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Endoscopists use anatomical landmarks to help them identify where their scope is at any given point of the procedure to ensure pathology is correctly located for the purposes of documenting disease extent, for future re‐examination, or for locating pathology for possible surgical intervention. Another method to identify location is by checking the scope length markings at the anal verge. These markings will inform the endoscopist how many centimeters of scope are inside the patient. However, relying on the scope length from the anal verge is unreliable, especially in the right colon where variances in the anatomic length of an individual's colon segments can lead to marked variability. Also, using the scope markings during the insertion phase of the exam is also prone to substantial error as looping can greatly increase the length of scope inserted and greatly alters the reliability of these numbers as they correlate to the location. As a rule, these numbers are only used as a crude estimate of the location, typically only in the left half of the colon and only during the withdrawal phase of the exam. Instead, the anatomical landmarks of the colon are in general more reliable markers of the location than scope depth.

Figure 6.4 Endoscopic view of the transverse colon. The transverse colon is easily identified by the triangular appearance of the lumen. Externally, the tinea coli are located at the apexes of the triangular folds.

Figure 6.5 Endoscopic view of the hepatic flexure. At the splenic and hepatic flexure, the purplish hue of the spleen or liver can often be seen through the wall of the colon.

Figure 6.6 Endoscopic view of the cecum. This view of the cecum demonstrates the small semilunar os of the appendiceal orifice (AO) as well as the indentations of the tinea coli coming together externally to make up the “Crow's foot (CF).” The ileal–cecal valve (ICV) can be seen at the thickening of the first major fold above cecal base.

The major landmarks during withdrawal start with the appendiceal orifice, crow's foot, and ileocecal valve of the cecum. The next major landmark is the acute angulation in the colon with the purplish hue of the liver representing the hepatic flexure. The triangular folds of the transverse colon make it readily identifiable. At the distal end of the triangular lumen is a second acute angulation with a purplish hue of the spleen, which signifies the splenic flexure, and is located at roughly 50 cm from the anal verge. Just past this acute turn, one often encounters a collection of retained liquid stool that collects at this point as the proximal segment of the descending colon is the most gravity‐dependent portion of the colon with the patient in the left lateral decubitus position. The descending colon is marked by a long straightaway from roughly 50 to 30–35 cm from the anal verge, followed by a number of acute turns and the more muscular haustra of the sigmoid colon. The rectosigmoid junction is located at roughly 15 cm from the anal verge. Distal to the junction, the rectum is identified by the increase in lumen caliber and the three prominent semilunar folds called the valves of Houston (Figure 6.3). The dentate line is seen on retroflexion in the rectum.