Risk Profile of CT Colonography
CT colonography is considered a safe, noninvasive method of examining the entire colon. Generally it is a less burdensome examination that is preferred by most patients over conventional colonoscopy or double-contrast barium enema. Nevertheless, there are a few factors that pose a very small risk for the patients and these must be taken into account. These include the risk of perforation and the radiation exposure as well as drug-induced adverse effects such as those related to laxatives, spasmolytics, and contrast media.
In extremely rare cases perforation can occur during bowel distension. Most often this happens in patients with pre-existing acute bowel inflammation such as diverticulitis or chronic inflammatory bowel disease; for this reason, pre-existing bowel disease is generally regarded as a contraindication to CT colonography. According to current figures, the average rate of perforation is very low, ranging from 0.009% to 0.05% depending on the study. The number of symptomatic perforations is much lower, because the escape of small volumes of air or gas may cause very little in the way of symptoms, and in such cases the patient need only be kept under observation. To the best of our knowledge, no deaths due to CT colonography have been reported.
Risk factors. Retrospective analyses have identified several risk factors for perforation during CT colonography. Perforation is more common with manual insufflation of air than with automatic insufflation of carbon dioxide. Rigid, large-diameter catheters are also associated with a higher perforation rate than are thin, flexible catheters. Overinflation of large-volume catheter balloons (up to 100 mL) can lead to rectal injuries and perforation. In addition, it may be possible that such balloons occlude the anus and thereby prevent rectal gas escape. The risk of perforation is much higher in symptomatic patients than in screening patients. Risk is higher in particular in patients with acute inflammation of the colon such as active colitis or diverticulitis, and in patients with colonic obstruction due to colorectal carcinoma, high-grade diverticulosis, postinflammatory stenosis, and hernias with involvement of the large intestine (Fig. 2.34) and after partial resection of the colon. There is also an increased risk of perforation in patients who have previously undergone colonoscopy with a deep colonic biopsy or polypectomy. A higher perforation risk is also present during rectal tube insertion and distension through a colostomy (see Chapter 4, “CT Colonography in Patients with a Colostomy,” p. 139) (Fig. 2.35).
CT Colonography Morphology
Perforation of the colon is identified on 2D views only; 3D views are not useful. Signs of perforation include the presence of extraluminal air or fluid. To detect the presence of extracolonic air, wide window settings such as the bone or lung window are useful (Fig. 2.36). The nature and site of the perforation may be inferred from the distribution of the escaped air. Perforations in intraperitoneal segments of the colon such as the transverse colon, sigmoid colon, and cecum more often lead to free intraperitoneal air and fluid. Perforations of retroperitoneal segments of the colon such as the descending colon and rectum more often lead to retroperitoneal air or fluid. They should be distinguished from purely intramural air.
Laceration and intramural pneumatosis. Laceration and pneumatosis of the bowel wall causes intramural pockets of air that surround the colonic lumen but do not pass through the wall to the intraperitoneal or retroperitoneal space (Fig. 2.37). There is therefore no free intraperitoneal or retroperitoneal air. The cause is likely to be increased permeability of the mucosa or a small mucosal defect that does not involve all layers of the wall and thus does not allow transmural distribution of air. Patients with intramural air often remain asymptomatic. Intramural air can also be an incidental finding in asymptomatic patients (CT colonography-related asymptomatic colonic pneumatosis). The working group of Pickhardt and collegues reported detecting intramural air in 0.11% of 5368 screening patients. In this study, an association with colonic distension by carbon dioxide was presumed. The intraluminal air often has a curvilinear configuration and is more often located in the right colon (Fig. 2.38). These pockets of air are usually self-limiting and do not require treatment, but careful correlation with the patient′s clinical symptoms and close monitoring are nevertheless indicated. CT colonography-related asymptomatic colonic pneumatosis can be differentiated from idiopathic pneumatosis cystoides of the colon, which is characterized by multiple pearl-like or macrocystic air bubbles in the colon wall without any recognizable cause. On conventional colonoscopy, this typically simulates polyposis.
Complete perforation. Complete perforation involves all the layers of the bowel wall. The sign of complete perforation of the colon is free intraperitoneal and/or retroperitoneal air. The amount of free air can vary. Frequently, only small amounts of extracolonic air are detected (Fig. 2.39).
Contained perforations. In patients with inflammatory bowel diseases—typically diverticulitis—“contained” perforations may occur. Perforation may already be present before the CT colonography if the examination is performed during the acute or subacute stages of inflammation. The bowel distension often makes it easier to detect such perforations, as intraluminal gas moves into extraluminal areas due to the pressure gradient. On 2D images, a circumscribed pocket of air can be seen in the pericolic fat of a bowel segment showing inflammatory changes. Often the signs of inflammation thickening are detected in the surrounding of fatty tissue.
CT colonography is contraindicated in patients with acute inflammatory bowel disease. Distension of the colon is not required for CT diagnosis or monitoring of acute diverticulitis.
A sufficient amount of time should be allowed to pass between an episode of acute diverticulitis and CT colonography (Fig. 2.40).
Reducing the Risk of Perforation
(See also “Contraindications,” p. 6, and “Colon Distension,” p. 18)
It is better to use a thin flexible tube rather than a rigid large-diameter rectal catheter with a large-volume catheter balloon. Automatic insufflation of carbon dioxide is preferable to manual insufflation of air. The bowel distension, especially manual delivery of air, should be done carefully. Extra care is needed when performing CT colonography in patients with obstructive colonic disease or a hernia involving the colon. A scout scan may be performed whenever the insufflation is complicated or painful, to identify overdistended poststenotic colonic segments.
Additional CT colonography immediately following an incomplete colonoscopy should only be done if no polypectomy or deep biopsy was carried out. If either polypectomy or deep biopsy was performed, CT colonography should be done no earlier than 10 days after the conventional colonoscopy.