What is Crohn's disease?

What is Crohn's Disease?

Crohn's disease is a chronic inflammatory bowel disease characterized by patchy, transmural inflammation that can affect any part of the gastrointestinal tract from mouth to anus, most commonly involving the terminal ileum and colon. 1

Pathophysiology and Disease Characteristics

Crohn's disease results from an inappropriate inflammatory response to intestinal flora in genetically susceptible individuals exposed to environmental triggers. 1 The hallmark pathologic feature is transmural granulomatous inflammation with discontinuous "skip lesions"—areas of inflamed tissue interspersed between normal-appearing mucosa. 1

The disease differs fundamentally from ulcerative colitis in several ways:

• Inflammation extends through the full thickness of the bowel wall (transmural), not just the mucosa 1
• Can affect any segment of the GI tract, whereas ulcerative colitis is limited to the colon 1
• Characterized by patchy distribution with skip lesions rather than continuous inflammation 1
• Rectal sparing is common, unlike ulcerative colitis 1

Disease Location and Patterns

The disease can be classified by anatomic location: 1

• Small bowel alone: approximately one-third of patients 1, 2
• Colon alone: slightly higher percentage 1, 2
• Combined ileocolonic involvement: less than one-third 1
• Upper gastrointestinal tract: less common 1

Disease patterns include: 1

• Inflammatory (non-stricturing, non-penetrating)
• Stricturing (causing intestinal obstruction)
• Penetrating/fistulizing (forming abnormal connections between organs)
• Perianal disease (affecting 15-25% of pediatric patients and up to one-third of adults) 1, 3

Clinical Presentation

Cardinal Symptoms

The typical presentation involves: 3, 2, 4

• Abdominal pain (colicky, often daily during flares) 3, 4
• Chronic diarrhea (at least 10 loose stools daily in severe disease) 4
• Weight loss 2, 4
• Fatigue 5

Systemic Manifestations

Systemic symptoms are more prominent in Crohn's disease than ulcerative colitis: 3, 4

• Fever 4
• Malaise and anorexia 4

Anorectal Complications

Perianal manifestations include fistulas, abscesses, pain, bowel urgency, fecal incontinence, and perianal discharge. 3, 4

Epidemiology

• Incidence: 5-10 per 100,000 per year 1
• Prevalence: 50-100 per 100,000 (likely underestimated) 1
• Peak age of onset: 10-40 years, though 15% are diagnosed after age 60 1
• 25% of all IBD patients are diagnosed before age 20 1
• Incidence may be increasing globally 1, 5
• Marked ethnic variation, with higher rates in Ashkenazi Jews 1

Disease Course and Natural History

The disease follows a chronic relapsing-remitting pattern with periods of exacerbation and remission. 3, 2 Key prognostic features include:

• Approximately 50% of patients experience relapse in any given year 3, 2
• At least 50% require surgical intervention within the first 10 years 3, 2
• 70-80% will require surgery within their lifetime 2
• Surgery is not curative—disease recurrence is common 2
• Progressive structural damage occurs over time despite treatment 1

Complications and Long-Term Impact

Structural Complications

Recurrent inflammation leads to: 1, 3

• Stricture formation causing intestinal obstruction 3
• Penetrating disease with fistulas and sinuses 1
• Abscess formation 4

Quality of Life Impact

• 75% of patients are fully capable of work in the year after diagnosis 3
• 15% may be unable to work after 5-10 years 3
• High incidence of psychological morbidity in both children and adults 3, 2
• Significant impact on activities of daily living during moderate-to-severe disease 1

Mortality and Cancer Risk

• Slightly higher mortality than the general population, greatest in the first 2 years after diagnosis or with upper GI involvement 3
• Increased risk of colorectal cancer in patients with Crohn's colitis 3

Diagnosis

Diagnosis requires a combination of clinical, laboratory, endoscopic, histological, and imaging findings—no single test is definitive. 1, 2

Key Diagnostic Features

• Ileocolonoscopy with biopsy is the primary diagnostic tool 2
• Multiple biopsies from six segments (terminal ileum, ascending, transverse, descending, sigmoid, rectum) should be obtained 1
• Granulomas on histology are pathognomonic but not always present 1
• Cross-sectional imaging (MR enterography or CT enterography) assesses small bowel and extraluminal disease 1, 2
• Fecal calprotectin has high sensitivity and specificity for distinguishing IBD from irritable bowel syndrome 2

Laboratory Markers

Inflammatory markers during active disease include: 4

• Elevated C-reactive protein (CRP) 4
• Low albumin 4
• Anemia 4

Disease Severity Classification

The Crohn's Disease Activity Index (CDAI) defines severity: 1, 4

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

The International Organization for the Study of Inflammatory Bowel Disease characterizes severe disease based on high risk for complications including surgery, hospitalization, and disability. 1 Indicators include large or deep mucosal lesions, fistulas, strictures, prior resections >40cm, extensive disease, anemia, elevated CRP, low albumin, and systemic corticosteroid use within the prior year. 1

Critical Clinical Pitfalls

Active inflammation frequently persists without gastrointestinal symptoms, leading to progressive bowel damage even when patients feel well. 4 This underscores the importance of objective monitoring beyond symptom assessment.

Intestinal obstruction from strictures may mimic an inflammatory flare but requires different management—potentially surgical rather than immunosuppressive therapy. 4

Abscess formation presents with fever and abdominal pain but needs drainage rather than immunosuppression alone. 4

In endemic areas, differentiating intestinal tuberculosis from Crohn's disease can be challenging—features suggesting tuberculosis include night sweats, concomitant pulmonary TB, and abdominal lymphadenopathy. 2