• Clinical science

Crohn disease


Crohn disease (CD) is an inflammatory bowel disease (IBD), the pathogenesis of which is not fully understood. The clinical presentation of CD may be similar to ulcerative colitis (UC), the other most common IBD. CD mostly affects young adults and adolescents between the ages of 15 and 35. It typically affects the terminal ileum, but can discontinuously affect the entire gastrointestinal tract and commonly leads to complications such as fistulas, abscesses, and stenosis. Clinical features include diarrhea, weight loss, and abdominal pain in the right lower quadrant (RLQ), as well as extraintestinal manifestations in the eyes, joints, or skin. Diagnosis is based on the patient's medical history, physical examination, lab tests, imaging (e.g., MRI), endoscopy, and serological testing. Acute episodes are treated with corticosteroids; immunosuppressants may be indicated in severe cases. Antibiotics and surgical intervention may be needed to help treat complications. As Crohn disease is not localized to a specific region of the GI tract, surgical resection is not a curative option (unlike in UC), and treatment instead focuses on limiting the progression and recurrence of inflammatory episodes.


  • Prevalence: 200 cases per 100,000 population
  • Incidence: ∼ 6 cases per 100,000 population per year [1]
  • Sex: =
  • Typical age of onset: bimodal distribution with one peak at 15–35 years and another one at 55–70 years [2][3]
  • Populations with higher prevalence [4]
    • Individuals of Northern European descent
    • Individuals of Ashkenazi Jewish descent

Epidemiological data refers to the US, unless otherwise specified.


  • Cause: Immune dysregulation and dysbiosis, which promotes chronic inflammation, the ultimate cause of which is not fully understood.
  • Risk factors [4]
    • Active and passive smoking of tobacco
    • Familial aggregation
    • Genetic predisposition (e.g., mutation of the NOD2 gene, HLA-B27 association)

Nicotine consumption is the only (known) controllable risk factor for Crohn disease. Therefore, smoking cessation is especially important in patients with CD.

Crohn disease activity index (CDAI)

  • Definition: A validated score used to assess disease activity in Crohn disease calculated using the following variables, assessed over the course of one week:
  • Interpretation
    • 0–149: asymptomatic remission
    • 150–220: low to moderate activity
    • 221–450: moderate to high activity
    • 451–1100: high activity, fulminant disease



Inflammation is most likely caused by immune dysregulation.

  • Dysregulation of IL-23-Th17 signaling → unrestrained Th17 cell function → inflammation local tissue damage (edema, erosions/ulcers, necrosis) → obstruction, fibrotic scarring, stricture, and strangulation of the bowel [5]
  • There is evidence that mutations in the nucleotide oligomerization binding domain 2 (NOD2) protein are involved in the development of Crohn disease, but the exact mechanism is not fully understood. [5]
    • Loss of function mutations in NOD2 ↑ susceptibility for bacterial invasion of the intestinal mucosa unregulated inflammation
    • Dysfunctional NOD2 overactivity of NF-κB signaling pathway → ↑ production of proinflammatory cytokines and antimicrobial peptides chronic autoinflammation

Abscess and fistula formation

Intestinal aphthous ulcers transmural fissures and inflammation of the intestinal walls → adherence of other organs or the skin penetration → microperforation and abscess formation → macroperforation into these structures → fistula formation

Clinical features

CD typically occurs episodically with a 30%-risk of recurring inflammation over the span of one year. If symptoms persist for six months, the disease is considered chronic. Without treatment, relapses and complications are to be expected.

Constitutional symptoms [6]

  • Low-grade fever
  • Weight loss
  • Fatigue

Intestinal symptoms [6]

CD most commonly affects the terminal ileum and colon, but involvement of any part of the GI tract (from mouth to anus, except rectum) is possible.

Perianal fistulas and abscesses are often the first signs of Crohn disease.

Extraintestinal symptoms [8]



Diagnosing CD requires the integration of clinical presentation, laboratory tests, and endoscopic, histologic, pathologic and radiologic findings.

