What are the key features of Crohn's disease?

Key Features of Crohn's Disease

Crohn's disease is a chronic inflammatory disorder characterized by transmural inflammation that can affect any part of the gastrointestinal tract, typically presenting with skip lesions, stricturing or penetrating complications, and perianal involvement. 1

Anatomical Distribution and Location

• Can involve any portion of the GI tract from mouth to anus, distinguishing it fundamentally from ulcerative colitis which is limited to the colon 2, 3
• Terminal ileum is the most commonly affected site, with approximately one-third of patients having small bowel involvement alone, slightly more having colonic disease alone, and less than one-third having combined small bowel and colonic disease 1, 2
• Demonstrates patchy, discontinuous, segmental inflammation with skip lesions—meaning normal bowel segments separate inflamed areas 2, 3
• Rectal sparing is common, which strongly favors Crohn's over ulcerative colitis 1, 3

Pathologic Characteristics

Transmural Inflammation

• Extends through all layers of the intestinal wall (mucosa, submucosa, muscularis propria, and serosa), not just the mucosa as in ulcerative colitis 2, 3
• This full-thickness involvement is the defining pathologic feature that drives development of stricturing and penetrating complications 2
• Granulomatous inflammation is characteristic, though non-caseating granulomas are present in only a minority of biopsy samples 1

Pattern of Inflammation

• Asymmetric involvement is typical, particularly along the mesenteric border, rather than circumferential 2, 3
• Focal crypt architectural irregularity and chronic inflammation that varies in intensity within and between biopsies 1, 2
• Chronic histological changes include architectural distortion, crypt atrophy, increased lamina propria chronic inflammatory cells, and Paneth cell metaplasia 1

Disease Complications and Phenotypes

Three Main Disease Phenotypes

• Inflammatory phenotype: Active mucosal and transmural inflammation without stricturing or penetrating behavior 1
• Stricturing phenotype: Progressive structural damage leads to luminal narrowing with upstream bowel dilation 1
• Penetrating phenotype: Fistulas, sinuses, abscesses, or inflammatory masses 1

Penetrating Complications

• Fistulas represent a hallmark complication, arising from transmural inflammation that creates abnormal connections between bowel loops or to adjacent structures 2, 3
• Approximately one-quarter of perianal fistulas present at or before diagnosis 2, 3
• Fistulas typically arise from within or just proximal to strictures and can be simple or complex 2

Stricture Formation

• Defined as luminal narrowing with unequivocal upstream dilation (upstream lumen >3 cm) 1
• Most Crohn's strictures contain both inflammation and fibrosis 1
• Progressive structural damage occurs over time with recurrent inflammation, even when bowel returns to normal appearance after acute flares 1

Perianal Disease

• Occurs in up to one-third of patients as a distinct manifestation 1
• Can include perianal fistulas, abscesses, and fissures 1

Imaging and Endoscopic Features

Cross-Sectional Imaging Findings

• Asymmetric wall thickening, hyperenhancement, and mural edema (T2-weighted signal on MRI) are specific for active Crohn's disease 1
• Ulcerations appear as focal breaks in the bowel wall surface and correlate with severe endoscopic inflammation 1
• Mural stratification (layered enhancement pattern) can be seen with active inflammation 1
• Perienteric fat stranding and mesenteric lymphadenopathy indicate more severe disease 1

Endoscopic Characteristics

• Skip lesions with intervening normal mucosa 1, 3
• Deep, linear, or serpiginous ulcerations that can lead to a "cobblestone" appearance 1
• Discontinuous distribution between anatomical sites 1

Clinical Course

• Characterized by episodic flares and periods of remission with increasing incidence over recent decades 1
• Active inflammation can exist despite clinical resolution of symptoms, making endoscopy and imaging central tools for disease monitoring 1
• Mucosal healing represents a better treatment target for long-term outcomes than reliance on clinical symptoms alone 1
• Most patients will eventually require surgery, though surgery is not curative and ongoing therapy is needed for disease recurrence 4

Diagnostic Approach

• No single diagnostic test allows unequivocal diagnosis—requires combination of clinical, laboratory, endoscopic, histological, and imaging findings 1
• Complete ileocolonoscopy with biopsies from at least five different sites (at least two from terminal ileum, four colonic segments, and rectum) is essential 1, 5
• Cross-sectional imaging (CT or MR enterography) is complementary to endoscopy, allowing evaluation of disease proximal to the ileum and detection of transmural disease with normal-appearing mucosa 1, 5