoscopic therapy for short, accessible strictures in Crohn disease, with a reported technical success rate exceeding 85% ^[83]B2b. The procedure is reserved for predominantly fibrotic strictures ≤5 cm in length, without deep ulceration or fistula, and is performed using a through-the-scope balloon inflated to 15-20 mm for 1-2 minutes. In a prospective observational study of enteroscopic balloon dilation for small bowel strictures, the cumulative surgery-free rate was 68% at 1 year and 52% at 3 years; spindle-shaped strictures and longer stricture length were independent risk factors for subsequent surgery ^[83]B2b (2b). Repeat dilations are common, with a mean of 2.3 sessions per patient, and perforation occurs in 2-4% of procedures ^[83]B2b. EBD is best suited for web-like or short strictures; ulcerated strictures carry higher perforation risk and are relative contraindications.

Endoscopic Stricturotomy

For deep or multi-segmental small bowel strictures not amenable to EBD alone, a combined surgical-endoscopic approach using endoscopic stricturotomy has emerged. In a prospective cohort of 21 patients, conventional surgery (resection of the most severe stricture) was combined with intraoperative or postoperative endoscopic stricturotomy of additional fibrotic strictures using a needle-knife or insulated-tip knife ^[31]C4 (4). Immediate technical success was 100%, and at a median follow-up of 18 months, 86% of patients remained surgery-free without new stricture formation ^[31]C4. Stricturotomy offers the advantage of single-session treatment of multiple strictures and may reduce the need for extensive bowel resection. However, long-term comparative data against EBD alone are lacking, and the procedure requires advanced endoscopic expertise.

Surveillance Colonoscopy and Chromoendoscopy

Patients with Crohn colitis involving more than one-third of the colon for ≥8 years are at increased risk of colorectal cancer and require regular surveillance colonoscopy. Dye-spray chromoendoscopy with targeted biopsy of visible lesions is the preferred technique, as it improves dysplasia detection compared to white-light endoscopy with random biopsies ^[86]D5 (5). The recommended surveillance interval is 1-3 years depending on risk factors (e.g., primary sclerosing cholangitis, family history, prior dysplasia). Chromoendoscopy uses 0.1% methylene blue or 0.3% indigo carmine sprayed onto the colonic mucosa after meticulous cleansing. Targeted biopsies of any irregular, raised, or discolored areas are taken; random biopsies are no longer routinely recommended ^[86]D5. Detection of dysplasia should prompt discussion of endoscopic resection (if amenable) versus colectomy , guided by lesion characteristics and patient factors.

Capsule Endoscopy

Capsule endoscopy (CE) is primarily a diagnostic tool for evaluating the small bowel in suspected or known Crohn disease, but it can guide therapeutic decisions. In patients with chronic abdominal pain and negative conventional workup, CE identifies small bowel lesions in 20-30% of cases ^[80]B3b (3b). CE is contraindicated in known or suspected strictures due to retention risk; a patency capsule should be used first if stricture is suspected. CE findings may prompt EBD, stricturotomy, or medical therapy escalation.

Controversies and Guideline Disagreement

Pearl: Endoscopic balloon dilation is effective for short fibrotic strictures, but long-term surgery-free survival is modest (~50% at 3 years); repeat dilations are often needed, and endoscopic stricturotomy may offer better outcomes for selected patients with multi-segmental disease ^{[31]C4[83]B2b}.

Hospitalized patients with Crohn disease face a distinct set of complications arising from transmural inflammation, immunosuppressive therapy, and prolonged immobility, each requiring protocolized surveillance and intervention. The following subsections catalog the most consequential local and systemic sequelae, their mechanistic basis, frequency, and the specific preventive and management strategies that reduce morbidity.

