For resecting colorectal lesions, underwater endoscopic mucosal resection is more effective than conventional EMR. That’s according to a recent meta-analysis published in Gastrointestinal Endoscopy by Saurabh Chandan, MD, and his colleagues.

“We concluded that while both techniques have comparable rates of R0 and piecemeal resection, underwater EMR outperforms conventional EMR in terms of successful en bloc resection, incidence of polyp recurrence and lower rates of incomplete resection,” Dr. Chandan, an assistant professor of gastroenterology and hepatology at CHI Health Creighton University Medical Center in Omaha, Neb., said.

The utility of conventional EMR is limited by a reduced ability to accomplish en bloc resection for lesions larger than 20 mm, thus leaving a risk for residual adenomas and recurrent polyps. Indeed, the European Society of Gastrointestinal Endoscopy advises that en bloc EMR should be limited to lesions no larger than 20 mm in the colon and no larger than 25 mm in the rectum (Gastrointest Endosc 2020;49:270-297).

“Underwater EMR has emerged as an alternate technique for resecting large lesions. It’s easily learned and doesn’t require additional or new equipment,” Dr. Chandan said.

Advantages of Underwater EMR

With underwater EMR, the colon is filled with distilled water or a saline solution instead of air or carbon dioxide, which decreases tension in the wall of the colon. The technique eliminates the need for submucosal injection, as the wall retains its native thickness and the “floating” effect of water submersion separates the mucosa and submucosa from the muscularis propria. Visualization of polyps is also enhanced by the magnification effect of refraction with the water and 3D imaging.

Water also facilitates maneuverability of the endoscope. Sessile or flat mucosal lesions become more contracted and polypoid, which facilitates successful snare resection and yields higher rates of en bloc resection. Because of these features, underwater EMR seems to be more effective and safer than conventional EMR, Dr. Chandan said.

The systematic review and meta-analysis included 11 studies, of which four were randomized controlled trials and seven were retrospective or prospective cohort studies. Only studies that reported results with both underwater and conventional EMR were included. The final analysis was based on 1,851 patients who had 1,071 lesions removed by underwater EMR and 1,049 by conventional EMR.

The primary outcomes were pooled odds ratio (OR) and proportions of successful R0 resections (complete en bloc resection of a lesion with tumor-free lateral and vertical margins), en bloc resection for colorectal polyps greater than 20 mm and lesion recurrence (adenoma or cancer at resection site on follow-up colonoscopy).

For colorectal lesions of all sizes, the underwater technique was superior in terms of the rates of successful en bloc resection and incomplete resection and lesion recurrence in pooled analyses:

  • Lesion recurrence: 11.3% versus 27.0% (OR, 0.3; P=0.01)
  • Incomplete resection: 5.8% versus 19% (OR, 0.4; P=0.001)
  • Successful en bloc resection: 58.7% versus 49.7% (OR, 1.49; P=0.04)

The two approaches were similar with regard to en bloc resection of polyps larger than 20 mm, achievement of R0 resection, piecemeal resection and diagnostic accuracy for colorectal cancer:

  • Successful R0 resections across all studies: 95.3% versus 81.5% (OR, 3.1; P=0.14)
  • En bloc resection of lesions larger than 20 mm: 31.0% versus 35.4% (OR, 0.8; P=0.75)

The two approaches were also comparable in terms of resection times and adverse events, including immediate and delayed bleeding and perforation, according to the researchers.

When the analysis was restricted to randomized controlled trials, underwater EMR performed significantly better than conventional EMR with regard to successful en bloc and piecemeal resection at index colonoscopy, Dr. Chandan said.

“While no one technique can be applied to all polyps, selection of polyp removal technique should be based on polyp size, its location, quality of bowel preparation as well as endoscopist experience level,” Dr. Chandan said. “Our study will definitely add weight to the growing body of literature indicating safety and efficacy of underwater EMR.”

Learning Underwater EMR

Kenneth F. Binmoeller, MD, the director of interventional endoscopy services at California Pacific Medical Center, in San Francisco, said underwater EMR has multiple advantages. Its main drawback, he said, is poor visualization of a poorly prepped colon. “Fortunately, vigorous water exchange will usually clear this,” he said.

Regarding the meta-analysis, he said he was surprised that despite a significantly higher rate of en bloc resection for all polyps with underwater EMR, its resection rate for large lesions was not higher. “A key advantage of underwater EMR is the ability to achieve en bloc resection of larger polyps. In my experience, around 50% of lesions larger than 2 cm can be removed en bloc with a single ensnarement,” he said.

Endoscopists interested in performing EMR need first to become “‘at home’ working in a water-filled lumen,” Dr. Binmoeller said. “Start with a small 1- to 2-cm lesion in the rectum or left colon, as these can probably be removed en bloc, then work up to larger granular-type lesions. The last learning step is removal of flat [IIA] nongranular lesions. The learning curve is very short for someone already performing EMR. The ideal is to observe a few cases with someone who is already performing underwater EMR.”

Dr. Binmoeller is the author of a comprehensive review of underwater EMR, which provides guidance for endoscopists (J Interv Gastroenterol 2014;4:113-116).

—Caroline Helwick


Dr. Binmoeller is the founder of Endeau Inc., a company that is developing tools to facilitate underwater endoscopy. Dr. Chandan reported no relevant financial disclosures.

This article is from the June 2021 print issue.