Crohn disease

Crohn disease, also known as regional enteritis, is an idiopathic inflammatory bowel disease characterized by widespread discontinuous gastrointestinal tract inflammation. The terminal ileum and proximal colon are most often affected. Extraintestinal disease is common.

The diagnosis is typically made between the ages of 15 and 25 years with no gender predilection 5. There is a familial component and incidence varies geographically.

Patients typically present with chronic diarrhea and recurrent abdominal pain, although occasionally the presentation is with a complication or an extraintestinal manifestation. Anemia may be present and C-reactive protein may be elevated 29.

Fecal calprotectin has been increasingly used in recent years to:

Crohn disease remains idiopathic, although infective agents have been gaining in popularity as a possible cause, including the measles virus and atypical mycobacterium. As there are definite genetic factors at play, multiple factors are likely to contribute 1. Incidence is higher in people with first degree relatives having IBD, reaching up to 10%. Also, there has been shown 30-50% chance of developing the disease in mono- or heterozygous twins.

Initially, the disease is limited to the mucosa with neutrophilic cryptitis and lymphoid hyperplasia, lymphedema and shallow aphthoid ulceration. As the disease progresses, the entire bowel wall becomes involved, with linear longitudinal and circumferential ulcers extending deep into the bowel wall, predisposing to fistulae. Inflammation also extends into the mesentery and over time leads to chronic fibrotic change, and stricture formation 5.

Inflammation can occur anywhere along the digestive tract, including the mouth and esophagus:

  • mucogingivitis, mucosal tags, deep ulcerations, cobblestoning, lip swelling and pyostomatitis vegetans, esophageal ulcers and strictures

Extraintestinal manifestations include 3,15-17:

The characteristic of Crohn disease is the presence of skip lesions and discrete ulcers. The frequency with which various parts of the gastrointestinal tract are affected varies widely 5:

  • small bowel: 70-80% 5,6; the terminal ileum is usually affected first 33
  • small and large bowel: 50%
  • large bowel only: 15-20%

The choice of investigation modality depends on local expertise and availability. CT and MR enterography are similar in sensitivity for active inflammation (89% vs 83% respectively) and both are better than small bowel follow-through (67-72%) 6. The lack of ionizing radiation from MRI would make it a better option, however, the availability of MRI is limited in many countries.  

Ultrasound, which has been largely applied by gastroenterologists, is also an option for diagnosing active disease, following-up treatment response, and assessing complications 20. Reported sensitivity of 75-94% and specificity 67-100% 20.

Features on barium small bowel follow-through include:

  • mucosal ulcers
  • widely separated loops of bowel due to fibrofatty proliferation (creeping fat) 2
  • thickened folds due to edema
  • pseudodiverticula/pseudosacculation formation: due to contraction at the site of ulcer with ballooning of the opposite (usually antimesenteric) site
  • string sign: tubular narrowing due to spasm or stricture depending on the chronicity
  • partial obstruction
  • on control films presence of gallstones, renal oxalate stones, and sacroiliac joint or lumbosacral spine changes should be sought

Ultrasound has a limited role, but due to it being cheap, available, and not involving ionizing radiation, it has been evaluated as an initial screening tool for active disease and also for follow-up and to assess complications 4,20. Features which may be discovered during transabdominal sonography include 39:

  • small bowel wall thickening (>3-4 mm) 
    • needs to be interpreted in the context of pretest probability
    • affected loops often non-compressible and are difficult to displace with transducer pressure
  • affected segments lose peristaltic activity 38
  • loss of mural stratification
    • the gut signature is characteristic of small bowel
    • chronic disease activity may result in collagen deposition within the wall, which imitates the appearance of the normal submucosa
  • bowel wall hyperemia
  • hyperechoic, circumferential layer external to the bowel wall
    • represents fibrofatty proliferation, thought to represent active inflammation
    • interruption by hypoechoic mesenteric streaks imply a greater degree of inflammation
  • mesenteric lymphadenopathy
    • three or more mesenteric lymph nodes enlarged, measured over 4 mm (width) or 8 mm (length)
  • free intraperitoneal fluid
    • thought to be secondary to transmural inflammation

On pulsed-wave Doppler evaluation, increased superior mesenteric artery (SMA) flow volume and decreased SMA resistive index (SMA RI) also correlate with disease activity. Successful treatment may result in the normalization of these imaging parameters 12.

