A well-distended colon is a basic requirement for performing CT colonography, since it can be very difficult or even impossible to evaluate intraluminal lesions located in collapsed sections of the large intestine. Gas insufflation is through a rectal catheter. Distension may be performed using either a manual insufflation bulb or an automatic insufflation device. Either air or carbon dioxide may be used. Proper bowel distension requires appropriately trained and experienced personnel.
Basic Principles and Preparation
Patient Information and Preparation for Examination
Before beginning the examination, the process and sequence of events in CT colonography should be explained to the patient. It is not uncommon for patients to feel they have received too little information about the bowel preparation and the examination itself from their referring physician. Some patients may even not have been told that a rectal catheter or colonic distension is needed. Explaining to the patient clearly why bowel distension via a rectal tube is necessary, and the direct relationship between distension and the quality of the examination, can often help increase patient compliance.
To put any fears to rest, it can be useful to show the patient a thin, flexible rectal tube, and to fill a latex glove with air to demonstrate the necessity of distension.
Before beginning the examination, the patient should be asked to visit the bathroom to empty the bowels one last time. Ideally, there should be almost no fecal residue.
Antispasmodic agents (spasmolytics) relax the smooth muscle layers of the intestinal wall and reduce intestinal peristalsis. This enhances distension and helps prevent spasms, which in turn increases patient comfort. Spasmolytics can also cause the ileocecal valve to open, allowing air to enter the small intestine (Fig. 2.12).
For CT colonography, the spasmolytics butylscopolamine and glucagon are commonly used. Their efficacy and use in CT colonography have been a subject of controversy in the international literature. According to the consensus statement by ESGAR, the use of spasmolytics before colonic distension is preferable, as long as specific contraindications are observed (Table 2.1). The decision whether or not to administer spasmolytics should be also based on the patient′s history (diverticular disease, previous incomplete colonoscopy for strictures, etc.). The agent of choice is butylscopolamine. Alternatively, glucagon may be used in patients in whom butylscopolamine is contraindicated; it has been reported, though, that its antiperistaltic effect on the large bowel is much less pronounced than that of butylscopolamine.
In the United States, many authors are opposed to the routine use of spasmolytics. In their view, intravenous administration of a spasmolytic decreases patient comfort and can lead to side effects. The use of spasmolytics also increases the cost and duration of the examination.
Butylscopolamine (Buscopan; Boehringer Ingelheim, Ingelheim, Germany) is an anticholinergic agent that relaxes the smooth muscle layers of the intestinal wall; it also has a relaxing effect on the ciliary muscle and other smooth muscles. Several studies have shown that butylscopolamine can be used to enhance colonic distension in CT colonography. The drug presumably also enhances patient comfort. Butylscopolamine is much less expensive than glucagon and is also more effective. It is not approved for use in patients in the United States.
Administration. Butylscopolamine is administered intravenously. The usual dose is 20 mg (one ampoule). The drug should be given immediately before the start of bowel distension prior to the first scan. Side effects are usually self-limiting and include a dry mouth, tachycardia, dizziness, and urinary retention.
Butylscopolamine can cause accommodation disorders (Warning: This can have implications for the ability to drive after the examination). It is essential to warn patients of this risk before administering the drug (ideally, when the examination appointment is made).
Very rarely, allergic reactions or anaphylactic shock may occur. Butylscopolamine is contraindicated in patients with a history of hypersensitivity, glaucoma, heart disease or tachycardia, mechanical stenosis of the gastrointestinal tract, prostatic hypertrophy, or myasthenia gravis.
The following recommendations for administration of butylscopolamine have been published by Dyde et al. (2008):
Enquire whether there is an allergic history.
Ensure that patient literature warns: “In the unusual event that you develop painful, blurred vision in one or both eyes following the examination, you must attend hospital immediately for assessment.”
Warn patients to expect blurred vision and not to drive until this has worn off.
Remind clinicians that special consideration needs to be given as to the method of investigating patients with cardiac instability (such as those recently admitted with acute coronary syndrome, recurrent cardiac pain at rest, uncontrolled left ventricular failure, and recent ventricular arrhythmias) and prostate hypertrophy.
