Crohn's disease

(Burrill Bernard Crohn, 1884 - 1983, American physician), chronic inflammatory condition of the gastrointestinal tract from unknown origin, belonging together with ulcerative colitis (UC) to the group of inflammatory bowel disease (IBD). The epidemiology and aetiology of UC and Crohn's disease (CD) share many features.

From a pathological point of view, however, CD in contrast with UC is characterized by chronic inflammation extending through all the layers of the intestinal wall and involving the mesenteric as well as the regional lymph nodes. CD may exclusively affect the small bowel, which is the most common location, or it may involve the small bowel and the colon. In some patients the involvement is limited to the colon. In more rare instances other segments of the alimentary tract (stomach, duodenum and oesophagus) may be affected by CD but this usually occurs in association with involvement of the distal ileum. Early pathological features of CD include hyperaemia and oedema of the bowel mucosa and of the mesentery associated with enlarged inflammatory lymph nodes. Another early, typical gross pathological lesion of CD is a small superficial aphthous mucosal ulceration, commonly located over Peyer patches in the small intestine and over the lymphoid aggregates in the colon.

Later during the course of the disease the ulcerations tend to enlarge and to coalesce. Typically, long linear longitudinal ulcerations develop along the long axis of the bowel (Fig.1). These multiple ulcers associated with the intervening remaining islands of submucosal wall thickening produces the so-called typical "cobblestone" pattern. Fissures develop from the base of the ulcers destroying the full thickness of the bowel wall, but will not result in free perforations because a fibrinous reaction of the serosa induces adherence of other intestinal loops into which the fissure ends. However, the circumscribed inflammatory process commonly results in abscess formation and fistulization. Extraluminal fistulous tracts may interconnect small bowel loops or a small bowel loop with a segment of the colon. It is not uncommon that fistulas may develop with adjacent nonintestinal pelvic organs such as the vagina and the bladder or with the skin. The fibrosing process which develops in long-standig CD results in narrowing of the lumen of the bowel and a generalized loss of flexibility of the bowel. The stenosing segments may be very long and extend over several cm. The fibrosing process also involves the mesentery which becomes considerably thickened and tends to retract, resulting in fixed loops of bowel.

In the large bowel "skip" areas of apparently normal bowel separate severely involved segments. Whereas in UC the rectum is almost always involved in patients with CD of the colon the rectum is frequently spared. Similarly to UC inflammatory pseudopolyps may develop in long standing disease (Fig.2). Histologically CD is characterized by lymphoid infiltration of the submucosa and to a lesser degree of the muscularis mucosa. Moreover noncaseating granulomas are commonly present in the mucosa and submucosa of the bowel as well as in the mesentery, peritoneum and lymph nodes.

The predominant clinical symptoms observed in patients with CD are: diarrhoea, abdominal pain and weight loss. Duodenal localization of CD is often characterized by upper abdominal discomfort or pain and by postprandial vomiting due to stenosis and obstruction of the duodenum. The clinical course of CD in most patients is characterized by periods of relapses and remissions. Spontaneous improvement and disappearance of clinical symptoms may be seen in some patients without medical or surgical treatment. A considerable number of patients with CD will require a surgical intervention at one or another phase during the course of their disease. It is not exceptional that a second or even multiple surgical in the lumen due to fibrosis. The "string" sign (Fig.4) is most frequently visible in the terminal ileum. It is caused by incomplete filling of a bowel segment due to irritability and spasm associated with marked ulceration. Displacement of intestinal loops may be due to abscess formation or to mesenteric fatty tissue increase and induration. The discontinuity of the ulcerations as well as the asymmetric involvement of the walls of the bowel with intervening areas of normal mucosa called "skip "areas are typical features of CD.

In the advanced stage of the disease serpiginous longitudinal and transverse ulcerations are present. Pseudopolypoid lesions composed of islands of remnant inflamed tissue or of hypertrophic inflammatory tissue proliferations may lead to the appearance of filling defects of varying size and configuration. Internal fistulas are well demonstrated by barium study.

Rectal involvement in CD is characterized by patchy, discontinuous mucosal changes, by deep or collar-button ulcers and by sinus tracts.

The radiological differential diagnosis between UC and CD of the colon or granulomatous colitis is difficult and frequently impossible, particularly in chronic long-standing disease. Granular mucosa, diffuse rectal disease, continuous inflammation and a normal terminal ileum are in favour of UC. Patchy rectal involvement with collar-button ulcers on a background of normal mucosa, discontinuous involvement associated with lesions of the terminal ileum are features favouring CD.

CT and MRI features

The main contribution of CT in the evaluation of patients with Crohns disease is its ability to display the degree of bowel wall thickening and the extraluminal bowel changes in the mesentery and the retroperitoneum (Fig.5). In patients with granulomatous colitis the colon wall tends to be thicker and more homogeneous than in patients with UC. Furthermore, CT demonstrate mesenteric or retroperitoneal abscess more appropriately than barium studies (Fig.6) and displays its exact anatomic location and extent. In the chronic phase of the disease one can note an important proliferation of fibrofatty tissue representing a reaction to the chronic inflammation of the mesenteric fat. The "comb" sign is based on the prominence, dilatation and tortuosity of the vasa recta coursing through proliferative mesenteric fat (Fig.7). This condition may cause displacement of the bowel loops as visualized on the barium study and the CT findings will be most helpful for detecting the cause of the bowel displacement. When the degree of liquefaction in an abscess is still limited minimal or no difference in attenuation value between an inflammatory phlegmonous mass and an abscess will be noted and both conditions cannot be distinguished.

CT can in some instances visualize fistulous tracts (Fig.8) but in general MRI, because of its multiplanar capabilities, is better suited to display optimally the exact topographical course and the extent of these fistulas, particularly in the perirectal fossa and the perineum. Also, see abscess paracolic.

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