Congenital positional anomalies of the colon: Persistent descending mesocolon

A patient with no specific complaints, presented to the outpatient department for a routine health check-up. Past medical history was unremarkable.

A contrast-enhanced abdominal CT scan revealed a medial shift of the descending colon, and small bowel loops were positioned lateral to it, in the left paracolic gutter. No signs of bowel obstruction or bowel wall pathology were detected.

Persistent descending mesocolon (PDM) refers to a congenital anomaly of the colon, in which the mesocolon remains present in the descending colon [1,2]. It is characterized by the failure of the fusion of the mesentery with the posterior and lateral parietal peritoneum, during embryonic development [1].

A mesentery of varying length persists and may extend from the region of the splenic flexure to the sigmoid colon [1]. This persistent mesentery allows for increased mobility of the descending colon, leading to considerable variation in its position, and making it more vulnerable to torsion. The descending colon moves medially, vacating the left iliac fossa and the left paracolic gutter, occupied by all or a portion of the small bowel [2]. The small bowel can enter either anterior to the descending colon or through an opening in the persistent mesocolon. Entrapment of the small bowel and the development of an incarcerated hernia can occur, due to adhesions formed between the ileal mesentery and sigmoid mesocolon, or through persistent congenital openings in the descending mesocolon [1,2].

PDM is usually asymptomatic. Symptoms include recurrent episodes of crampy abdominal pain, occasionally accompanied by nausea and vomiting [2]. Its clinical significance is due to potential complications such as intestinal obstruction, volvulus, intussusception, or internal hernia with or without a mesenteric defect [2,3].

On Imaging, PDM can be identified on computed tomography scans or magnetic resonance imaging. The typical finding is the medialisation and oblique course of the descending colon, with small bowel loops seen lateral to it [3]. Historically, abdominal plain films and contrast studies of the colon were also used to identify this condition [1].

The significance and therapeutic options of a persistent descending mesocolon depend on individual cases and whether it causes any symptoms or complications. Patients present with symptoms may benefit from surgical intervention, to reduce the small bowel displacement [2].

It is also crucial to identify it preoperatively to plan a safe surgery and prevent inadvertent iatrogenic injury [4]. PDM was also found to be an independent risk factor for prolonged operative time and colonic anastomotic failure [5].

In conclusion, finding and reporting a PDM is important, even if the patient is asymptomatic, to prevent its potential complications and, in case of a needed colonic surgery, to allow the surgeons to better handle this rare congenital variant.