  1. If a patient presents with symptoms suggestive of CD, conduct blood tests and stool tests (see “Laboratory tests” below) to rule out other possible causes for bowel inflammation/GI symptoms. [9]
  2. Confirm diagnosis with endoscopy and/or radiographic imaging and/or biopsy.
  3. Perform contrast radiological studies and/or ultrasonography to assess extent, severity, and complications (e.g., abscesses, fistulas, and stenoses)

Laboratory tests

Blood [9]


Imaging [13][14]

  • Plain x-ray abdomen: may show bowel distention or pneumoperitoneum
  • Upper GI series with barium swallow and small bowel follow-through (enteroclysis): used to detect fistulas or stenoses, characteristic findings are:
    • Procedure
      • Water-soluble contrast medium is inserted into the small intestine via a nasopharyngeal tube.
      • Multiple x-rays are taken in a chronological sequence to evaluate each section.
    • Findings
  • Ultrasound findings
  • MR enterography: noninvasive, highly sensitive and specific imaging technique that involves the visualization of an oral contrast medium on MRI and is used in the diagnosis of IBD.
    • Used to assess the extent and pattern of intestinal inflammation, detect perianal and pelvic disease, and to predict disease activity
    • Characteristic findings are an edematous thickening of the intestinal wall and enlarged lymph nodes.
    • Can be done as invasive MRI enteroclysis, during which contrast medium is applied via nasoduodenal tube and the small bowel is distended via an electric infusion pump.

Endoscopy [16]

Endoscopy confirms the diagnosis, assesses the extent of the disease, differentiates CD from other diseases (e.g., ulcerative colitis, peptic ulcers, etc.), and may also be used as a therapeutic tool (e.g., dilatation of ducts, intestinal loops).

  • Ileocolonoscopy: endoscopic examination of the rectum, colon, and terminal ileum that allows for direct visualization of the intestinal mucosa and sampling of tissue
    • Procedure: ileocolonoscopy with biopsies at various locations throughout the terminal ileum, colon, and rectum
    • Characteristic macroscopic findings
      • Segmental/discontinuous pattern of involvement
      • Snail trails: longitudinal ulcerations
      • Pinpoint lesions: small, aphthous hemorrhagic mucosal defects
      • Cobblestone sign: inflamed sections followed by deep ulcerations that resemble cobblestones
      • Erythema and transmural inflammation (all mucosal layers of the intestinal wall are involved)
      • Fissures, fistulas
  • Esophagogastroduodenoscopy
  • Video capsule endoscopy [13]


Differential diagnoses

Crohn disease and ulcerative colitis

Crohn disease

Ulcerative colitis

  • Mediated by dysfunctional IL-23-Th17 signaling
Frequency/type of defecation
  • Increased
  • Typically nonbloody, watery diarrhea
  • May be bloody in more severe cases
Nutritional status
  • Mostly normal, but weight loss and malnutrition may occur in severe disease [18]
Physical examination
  • Mostly constant pain in RLQ
  • Palpable abdominal mass
  • Low-grade fever
Extraintestinal manifestations
  • Rare
Other complications
  • Abscess
  • Strictures (obstructions)
  • Perianal fissures
Cancer risk
Endoscopy and imaging
  • Typical location: terminal ileum and colon with rectal sparing
  • May affect the entire GI tract
Pattern of inflammation
  • Continuous
Typical diagnostic findings
  • Noncurative surgery may become necessary to alleviate symptoms
  • Curative surgery possible (proctocolectomy)

To remember the most important features of Crohn disease (creeping fat, granuloma, skip lesions, rectal sparing, cobblestone sign), think: The crone and the fat granny skipped over the wrecked cobblestones.

Other differential diagnoses

The differential diagnoses listed here are not exhaustive.


