Since its introduction, colonoscopy has generally been accepted to be the most effective method for examining the colon. Colonos­copy became a popular diagnostic and therapeutic modality for the large bowel shortly after it was introduced. Because of its ability to visualize the mucosal surface of the colon and delineate stool from polyps, colonoscopy has displaced the barium enema as the primary imaging tool of the large bowel. Colonoscopy has the ability to suc­tion pools of fluid from the large bowel to visualize the surface in full color, wash away debris such as fecal material or seeds, identify any protrusion as mucosal or submucosal in origin, and, with a high degree of certainty, distinguish benign from malignant lesions. Because of the visual clarity of colonoscopy, it has been hailed as the standard for colonic imaging since it was first introduced. In addi­tion to the diagnostic superiority of the colonoscope, colonoscopy has the ability to biopsy tissue, remove polyps, and control bleeding.

However, in spite of the known diagnostic accuracy of colo­noscopy, it is not an infallible examination for the discovery of colonic lesions. When the first tandem study of the miss rate of polyps during colonoscopy was published, it was met with skepti­cism by the gastrointestinal community, which had fully embraced the overriding accuracy of colonoscopy. This type of study was not repeated for another 6 years, when the Indiana group reported similar findings. The third, and most recent, tandem study was published 11 years later.
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In the earlier studies, the overall miss rates for adenomas were 15—24%.2,3 The most recent multicenter study reported that the miss rate for polyps of all types and sizes was 28%. The investigators found that 31% of polyps were overlooked, as were 21% of adenomas. However, the miss rate for all polyps equal to or larger than 5 mm was 12%, whereas the miss rate of adenomas was 9%. Among the 14 polyps and 6 adenomas larger than 5 mm that were missed during the first examination, 6 were sessile and 14 were flat. The missed lesions ranged from 1 mm to 18 mm, with 37 adenomas (median size, 3 mm) overlooked in 286 patients. In this European study, 3 advanced adenomas, varying in size from 15 mm to 18 mm, were missed. Overall, there was a 27% miss rate for small adenomas (<5 mm in diameter) and 9% for larger lesions. In a previous study of 183 patients who underwent tandem colonoscopy, Rex and colleagues reported a 27% miss rate for polyps smaller than 6 mm in diameter but a rate of only 6% for polyps larger than 9 mm. There was no significant difference in the distri­bution of the missed polyps, with 27% being missed in the right side of the colon and 21% of the polyps being overlooked in the left side of the colon. As expected, the larger the lesion, the greater the chance of detection, and the inverse was true as well. It is possible that the actual miss rate is higher than that reported, as the second exam­iner could have overlooked some polyps. In the summary of the report, Rex and associates pointed out the techni­cal limitations of colonoscopy and recommended that technology be developed to solve the problem of missed lesions in the colon.

Colonoscopic neoplasms are missed primarily due to their location on the proximal aspect of folds. The technique of an examination is critically important in the discovery of colonic polyps. It has been shown that not all missed lesions are hidden; for example, flat neoplasms can elude detection by the casual or untrained observer even when they are in the field of view of a straight forward- viewing standard colonoscope. These flat neoplasms may contain early invasive cancer or may be frank carcinomas, even though they may not reveal any specific endoscopic red flags such as erythema, friability, or ulcerations. They often appear as a slightly thickened patch of mucosa, or they may look slightly opaque, with their presence her­alded by a lack of normal vascular pattern in the colon.
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The clinical consequence of missed lesions is the progression of the adenomas to carcinomas. The National Polyp Study reported that the incidence of cancer can be markedly diminished by polyp removal, a prime reason for the popularity of screening colonoscopy in a popu­lation at risk for colorectal cancer. There have been 2 reports from Canada that colonoscopy protects against the development of cancer in the left side of the colon but does not diminish the death rate for right-sided colorectal cancer, presumably because of missed adenomas.