and used with permission of Mayo Foundation for Medical Education and Research.)
Figure 6.2 Rectal anatomy. Just inside the anal sphincter muscles, the dentate line marks the transition from the squamous mucosa lining the anal canal to the columnar epithelium of the rectum. The rectum is notable for its semilunar (half‐circle) folds called the Valves of Houston.
(Copyrighted and used with permission of Mayo Foundation for Medical Education and Research.)
Figure 6.3 Endoscopic view of the rectum. This endoscopic view shows the semilunar folds of the rectum (Valves of Houston).
Basics of endoscopic anatomy
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.
Preparation
One of the most important steps in a successful colonoscopy is adequate cleaning of the colon. Without this, polyp detection or pathology identification may be hindered and adequate visualization to enable safe scope advancement may not be possible.
There are a number of different methods that have been used to prepare the colon. These fall under two main groups: osmotic and nonosmotic agents. Ingestion of highly osmotic agents, such as sodium phosphate, magnesium citrate, or mannitol, function by creating a large osmotic gradient between the bowel lumen and interstitial tissue, resulting in a large influx of fluid from the bowel lining into the lumen. This fluid is then passed, cleansing the colon in the process. These agents have been used successfully; however, they do have significant drawbacks that have led to limitations or even the discontinuation of their use in some instances. As a group, the osmotic agents result in large intravascular and intracellular fluid shifts. In healthy individuals, symptoms of dehydration are not uncommon, however, in patients with significant heart or renal disease, these fluid shifts can lead to significant heart failure or worsening of renal function. Additionally, magnesium citrate and sodium phosphate result in some absorption of their elements and can lead to dangerous elevations of magnesium or phosphate in patients. This is especially true in the elderly or patients with renal insufficiency. Mannitol, on the other hand, is a nondigested carbohydrate and does not get absorbed, which would limit the problem of electrolyte disturbances; however, colonic bacteria can metabolize this carbohydrate, leading to the production of methane and hydrogen gases within the colon. Not only do these gases lead to distention and greater patient discomfort but they are also extremely flammable and can lead to combustion with the use of electrocautery during polyp removal.
Lavage of the colon using nonosmotic agents has become the most common method of preparation. These agents use various preparations of polyethylene glycol (PEG) in electrolyte solutions. Like mannitol, PEG is a nondigestible and nonabsorbable molecule; however, it cannot be fermented by colonic bacteria. PEG preparations are formulated so that concentrations of electrolytes and PEG are isoosmotic to patients' interstitial fluids. This results in no significant fluid shifts in either direction, leaving the patient's fluid status largely unaffected by the preparation with no dangerous electrolyte imbalances. This makes PEG a safer alternative to cleanse the colon, especially in patients with cardiac or renal disease. The strengths of these PEG preparations, however, are also their weaknesses. Since there is no fluid recruitment from the patient's interstitial fluid as seen with the osmotic agents, the entire volume of cleansing fluid must be ingested. This requires drinking large volumes of solution to achieve the preparation goals. These solutions are generally not very palatable, also limiting the tolerability of these. For many years, PEG‐based preparations consisted of 4 liters of solution taken the night before a procedure; however, preparations are now commonly split into two 1‐liter doses, one taken the night before and the second the morning of the procedure. Split‐dose preparation has been found to be superior in cleansing the colon and better tolerated and is currently the recommended method. Other variations have tried to reduce the volume of PEG solution needing
