Relevant Anatomy and Embryology
- The parietal peritoneum separates the peritoneal cavity from the retroperitoneum. The retroperitoneum contains all the abdominal contents located between the parietal peritoneum and the transversalis fascia. It is divided into 3 compartments by 2 well-defined fascial planes: The renal and lateroconal fasciae.
- The perirenal space contains the kidney, adrenal, proximal ureter, and abundant fat, and it is enclosed by the renal fascia, a.k.a. Gerota fascia. The 2 perirenal spaces do not communicate across the abdominal midline.
- The anterior pararenal space contains the pancreas, duodenum, colon (ascending and descending), and a variable amount of fat.
- The posterior pararenal space contains fat but no organs; it is contiguous with the properitoneal fat along the flanks.
- The anterior renal fascia separates the perirenal space from the anterior pararenal space, and the posterior renal fascia separates the perirenal space from the posterior pararenal space.
- The lateroconal fascia separates the anterior from the posterior pararenal space and marks the lateral extent of the anterior pararenal space.
- The renal fascia joins and closes the perirenal space, resembling an inverted cone with its tip in the iliac fossa. Caudal to the perirenal space, in the pelvis, the anterior and posterior pararenal spaces merge to form a single infrarenal retroperitoneal space. Due to an opening in the cone of the renal fascia caudally, the perirenal space communicates with the infrarenal retroperitoneal space, which in turn communicates directly with the extraperitoneal pelvic spaces. Thus, all 3 retroperitoneal compartments communicate with each other within the lower abdomen and pelvis. All of the pelvic retroperitoneal compartments, such as the prevesical (space of Retzius), perivesical, and presacral spaces, communicate with each other, which is evident and clinically relevant in cases of pelvic hemorrhage or tumor as well as with extraperitoneal rupture of the urinary bladder.
- The renal and lateroconal fascia are laminated planes, which can split to form potential spaces as pathways of spread for rapidly expanding fluid collections or inflammatory processes, such as hemorrhage or acute pancreatitis. Splitting of the anterior renal fascia creates a "retromesenteric plane" that communicates across the midline; splitting of the posterior renal fascia creates a "retrorenal plane," which also communicates across the midline and anteriorly. There are also radial bridging septa in the perirenal space that allow for pathology to cross from the perirenal space to the pararenal spaces or vice versa.
Imaging Techniques and Indications
- Multiplanar CT and MR are ideally suited to display the anatomy and pathology of retroperitoneal disease processes. Use of IV contrast material allows easier recognition of fascial plane landmarks and pathology and should be used unless contraindicated.
Approach to Retroperitoneal Abnormalities
- Disease within the perirenal space is usually the result of diseases of the kidney. Common disease states include hemorrhage, infection, inflammation, and neoplasia.
- The renal fascia is very strong and is usually effective in containing most primary renal pathology within the perirenal space. Similarly, it usually excludes most other processes from invading or involving the perirenal space.
- Perirenal fluid may represent blood, urine, or pus or may be simulated by inflammation of the perirenal fat (a common, benign, and often age-related finding). Hemorrhage is often due to trauma but may occur due to anticoagulation, rupture of a renal tumor, or vasculitis. Pus or inflammation usually originates from acute pyelonephritis, which may be associated with an abscess. Perirenal urine collection ("urinoma") may result from trauma with laceration through the renal collecting system. Acute urine leak may also accompany ureteral obstruction by a calculus due to forniceal rupture.
- Renal cell carcinoma is common, and the renal fascia usually confines the tumor, preventing invasion of contiguous structures. However, renal cell carcinoma can invade through the perirenal fascia (T4 tumor).
- Other neoplasms can involve the perirenal space, classically lymphoma or rarely extramedullary hematopoiesis. Inflammatory processes may also rarely be seen, such as Erdheim-Chester, IgG4, and retroperitoneal fibrosis.
- Disease within the anterior pararenal space is common. For example, acute pancreatitis results in peripancreatic inflammation, necrosis, &/or fluid collections that often first spread to the anterior pararenal space, surrounding the duodenum and ascending and descending colon segments that share this anatomic compartment. The spread of inflammation is usually limited posteriorly by the anterior renal fascia and laterally by the lateroconal fascia. Thickening of these planes is a reliable clue as to the presence of inflammatory diseases, which might otherwise be occult on imaging. The perirenal space is usually not involved in acute pancreatitis, sometimes resulting in the striking appearance of a perirenal "halo" of fat density, while other retroperitoneal spaces and planes are infiltrated. As the collections become large (often in necrotizing pancreatitis), spread occurs from the anterior pararenal to posterior and infrarenal spaces, occasionally reaching the pelvis. The root of the mesentery and transverse mesocolon originate from just ventral to the 3rd portion of duodenum and pancreas, and pancreatitis can dissect along these planes beneath the parietal peritoneum.
