Pelvic extension of retroperitoneal fluid: analysis in vivo

The purposes of this study were to describe the pathway of fluid flow from the retroperitoneal space into the pelvic extraperitoneal space on CT in vivo, to clarify the relation between its occurrence and the site or amount of retroperitoneal fluid, and to delineate the anatomic relation between the retroperitoneal spaces and the pelvic extraperitoneal space. We reviewed the CT scans of 37 patients with retroperitoneal fluid collections. Patients who had undergone pelvic laparotomy and patients who had either fascial thickening alone or fluid within muscle (such as the psoas muscle or iliac muscle) alone were excluded. Fluid extension into the pelvic extraperitoneal space was seen in six patients (16%). Extension by the infrarenal extraperitoneal space was seen in all six of these patients, but extension by properitoneal fat was seen in only one of the six patients. In patients with large amounts of fluid in the infrarenal extraperitoneal space, we frequently saw extension into the pelvic extraperitoneal space. Extension of pancreatic fluid into the infrarenal extraperitoneal space occurred in only 15% of the 37 patients. However, it occurred in both patients with ruptured abdominal aortic aneurysms. Three pathways from the infrarenal extraperitoneal space into the pelvic extraperitoneal space were seen: extension dorsally medial to the iliac vessels (n = 6), extension dorsally lateral to the iliac vessels (n = 1), and extension medially into the prevesical space (n = 2). Coexistence of two of these three pathways was seen in three patients. In vivo, extension of retroperitoneal fluid into the pelvic extraperitoneal space is not rare and occurs more often by the infrarenal extraperitoneal space than by properitoneal fat. Extension of retroperitoneal fluid to the infrarenal extraperitoneal space can be attributed less frequently to sources distant to the pelvic cavity such as pancreatic fluid. Such extension often derives from sources that can produce large amounts of retroperitoneal fluid such as ruptured abdominal aortic aneurysms. Of the three pathways from the infrarenal extraperitoneal space to the pelvic extraperitoneal space, dorsal extension medial to the iliac vessels is the most common, and multiple pathways often coexist.