Approach [13]

  • Therapy of CD is based on the following steps:
    1. Treating acute disease
    2. Inducing clinical remission
    3. Maintaining response/remission
  • Patients should be stratified according to their specific prognostic risk factors.
  • For optimal results, therapy should be as individually tailored as possible.
  • Disease activity should be monitored regularly based on objective markers.
  • Patients should be motivated to engage in lifestyle modifications (see below).


Treatment of Crohn disease can be approached in two different ways: step-up therapy and top-down therapy.

Overview of pharmacotherapy for Crohn disease [13][20]


Substance class


Symptomatic treatment
  • Topical 5‑aminosalicylic acid derivatives (5-ASAs) (e.g., suppository, foam, enema)
Acute episode

Mild-to-moderate disease

Moderate-to-severe disease

  • Steroid-sparing: thiopurine analogs
  • Alternatively
    • Anti-p40 antibodies
    • Alpha 4 integrin inhibitors
Severe/fulminant disease
Steroid-refractory disease
  • First‑line: TNF-α antibodies, if necessary in combination with thiopurine analogs
Maintenance therapy

Corticosteroids should not be used for long-term maintenance therapy!



  • Minimally-invasive resection of affected and nonfunctional intestinal loops while preserving as much intestinal length and function as possible
  • Indicated when medical therapy fails or patient develops severe complications (e.g., obstruction, stricture, abscess)


  • Balloon dilatation: to treat intestinal stenosis
  • Percutaneous drainage: prevents retention of secretions and abscessation
  • Surgical drainage: when application of percutaneous drainage fails
  • Strictureplasty
    • A surgical procedure that opens up a bowel stricture without having to resect the bowel (bowel-sparing technique)
    • Indicated after multiple resections
  • Limited resection (e.g., proctocolectomy): in case of obstructions or strictures

Crohn disease is mainly treated with medication, but surgical interventions may be required to treat complications or if medical therapy fails.

Surgical intervention alone cannot cure Crohn disease and should therefore be considered as a last resort to avoid complications in which significant amounts of bowel are lost (e.g., short bowel syndrome)!

Additional considerations

Lifestyle modifications

  • Smoking cessation
  • Avoiding certain drugs (e.g., NSAIDs )
  • Minimizing stress

Management of complications and comorbidities

Prevention of malignancies

  • Individuals with CD have an increased risk of cancer (especially of the small intestine, colon , and lymphatics).
  • Regular colonoscopies should be performed to minimize risk.
  • People without major colonic involvement are managed according to the general screening guidelines (see ”Screening for colorectal cancer”)
  • Nutrition
    • Enteral nutrition always take preference over parenteral
    • If oral food intake is not possible, nasogastric or nasoenteric tube should be used
    • Secondary lactose intolerance (approx. 30% of cases): lactose-free diet [22]
    • During acute episodes: avoid dietary fiber


Intestinal complications

Intestinal fistulas and abscesses [23]

  • Overview
  • Etiology
  • Pathophysiology [24]
    • Inflammation epithelial defects → epithelial-mesenchymal transition → deeper penetration of cell layers by epithelial cells → tissue damage that organizes as tubular structures that ultimately connect to other organs or the surface
    • After surgery or percutaneous drainage: deficient anastomoses/sutures or improper healing following intervention (e.g., due to reduced organ blood flow) → leakage of intestinal contents → local infection → abscess formation and/or erosion fistula formation
  • Clinical features: depend on location of the fistula
    • Enterovesical/colovesical fistula pneumaturia; (passing of urine together with air)/fecaluria; (passing of stool together with urine) → recurrent urinary tract infections (UTIs)
    • Enterocutaneous fistula drainage of intestinal content through the skin
    • Gastrocolic fistula abdominal pain, weight loss, foul‑smelling (feculent) belching

Systemic complications

We list the most important complications. The selection is not exhaustive.


  • CD is a chronic disease that is currently not curable.
  • Life expectancy is normal with proper treatment. [25]
  • 70–90% of all patients will require surgery at some point during their lifetime. [26]
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last updated 11/04/2020
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