Colonoscopy

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Colonoscopy in this section refers to optical colonoscopy, to be distinguished from "virtual colonoscopy" that is an earlier term for CT colonography. There is a strong biologic argument that colonoscopy, with direct visualization of the colonic mucosa and the ability to biopsy or excise polyps and localized cancers, can prevent colorectal cancers and deaths.

Colonoscopy has the added benefit that lesions can be removed during the same procedure. Furthermore, colonoscopy detects proximal lesions that would be missed by screening sigmoidoscopy [61-63]. As an example, in a study of screening colonoscopy in 1994 asymptomatic adults, 5.7 percent had advanced neoplasms; 1.6 percent had only a proximal advanced neoplasm. Thus, it would be necessary to screen 64 patients with colonoscopy rather than sigmoidoscopy to find one additional person with an advanced neoplasm.

There are no published randomized trials of the effectiveness of screening colonoscopy in average risk patients. A study of patients with polyps in which the colon was cleared of polyps by colonoscopy showed a large reduction in the development of advanced lesions (76 to 90 percent) compared with patients in other studies who did not have polyps removed or compared with the general population [64].

Although there is no direct evidence that colonoscopy reduces mortality in people without symptoms or known polyps, studies showing that sigmoidoscopy reduces CRC deaths provide indirect evidence that colonoscopy should also be effective [47-49].

The effectiveness of colonoscopy in detecting proximal lesions, as performed in the community setting, has been called into question, however. A case control study in Canada compared colonoscopy rates for 10,292 patients who died of colorectal cancer and 52,450 matched controls. Completed colonoscopy was associated with decreased deaths from left-sided colorectal cancer (OR 0.33, 95% CI 0.28-0.39) but not from right-sided cancer (OR 0.99, 0.86-1.14) [65]. This study had limitations, including the inability to distinguish screening and diagnostic colonoscopies, and that one third of colonoscopies were performed by clinicians other than gastroenterologists or surgeons.

The risk associated with screening colonoscopy is higher than the risk with sigmoidoscopy (OR 1.8) [46]. The rate of major complications (perforation and major bleeding) is about 1 per 1000 colonoscopy procedures [66-68]. Comorbidities, increasing age, polypectomy, and less experienced endoscopists increase the risk of perforation [67,68]. In a series of over 16,000 diagnostic colonoscopies in one health care system, serious complications occurred at a rate of 0.8 per 1,000 procedures without biopsy and 7.0 per 1,000 colonoscopies with biopsy or polypectomy [69]. Increased morbidity in patients undergoing diagnostic tests may have resulted in a higher adverse event rate than would be expected in a screening population.

In an earlier report of a Medicare population, the risk of perforation was 1.96 per 1000 procedures, including both inpatient and outpatient colonoscopies [46]. A perforation rate of 0.6 per 1,000 outpatient procedures in a sample of Medicare patients (age 65 to 95 years) was reported more recently [68]. In this report, the risk of perforation was four times greater for patients who had a polypectomy. The risk of bleeding was 8.7 per 1,000 colonoscopies in which polypectomy was performed. Adverse events within 30 days of colonoscopy occurred more frequently in patients who were older, or had a history of diabetes mellitus, stroke, chronic obstructive pulmonary disease, atrial fibrillation, or heart failure.

Colonoscopy is expensive. Patients usually receive conscious sedation for comfort and must be accompanied home after the test and cannot return directly to their usual activities. Many patients find the rigorous bowel preparation worse than the procedure itself. (See "Bowel preparation for colonoscopy").

The sensitivity of colonoscopy for detection of adenomas and carcinomas is dependent upon the experience and technique of the colonoscopist. One study of screening colonoscopies by 12 experienced gastroenterologists in a community-based practice found the detection rate for adenomas to range from 9.4 to 32.7 percent among clinicians [70]. Detection rates for neoplasms were significantly higher for clinicians who, on average, took longer than six minutes for withdrawal of the colonoscope, compared to those who took less than six minutes.

The sensitivity of colonoscopy has typically been estimated by using it as its own reference standard (results of two colonoscopies performed shortly after one another). A systematic review of studies of tandem colonoscopies (n = 465 patients) found a miss rate of 2 percent for adenomas ≥10 mm, 13 percent for adenomas 5 to 10 mm, and 25 percent for adenomas <5 mm [71]. The overall miss rate for polyps of any size was 22 percent.

In contrast, a large study (n = 614) of patients at high risk for colon lesions found the sensitivity of colonoscopy to be 98 to 99 percent for lesions ≥6 mm in size [72]. In this study, the reference standard was an aggregate view of the colon based on double-contrast barium enema (DCBE), two-dimensional computed tomographic colonography (CTC), and repeat colonoscopy of the involved bowel segment if a lesion was found on imaging that was not seen on the initial colonoscopy. Colonoscopy sensitivity in this study was substantially higher than reported in other studies.

Some polyps and cancers may be difficult to detect because of their location, and thus may be missed on both initial and subsequent colonoscopy studies, making repeat colonoscopy less than an ideal gold standard. It appears that flat or depressed lesions occur more commonly than previously thought, are at least as likely to represent advanced neoplasia even if small, and can be difficult to detect by either optical colonoscopy or CTC [4].

In a study of patients receiving both CTC and colonoscopy, 14 of 15 nonrectal neoplasms missed by colonoscopy were located on a fold [73]. Five of six missed rectal lesions were located within 10 cm of the anal verge. The authors estimated that when state-of-the-art three-dimensional CTC was used as a reference standard, the miss rate of colonoscopy for large adenomas (≥10 mm) was 12 percent, significantly higher than the previous estimates discussed above.

Frequency of testing — Colonoscopy has become increasingly popular for screening. The American College of Gastroenterology now considers colonoscopy the "preferred" screening test when it is available [74], but other expert groups simply list is as an option.
There is limited evidence to determine the optimal frequency for screening with colonoscopy.

No cancers were found in follow-up colonoscopy testing in a group of 1256 average risk patients who had had an initial screening colonoscopy five years earlier (detection rate 0 percent; 95% CI 0-0.24%) [75]. Nineteen advanced adenomas were found among 16 persons (1.3 percent), with risk greater in men.
A study from Canada followed a cohort of patients who had a negative colonoscopy (n = 35,975), and compared the subsequent incidence of colorectal cancer in this cohort to the general population [76]. The subsequent incidence of colorectal cancer following a negative colonoscopy was 31 percent lower than expected at six months, and remained reduced beyond 10 years (72 percent lower than expected).
Follow-up of over 100,00 patients who had a negative colonoscopy (either screening or diagnostic) found a decreased risk of distal colorectal cancer at 14 years, compared to the general population (RR 0.21, 95% CI 0.05-0.36), and a decreased risk for proximal colorectal cancer that persisted for 7 years [77].
These findings suggest that a 10 year interval for colonoscopy following an initial negative study is an acceptable strategy. When an adequate screening colonoscopy is accomplished, additional screening with intercurrent stool tests between colonoscopy examinations is not necessary.