Sigmoidoscopy or colonoscopy?

Summarized by Robert W. Griffith, MD
July 31, 2000 (Reviewed: December 13, 2002)

Introduction

Colorectal cancer is the second leading cause of cancer deaths in the USA. Fecal occult blood testing and sigmoidoscopy are recognized screening methods for this condition in asymptomatic subjects; another recommendation is double-contrast barium enema. If any screen gives a positive result, colonoscopy is usually the next step. Since occult blood tests and barium enemas are not considered sensitive enough, and a number of tumors are situated beyond the view of the sigmoidoscope, some experts have recommended that colonoscopy should replace other screening methods, and be done routinely in all persons over 50 years of age. Dr Lieberman and his colleagues have determined the advantages of colonoscopy over sigmoidoscopy in early detection of colorectal tumors.

Method

Colonoscopy was done on 3,121 asymptomatic subjects recruited from Veterans Affairs medical centers. A polyethylene glycol-based electrolyte solution was used for bowel preparation. Examination up to the cecum was done under conscious sedation using Intravenous agents. Measurement, photography and a biopsy were done of the majority of lesions found. An independent, blinded pathologist reviewed the pathology specimens. Subjects were classified on the basis of their most advanced lesion.

Results

The mean age of the subjects was 62.9 years. Of the 3,121 subjects, 97% were male, and 84% were white. Colonoscopy showed that 46% had no polyps, 12.5% had a hyperplastic polyp or polyps, and 3.8% had a miscellaneous lesion that was benign on pathologic examination. This left 37.5% (1,171) who had one or more adenomas or an invasive cancer: 32% with tubular adenomas, 3% with villous adenomas, 1.5% with high-grade dysplasia, and 1% (30 subjects) with invasive cancer. Of those with invasive cancer, six had nodal involvement and two had metastatic disease.

Subjects were further classified as having advanced disease if they had an adenoma at least 10 mm in diameter villous features, high-grade dysplasia or cancer; these amounted to 329 subjects (10.5%). The splenic flexure was taken as the dividing point between proximal and distal colon; the distal colon comprised, therefore, the rectum, the sigmoid and the descending colon. A further division was made in the distal colon according to whether the descending colon was included or not.

The most advanced lesions for the proximal and distal colon were identified separately for each subject. A total of 228 subjects (7.3%) had an advanced lesion in the distal colon, and 128 (4.1%) had a lesion in the proximal colon. When the dividing point was set between the descending colon and the sigmoid, there were 188 (6.0%) in the distal part, and 169 (5.4%) in t he proximal part.

Further analysis showed that the presence of distal hyperplastic polyps was not associated with an increased risk of advanced lesions in the proximal colon. However, among subjects with no adenomas distal to the splenic flexure, 48 (2.7%) had advanced lesions in the ascending or transverse colon; on the other hand, those with large (greater than 10 mm) or small adenomas (less than 10 mm diameter) distal to the splenic flexure were significantly more likely to have an advanced lesion in the proximal colon - odds ratios of 3.4 and 2.6, respectively. The presence of more than two distal adenomas was not increase the risk of finding a proximal advanced lesion.

Comment

One determining factor in diagnosing advanced lesions in the colon are clearly dependent on how much of the colon is viewed on endoscopy. Sigmoidoscopy limits the length of the colon that can be inspected - it may be confined to the rectum and sigmoid colon, or it may extend to include part or the entire descending colon. While the majority of advanced lesions detected in this study were distal to the splenic flexure, more than half the cases of advanced proximal lesions would not have been detected by sigmoidoscopy up to the flexure.
The findings in this study compliment those from another trial in the same issue of the Journal¹ . Here, almost 2,000 asymptomatic employees of a company had colonoscopy screening, and 50 persons were found to have advanced proximal neoplasms that would not have been found on sigmoidoscopy.

In the USA, most standard screening recommendations for colorectal cancer include fecal occult blood testing and sigmoidoscopy in persons over 50. Barium-enema is an alternative to sigmoidoscopy, at the discretion of the physician. (In other countries, such recommendations are probably even less stringent.) Unfortunately, aversion to the problem and the methods used result in less than 30% of the candidates for screening actually being screened². To make matters worse, fecal occult blood testing has limited sensitivity, while sigmoidoscopy does not extend far enough proximally to detect a substantial proportion of asymptomatic tumors.

The present studies support the instinctive assumption that colonoscopy is necessary to provide efficient screening for lesions involving the colon proximal to the splenic flexure. Of course, before such a procedure can be adapted universally for people over a given age, the cost and attendant risks have to be analyzed. This debate will no doubt occupy health professionals and providers for some time. By the time it is settled, there may be cheaper, more effective ways of visualizing the whole colon. In the meantime, efforts must be intensified to educate the public, and their health professionals, of the importance of undergoing one or another screening procedure for colorectal cancer, as a minimum. Many of those who pay for their own health care may well adapt the strategy of having a colonoscopy every 5-10 years after 50.

Source

• Use of colonoscopy to screen asymptomatic adults for colorectal cancer DA. Lieberman, DG.  Weiss, JH. Bond,   et al., N Engl J Med, 2000, vol. 343, pp. 162--168

Footnotes
1. Risk of advanced proximal neoplasms in asymptomatic adults according to the distal colorectal findings. T. Imperiale, DR. Wagner, CY. Lin,  et al., N Engl J Med, 2000, vol. 343, pp. 169--174
2. Going the distance - the case for true colorectal-cancer screening. DK. Podolsky, N Eng j med, 2000, pp. 207--208

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