Intra-mural gas refers to abnormal gas in the wall of hollow abdominal viscus. Intra-mural gas is one of the most serious findings on abdominal plain film requiring timely surgical intervention in adults. Intra-mural gas is not a difficult pattern recognition challenge. Radiographers should be able to identify mural gas to facilitate timely treatment.

Linear streaks of intra-mural gas indicate infarction of the bowel wall. Gas in the wall of the bowel in the neonatal period, whatever its shape, is diagnostic of necrotising enterocolitis. (http://www.medstudents.com.br/radio/radio2.htm)

Toxic megacolon should be considered one of those life-threatening conditions that can be diagnosed on plain abdominal film. If untreated, the mortality rate is between 20 and 30%. (Stephen R. Baker, The Abdominal Plain Film, Appleton and Lange, 1990 p203) Toxic megacolon is a fulminant form of colitis. Plain film appearance is of moderately dilated bowel with abnormal haustral pattern. The moderately dilated bowel will not dilate to the extent seen in sigmoid volvulus. There is usually no faeces in the bowel. The presence of intra-mural gas can quickly advance to perforation. These changes are most often demonstrated in the transverse colon.

Ulcerative colitis is the most common cause of toxic megacolon.

The characteristic features of toxic megacolon are acute dilation of a segment of colon, florid inflammation of the wall of the colon. The colon wall may be thickened in areas marked inflammation or thin and fragile in areas of colon where the mucosa has broken down and sloughed off. The colon wall in affected area may lose its characteristic pattern of haustra and plicae. The wall may be thickened and pseudopolyps may protrude into the lumen.

The transverse colon was thought to be a common site of toxic megacolon. This may not be the case- it may be that the transverse colon is where the disease is visualised on plain film because of the presence of gas (the transverse colon is anterior and tends to fill with gas when the patient is in the supine position). This would suggest that a decubitus projection would be advantageous to demonstrate the full extent of the diseased colon- left and right decubitus abdomen projections may also be warranted.

Barium enema is contra-indicated in fulminant colitis rendering the plain film an even more important diagnostic tool.

The transverse colon is dilated and shows evidence of thumbprinting (black arrow) The descending colon has a thickened featureless wall and possible pockets of intra-mural gas (white arrow). It is not clear whether this appearance is due to intra-mural gas, or properitoneal fat interposed between the descending colon and abdominal wall. Patient has known Crohn's disease Appearances are consistent with toxic megacolon

Dilated stomach with abnormal wall pattern (effaced). Intra-mural gas seen along the greater curve of the stomach (white arrow) Note also splenic artery calcification basal lung disease on right lumbar scoliosis concave to left left hip orthopaedic hardware ? bladder catheter/flatus tube

hepatic portal venous gas unknown history •Portomesenteric vein gas is most commonly caused by mesenteric ischemia but may have a variety of other causes. •When CT demonstrates portomesenteric vein gas and clinical findings suggest the presence of mesenteric ischemia, surgery is mandatory Where hepatic venous gas is demonstrated a deliberate focused search for intra-mural gas should be undertaken and Where intramural gas is demonstrated a deliberate focused search for hepatic venous gas should be undertaken

http://myweb.lsbu.ac.uk/dirt/museum/margaret/758-267c-1080241.jpg

"Clinical presentation: Male 3 weeks old, blood and mucus in stool. There is gas distension of transverse colon and splenic flexure, proximal to an area of abnormal bowel in the descending colon. There is the irregular double line of gas in the bowel wall (white arrow), separate from bowel contents. Immediately above the left iliac crest, there are two biconcave triangles of gas density. These extend above to a line of gas that is (left) lateral and separate from the line of intramural gas. Gas is seen on both sides of the abnormal colonic wall. The anterior location and the AP. projection mean that the transverse colon can be projected above the more posterior fundus of the stomach." myweb.lsbu.ac.uk/dirt/museum/p7-abdo.html

	"Clinical presentation: 67 year old male with renal failure and previous ileostomy. Recent SMA stenting. ? SBO There is evidence of hepatic-portal venous gas (solid white arrows). There are dilated loops of small bowel. There is a prominant loop of gas-filled small bowel on the left with evidence of wall thickening. The black arrow identifies what is probably fat within the gall-bladder bed. The white outline arrow identifies what may be portal venous gas.
	This is a magnified view of the supine abdominal plain film. In addition to the hepatic-portal venous gas (white arrows) there is evidence of intramural gas (black arrows). The lucent structure identified by the bottom white arrow is probably fat around the gall bladder.
	The erect abdominal plain film demonstrates extensive hepatic-portal venous gas (white arrows). There are multiple airfluid levels within the bowel suggesting a motility disorder.
	This is a magnified view of the erect abdominal plain film RUQ. The hepatic portal venous gas is clearly seen as well as intramural gas (black arrow) and surgical sutures (white arrow).

Intra-mural gas is one of the more serious findings on abdominal plain film. A timely report by the radiographer to the referring doctor of the possibility of intra-mural gas could improve the prognosis for the patient. Decubitus views should be considered where there is a possibility of more extensive disease and where the additional information will affect patient management.