Flexible sigmoidoscopy is reasonably inexpensive, safe, relatively easy to perform, and accurate in identifying and excluding lesions within its reach. Though the best mortality-improvement data actually come from the era of rigid proctoscopy, investigators have uniformly concluded that flexible sigmoidoscopy is associated with better patient acceptance. In principle, the greater depth of insertion should also achieve the identification of an even higher proportion of lesions. In reality, this last advantage points up the main limitation of flexible sigmoidoscopy, namely its complete lack of sensitivity for lesions beyond its reach. In the most widely cited trial (Selby et al, 1992), no improvement in mortality was observed in subjects ultimately diagnosed with lesions beyond the reach of the (in that study, rigid) scope. This has led to recommendations for combined use of FOBT and flexible sigmoidoscopy, thought to perhaps achieve a mortality reduction of as much as 80%. A tremendous amount of study has attempted to determine whether a flexible sigmoidoscopy finding positive for adenomas is predictive of proximal neoplasia. Controversy has existed over whether distal hyperplastic polyps predict proximal neoplasia. The National Polyp Study rather convincingly indicated no increased risk of proximal neoplasia if the only flexible sigmoidoscopy finding is 1 or more hyperplastic polyps (Winawer et al, 1993). The risk of proximal neoplasia is increased if distal adenomas are large (>1cm) or multiple or show villous architecture. Some controversy persists over whether small (<1 cm) adenomas represent a risk of proximal neoplasia high enough to warrant colonoscopy. In the United States, the standard of care has been to evaluate the entire colon if any adenoma is identified by flexible sigmoidoscopy. Because doubt has been expressed about the magnitude of this risk (Atkin et al, 1992), the issue remains unresolved.
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