Three-dimensional morphology of the Colon
The ideal condition for the endoscopist would be to explore a straight colon, extended on a single plane, without bends or angles, so as that the “push” technique could allow him/her to intubate the viscus up to the cecum without causing pain or discomfort to the patient.
In real life, unfortunately, the colon has a more or less winding shape, mostly characterized by variably marked angles, between the rectum and the sigmoid (1), the sigmoid and the descending colon (2), the descending and the transverse colon (3), the distal and the proximal transverse colon (4), and the proximal transverse and the ascending colon (5) (fig. 1). Therefore, the skill of the endoscopist is to achieve, during the intubation phase, the conditions that favor the straightening of bends and the progression to the cecum with the colon as straight as possible.
Fig. 1: throughout the colon there are variably marked bends, in different segments: rectum and sigmoid (1), sigmoid and descending colon (2), descending and transverse colon (3), distal and proximal transverse colon (4), and proximal transverse and ascending colon.
Three-dimensional view of the colon show the tendency of the various colonic segment to arrange themselves within the abdomen in a postero-anterior direction. For example, due to the curvature of the sacrum in the pelvis, the sigmoid colon extends from the posterior to the anterior abdominal space in the tract passing over the sacrum promontory (A); or the transverse colon (B), which from splenic flexure (1) to the hepatic flexure (2), has an arched shape toward the anterior abdominal wall (b) (fig. 2).
Fig. 2: from the recto-sigmoid junction, the first tract of the sigmoid colon extends toward the anterior area of the abdomen (A). The transverse colon, which links the splenic (1) to the hepatic flexure (2), shows an arched development toward the anterior abdominal space.
Consequently, the progression of the colonoscope along the sigmoid makes a “spiral” movement, which explains the utility to associate to the “push” maneuver both the angulation of the distal tip of the scope to “flatten” a coil and the “rotation” of the shaft of the scope to prevent re-looping of the colon once the bend has been passed.
The knowledge of these anatomic details, is useful when the progression of the colonoscope is laborious, and the assistant needs to exert a specific “compression” on the abdomen to reduce or limit the bends of the scope (Fig. 3)
Fig. 3: the specific points of compression to facilitate the progression of the colonoscope are: on the sigmoid colon (left quadrant of the abdomen, under the umbilicus – red arrow), on the transverse colon (soon under the sternum – yellow arrow), on the proximal part of the ascending colon (the so-called “Waye’s point”, right posterior lumbar region – orange arrow).
The maneuvers of specific compression should always be preceded by a gentle retraction of the colonoscope for the whole tract that does not determine advancement, avoiding excessive retraction that may cause retrogression. On the sigmoid colon, the left hand of the nurse assistant, slightly arched, is positioned to “limit” the formation of an excessive curve, painful for the patient, and to achieve an efficacious progression of the colonoscope (fig. 4).
Fig. 4: Positioning of the left hand of the nurse assistant to obtain a specific compression that facilitates the progression of the colonoscope (patient in supine position).
In the transverse colon, after the calibrated retraction of the instrument and after suctioning of excessive air to obtain maximal shortening of the colon, with the patient in supine position, the nurse assistant exerts a stretch-handed compression on the central upper part of the abdomen, soon below the sternum, checking that such maneuver causes a slight advancement of the colonoscope (fig. 5).
Fig. 5: position of the left hand of the nurse assistant to obtain a specific compression on the transverse colon (patient in supine position).
It is very important for the endoscopist to know which colonic segments are movable during the intubation, in order to prevent excessive curves, or to apply maneuvers to reduce them. The sigmoid coil should be considered first; this can show different shapes according to the tension exerted by pushing of the scope and its own anatomical length. This because the so-called meso-sigmoid (A), a ligament that fastens the sigmoid colon to the posterior abdominal wall, can have very variable length and looseness (Fig. 6). Second, the transverse colon is another colonic segment that can be mobilized during the intubation, widely variable in length among subjects, held up in the abdominal cavity by a ligament called transverse mesocolon (B). Also the cecum is an intra-peritoneal movable segment but, being the terminal end of the progression, does not represent a key element in the strategy of intubation.
Fig. 6: the colon has movable segments in its extension from the rectum to the cecum. The sigmoid can have a variable length, is fastened to the posterior abdominal wall by the meso-sigmoid (A), which for educational convenience is shown in the figure in a position opposed to the real one, marked by the asterisk (*). The transverse colon, intra-peritoneal and of variable length as the sigmoid, is held up by the meso-transverse ligament (B). The phreno-colic ligament (C) extends between the splenic flexure and the costal insertions of the diaphragm. The ascending and descending colon are both retro-peritoneal colonic segments (blue lines) and therefore not movable during the colonoscopy.
If we consider the many possible inter-personal variations of the colon anatomy, the three-dimensional representation of the colon helps us to comprehend how the endoscopist should adopt different techniques of intubation, tailored to each morphologic type. As an example, a sigmoid colon with an exceedingly wide coil and a longer meso-sigmoid, obligates a longer length of the colonoscope in the coil soon above the recto-sigmoid junction. Consequently, as the instrument is inserted, according to the modality of insertion, either an alpha loop (fig. 5), or a wide N loop can occur.
Fig. 7: a wide sigmoid loop and a longer meso-sigmoid favor the formation of different types of colonic loops, such as an alfa loop, which require a different technique of intubation compared to a “standard” straight sigmoid.
Also the transverse colon often presents a lengthened shape, with such a long meso-transverse to engage the pelvis (Fig. 8).
Fig. 8: the so-called “ptosic” transverse colon or deep transverse can reach the pelvis and require particular maneuvers to achieve the progression of the colonoscope.
In such cases, the possibility to reach the hepatic flexure often requires sequential or combined maneuvers: beside suctioning any excess air and exerting a manual compression on the upper part of the abdomen, both stiffening of the scope (if a variable stiffness colonoscope is available) and changing the position of the patient to the right lateral decubitus may be useful.