A Visual Tour of Effective Colonoscopy The quality of colonoscopy for colorectal cancer (CRC) screening depends on the operator's ability to detect precancerous lesions (including both conventional adenomas and serrated lesions), remove precancerous lesions effectively, and observe appropriate screening and surveillance intervals. This activity reviews the elements necessary for optimizing CRC screening with colonoscopy.

The quality of colonoscopy for colorectal cancer (CRC) screening depends on the operator's ability to detect precancerous lesions (including both conventional adenomas and serrated lesions), remove precancerous lesions effectively, and observe appropriate screening and surveillance intervals. This activity reviews the elements necessary for optimizing CRC screening with colonoscopy.

New guidelines recommend that colonoscopists be able to achieve adequate preparation in 90% or more of all colonoscopies.[1] Key to increasing rates of effective preparation is to split the dose -- half the preparation is consumed the evening before colonoscopy and half is consumed on the morning of colonoscopy. For afternoon procedures, consuming the entire preparation on the day of colonoscopy is as effective as split dosing. American Society of Anesthesiologists guidelines allow clear liquids to be consumed up until 2 hours before sedation.[2] In split-dose regimens, patients are instructed to drink the second half of the preparation 4 to 6 hours before their procedure is scheduled and to finish by 2 to 3 hours before the scheduled time.

Colonoscopy is less effective at preventing right-sided cancers compared with left-sided colon cancers.[3,4] Less-effective preparation on the right side of the colon may have a role in this discrepancy. Bowel preparation completed the evening before colonoscopy may result in the appearance of the right colon (left image) with adherent chyme and debris in the cecum and ascending colon. A long interval between completion of the preparation and insertion of the colonoscope allows this material to move from the small intestine and adhere to the cecum and ascending colon. Cleaner mucosal surfaces (right image) are seen when a portion of the bowel preparation is given closer to the time of the colonoscopy.

No bowel preparation is ideal for all patients No bowel preparation is ideal for all patients. Low-volume preparations are better tolerated and highly appreciated by patients. They are appropriate for nonconstipated patients who are not receiving agents known to slow bowel motility, such as opioid analgesics. A high-volume preparation such as 4 L of polyethylene glycol-electrolyte lavage solution should be considered in patients who are constipated and in others at higher risk for preparation failure. Some patients will require preparation on 2 consecutive days to achieve effective cleansing.

Approximately two thirds of CRCs arise through the chromosomal instability pathway, are microsatellite stable, have the conventional adenoma (tubular adenoma, tubulovillous adenoma, villous adenoma) as their precursor, and transition through the adenoma-carcinoma sequence slowly, in many cases over an interval of 10 to 15 years.[5] The CpG island methylator phenotype (CIMP) pathway accounts for up to one third of CRCs. The sessile serrated polyp (also known as sessile serrated adenoma) is the primary precursor in this pathway.[5] The least common pathway is the Lynch syndrome, which accounts for 3% of CRCs.[5]

Serrated lesions represent a separate class of precancerous lesions, of which the most important is the sessile serrated polyp (sessile serrated adenoma), the main cancer precursor in the serrated group, with a prevalence of 2% to 4% in screening colonoscopy studies.[5] The serrated class includes hyperplastic polyps, which are common in the rectosigmoid colon, where they appear as small sessile or flat, pale lesions. Hyperplastic polyps have very low malignant potential. The sessile serrated polyp is flat or sessile, can be large, is located primarily in the proximal colon, and shares molecular features with CIMP-positive tumors, including hypermethylation and mutation of the BRAF gene.[5] The traditional serrated adenoma is rare, can be bulky, and is located almost exclusively in the distal colon.[5]

Several studies have highlighted the wide range of detection in patients with adenomas among members of the same gastroenterology groups, which commonly ranges from 3- to 6-fold.[6-9]

Initial studies of the range of detection among members of the same gastroenterology groups for proximal colon serrated lesions (defined here as sessile serrated polyps and hyperplastic polyps proximal to the sigmoid colon) show a greater range than demonstrated in the previously noted studies of variable detection of conventional adenomas.[10,11] This suggests that miss rates for serrated lesions are higher than those seen for conventional adenomas. This observation is consistent with the observation that cancers that occur after a colonoscopy are more likely to be located in the proximal colon, to be CIMP-high, and to be microsatellite unstable.

The subtle shapes taken by precancerous lesions are demonstrated by the Paris classification of precancerous lesion shape, which provides a common vocabulary for discussion of these lesions in the medical literature.[12] Type 1 lesions (polyps) can be either pedunculated (type 1p) or sessile (type 1s). Type 2a and 2b lesions are called “flat.” Type 2a lesions project into the lumen of the gastrointestinal tract but less than the diameter of a standard biopsy forceps (2.5 mm). Type 2b lesions are completely flat and are rare. Type 2c lesions are depressed; they are rare and are associated with a high rate of high-grade dysplasia and invasive cancer.

Skilled detectors must understand and identify a range of precancerous lesions.[13] The top 3 rows represent the conventional adenomas. The more subtle conventional adenomas, including the flat 2a lesions and the depressed 2c lesions, are more prevalent in the proximal colon. The bottom 2 rows represent the precancerous serrated lesions. The sessile serrated polyp is also distributed more toward the right colon. Thus, all of the flat and subtle lesions (Paris type 2) are distributed toward the right colon. These findings emphasize the importance of high-quality bowel preparation and a detailed examination of the proximal colon. Lesions not on the slide that occur commonly are the distal colon hyperplastic polyps, which are considered to have no malignant potential.

These 2 images of flat conventional adenomas emphasize the importance of high-quality bowel preparation and a detailed examination that will allow detection of flat lesions. The examiner should look for subtle changes in color, shape, and texture of the colon surface, as well as interruption of the normal background vascular pattern.

Sessile serrated polyps (left image) typically have a paucity or absence of blood vessels on the surface and sometimes have an adherent mucus cap (not shown). Dark spots across the surface are the typical pits of a serrated lesion. Edges of serrated lesions often are indiscrete, and these lesions often are incompletely resected during colonoscopy. The image at right shows a normal colonic wall vascular pattern toward the top and left direction and loss of the normal vascular pattern in the lower right. Note the slight change in texture of the surface in the lower right. The lower right portion is the edge of a very large, flat sessile serrated polyp.

The left image of a flat sessile serrated polyp has no mucus cap and a few tiny, thin blood vessels. On the right is a lesion with a small amount of debris at the upper left corner and a thin mucus cap over its surface.

The operator's cecal intubation rate and ADR provide data on which to base the quality of colonoscopy. The cecal intubation rate should exceed 95% in screening patients and 90% overall.[14] The ADR (percentage of patients aged 50 years and older undergoing a first screening colonoscopy who have 1 or more adenomas) should exceed 25% in male patients and 15% in female patients. Appropriate use of screening and surveillance intervals is another measure of quality.[14]