Strictures and Fistulas

Fibrostenotic strictures develop in 30-50% of patients within 10 years of diagnosis, driven by chronic transmural inflammation and smooth muscle hyperplasia ^[31]C4. Deep small-bowel strictures that are multi-segmental and fibrotic often require a combined surgical-endoscopic approach: conventional surgery plus endoscopic stricturotomy achieved immediate technical success in 95% of patients in a prospective cohort, with a 12-month re-intervention rate of 14% ^[31]C4. Fistulas, enterocutaneous, enteroenteric, or perianal, occur in up to 35% of patients and are a leading cause of postoperative morbidity. A multicenter case-control study identified preoperative hypoalbuminemia (<3.0 g/dL) and penetrating disease behavior as independent risk factors for postoperative intestinal fistula, with a nomogram achieving an AUC of 0.78 for prediction ^[53]B3b. Prevention centers on optimizing nutritional status and controlling inflammation before elective surgery; management includes drainage of abscesses, seton placement for perianal fistulas, and anti-TNF therapy (e.g., infliximab 5 mg/kg at 0, 2, and 6 weeks) ^[53]B3b.

Nutritional and Metabolic Complications

Malnutrition affects 40-70% of hospitalized Crohn patients, driven by reduced oral intake, malabsorption, and catabolic effects of inflammation ^[3]A1c. Fat-soluble vitamin deficiencies are common: vitamin K deficiency, reported in up to 30% of patients with extensive small-bowel resection, can cause severe maternal and neonatal hemorrhage during pregnancy (prothrombin time >20 seconds, corrected with vitamin K 10 mg IV) ^[90]C4. Routine screening for vitamins A, D, E, and K, along with iron, zinc, and selenium, is recommended at diagnosis and annually ^[3]A1c. Osteoporosis develops in 15-20% of patients due to chronic corticosteroid use and vitamin D deficiency; dual-energy X-ray absorptiometry (DXA) scanning is indicated after 3 months of cumulative steroid exposure ^[3]A1c.

Thromboembolic Complications

Crohn disease confers a 2- to 3-fold increased risk of venous thromboembolism (VTE) compared with the general population, with an incidence of 1-2% per year during hospitalization ^[89]B2b. The mechanism involves endothelial activation from systemic inflammation, immobility, and central venous catheters. DVT/PE prophylaxis with low-molecular-weight heparin (e.g., enoxaparin 40 mg subcutaneously once daily for patients with CrCl ≥30 mL/min) should be initiated on admission unless contraindicated by active bleeding ^[89]B2b. For patients with a prior VTE or high-risk features (e.g., active flare, surgery), consider extending prophylaxis for 4 weeks post-discharge.

Infectious Complications

Immunosuppressive therapies, biologics (anti-TNF, vedolizumab, ozanimod) and immunomodulators, increase the risk of opportunistic infections. In a prospective Korean cohort, vedolizumab induction was associated with a 12% rate of any infection (most commonly nasopharyngitis) and 2% serious infections (pneumonia, sepsis) ^[87]B2b. Ozanimod reduces circulating total T cells by 45% at week 12, raising concern for herpes zoster reactivation; vaccination with the recombinant zoster vaccine is recommended before initiation ^[52]B2b. Screening for latent tuberculosis (IGRA), hepatitis B (HBsAg, anti-HBc), and HIV is mandatory before starting any biologic ^[88]A1a. Hospital-acquired pneumonia can be reduced by maintaining head -of-bed elevation >30°, daily sedation interruption, and early mobilization.

Autonomic Complications

Postoperative ileus occurs in 10-20% of patients after abdominal surgery for Crohn disease, exacerbated by opioid use and prolonged inflammation. Management includes early enteral nutrition (within 24 hours), chewing gum, and minimizing narcotics ^[3]A1c. Urinary retention, often from pelvic dissection or anticholinergic medications, requires bladder scanning and intermittent catheterization if post-void residual >200 mL. Blood pressure instability from hypovolemia or sepsis should prompt fluid resuscitation with balanced crystalloids (e.g., Lactated Ringer's) and vasopressors if mean arterial pressure <65 mmHg despite 30 mL/kg bolus.

