This 55 years old male patient with known ulcerative colitis presented with fever and malaise suggesting a flare up. There is gross dilatation of the transverse colon which measures approximately 10cm in maximum diameter. There is also evidence of pseudopolyp formation (black arrows). This is typical of toxic megacolon and is a surgical emergency.

If toxic megacolon is clinically suspected, patients are usually followed up with plain abdominal radiography every 12-24 hours, depending on the patient’s clinical condition. A single abdominal radiograph may not be sufficient and should be combined with a horizontal-beam radiograph because it may better depict large, dilated bowel loops with fluid levels. Also, abdominal perforation is less likely to be missed.
Pancolitis (including changes such as strictures and mucosal abnormalities) may be seen in association with toxic megacolon. Toxic megacolon in the setting of Crohn disease is less common, and the plain radiographic findings of toxic megacolon in ulcerative colitis and Crohn disease overlap. However, with Crohn disease, the colonic wall tends to be thicker, thus a thicker colonic wall in the setting of toxic megacolon in a patient with no previous disease should suggest Crohn disease rather than ulcerative colitis.

Marked dilatation is observed in the transverse colon; the upper range of normal for the transverse diameter is 5.5-6.5 cm. This finding led to the belief that the transverse colon is the area most severely affected. However, if a prone radiograph is obtained in the same patient, the greatest distension is observed in the ascending and descending colon. The apparent prominent involvement simply reflects the movement of the retained gas to the least dependent part of the colon. Serial radiographs may show increasing dilatation of the transverse diameter of the colon.

Images may show a coarse, irregular, mucosal pattern of the large bowel. This thumbprinting is caused by mucosal edema due to inflammatory infiltration. The normal haustral pattern is absent in the involved segments, and pseudopolyps often extend into the lumen.

These represent mucosal islands in denuded ulcerated colonic wall in ulcerative colitis. Pneumatosis coli is an occasional finding. If perforation occurs, radiographic signs of a pneumoperitoneum may be apparent on the supine and or lateral decubitus radiographs.

Reference: Imbriaco M, Balthazar EJ: Toxic megacolon: role of CT in evaluation and detection of complications. Clin Imaging 2001 Sep-Oct; 25(5): 349-54
Credit: Dr Abhijit Datir