Crohn disease is an idiopathic inflammatory bowel disease (IBD) characterised by widespread gastrointestinal tract involvement typically with skip lesions. It is also known as regional enteritis, and frequently there is systemic involvement.
The diagnosis is typically made between the ages of 15 and 25 years of age, with no gender predilection 5. There is a familial component and incidence also varies with geographical location.
Clinical presentation is typically with chronic diarrhoea and recurrent abdominal pain. Alternatively, patients may present with one of the many complications or extraintestinal manifestations (see below).
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 relative having IBD and it reaches up to 10%, also there has been shown 30-50% chance of developing disease in mono- or heterozygous twins.
Initially the disease is limited to the mucosa with neutrophilic cryptitis and lymphoid hyperplasia, lymphoedema 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.
Extraintestinal manifestations include 3,15-17:
anywhere along digestive tract, including mouth and oesophagus
- mucogingivitis, mucosal tags, deep ulceration, cobblestoning, lip swelling and pyostomatitis vegetans, esophageal ulcers and strictures
- uveitis (acute anterior uveitis [AAU])
- liver and biliary system
- renal calculi containing oxalate due to poor fat absorption results in binding of calcium by fats, which in turn reduces the amount of calcium that can bind to oxalate, therefore increasing the amount of unbound oxalate available for resorption; this resorption occurs in the colon, and therefore patients with an ileostomy do not have the same increased risk
- pulmonary and thoracic associations
The characteristic of Crohn disease is the presence of skip lesions and presence of discrete ulcers. The frequency with which various parts of the gastrointestinal tract are affected varies widely 5:
- small bowel: 70-80% 5-6
- small and large bowel: 50%
- large bowel only: 15-20%
The choice of investigation modality depends on local expertise and availability. CT and MR enteroclysis are similar in sensitivity for active inflammation (89% vs 83% respectively) and both are somewhat 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 is also an option for diagnosing active disease, follow-up and assessing complications20. Reported sensitivity 75-94% and specificity 67-100%20.
Features on barium small bowel follow-through include:
- mucosal ulcers
- widely separated loops of bowel due to fibro-fatty proliferation (creeping fat) 2
- thickened folds due to oedema
- pseudodiverticula formation: due to contraction at the site of ulcer with ballooning of the opposite site
- string sign: tubular narrowing due to spasm or stricture depending on chronicity
- partial obstruction
- on control films presence of gall stones, renal oxalate stones, and sacroiliac joint or lumbosacral spine changes should be sought
CT examination can be carried out with both intravenous and intraluminal contrast (positive or negative) 5:
- fat halo sign
- comb sign
- bowel wall enhancement
- bowel wall thickening (1-2 cm) which is most frequently seen in the terminal ileum (present in up to 83% of patients) 8.
- strictures and fistulae
- 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
MRI enterography has no ionising radiation and an ability to evaluate both mural and extramural involvement. It has become an increasingly important part of management of patients with Crohn disease. MRI enteroclysis may be attempted in select patients.
MRI enterography (MRE)
MR enterography can be a useful technique for evaluation of the bowel. Inflamed loops of bowel demonstrate thickening and contrast enhancement.
Extramural disease is where MRI excels:
- fibrofatty proliferation:
- thickening of extramural fat, which separates bowel loops
- equivalent to the fat halo sign on CT
- vascular engorgement: comb sign
- stenoses and strictures
Coronal cine sequences (bSSFP) can also be useful in diagnosis. Inflamed loops of bowel frequently demonstrate decreased peristalsis.
MRI enteroclysis requires the placement of a nasojejunal catheter through which 1.5-2 L of contrast solution (e.g. water with polyethylene glycol and electrolytes) are injected 2.
Spatial resolution is not as good as with conventional fluoroscopic enteroclysis, and thus minor mucosal changes are not apparent. When disease is transmural, with cobblestone appearance, the abnormalities are evident as high T2 signal linear regions, provided adequate distension is achieved 2.
Routine MRI can also give valuable information:
- perianal disease
- liver disease
- sacroiliac joints and spine
Ultrasound has a limited role, but due to it being cheap and 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. Typically examination is limited to the small bowel and wall thickness assessed:
- bowel wall thickness should be <3 mm
The usefulness of this finding needs to be interpreted in the context of pretest probability (i.e. thickness of less than 3 mm helps exclude the disease in a low risk patient, and a thickness of greater than 4 mm helps establish the diagnosis in a high risk patient) 4. As it has difficulty examining the whole bowel, it is not appropriate as a true diagnostic test.
On Doppler evaluation, increased SMA flow volume and decreased SMA resistive index (SMA RI) also correlate with disease activity. Successful treatment may result in normalization of these imaging parameters 12.
Other features on ultrasound20:
- non compressible, rigid, fixed bowel wall
- perienteric fluid
- creeping fat - echogenic area (representing proliferation of adipose tissue that extends around active inflammation) separating bowel loops
- gut signature - lost or preserved
- strictures - fibrotic (maintains gut signature) or inflammatory (loss of gut signature)
Ultrasound does of course have a significant role to play in the assessment of:
- perianal disease: rectal ultrasound
- hepatobiliary disease
Treatment and prognosis
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:
- adhesions and bowel obstructions
- perianal disease
History and etymology
It is named after Burrill Bernard Crohn, American gastroenterologist 11.
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:
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