In virtual colonoscopy, nothing beats a good prep

The colonoscopy may be virtual, but discussions of bowel prep can seem all too real for radiologists more comfortable with talk of mAs and multiplanar reconstructions.

But discuss it they must if VC is to become a leader among colorectal cancer screening techniques. Certainly there are alternatives to so-called cathartic bowel cleansing, including new stool-tagging agents and fecal digital subtraction techniques. But they're not ready for prime time.

"A clean, distended colon is really essential for optimal virtual colonoscopy," said Dr. Michael Macari from the New York University Medical Center in New York City. Fact is, a good prep won't hurt the patients either. "If you look at the average American, they could use a good bowel preparation, (and to) lose a little weight, fast a little bit."

In a presentation at the 2002 Virtual Colonoscopy Symposium in Boston, Macari said it's important for radiologists to understand the pros and cons of different cleansing methods. The principal agents include:

  • GoLyteLy (Braintree Laboratories, Braintree, MA) contains 236 g of polyethylene glycol and an electrolyte solution in 4 liters of fluid. The preparation is now available in different flavors and reduced fluid volumes. Administered the evening before VC, GoLyteLy induces osmotic diarrhea, which produces a satisfactory preparation in 95% of VC patients. However, high residual fluid levels can limit the visualization of polyps, and is thus a potential limitation.
  • Phospho-soda prep (C.B. Fleet Company, Lynchburg, VA) is available in liquid and pill forms in various doses. The Fleet 1 kit contains 45 ml of sodium phosphate (an osmotic cathartic agent) as well as 4 bisacodyl laxative tablets, and 1 bisacodyl suppository (for colonic stimulation). Because it can elevate serum sodium, this preparation is contraindicated in patients with congestive heart failure or renal failure.

    "We found in a relatively small study that there is considerably less fluid in a single-dose phospho-soda (prep) compared to GoLyteLy," Macari said (Radiology, January 2001, Vol. 218:1, pp. 274-277).

  • LoSo (E-Z-EM, Westbury, NY). This magnesium citrate preparation has a very low sodium content, and can thus be used in patients with sodium restrictions. Used in combination with the firm's NutraPrep nutritional kit, LoSo produces clean, dry colons for imaging, Macari said.
  • Dulcolax (laxative and suppositories, Novartis, Basel, Switzerland) is a bisacodyl preparation that Macari's group administers in a single 45-mg oral dose. It provides a relatively fluid-rich prep similar to GoLyteLy; however, many patients prefer the Dulcolax prep because it requires less fluid intake, he said.

Even when there's significant residual fluid, turning the patient over usually redistributes the fluid sufficiently to visualize all of the polyps. Even so, Macari said, prep sometimes fails no matter which agent is used, leaving a residual coating of fecal material in the colon.

"When you find a case like this, don't give up on the patient," Macari said. "You never know when you may find a large cancer. You've got to tell the patient to reprep."

Fecal tagging agents have also shown some advantages, he said. Stool generally moves when the patient is turned over, so it can be distinguished from a polyp by simply comparing prone and supine images. If the stool doesn't move, however, fecal tagging agents can help by producing high attenuation in the stool that absorbs it. Polyps do not absorb the barium tagging agents, and thus have comparatively lower, more homogeneous attenuation, he said.

Compared with radiologists, gastroenterologists have generally done a better job of communicating with patients about bowel prep. But radiologists need to spend time discussing prep with patients in order to ensure the best results.

"Know about the preparations, and know about the limitations of them," Macari said.

By Eric Barnes
AuntMinnie.com staff writer
July 16, 2002

Copyright © 2002 AuntMinnie.com

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