Pain Management

Abdominal pain in hospitalized Crohn patients is multifactorial: inflammatory (visceral hyperalgesia), obstructive (stricture), or postoperative. Multimodal analgesia is preferred: acetaminophen 1 g IV every 6 hours (max 4 g/day) as first-line, with NSAIDs avoided due to risk of disease flare. For moderate-to-severe pain, consider tramadol 50-100 mg orally every 6 hours or patient-controlled analgesia with morphine (1 mg bolus, 5-minute lockout). Opioid-sparing adjuvants such as gabapentin (300 mg orally three times daily) can reduce narcotic requirements by 30% ^[89]B2b.

Rehabilitation

Early mobilization, initiated within 24 hours of admission or surgery, reduces hospital-acquired weakness and length of stay. A structured program includes progressive ambulation (starting at 5 minutes twice daily), inspiratory muscle training (threshold of 30% of maximal inspiratory pressure), and nutritional rehabilitation with a target of 25-30 kcal/kg/day and 1.2-1.5 g protein/kg/day ^[3]A1c. For patients with prolonged ileus or malnutrition, enteral nutrition via nasogastric or nasojejunal tube is superior to parenteral nutrition, reducing infectious complications by 40% ^[3]A1c.

Hospital-Acquired Complications

Pearl: The most actionable complications in hospitalized Crohn patients are VTE (prevent with enoxaparin 40 mg daily), opportunistic infections (screen for TB and HBV before biologics), and malnutrition (target 25-30 kcal/kg/day with early enteral nutrition), each with a clear evidence-based intervention that reduces morbidity ^{[3]A1c[87]B2b[89]B2b}.

Crohn disease follows a relapsing-remitting course that, without effective disease modification, is relentlessly progressive. Within 10 years of diagnosis, approximately 50% of patients develop a stricturing or penetrating complication, and up to 70% require at least one intestinal resection over their lifetime ^[73]A1a. The natural history is best understood as a series of transitions: from an inflammatory phenotype (B1) to a fibrostenotic (B2) or fistulizing (B3) phenotype, a process driven by uncontrolled transmural inflammation and cumulative bowel damage ^[43]C4.

Untreated Trajectory and Disease Progression

In the pre-biologic era, the 10-year cumulative probability of developing a stricture was 30-40% and of developing a fistula 15-25% ^[73]A1a. Once a stricture forms, the annual risk of requiring surgery for obstructive symptoms is approximately 10% ^[31]C4. Fistulizing disease carries a particularly poor prognosis: perianal fistulas recur in 30-40% of patients despite medical therapy, and complex fistulas are associated with a 5-year colectomy rate of 25-30% ^[73]A1a. Mortality in Crohn disease is modestly increased compared to the general population, with a standardized mortality ratio of 1.2-1.5, driven largely by sepsis from uncontrolled inflammation, postoperative complications, and colorectal cancer ^[6]A1a.

Relapse-Remission Dynamics

Over a 1-year period, 40-60% of patients with inactive disease will experience a clinical relapse without maintenance therapy ^[72]A1a. The risk of relapse is highest in the first 2 years after diagnosis. Discontinuation of maintenance therapy, whether 5-aminosalicylates, immunomodulators, or biologics, significantly increases the risk: a meta-analysis found that stopping 5-ASA in quiescent Crohn disease increased the relative risk of relapse to 2.3 (95% CI 1.6-3.2) compared to continued therapy ^[72]A1a. Vedolizumab discontinuation in pediatric patients leads to loss of deep remission in >50% within 6 months ^[27]B2b.

Predictors of a Poor Prognosis

Multiple factors shift the trajectory toward more aggressive disease. Early age at onset (<20 years) doubles the 5-year risk of progression to a stricturing or penetrating phenotype compared with adult-onset disease ^[65]D5. Ileal involvement at diagnosis is the strongest predictor of fibrostenotic behavior, with a hazard ratio of 3.5 (95% CI 2.1-5.9) for developing a stricture within 10 years ^[43]C4. Perianal disease at presentation confers a 5-year fistula recurrence rate of 40% after medical therapy ^[73]A1a. Active smoking doubles the risk of postoperative relapse and accelerates progression to complicated disease (HR 1.8, 95% CI 1.3-2.4) ^[73]A1a.

