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One of the hardest skills for trainees to master is the prevention and adequate reduction of loops that develop during scope advancement. This section will address the common types and locations of loops that form and how to manage these effectively. One key concept to understand with loop formation is that of “force vectors.” Force vectors are where and in what direction the majority of the pushing force is being delivered in the colon. If a scope is perfectly straight, all of the force is being translated directly into tip advancement. If there is a 90° or greater turn, much of this force is directed at the wall of the colon on the outside part of the turn (Figure 6.21). As the wall pushes back, some of this opposing force gets delivered forward to the scope tip, resulting in advancement, some back to the operator as resistance and some absorbed by the elastic nature of the colon wall as a loop. Reducing and straightening these loops not only removes the force against the colon wall but with correct technique can also result in prevention of recurrent loop formation and better delivery of the pushing force to the scope tip.

Figure 6.21 Force vector. In this illustration, the tip of the scope is deflected greater than 90° around an acute turn in the colon. In this configuration, the force vector (FV) of any attempts to advance the colon will be directed against the wall of the colon on the outside turn. This will be felt by the endoscopist as resistance to advancement or will result in looping of the scope shaft here or elsewhere in the colon.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

The sigmoid colon is the location where most loops occur. This is due to the serpentine nature of this section of colon accompanied by the fact that it is freely mobile within the abdominal cavity. The most common natural course of the sigmoid is a clockwise spiral between the rectum and descending colon. As the tip of the scope makes the first acute turn from the rectum into the sigmoid and the scope is advanced, the shaft of the scope behind the flexible scope tip tends to be pushed upward in the abdomen as the force vector of the scope is still relatively straight‐in from the anus and rectum (Figure 6.22). This results in loss of “one‐to‐one” motion; a term meaning the scope is not advancing as much (or not at all) as the shaft of the scope is being pushed in. This extra inserted scope that is not resulting into tip advancement is instead contributing to the development of a loop. Looping can also result in “paradoxical movement” of the scope, which is when the scope tip actually moves in the opposite direction as the shaft is pushed or pulled.

Figure 6.22 Sigmoid loop. As the scope makes multiple turns in the sigmoid colon, advancing the scope frequently results in the force being transmitted laterally against the sigmoid walls (arrows) resulting in loop formation.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

There are three main types of loops that develop in the sigmoid colon. The “alpha‐loop” is one of the most common and is termed this because the sigmoid is looped in a counterclockwise spiral in the shape of the Greek letter of its name (Figure 6.23) ( Video 6.5). The second most common loop is the “N‐loop” called this because it too is shaped like the letter of its name (Figure 6.24) ( Video 6.6). This loop follows and exaggerates the normal S‐shaped spiral of the sigmoid colon. Less common than these two is the “reverse alpha‐loop” that follows a similar spiral to the alpha‐loop but the more proximal sigmoid passes behind the more distal sigmoid (Figure 6.25) (Video 6.7). Which type of loop forms is likely due to variances in sigmoid anatomy? The method best suited for reduction of these sigmoid loops varies depending on the type of loop. In each of the loops, it is generally advisable to attempt to advance the scope beyond the splenic flexure, or other acute turn, if possible before attempting reduction. This allows the flexible portion of the scope tip to hook around this flexure and act as an anchor. This allows for greater direct force on the loop itself during reduction and torques maneuvers. Once anchored, the dials are held with the left thumb to prevent the scope tip from straightening out and slipping back into the descending colon. The two most common types of loops (alpha‐ and N‐loop) respond to slow scope withdrawal augmented by clockwise scope torque (Figures 6.23 and 6.24). During this maneuver, the tip of the scope may advance or simply remain motionless as the scope shaft is withdrawn. Clockwise torque and withdrawal of the shaft are continued until the scope tip begins to respond by starting to move backward in a one‐to‐one manner. This is evidence that the loop has been reduced. Torque is key to the maneuver as this will untwist the spiral nature of the sigmoid and create a straighter lumen if done correctly. The most common cause for failure of this technique is either failure to withdraw enough scope to re‐establish one‐to‐one motion or inadequate clockwise torque of the scope shaft during withdrawal. It is not uncommon to require 360° of torque or more during sigmoid reduction to adequately remove the spiral nature of this segment of the colon. Another cause of failure is the presence of a reverse alpha‐loop. Suspicion of this should arise if the usual clockwise maneuver repeatedly fails.

Figure 6.23 Alpha‐loop. One of the most common types of sigmoid loop formation is the alpha‐loop. This can be reduced with clockwise torque of the scope shaft as it is slowly pulled back. Once the loop is reduced, the scope shaft is again straight and can be readily advanced again.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

Figure 6.24 N‐loop. The N‐loop is also a common type of loop formation in the sigmoid and can also be reduced with clockwise torque and slow withdrawal like the alpha‐loop.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

Figure 6.25 Reverse alpha‐loop. A reverse alpha‐loop follows a similar configuration as an alpha‐loop; however, the loop passes posteriorly to the scope shaft. Attempts at clockwise torque of the scope shaft will typically result in tightening of this loop and a sensation of increasing resistance to torque attempts by the endoscopists. Instead, counterclockwise torque and withdrawal is needed to reduce this type of loop.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

In cases such as this, attempts at counterclockwise torque during scope withdrawal may result in successful loop reduction. Other clues that the direction of required torque should be reversed are if one experiences increasing resistance to scope shaft rotation during torque attempts, or if the tip of the scope moves backward with the torque maneuver. In general, the correct direction of torque should result in a sensation of decreasing resistance to shaft rotation and modest scope tip advancement. Once a loop is reduced and the scope is straight, the torque that was used in the reduction can be undone. If the scope is straight, this should not result in any reproduction of the spiral but rather simply rotate the entire shaft of the scope back to a comfortable position. Some scopes are equipped with a variable stiffness feature that is controlled by a dial at the base of the handle. If this feature is available, increasing the stiffness of the scope, now that the loops are removed, can help prevent the reformation of these loops as the scope is advanced. This increased stiffness should be removed during subsequent attempts at loop reduction and reengaged when pushing forward. External pressure can also help prevent the reformation of loops and will be discussed below.

Figure 6.26 Transverse colon loop. Like the sigmoid, the transverse colon is also typically very mobile and can result in an assortment of loops. Reduction techniques vary but often require a combination of torque with slow shaft withdrawal. The direction of torque required will depend on the nature of the loop.

(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)

The other mobile section of the colon, the transverse colon, also frequently requires loop reduction. Looping here is mainly caused by redundancy of this section of the colon looping down, resulting in multiple changes in the force vector of the scope. The reduction technique is similar to loop reduction in the sigmoid. If possible, the flexible tip of the scope should be advanced around the hepatic flexure and hooked around this turn by holding the scope dials in place. With appropriate torque and withdrawal, the transverse colon will be straightened out. The scope tip may advance considerably down the right colon during this maneuver (Figure 6.26) (Video 6.8). The direction of torque here can vary, however, like the sigmoid; the correct direction of torque should result in a sensation of decreasing resistance and modest scope tip advancement.