Glucagon is a polypeptide that is secreted by the islets of Langerhans in the pancreas. It has a relaxing effect on the smooth muscle lining the gastrointestinal tract. Glucagon inhibits motility and relieves muscle spasms. Although studies have shown that butylscopolamine can enhance distension of the colon, there is no evidence as yet of a similar effect for glucagon. Recently, Rogalla et al. (2005) reported a significant reduction in the number of collapsed segments of colon when glucagon was administered. The effect was less pronounced, however, than that seen with butylscopolamine. It is generally assumed that glucagon enhances patient comfort.
Administration. Glucagon may be used in patients in whom butylscopolamine is contraindicated. Glucagon may be administered subcutaneously, intramuscularly, or intravenously. Customary doses are 1 mg subcutaneously or intramuscularly or 0.5–1 mg intravenously. For subcutaneous or intramuscular administration, glucagon is injected about 10 minutes before insufflation begins; when given intravenously it is slowly injected about 1–2 minutes prior to insufflation. Uncommon adverse effects include gastrointestinal symptoms such as nausea, vomiting, diarrhea, dizziness, and hypokalemia. Rarely, arrhythmia and allergic reactions can occur. Contraindications include pheochromocytoma, insulinoma, and diabetes mellitus.
Catheter types. Any catheter that is suitable for rectal insertion and insufflation may be used for CT colonography (Fig. 2.13). In general, however, the use of thin, flexible rectal tubes is recommended (ESGAR consensus) (Table 2.1). The standard, rigid, large-caliber catheters often used for administering a barium enema should not be used for CT colonography.
With the exception of pediatric rectal catheters, the standard rigid catheters used for barium enemas are uncomfortable for the patient due to their inflexibility and larger diameter. They also carry a higher risk of injuring the rectum.
Because of the minimal flow of gas required for CT colonography, a large-diameter rectal tube is not needed for colonic distension. This allows the use of a thinner and more flexible catheter instead. The simplest option is to use a thin, flexible rubber catheter or a Foley catheter (14–20 Fr) with a small inflatable balloon at the tip. The catheter is readily connected to a hand-held squeeze bulb or a tube system. The soft, thin tip allows safe rectal insertion. There are also specially made thin, latex-free balloon catheters for automatic distension with carbon dioxide (e.g., Bracco).
Inflating the balloon. Distension of the catheter balloon remains controversial, and some centers refuse to use it. The use of an inflatable balloon is not essential for CT colonography. However, the presence of an inflatable balloon on the catheter tip can help keep it positioned correctly, and also helps the patient retain the gas, which is particularly useful in incontinent patients. Experience with barium enemas and CT colonography has shown that overdistending the balloon can lead to rectal injuries and perforation. These complications may be avoided by proper positioning of the catheter and careful inflation of the balloon. In addition, the balloon can obscure deep rectal lesions. To avoid this, some examiners deflate the balloon or even remove the catheter before performing the second scan. The volume of the balloon attached to a Foley catheter (5–10 mL) or of a catheter specifically intended for CT colonography that is used for automatic carbon dioxide distension (30 mL) is much smaller than the volume of the balloon on a barium enema catheter (100 mL), and hence using a thin catheter carries a smaller risk of injury to the bowel or of obscuring rectal lesions. The authors, therefore, do not deflate the small balloon and leave the catheter in place.
Choice of Gas for Distension
The colon may be distended with normal air or carbon dioxide. According to the ESGAR consensus statement, carbon dioxide is the preferred gas for bowel distension in CT colonography, but air is an acceptable alternative (Table 2.1).
Air. Until about 2005, colonic distension in CT colonography was primarily performed using air. Advantages are that not only is air freely available, it is also easily delivered using a hand-held insufflation bulb. A disadvantage is that using air diminishes patient comfort because the feeling of bloating after the examination lasts longer. This is due to the higher proportion of nitrogen in air, which is not absorbed by the intestinal wall. Nitrogen remains in the bowel for longer and is mostly expelled through the rectum.
Carbon dioxide. Unlike air, carbon dioxide, with its much higher diffusion coefficient, quickly diffuses through the bowel wall into the blood and is expired through the lungs. Studies have shown that insufflation with carbon dioxide is better in terms of patient comfort during and after the examination. In addition, with automatic administration of carbon dioxide, there are better results for distension of the colon, especially of the left colon when the patient is supine. In addition to manual distension of the colon with carbon dioxide, there are also devices available for automatic insufflation.