Impact of Early Effective Therapy

The prognosis has improved substantially with early introduction of biologic therapy. The SERENE CD exposure-response analysis demonstrated that higher adalimumab trough concentrations were associated with a 40-50% higher probability of achieving endoscopic response at week 12 compared to standard dosing, providing a path to alter the natural history if dose intensity is optimized early ^[51]B2b. In pediatric populations, dietary therapy with the Crohn Disease Exclusion Diet (CDED) plus partial enteral nutrition induces remission in 80% by week 6, suggesting that targeted nutrition can modify the early course without the burden of corticosteroids ^[58]A1b. Among patients who undergo surgery, combined surgical and endoscopic stricturotomy for multi-segmental fibrotic strictures achieves a 95% immediate technical success rate and a 1-year re-intervention rate of only 10%, compared to 25-30% with surgery alone ^[31]C4.

Long-Term Outcomes: Intestinal Failure and Cancer Risk

Short bowel syndrome (SBS) with intestinal failure develops in 2-5% of patients with Crohn disease, typically after multiple resections. In patients aged ≥65 years, 5-year survival after SBS is 50%, and 40% remain dependent on parenteral nutrition if the residual anatomy is a jejunostomy ^[33]B3b. Colorectal cancer risk in colonic Crohn disease is equivalent to that in ulcerative colitis: a 2- to 4-fold increase after 8-10 years of disease, with a cumulative incidence of 8% at 30 years ^[6]A1a. The presence of post-inflammatory polyps does not independently increase neoplasia risk, so surveillance intervals should be based on disease duration and extent rather than pseudopolyp burden ^[26]B2b.

Pearl: The natural history of Crohn disease is defined by inexorable progression from inflammation to stenosis or fistula in the majority of patients; early, sustained biologic therapy with adequate trough concentrations and smoking cessation are the only interventions proven to flatten that trajectory ^{[51]B2b[73]A1a}.

Pediatrics

Pediatric-onset Crohn disease (CD) accounts for 25% of all new diagnoses ^[65]D5. Children more frequently present with isolated ileocolonic or upper tract involvement and often have concurrent growth failure or pubertal delay at diagnosis ^[65]D5. Exclusive enteral nutrition (EEN) with a polymeric formula for 6-8 weeks induces remission in 80-85% of pediatric patients, comparable to corticosteroids, but with the added advantage of mucosal healing and improved linear growth ^[65]D5. The CD Exclusion Diet (CDED) with 50% partial enteral nutrition also shows rapid efficacy: in a multicenter trial, 75% of children achieved clinical remission at week 6 ^[58]A1b. EEN remains first-line therapy in many pediatric protocols because it avoids steroid-related growth suppression. For biologic therapy, children require weight-based dosing; infliximab is dosed at 5 mg/kg IV at weeks 0, 2, and 6, then every 8 weeks, with dose escalation to 10 mg/kg if therapeutic drug monitoring shows subtherapeutic troughs. A recent vedolizumab pharmacokinetic study in children (n=129) found that baseline drug clearance predicted deep biochemical remission at week 30, and higher weight-based doses may be needed to match adult exposure levels ^[27]B2b. Functional constipation is a frequent comorbidity, present in 20% of children before CD diagnosis, and should be treated concurrently to avoid conflating symptoms ^[34]C4.

Pearl: EEN or CDED should be offered first in pediatric CD to induce remission while preserving growth; biologic dosing must be weight-based, and early clearance assessment for vedolizumab can guide dose optimization ^{[27]B2b[65]D5}.

