retroperitoneal space

**Definition/anatomy:** The space between the posterior parietal peritoneum anteriorly and the transversalis fascia posteriorly, extending from the diaphragm to the pelvic brim (continuous with the pelvic extraperitoneal space).

**Primary retroperitoneal organs (SADPUCKER):** - **S**uprarenal (adrenal) glands - **A**orta / IVC - **D**uodenum (2nd, 3rd, 4th parts; the 1st part is intraperitoneal) - **P**ancreas (head, neck, body; the tail is intraperitoneal, within the splenorenal ligament) - **U**reters - **C**olon (ascending and descending) - **K**idneys - **E**sophagus (partly) - **R**ectum (partly)

**Secondary retroperitoneal:** organs that were originally intraperitoneal but fused to the body wall during development (e.g., duodenum, ascending/descending colon, pancreas). Truly intraperitoneal organs include the stomach, liver, spleen, jejunum/ileum, transverse and sigmoid colon, and the cecum/appendix (variable).

**Compartments:** Anterior pararenal space (pancreas, duodenum 2nd–4th parts, ascending/descending colon); perirenal space (kidneys, adrenals, ureters, perirenal fat, enclosed by Gerota/renal fascia); posterior pararenal space (fat only, no organs).

**Clinical significance:** - **Retroperitoneal hemorrhage:** Causes include ruptured AAA, anticoagulation, pelvic/renal trauma, and femoral artery catheterization (high puncture above the inguinal ligament). Presents with hypotension, flank ecchymosis (Grey Turner sign), and back/flank pain. Hematoma can track widely and hide significant blood loss. - **Acute pancreatitis:** Pain radiates to the back because the pancreas is retroperitoneal. Hemorrhagic pancreatitis can cause Grey Turner (flank) and Cullen (periumbilical) signs. - **Ruptured AAA:** Contained retroperitoneal rupture gives abdominal/back pain, a pulsatile mass, and hypotension. - **Renal and ureteral disease:** Kidney stones cause colicky flank pain that radiates to the groin. - **Retroperitoneal fibrosis (Ormond disease):** Fibroinflammatory tissue encases the aorta and ureters, causing hydronephrosis and renal failure. Associated with IgG4-related disease, methysergide, and malignancy. CT shows a periaortic soft-tissue mass. - **Psoas abscess/sign:** Psoas muscle lies in the retroperitoneum; infection (e.g., Crohn disease, appendicitis, vertebral TB/Pott disease) causes pain with hip extension. - **Duodenal injury:** Blunt trauma can cause retroperitoneal duodenal rupture or hematoma. It is easy to miss because there is no early peritonitis. Look for retroperitoneal air on CT. - **Tumors:** Sarcomas (liposarcoma), lymphoma, and germ cell tumor lymph node metastases present late with vague symptoms. - **Pancreatic cancer:** Head tumors cause painless jaundice and back pain from posterior invasion. - **Imaging/exam pearls:** Retroperitoneal injuries are not detected by FAST (which looks at the peritoneal cavity), so CT is needed in stable patients. A hemodynamically unstable patient with a pelvic fracture may bleed into the retroperitoneum.

**Embryology:** Organs become secondarily retroperitoneal when their mesentery fuses with the posterior body wall (the duodenum, ascending/descending colon, and pancreas). Gut malrotation does not change this classification, but it affects fixation.

**Boards tip:** Retroperitoneal organs have no mesentery (or only a fused one) and are fixed in position. Pain from retroperitoneal structures often radiates to the back, and peritoneal signs are usually delayed or absent.