Laparoscopic Sigmoid Colectomy
CT exam of a patient 10 days post-laparoscopic sigmoid colectomy.
Question: What abnormality is marked? What may account for these appearances? How long should be present on a post-operative CT?
Answer: Free gas is noted in the right anterior peri-hepatic space (arrow) along with multiple smaller foci in the left upper quadrant and adjacent to the gastro-esophageal junction. This was due to a perforated duodenal ulcer (not shown) but the differential includes normal post-operative free gas and anastomotic break down.
Post-operative free gas
Free intraperitoneal gas on a post-operative CT can be problematic for the radiologist when trying to decide whether all can be attributed to retained post surgical air alone, or whether the air is a consequence of perforation, anastomotic leak or infection. The traditional radiological literature on post-operative intraperitoneal gas is based on plain abdominal radiograph findings (EARLS et al. 1993). CT is significantly more sensitive in detecting post-operative gas than conventional radiographs especially in the obese patient or in the presence of a post operative ileus, with CT demonstrating free gas in 87% on day 3 and 50% on day 6 compared with 53% and 8% with plain films taken at the same time (EARLS et al. 1993). The reported average prevalence of persisting free gas on a CT is approximately 44% on post-operative day 3, and 30% between days 4 to 18 (GAYER et al. 2000). However, small traces of free gas can be demonstrated for up to 24 days following surgery indicating a potential extended time line for this finding.
The advent of laparoscopic surgery has complicated interpretation by introducing an alternative time line due to carbon dioxide dissipating at a significantly faster rate than room air. Animal studies have suggested that gas should be expected to resolve by day 2, with any persisting gas after this time likely to reflect a pathological process. Nevertheless, interpretation is often a matter of judgement as to whether an excess of free gas remains. As a rule of thumb, gas from an open procedure should have largely resolved by 7-10 days and for a laparoscopic procedure by day 2. When doubt remains, the option of a follow up CT study may help. Uncomplicated post-operative gas should always diminish with time. Any increasing free gas volume should alert the radiologist to consider infection or an anastomotic leak as a likely explanation.
Distinction should be made between intraperitoneal and retroperitoneal gas. Unless drainage catheters are located in the retroperitoneal spaces, gas here generally indicates pathology. Examples would include perforation from the duodenum and rectosigmoid, retroperitoneal infection and trauma either resulting from iatrogenic injury (e.g. ERCP or endoscopy) or another cause.
Earls JP, Dachman AH, Colon E, Garrett MG, Molloy M (1993) Prevalence and duration of postoperative pneumoperitoneum: sensitivity of CT vs left lateral decubitus radiography. AJR Am J Roentgenol 161:781-5
Gayer G, Jonas T, Apter S, Amitai M, Shabtai M, Hertz M (2000) Postoperative pneumoperitoneum as detected by CT: prevalence, duration, and relevant factors affecting its possible significance. Abdom Imaging 25:301-5
These are example cases taken from ‘Radiology of the Post Surgical Abdomen’ edited by John Brittenden & Damian J.M. Tolan