Pan colitis and perforation
The whole of the colon is distended. There is thickening of the mucosa which is oedematous. In the centre of the film there are several dilated loops of small bowel and their inner and outer walls are both visible. This latter feature indicates free gas within the peritoneal cavity.
The appearances of the bowel are characteristic of a pan colitis (affecting the whole colon) typical of ulcerative colitis. The bowel has clearly perforated. The term megacolon is frequently applied in cases of transmural fulminant colitis when the bowel looses motor tone and dilates to a transverse diameter of greater than 5.5cm. The term toxic megacolon 93
should be reserved for cases of dilatation with systemic toxicity, abnormal clinical signs (peritonism, fever) and abnormal laboratory indices (raised inflammatory markers, leukocytosis and left shift). The clinical setting is usually accompanied by profuse bloody diarrhoea. Mortality is up to 20%, barium enema is contraindicated. Ulcerative colitis is the commonest cause but others include Crohn's disease, amoebiasis, Salmonella, pseudomembranous and ischaemic colitis.
Normally bowel gas is only present within the bowel lumen. This results in a clear image of the inner margin of the bowel on the abdominal X-ray. This is due to the air-mucosa interface which has different densities. The outer margin, however, is not clearly seen since the serosal surfaces merge with other adjacent bowel wall loops of similar density. However, free intra-peritoneal gas will also clearly outline the outer serosal margin of the bowel. The bowel wall thus appears as a thin 'pencilled' line with gas on either side. This appearance is known as Rigler's sign. Gas may be visible under the hemidiaphragms on an erect chest or abdominal film (Fig. 2.15).
Free gas may be seen after bowel perforation or following laparotomy. In adults, post-laparotomy pneumoperitoneum persists for up to 7 days but is absorbed very much more quickly in children, usually by 24 hours.
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