Glucagon is a polypeptide hormone normally produced by the pancreatic islets of Langerhans. It causes an increase in blood glucose, but is perhaps better known for its clinical use as a hypotonic agent for the stomach, small bowel, and colon. Glucagon relaxes the smooth muscle of the gastrointestinal tract and is thought to improve bowel distension and decrease patient discomfort due to spasm. The effectiveness of glucagon is dependent upon location, and it has been found to be most effective on the duodenum and least effective on the colon (Chernish and Maglinte 1990). Although uncommon, the most frequently encountered side effects of glucagon are nausea, vomiting and headache. One study found that 4% of patients experienced nausea following the intravenous administration of gluca-gon prior to CT colonography.(Morrin et al. 2002). Rarely, generalized allergic-type reactions such as urticaria, respiratory distress and hypotension may occur. Glucagon is contraindicated in patients with pheochromocytoma, insulinoma, poorly controlled diabetes or a known hypersensitivity to glucagon.

Glucagon was used in the past, and is still being used at some sites, as a spasmolytic agent for CT colonography. Many of the older published trials evaluating the performance of CT colonography for polyp detection were performed on subjects who had received 1 mg of glucagon intravenously. The routine use of glucagon for colonic evaluation had been adopted from barium enema practice. Some studies have found decreased discomfort during and after barium enema when glucagon is given prior to the procedure (Bova et al. 1993; Meeroff et al. 1975). However, it has also been reported that there was no improvement in colonic distension on double-

contrast barium enema after the administration of glucagon [33] and that there was also no improvement in colon polyp detection rates on double-contrast barium enema with glucagon administration (Thoeni et al. 1984). Important differences exist between the barium enema examination and CT colonography when considering the usefulness of glucagon for these studies. Liquid barium may cause colonic spasm during the barium enema examination, but this does not occur during CT colonography. Additionally CT colonography is a much more rapid study than barium enema. Colonic insufflation with air is important during the scan phase of the CT, which is very short and occupies less than 15_s in each position, whereas a distended colon is needed for at least 15 min after glucagon is administered for the barium enema examination.

Trials specifically evaluating the value of intravenous glucagon for CT colonography have been conducted. CT colonography was performed in 60 patients following manual air insufflation of the colon up to maximum patient tolerance. Thirty-three patients received 1 mg of glucagon immediately prior to the CT scan and the remaining patients did not (Yee et al. 1999a). Segmental as well as overall colonic distension was evaluated. The colon was divided into eight segments in both supine and prone positions for a total of 16 segments per patient. It was found that glucagon administration did not significantly improve colonic distension in supine or prone positions. In patients receiving glucagon, 222 segments (84.1%) were considered adequately distended. In patients not receiving glucagon, 187 segments (86.6%) were adequately distended. No statistically significant differences were identified between the glucagon group and the non-glucagon group for overall colonic distension scores in the prone, supine, or combined positions.

Another study also found that colonic distension at CT colonography is improved by dual positioning but not by the administration of intravenous glucagon (Morrin et al. 2002). In a study of 96 patients, 74 subjects received 1 mg of glucagon intravenously immediately prior to CT scanning and 22 patients did not. A five-point scale was used to score adequacy of distension, with 1 = collapsed and 5 = excellent distension. There was no statistically significant difference between the glucagon and non-glucagon groups (mean distension scores 3.6 and 3.9, respectively). We do not administer glu-cagon routinely for CT colonography at our institution, but we use it in specific cases where there is significant patient discomfort or evidence of colonic spasm on the scout CT view. Initial investigation has also found that glucagon does not appear to improve the sensitivity of CT colonography for detection of colorectal polyps (Yee et al. 1999b).

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