Colonoscopy: A Guide to Endoscopic Screening and Therapy
Colonoscopy: A Guide to Endoscopic Screening and Therapy
Colonoscopy has become the preferred method for colorectal cancer screening in the United States. For patients at average risk, the procedure is typically performed every 10 years, beginning at age 50. Colonoscopy reduces colorectal cancer mortality by 2 mechanisms: diagnosis of cancer at an earlier (presymptomatic) stage and prevention of cancer development by resection of premalignant adenomas. In addition to colorectal cancer screening, colonoscopy is also used to diagnose and treat GI bleeding, obstruction, colitis, and other disorders of the colon and rectum (Table 1).
In the United States, most colonoscopies are performed by gastroenterologists and surgeons. However, adequately trained primary care physicians, physician assistants, nurse practitioners, and nurses perform colonoscopy in a variety of set-tings throughout the country. Case series from several medical centers have documented the safety and efficacy of colonoscopies performed by experienced family physicians in these centers.1-3
PREPARATION OF THE BOWEL
Although colonoscopy can be performed without a bowel preparation in certain emergency situations, adequate visualization demands a thorough cleansing of the colon to remove solid and liquid stool. A variety of regimens can be used. These usually consist of a period of dietary restriction in combination with ingestion of a preparation agent.
Dietary restriction. An example of typical dietary restriction is consumption of a light breakfast of white bread and jelly the day before the procedure, followed by a clear liquid diet for the rest of the day. Alternatively, many centers prefer that patients take only clear liquids for the entire day before the procedure. Because most patients in the United States receive conscious sedation for colonoscopy, they should not eat during the 6 hours before the procedure or have any liquids for at least 2 hours before.
Discontinuation of certain medications. Iron supplements should be stopped at least 2 days before colonoscopy because iron can impair visualization. Decisions regarding the use of platelet inhibitors and anticoagulants can be complex. Many endoscopists are wary of performing polypectomy in patients who are currently taking these agents. Guidelines are available (for example, at http://www.asge.org); however, in many cases the decision to discontinue therapy may need to be individualized.
Preparation agents. The afternoon before the procedure, the preparation agent is ingested. One commonly used regimen is 1 gallon of polyethylene glycol, which is consumed by drinking 1 cup every 10 minutes. Flavored polyethylene glycol preparations, sulfate-free preparations, and half-volume preparations with various additives are also commonly used. Polyethylene glycol is a nonabsorbable, osmotically balanced solution. Because it causes comparatively few electrolyte derangements and fluid shifts, it is relatively safe even for patients with renal insufficiency and other comorbidities. Major disadvantages of polyethylene glycol are the large volume that must be consumed and its poor palatability.
Another popular preparation is sodium phosphate (phosphosoda), which is available as a concentrated liquid or as tablets. Many patients find sodium phosphate more palatable than polyethylene glycol, but this agent can result in kidney injury, fluid shifts, and electrolyte imbalances.
Because colonoscopy is often moderately painful, it is usually performed under conscious sedation. Midazolam with either fentanyl or meperidine are commonly used, but deeper sedation with propofol is favored by some practitioners. Because propofol has a short half-life, patients typically recover quickly from the procedure; however, apnea can occur, and the treating team must be prepared to ventilate the patient if necessary.
A registered nurse typically administers sedation and monitors the patient. However, if propofol is used, then an anesthesiologist may be preferable or even required. A second assistant is usually present to assist with endoscopic accessories, to administer abdominal pressure to help advance the scope effectively, and to manage the endoscope shaft if you prefer to delegate this task.
FIRST STEPS Start the procedure by performing a digital rectal examination while applying lubricant to the anus—usually with the patient lying on his or her left side. Then insert the endoscope into the rectum. To allow visualization, insufflate air into the rectum by covering the air/water button; if the lens is covered by debris, pressing down on the air/water button will clean it by streaming a small jet of water across the lens. Then advance the endoscope while maintaining a view of the lumen in front of the scope. Two dials on the scope allow you to steer the tip up and down and left and right.
DEALING WITH LOOPS
It is rarely possible to advance safely through the entire colon by simply steering the tip toward the center of the lumen and advancing the scope. This is because, after traversing several turns, the scope will no longer move forward smoothly. When the scope is simply advanced in this situation, there is a tendency for the loops in the colon to enlarge and for the tip to remain stationary rather than move forward as desired. This phenomenon is colloquially referred to as “looping.”
Figure 1 – These 2 pairs of images, obtained from a 3-dimensional magnetic scope device, are examples of scope looping and subsequent successful reduction of loops. All are standard anteroposterior views with the patient’s head at the top of the screen and the patient’s left side on the right of the screen. (A) Here the scope tip is at the splenic flexure; there is significant looping in the sigmoid colon. (B) After withdrawing a short length of scope while applying torque in a clockwise direction, the sigmoid loop has been eliminated and the scope tip has moved forward just past the flexure. (C) This is an example of more significant looping in the sigmoid colon. (D) Here the loop has been reduced by withdrawing a short length of scope while applying torque in a counterclockwise direction. The scope tip is then easily advanced to the proximal transverse colon.
A variety of maneuvers can be used to reduce the loops. One is to withdraw the scope while applying torque to the shaft—in either a clockwise or a counterclockwise direction (Figure 1). When this maneuver (called a reduction) is successful, it results in a pleating of the colon over the scope, like a curtain being fitted over a curtain rod. Once the pleating occurs, it is often possible to start advancing the scope again without the loop redeveloping.
Another technique for avoiding looping is to reposition the patient onto his back, stomach, or right side. Having the assistant apply pressure strategically to various locations on the abdomen can also be helpful.
With these techniques, the cecum can be reached in about 98% of patients, although some patients require changing to a thinner scope (pediatric scope) or occasionally a longer scope (enteroscope). In most cases the cecum is reached in 3 to 10 minutes; however, significantly more time is needed in a minority of patients.