Abdominal X-ray - Abnormal bowel gas pattern Large bowel obstruction
Key points
Dilatation of the caecum >9cm is abnormal
Dilatation of any other part of the colon >6cm is abnormal
Abdominal X-ray may demonstrate the level of obstruction
Abdominal X-ray cannot reliably differentiate mechanical obstruction from pseudo-obstruction
The most common causes of large bowel obstruction are colo-rectal carcinoma and diverticular strictures. Less common causes are hernias or volvulus (twisting of the bowel on its mesentery). Adhesions do not commonly cause large bowel obstruction.
Radiological appearances of large bowel obstruction differ from those of small bowel obstruction, however, with large bowel obstruction there is often co-existing small bowel dilatation proximally.
Dilatation of the caecum >9cm, and >6cm for the rest of the colon is considered abnormal.
Here the colon is dilated down to the level of the distal descending colon. There is the impression of soft tissue density at the level of obstruction (X). No gas is seen within the sigmoid colon.
Obstruction is not absolute in this patient as a small volume of gas has reached the rectum (arrow).
An obstructing colon carcinoma was confirmed on CT and at surgery.
Pseudo-obstruction
Pseudo-obstruction is a poorly understood functional abnormality of bowel, most often occurring in the elderly population, in those with underlying systemic medical conditions, or due to certain drugs. The clinical features can be similar to true obstruction, but no mechanical cause is found. An abdominal X-ray cannot reliably differentiate true mechanical obstruction from pseudo-obstruction.
