Crohn's Disease (CD)

SUMMARY

Crohn's disease (CD) is a chronic, relapsing, transmural inflammatory bowel disease (IBD) that can affect any part of the GI tract from mouth to anus, most commonly the terminal ileum. It is defined by segmental ("skip") lesions and full-thickness inflammation that progressively damages the bowel, producing fibrosis, strictures, fistulas, abscesses, and perforation.

• Etiology (Rule of 3):
  1. genetic susceptibility (NOD2/CARD15) + 2. environmental trigger (smoking — the strongest modifiable factor) + 3. a dysregulated Th1/Th17 immune response driven by TNF-α.
• Hallmarks: skip lesions, transmural inflammation, cobblestone mucosa, creeping fat, and noncaseating granulomas.
• Presentation: chronic (usually non-bloody) diarrhea, RLQ crampy pain, weight loss, and perianal disease.
• Diagnosis: ileocolonoscopy with biopsy (first-line) + CT/MR enterography for transmural disease and complications.
• Management: step-up medical therapy to induce and maintain remission; surgery treats complications but is not curative.

Definition

Definition

Crohn's disease is a chronic, relapsing, transmural inflammatory bowel disease that may involve any segment of the GI tract from mouth to anus.
It is characterized by:

• Segmental ("skip") lesions
• Transmural inflammation
• Progressive bowel damage leading to fibrosis, strictures, fistulas, abscesses, and perforation.

Key point: unlike ulcerative colitis, Crohn's disease is not cured by surgery because inflammation may recur in previously normal bowel segments.

Epidemiology

Epidemiology

Etiology & Pathophysiology

Crohn's disease results from the interaction of three forces — a useful "Rule of 3" for etiology and risk factors:

1. Genetic susceptibility. Over 200 loci are implicated, but the highest-yield gene is NOD2/CARD15 (with ATG16L1 and IL23R). These regulate bacterial recognition, autophagy, and innate immunity; loss of function → defective clearance of luminal bacteria and persistent mucosal immune activation.
2. Environmental trigger. Smoking is the single most important modifiable factor — it worsens disease severity and drives post-operative recurrence. (Note the paradox: smoking harms Crohn's but is protective in ulcerative colitis.)
3. Dysregulated immune response. A predominantly Th1 / Th17 reaction sustains chronic inflammation, with TNF-α as the pivotal effector cytokine (alongside IL-12 and IL-23). This axis drives macrophage activation, granuloma formation, tissue destruction, and fibrosis — and is the direct target of biologic therapy.

Two downstream features define the phenotype:

• Transmural inflammation — involvement of the full bowel-wall thickness produces deep linear ulcers, fissures, fibrosis, muscular hypertrophy, strictures, fistulas, abscesses, and perforation.
• Skip lesions — inflamed segments alternate with normal bowel, a hallmark that distinguishes CD from the continuous involvement of ulcerative colitis.

Pathological Hallmarks (Gross & Histology)

Gross pathology evolves along a characteristic sequence: Aphthous Ulcer → deep longitudinal ulcers → Cobblestone Mucosa → Bowel wall thickening → creeping fat → fibrotic strictures.

Creeping fat — mesenteric fat wrapping around the inflamed bowel — is highly characteristic of Crohn's disease.

Histology reflects the transmural, granulomatous nature of the disease:

• Transmural inflammation with lymphoid aggregates
• Fissuring ulcers, fibrosis, and neural hypertrophy
• Noncaseating granulomas — present in only 25–50% of cases

Exam pearl: the absence of granulomas does not exclude Crohn's disease. For the full quick-reference, see the Crohn disease feature summary covering skip lesions, transmural inflammation, cobblestoning, creeping fat, fistulae, and strictures.

Clinical Presentation, Complications & Extraintestinal Manifestations

Presentation depends on disease location and complications.

• Intestinal symptoms:
  - chronic diarrhea (usually non-bloody; may be bloody with colonic disease),
  - crampy RLQ pain (often postprandial),
  - weight loss (malabsorption + reduced intake + increased metabolic demand), fatigue, and fever.
• Obstructive symptoms (from fibrotic strictures): colicky pain, nausea, vomiting, and distention.
• Perianal disease (up to one-third of patients): skin tags, fissures, perianal fistulas, and abscesses. Perianal disease strongly suggests Crohn's over ulcerative colitis.

Fistulas: Transmural inflammation predisposes to enteroenteric, enterovesical, enterovaginal, enterocutaneous, and perianal fistulas.
Clues include pneumaturia, fecaluria, recurrent UTIs, and persistent cutaneous drainage.

Complications:

Intestinal: strictures, small-bowel obstruction, perforation, intra-abdominal abscess, fistulas, malnutrition, and short bowel syndrome after repeated resections.

Malignancy: increased risk of small-bowel adenocarcinoma and colorectal cancer (with long-standing colonic involvement).

Diagnosis & Differential (CD vs UC)

Diagnosis combines laboratory, endoscopic, imaging, and histologic data.