Complications may also be assessed with sonography, with the following features commonly detected in the following 37:

  • abscess formation
  • fistula creation
    • linear, anechoic tract extending from an affected bowel loop to another structure, often with scattered hyperechoic puncta with inhomogeneous posterior acoustic shadowing (air)
    • diameter of the fistulous tract should be <2 cm 41
  • luminal stenosis
    • segmental wall thickening, loss of peristalsis, luminal narrowing or obliteration, preceding segment dilated (>2.5 cm) 

CT is commonly the first imaging assessment of those patients in the setting of acute abdomen, or it can be also applied to the reassessment of complications in patients with known Crohn disease. Common features include:

  • mural hyperenhancement
  • fat halo sign: submucosal fat deposition
  • bowel wall thickening (1-2 cm), which is most frequently seen in the terminal ileum (present in up to 83% of patients) 8
  • comb sign: engorgement of the vasa recta
  • perienteric fat stranding
  • affected bowel loops separated by focal/regionally increased fat (fibrofatty proliferation; creeping fat)
  • strictures and fistulae, with upstream dilatation
  • mesenteric/intra-abdominal abscess or phlegmon formation 8
    • abscesses are eventually seen in 15-20% of patients 8

CT is also able to give valuable information on:

  • perianal disease
  • hepatobiliary disease

CT enterography is superior to standard CT studies when assessing for small bowel Crohn disease and is equivalent to MR enterography, the downside compared to the later is the ionizing radiation. CT enteroclysis may be attempted in select patients. For the imaging findings and descriptors in CT enterography, see the discussion below under MR enterography. 

MR enterography has become an increasingly important part of the management of patients with Crohn disease. MR enteroclysis may be attempted in select patients.

Features of small bowel Crohn disease to be assessed on dedicated MRI study include: 

  • segmental mural hyperenhancement
    • asymmetric
      • specific sign for Crohn disease
      • tends to more frequently involve the mesenteric border of a small bowel loop
    • stratified: layers of enhancement within the bowel wall
      • bilaminar: hyperenhancement of the inner layer
        • please note that the term “mucosal hyperenhancement” is discouraged given the mucosa itself is absent in the Crohn inflamed bowel segments 43
      • trilaminar: hyperenhancement of both the inner and outer layers of the small bowel wall
        • more frequently seen on MR enterography (cf. CT enterography)
        • it can be explained by a combination of intramural submucosal edema, granulation tissue, intramural fat deposition (fat halo sign on CT), or fibrosis
    • homogeneous or transmural hyperenhancement: involving the entire small bowel wall
      • less specific and, depending on distribution, other differentials should apply
  • DWI: restrict diffusion is seen within the small bowel wall segments of active inflammation 43
  • wall thickening 22,43: must be assessed in a loop adequately dissented by fluid
    • mild 3-5 mm
    • moderate >5-9 mm
    • severe > 9mm
    • please note that focal thickening over 15 mm is atypical for Crohn, and consideration for malignancy should be sought 43
  • intramural edema
    • better assessed on MRI T2 fat-saturated or low b-value DWI sequences
    • increased T2 signal in the thickened bowel wall is particularly helpful in evaluating for acute inflammation 25
  • stricture: focal narrow of the lumen with immediate upstream bowel dilatation >=3 cm. Bear in mind that sometimes the upstream loop is not dilated by size criteria due to decompression due to fistula/penetrating disease or limited bowel content inflow in the setting of two or more consecutive stricture segments 43
    • highly suspected stricture should be reported in persistent focal luminal narrowing across multiple MRI sequences
    • although they can be divided as fibrotic or inflamed strictures, the majority will have a component of active inflammation
    • location and length of the stricture should be reported
    • strong association of strictures and penetrating disease, so one should prompt the evaluation for the other
    • stricture vs obstruction: upstream dilatation over 4 cm is advised to be reported as small bowel obstruction 43
  • ulcerations: focal defect at the intraluminal aspect of the small bowel wall, confined within the serosa (cf. sinus tract)
  • decreased peristalsis: coronal cine MRI sequences (bSSFP) can also be useful in diagnosis as inflamed loops of bowel frequently demonstrate decreased peristalsis
  • pseudodiverticula/pseudosacculation formation: broadband outpouching along the antimesenteric border  

