Retroperitoneal

Retroperitoneal refers to the anatomical space in the abdomen and pelvis behind the peritoneum, lying between the posterior parietal peritoneum and the posterior abdominal wall. The retroperitoneum extends from the diaphragmatic crura superiorly to the pelvic brim inferiorly and is continuous with the retroperitoneal space of the pelvis. The space is classically subdivided into three compartments by fascia: the anterior pararenal space (between peritoneum and renal fascia, housing parts of the pancreas, duodenum, and the ascending/descending colon); the perirenal space (around the kidneys and adrenal glands, containing the proximal ureters); and the posterior pararenal space (containing fat and portions of the colon). The posterior abdominal wall forms its posterior boundary; the transversalis fascia contributes to the lining.

Organs and structures commonly described as retroperitoneal include the kidneys, adrenal glands, ureters, portions of the duodenum (second through fourth parts), the head and body of the pancreas (the tail being intraperitoneal), and the ascending and descending colon. The great vessels (aorta and inferior vena cava) lie within the retroperitoneum. Many retroperitoneal structures are primary retroperitoneal from development, whereas others are secondary retroperitoneal, having initially developed intraperitoneally and later becoming retroperitoneal as their mesenteries fused to the posterior abdominal wall.

Clinically, the retroperitoneum is important because retroperitoneal hemorrhage, infection, and tumors can arise in this space and may present with non-specific symptoms. Imaging with CT or MRI is commonly used to assess retroperitoneal disease, and retroperitoneal surgical approaches are used to reach organs such as the kidneys, pancreas, and portions of the colon.