The descending colon originates after the splenic flexure and ends at the origin of the sigmoid colon in the pelvis. By the time the stool reaches this region most of the water and retrievable products have been absorbed and the descending colon acts as a storage station. The length of the descending colon is between 10 and about 25 cm and is between 3 and 4cms in diameter. When empty it may have the same dimensions as the small bowel. The peritoneum covers the front and the sides and binds it to the posterior abdominal wall giving it very little mobility. It takes a straight and narrow path and is given no freedom to move about the abdomen. In this location it lies in the retroperitoneum and therefore is relatively protected from intraperitoneal processes but open to the spread of retroperitoneal disease the most common being acute pancreatitis.
It receives its blood supply from the left colic artery and drains into the left colic vein.
The following cases reflect diseases that can occur anywhere in the colon, but in these instances affected the descending colon.
Crohn’s disease is usually thought of as an inflammatory disease of the small bowel but it can affect any part of the gastrointestinal tract, and not uncommonly affects the large bowel. In the large bowel it is referred to as Crohn’s colitis or granulomatous colitis. It is called “granulomatous” because the most specific feature of Crohn’s disease for the pathologist, whether it occurs in the large or small bowel, is the finding of a granuloma or chronic inflammatory nodule in the submucosa.
It is important to remember that Crohn’s disease is a disease that affects all layers of the bowel wall so that in the acute stage the finding of pericolic fat involvement would be an important differentiating feature from ulcerative colitis which is a disease that usually only affects the mucosa. In the acute phases of Crohn’s there may be extensive ulceration of the mucosa so that only islands of tissue remain between the extensive ulceration. This appearance has reminded pathologists and radiologists of a cobblestone street and hence the descriptive term for this entity.
In the chronic phase of Crohn’s disease, fibrosis results causing shortening, loss of pliability, thickening of the wall and narrowing of the lumen. This leads to the rubber hose or lead pipe appearance.
Lymphoma is a disease that can present with involvement of the lymph nodes in a regional manner, so that it may present for example with involvement of the nodes of the neck as the presenting site. As the disease becomes more advanced, lymph nodes in other parts of the body such as mediastinum and then nodes on the other side of the diaphragm become involved. The involvement of regional nodes is the classical form of lymphoma. Sometimes however the lymphoid tissue in the submucosa is the site of origin of the disease and this may therefore occur anywhere where lymphoid tissue resides. In the gastrointestinal system the submucosa is the site where lymphoid tissue resides and its function is to protect the body from bacteria and other potentially harmful substances that may enter from the food or fluids we ingest. These lymphocytes can become neoplastic and develop into a localized form of lymphoma.. Generally lymphoma does not incite much inflammatory reaction and since it is usually a soft mass, it only occludes structures when it is extremely bulky. Thus when we identify a large mass in the intestinal system, that does not obstruct and is not associated with significant pericolic changes in the fat, lymphoma is a prime consideration.
In the opening passages of this module we described the fatty beads that hang on the outside of the colon called the appendices epiploica. We cannot usually identify the appendices epiploica since we cannot distinguish them from the other peritoneal fat that surrounds the bowel. When there is ascites, the mobile fat is displaced by the fluid and the fat attached to the bowel wall becomes apparent. In the case below the appendices epiploica of the sigmoid colon are floating in the ascetic fluid.
Since the appendices are mobile on a stalk and since there is continual peristalsis of the bowel, it is possible that they can twist on themselves and infarct resulting in an entity called appendagitis – inflammation of the appendage. It is important to make this diagnosis since it is treated conservatively while uncomplicated diverticulitis that has a similar clinical presentation and a not too dissimilar radiological presentation would be treated with antibiotics. Appendagitis is characterized by a bull’s eye appearance with the fatty central area being the appendage, with a surrounding rind of inflammation and associated stranding of the pericolic fat. It is truly an “aunt Minnie”, meaning that once seen never forgotten with the findings very characteristic of the condition.
While diverticulitis is more common than appendagitis the most common site for this entity is the sigmoid colon though it can affect any part of the colon. The following case of acute diverticulitis in the descending colon has been chosen in order to contrast it with the case of appendagitis above.
Posterior abdominal wall hernias
Since the descending colon is fixed in the retroperitoneum it is not usually a part of the colon that herniates, nor does it usually undergo neither volvulus nor torsion. Occasionally it may protrude into a lumbar hernia or through a surgical defect of the posterior abdominal wall such as might occur in patients who have had a nephrectomy.