• Serum: elevated CRP and ESR, anemia, leukocytosis, hypoalbuminemia; vitamin B12 deficiency (terminal ileal disease) and iron deficiency.
• Stool: elevated fecal calprotectin and lactoferrin (marker of mucosal inflammation; rule out infection).
• Endoscopy — first-line: ileocolonoscopy with biopsy shows aphthous and longitudinal ulcers, cobblestoning, skip lesions, and frequent rectal sparing. Biopsy both affected and normal-appearing mucosa.
• Imaging — preferred cross-sectional study: CT or MR enterography reveals bowel-wall thickening, mucosal hyperenhancement, mesenteric fat stranding, fistulas, abscesses, and strictures.
• Histology: transmural inflammation, lymphoid aggregates, fissuring ulcers, and noncaseating granulomas (when present).

Differential diagnosis: Crohn Disease vs Ulcerative Colitis

The single most tested differential is CD versus ulcerative colitis. The following is a Medical Comparison of the discriminating features:

See the Crohn disease vs ulcerative colitis comparison for the full breakdown across anatomy, microscopy, gross findings, clinical features, and complications.

Management & Prognosis

Goals: induce remission, maintain remission, prevent complications, avoid surgery where possible, and improve quality of life.

Medical therapy — step-up progression

Step 1 — Mild disease:

• budesonide (oral controlled-ileal-release 9 mg daily) is preferred for localized ileocecal disease because of high first-pass metabolism and few systemic effects.
• Aminosalicylates (5-ASA/mesalamine) have a limited role in Crohn's and are not recommended for induction or maintenance in current guidance.

Step 2 — Moderate–severe disease:

• Systemic corticosteroids (e.g., prednisone ~40 mg/day, then taper) to induce remission — never for long-term maintenance.
• Add a steroid-sparing immunomodulator for maintenance: azathioprine or 6-mercaptopurine, or methotrexate.

Step 3 — Biologics (moderate–severe or fistulizing disease):

• anti-TNF agents infliximab (5 mg/kg IV at weeks 0, 2, 6 then every 8 weeks) or adalimumab; anti-integrin vedolizumab; or anti-IL-12/23 ustekinumab.

Antibiotics (metronidazole, ciprofloxacin) are reserved mainly for perianal disease and intra-abdominal abscess.

Surgical therapy — decision framework

Principle: surgery treats complications but does not cure Crohn's disease; bowel preservation is the central goal because recurrence is common.

• Obstruction (most common indication) — usually a fibrotic stricture.
• Perforation — emergency surgery.
• Abscess — percutaneous drainage if feasible, delayed resection if needed.
• Fistula — operate if symptomatic, medically refractory, or associated with sepsis.
• Massive hemorrhage, and cancer/dysplasia (oncologic resection).
• Failure of optimized medical therapy.

Procedures:

• Ileocecal resection — most common operation; indicated for terminal ileal disease.
• Segmental small-bowel resection — for localized disease and fibrotic strictures.
• Stricturoplasty — preferred for multiple short fibrotic strictures in patients at risk for short bowel syndrome because it preserves bowel length; contraindicated with perforation, phlegmon, cancer, or severe active inflammation.
• Percutaneous abscess drainage — often performed before definitive surgical resection.
• Diversion/stoma — reserved for severe perianal disease, sepsis, or complex fistulas.

Post-operative recurrence & prognosis

Recurrence is common — endoscopic recurrence often within 1 year and clinical recurrence within 5 years.

Risk factors: smoking, penetrating disease, young age at diagnosis, and multiple prior resections. 

Approximately 70–80% of patients require surgery during their lifetime, and many need more than one operation.

Poor-prognosis features include young age at onset, extensive small-bowel involvement, perianal disease, penetrating phenotype, smoking, and early corticosteroid need. Modern biologic therapy and multidisciplinary care improve symptom control and reduce steroid exposure and hospitalization, but lifelong surveillance is usually required.

Key Points for Exams

High-yield surgical pearls — نقاط مهمة للامتحانات:

• The terminal ileum is the most common site of Crohn's disease.
• Transmural inflammation explains fistulas, abscesses, strictures, and perforation.
• Skip lesions, cobblestoning, creeping fat, and longitudinal ulcers are the classic gross findings.
• Noncaseating granulomas support the diagnosis but are absent in many patients (present in only 25–50%).
• CT or MR enterography is preferred for evaluating transmural disease and complications.
• Medical therapy induces and maintains remission; corticosteroids are for induction, not maintenance.
• Surgery is indicated for complications — obstruction, perforation, abscess, fistula, hemorrhage, dysplasia/cancer, or refractory disease.
• Ileocecal resection is the most common operation.
• Stricturoplasty preserves bowel length and is useful for multiple fibrotic strictures.
• Smoking is the strongest modifiable risk factor for disease progression and post-operative recurrence.
• Crohn's disease recurs after surgery, making bowel preservation a central surgical principle.