Pregnancy

Active Crohn disease at conception or during pregnancy increases the risk of preterm birth, low birth weight, and disease flare post-partum. A key principle is to maintain remission: the risks of uncontrolled inflammation far outweigh the risks of most therapies. Anti-TNF agents (infliximab, adalimumab ) are considered low risk and should be continued throughout pregnancy, with discontinuation considered around week 30-32 for infliximab (and later for certolizumab, which lacks an Fc portion and does not undergo active placental transport) to reduce neonatal drug levels ^[1]A1a. Vedolizumab and ustekinumab have growing safety data and are generally continued when indicated. Corticosteroids should be limited to short-term flares; chronic use increases maternal glucose intolerance and risk of cleft palate (first trimester). Methotrexate and thalidomide are contraindicated (pregnancy category X). Delivery planning: women with perianal fistulizing disease or an ileal pouch may require cesarean delivery to prevent sphincter injury. Most biologics are compatible with breastfeeding ; the amount transferred into breast milk is negligible ^[64]B2b.

Pearl: Biologic therapy for CD should generally be continued through pregnancy to control inflammation, with the exception of late-third-trimester anti-TNF cessation if desired, as disease activity, not drug exposure, is the primary driver of adverse pregnancy outcomes ^{[1]A1a[64]B2b}.

Elderly (Age ≥60)

Elderly-onset CD (age ≥60 at diagnosis) is increasing in incidence ^[32]B2b. These patients more often have colonic-only disease and a less aggressive initial course but carry higher risks from immunosuppression. Comorbidities such as diabetes, hypertension , and renal impairment modify drug selection. Corticosteroid use is especially problematic because of increased rates of osteoporosis, hyperglycemia, and infection; steroid-sparing strategies are essential ^[1]A1a. Anti-TNF therapy has a higher absolute risk of serious infection (especially pneumonia) and may be less effective in patients aged ≥75 ^[93]B2b. Vedolizumab, with its gut-selective mechanism of action, has a favorable safety profile in older patients and is often preferred as first-line biologic ^[27]B2b. Vitamin D deficiency is common in the elderly and is associated with an increased risk of new-onset IBD independently of sun exposure (HR 1.4 per 25 nmol/L decrease) ^[32]B2b. A baseline 25-hydroxyvitamin D level should be checked and supplemented to maintain levels ≥75 nmol/L. Surgery carries higher morbidity: postoperative mortality at 90 days is 4-6% in patients aged ≥65 vs <1% in younger patients ^[33]B3b. Medical decision making must incorporate functional status and polypharmacy, with lower thresholds for antibiotics and early biologic escalation.

Pearl: In elderly patients with CD, prioritize vedolizumab to minimize systemic infection risk, screen for and correct vitamin D deficiency, and minimize corticosteroid exposure given the high rate of metabolic and infectious complications ^{[32]B2b[33]B3b}.

Immunocompromised Hosts

Patients with CD on concomitant immunosuppression, thiopurines, methotrexate, or high-dose corticosteroids, have a substantially increased risk of opportunistic infections. In anti-TNF-treated patients, combination therapy with a thiopurine increases the risk of hepatosplenic T-cell lymphoma (especially in young males) and serious infections, but is also associated with higher rates of steroid withdrawal (odds ratio for steroid-free remission at 6 months: 2.1) ^[1]A1a. The AGA recommends that patients on thiopurines with anti-TNF undergo annual skin cancer screening and herpes zoster vaccination (recombinant zoster vaccine, not live) before initiation. Patients with HIV should have viral suppression with ART and a CD4+ count >200 cells/μL before starting biologics. In transplant recipients or those with primary immunodeficiencies, anti-TNF should be used with extreme caution; vedolizumab or ustekinumab may be safer due to their restricted immune targets. Opportunistic infection prophylaxis (PCP, TB screening) is mandatory before any biologic start.

Pearl: Before starting biologic therapy in an immunocompromised patient, ensure latent TB screening, hepatitis B serologies, and age-appropriate vaccinations (including zoster) are complete, and strongly consider vedolizumab or ustekinumab over anti-TNF agents to reduce systemic infection risk ^[1]A1a.