Mesenteric signs of inflammation include:

  • engorged vasa recta: enlarged mesenteric blood vessels related to an inflamed bowel loop (comb sign)
  • fibrofatty proliferation (creeping fat):
    • hypertrophy of the mesenteric fat, which separates bowel loops
  • ​mesenteric venous thrombosis
    • usually adjacent to an inflamed loop
    • acute: distended vein
    • chronic: narrowed or interrupted vein
  • lymphadenopathy  

Features of penetrating disease include:

  • sinus tract: blind-ended tract extending beyond the bowel serosa  
  • fistula tract: tract that communicates the bowel lumen to another epithelialize surface
    • usually seem proximal to a small bowel stricture
    • active inflammation is almost always present
    • complex fistula: more than one tract associated with angulation or tethering of affected loops 
  • inflammatory mass: mesenteric stranding (on CT) or increased T2 signal denoting inflammation but without fluid collections 
  • abscess: a fluid collection with rim-enhancement 

Extraintestinal involvement can be at least partially assessed on MR enterography, particularly, hepatobiliary disease (eg. gallstones) and sacroiliitis.

Crohn perianal disease is mostly characterized on imaging by perianal fistulas and is better evaluated using a dedicated MRI protocol for the anal canal

Management is complex as the condition is chronic with a relapsing-remitting course. Medical management includes corticosteroids, 5-ASA preparations, immunomodulation (e.g. azathioprine, cyclosporine, methotrexate) 7. Surgical management is reserved for complications including:

It is named after Burrill Bernard Crohn (1884-1983), an American gastroenterologist, who described the condition as 'regional ileitis' in his seminal 1932 paper 11,24. However, the first definite description (but see below) was nearly twenty years prior, by Sir T (Thomas) Kennedy Dalziel (1861-1924), a Scottish surgeon, in 1913 21,23.  

Antoni Leśniowski (1867-1940), a Polish surgeon, described a small bowel condition in 1904 in a small series of four patients, with similarities to Crohn disease, although it remains controversial if it was actually Crohn's 27,28. At least one of the patients probably actually had ileal tuberculosis. Nevertheless, Polish physicians and journals usually call the condition Lesniowski-Crohn disease.

The differential diagnosis depends on the presenting symptom. When terminal ileitis is the main presentation, then differentials (adjusted for patient's age) include 1:

When colonic involvement is the predominant feature then other considerations include:

Article information

rID: 6791
Synonyms or Alternate Spellings:
  • Lesniowski-Crohn disease
  • Crohn enteritis
  • Crohn's enteritis
  • Crohn's disease
  • Crohns enteritis
  • Crohn disease
  • regional enteritis
  • Lesniowski-Crohn enteritis
  • Lesniowski-Crohn's disease
  • Lesniowski-Crohn's enteritis
  • Lesniowski-Crohn ileitis
  • Lesniowski-Crohn's ileitis

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Cases and figures

  • Figure 1: gross pathology - cobblestone apparance
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  • Case 1
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  • Figure 2: histology - non-caeseating granuloma
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  • Case 2: with fat halo sign
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  • Figure 3: clinical photgraph - pyoderma gangrenosum
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  • Case 3: with duodenal involvement
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  • Figure 4: fat creeping
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  • Case 4: termial ileitis with perforation
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  • Colitis

Thickeni...
    Case 5
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  • Case 6: with comb sign
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  • Colitis
    Case 7
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  • Case 8
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  •  Case 9: MR enterography
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  • Case 10: with multiple strictures and 'comb sign'
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  • Case 11: with enterovesical fistula
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  • Case 12
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  • Case 13
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  • Case 14
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  • Case 15
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  • Case 16
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  • Case 17: with comb sign
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  • Case 18: with inflammatory strictures &amp; obstruction
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  • Case 19
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  • Case 20
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  • Case 21: on ultrasound
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  • Case 22: ileoileal fistula
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  • Case 23
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  • Case 24: mesenteric abscess
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  • Case 25: SBO secondary to ileal stricture from Crohn's disease
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  • Case 26: with colocolic fistula
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  • Case 27
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  • Case 28: pseudosacculations
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