The sigmoid colon pounces out of the retroperitoneum like a horse out of the barn, as it gains its freedom from the confines of the retroperitoneum. It acquires a cloak of long mesentery – the sigmoid mesocolon that fans out from the base to the apex of the sigmoid colon. The freedom does come with a price and it is at risk for torsion or volvulus in the same way as the cecum is predisposed, though sigmoid volvulus is far more common than cecal volvulus. In the perfect world the sigmoid colon holds on to the feces it receives from the straight and narrow descending colon. Its shape and mobility allows it to store fairly large volumes feces. Although it usually resides in the pelvis it has the freedom to move up into the abdomen because of its mobility. When opportunity knocks it passes its product onto the rectum which signals to its owner the need to move on and out. If this message is received at an inconvenient time, the stool is forced back into the sigmoid which holds onto the feces in storage until a more convenient time to evacuate is found.
The sigmoid is “s” or sigmoid shaped and hence its name. It is very different in shape to its predecessor in the chain – the descending colon which is straight and the rectum which comes after it which is also relatively straight. It measures about 40 cms. in length but can sometimes assume immense proportions both in diameter and length. (Bhatnagar) It is large in patients and cultures that have larger volumes of fiber in their diet and thus in Africa for example where dietary fiber is high the sigmoid colon is much larger. The incidence of sigmoid volvulus in these cultures is also much higher.
The sigmoid colon transitions to the rectum at about the level of the 3rd sacral vertebra. At the transition point the longitudinal muscles – taenia coli, spread and incorporate the entire rectum. The pleating effect therefore disappears and consequently the rectum does not have haustra.
The sigmoid colon is supplied by branches of the inferior mesenteric artery and the venous drainage is into the inferior mesenteric vein.
Megacolon and megarectum
This entity describes an enlarged colon that is not related to mechanical obstruction and is characterized by a cecum that is larger than 12cms, an ascending colon larger than 8cms, or a rectosigmoid that is greater than 6.5cms.
It has three forms. We have discussed toxic megacolon and the other two are acute megacolon (Ogilvie’s syndrome) and chronic megacolon. In the cases shown below the megacolon and megarectum are of the chronic form.
Although it is rather surprising that the patient above was not obstructed other complications such as stercoral ulcers due to chronic irritation of the feces on the mucosa, and bowel ischemia from vascular compromise must be considered in the appropriate clinical setting.
Diverticulosis is a very common entity in Western civilizations and is thought to be due to the lack of fiber bulk in our diets. In the USA approximately 50-60% of the elderly have evidence of diverticulosis and it is found predominantly on the left side of the colon and most commonly in the sigmoid colon. In the Asian population interestingly, diverticulosis occurs predominantly on the right side. The rectum is not affected presumably because of the extra layer of longitudinal muscle that completely encompasses the circumference. The pathogenesis of the entity is thought to relate to excessive pressure build up in the colon due to strong peristaltic action on the low bulk content, causing excessive intraluminal pressure, muscular hypertrophy, mucosal thickening, bowel shortening, and to some degree luminal narrowing. The increased luminal pressure is transferred to the walls of the bowel and blow out predominantly of the mucosa through weak regions in the bowel wall occurs. As stated in the histology section above, the regions of weakness occur where the blood vessels enter and leave the bowel wall. (Cleveland) (eMedicine)
The diverticuli are easily identified on any of the diagnostic studies performed including endoscopy barium enema and CTscan.
Feces and debris may get trapped in the narrow necked diverticulum and result in an inflammatory response. Mucus secretion and swelling of the mucosa result, which in turn cause further pressure build up in the diverticulum. The pressure on the walls may cause ischemia with microperforation with extension of the inflammatory process and infection to the surrounding fat. Complications include abscess formation, peritonitis, and fistulization to the bladder or skin. CTscan is the study of choice, and the diagnosis is made by finding inflammatory changes in the pericolic fat or of changes within the sigmoid mesocolon.