Persistent Descending Mesocolon

The entity, persistent descending mesocolon, although commonly known to European radiologists and frequently noted in their literature, has not previously been discussed in the American radiologic literature (5). This developmental anomaly is characterized by failure of fusion of the mesentery of the left side of the colon with the posterior and lateral parietal peritoneum. A mesentery of varying length persists and may extend from the region of the splenic flexure to the sigmoid colon. This mesentery permits mobility of the descending colon, resulting in considerable variation in its position. The absence of the descending colon, or a portion thereof, from it s usual domain adjacent to the left lateral abdominal wall leaves a space into which all or a part of the small bowel may migrate. This, in addition to the medial position of the descending colon, produces a characteristic roentgen picture on contrast studies of the gastrointestinal tract and occasionally on plain films of the abdomen (Fig. 1). No attempt will be made here to review all the gyrations and migrations of the gastrointestinal tract during its embryologic development. Suffice it to say that until the fourth month of gestation there is essentially no fixation of the intestinal mesenteries, all of which are derived from the primitive dorsal mesentery. The mesocolon, a term used to differentiate that portion of the mesentery from which the colon is suspended, is divided into ascending, transverse, and descending portions, corresponding to the attached region of the colon. Normally, by the end of the fifth month both the ascending and descending mesocolons are fused to the dorsal abdominal wall, causing permanent fixation of these limbs of the colon. The fusion, which is thought to occur by a process of lysis and agglutination, proceeds laterad from an almost midline position (2–5, 7). Should this process of fusion of the descending mesocolon with the parietal peritoneum fail to occur, a persistent descending mesocolon results (Fig. 2). It has been stated in a number of embryology texts that persistence of the descending mesocolon is much less common than persistence of the ascending mesocolon, and this finding has been confirmed by reports in the surgical literature (1, 8). One interesting note is that the condition as an anomaly is encountered only in anthropoid apes and man. There is no fusion of the primitive dorsal mesentery in quadrupeds; thus there is normally no fixation of the gut. This anomalous fixation with the persistence of the descending mesocolon permits the descending colon to move medially, thereby vacating the left iliac fossa and the region adjacent to the left lateral abdominal wall. It is this space which all or a portion of the small bowel may occupy. The route of entrance of the small bowel may be anterior to the descending colon or through an opening in the persistent mesocolon (Fig. 3).