What is Crohn's disease?

What is Crohn's Disease?

Crohn's disease is a chronic inflammatory bowel disease characterized by transmural inflammation that can affect any part of the gastrointestinal tract from mouth to anus, most commonly involving the terminal ileum and colon, with a relapsing-remitting course that carries substantial morbidity including high rates of hospitalization and surgery. 1

Pathophysiology and Nature

• Crohn's disease results from an inappropriate immune response to intestinal microbiota in genetically predisposed individuals, involving both innate and adaptive immune dysfunction 2, 3
• The inflammation is transmural (affecting all layers of the bowel wall), distinguishing it from ulcerative colitis which affects only the mucosa 1
• The disease is characterized by discontinuous or "skip" lesions with areas of normal bowel between inflamed segments 1

Clinical Presentation

Cardinal Gastrointestinal Symptoms

• Abdominal pain (typically colicky), chronic diarrhea, and weight loss are the classic triad 4, 5, 6
• Rectal bleeding, fever, anorexia, and fatigue commonly occur 7, 8
• Systemic symptoms (malaise, fever) are more prominent than in ulcerative colitis 1

Disease Location Patterns

• Terminal ileum and colon are most commonly affected sites 4, 5
• Small bowel alone: approximately one-third of patients 5
• Colon alone: affects a higher percentage of patients 5
• Any portion of the GI tract from mouth to anus can be involved 1, 4

Disease Phenotypes

The Montreal Classification categorizes Crohn's disease into distinct patterns 1:

• Inflammatory (non-stricturing, non-penetrating)
• Stricturing (causing intestinal obstruction)
• Penetrating (forming fistulas and abscesses)
• Perianal disease (occurs in 15-33% of patients) 1, 4

Disease Course and Prognosis

Natural History

• The disease follows a relapsing-remitting pattern with approximately 50% of patients experiencing relapse in any given year 4, 5
• Historically, 20% of patients required hospitalization annually 1
• At least 50% require surgical treatment within the first 10 years, and 70-80% will require surgery within their lifetime 4, 5
• Surgery is not curative—disease recurrence is common and ongoing medical therapy remains necessary 6

Complications and Associated Conditions

• Perianal disease including fistulas, abscesses, and perianal involvement affects up to one-third of patients 4
• Intestinal strictures develop from progressive structural damage, potentially causing obstruction 4
• Extraintestinal manifestations occur in nearly half of patients, including arthritis, uveitis, erythema nodosum, osteoporosis 3
• Increased risk of colorectal cancer in patients with Crohn's colitis 4
• Slightly higher mortality than general population, greatest in first 2 years after diagnosis or with upper GI involvement 4

Severity Classification

Moderate to Severe Disease Indicators

According to the International Organization for the Study of Inflammatory Bowel Disease, severe disease features include 1:

Structural/Inflammatory Markers:

• Large or deep mucosal lesions on endoscopy/imaging
• Presence of fistulas and/or perianal abscesses
• Strictures or prior intestinal resections (particularly >40cm)
• Extensive disease (ileal involvement >40cm or pancolitis)
• Elevated C-reactive protein, anemia, low albumin

Symptomatic Criteria:

• At least 10 loose stools daily
• Daily abdominal pain
• Anorectal symptoms (pain, urgency, incontinence, discharge)
• Systemic corticosteroid use within prior year
• Lack of response to prior biologics/immunosuppressants
• Significant impact on activities of daily living

Crohn's Disease Activity Index (CDAI)

• Remission: <150
• Mild to moderate: 150-220
• Moderate to severe: 220-450
• Severe: >450 1

Impact on Quality of Life

• Approximately 75% of patients are fully capable of work in the year after diagnosis 4
• However, 15% may be unable to work after 5-10 years 4
• High incidence of psychological morbidity affects both children and adults 4, 5
• Significant disability burden with substantial impact on daily activities 5

Key Clinical Pitfall

A critical disconnect exists between symptoms and inflammation—patients may have persistent intestinal inflammation without gastrointestinal symptoms, which can lead to progressive bowel damage, fistulae, abscesses, and strictures if not detected 9. This necessitates objective monitoring strategies using biomarkers (fecal calprotectin, CRP) and imaging rather than relying solely on symptom assessment 9.