- Since the 2nd through 4th portions of the duodenum are retroperitoneal, duodenal perforations (from ulcer, post-ERCP, etc.) may result in extraluminal gas and fluid collections in the right anterior pararenal space. As the collection grows, it may extend into other spaces similar to necrotizing pancreatitis collections. Only the duodenal bulb is intraperitoneal; as such, a perforation of the bulb (the most common type) shows pneumoperitoneum.
- Disease originating within the posterior pararenal space is uncommon, essentially limited to hemorrhage and tumor.
- "Retroperitoneal hemorrhage" is a misnomer since most spontaneous, coagulopathic hemorrhage originates within the abdominal wall, the iliopsoas compartment, or the rectus sheath. Only when hemorrhage extends beyond these fascial boundaries does it enter the retroperitoneum. Rectus sheath hematomas enter the extraperitoneal pelvic spaces through a defect in the caudal (infraumbilical) portion of the sheath. Iliopsoas hemorrhage often extends into any or all of the retroperitoneal compartments, predominantly along the main fascial planes. The hallmarks of coagulopathic hemorrhage are bleeding out of proportion to trauma, multiple sites of bleeding, and the presence of the hematocrit sign, a fluid-cellular debris level within the hematoma.
- Retroperitoneal sarcomas, most commonly liposarcoma, often originate within one of the retroperitoneal compartments, and the site of origin can be determined by the relative mass effect on various organs and structures, such as the kidneys, colon, and great vessels. Most liposarcomas have some identifiable fat within them and seem to be encapsulated, allowing for excision, although recurrent disease is common. Leiomyosarcomas originate typically around the IVC.
- If retroperitoneal nodes are included in the discussion, the most common retroperitoneal tumor is non-Hodgkin lymphoma (NHL). NHL often results in massive lymphadenopathy. This characteristically involves the mesenteric and retroperitoneal nodes that are confluent and anteriorly displace the aorta and inferior vena cava from the spine. Retroperitoneal nodes are also frequently involved by malignancies originating in pelvic organs, such as the prostate, rectum, and cervix.
- The other large (though uncommon) group of primary retroperitoneal tumors are of neurogenic origin, including schwannoma, paraganglioma, and other nerve sheath tumors. These often share the characteristics of appearing as well-defined, moderately enhancing masses that do not appear to arise from nodes nor abdominal viscera. Many, in fact, arise along the nerves or ganglia, while others are part of a syndrome, such as neurofibromatosis, that may involve multiple nerves in a paraspinal or presacral distribution.
- The aorta and IVC are located in the central retroperitoneum and are surrounded by fascia with the great vessel space. Although primary disease of the IVC is rare, it may be the site of primary tumor (leiomyosarcoma) or the site of intravascular spread of renal or adrenal carcinoma. Anomalies of the IVC are commonly seen incidentally in ~ 10% of the population, usually at or below the level of the renal veins, resulting in variations such as duplicated IVC and retro- and circumaortic renal vein. While these are uncommonly of clinical significance (limited to affecting surgical and interventional procedures), they may be mistaken for pathologic conditions, most commonly enlarged retroperitoneal lymph nodes.
- Abdominal aortic aneurysm is a major health concern, and rupture is usually fatal. Accurate diagnosis and precise mapping of the size and shape of an aneurysm allows effective, minimally invasive prophylactic treatment with endovascular stenting.
- Retroperitoneal fibrosis is an inflammatory disorder that may be misinterpreted as a malignant process, as it envelops the aorta and IVC, often causing displacement and encasement of the ureters. It most commonly occurs as part of IgG4 disease or less commonly due to medications or malignancy.
- Czeyda-Pommersheim F et al: Diagnostic approach to primary retroperitoneal pathologies: what the radiologist needs to know. Abdom Radiol (NY). 46(3):1062-81, 2021
- Al-Dasuqi K et al: Radiologic-pathologic correlation of primary retroperitoneal neoplasms. Radiographics. 40(6):1631-57, 2020
- Shaaban AM et al: Fat-containing retroperitoneal lesions: imaging characteristics, localization, and differential diagnosis. Radiographics. 36(3):710-34, 2016
- Osman S et al: A comprehensive review of the retroperitoneal anatomy, neoplasms, and pattern of disease spread. Curr Probl Diagn Radiol. 42(5):191-208, 2013
- Goenka AH et al: Imaging of the retroperitoneum. Radiol Clin North Am. 50(2):333-55, vii, 2012
- Tirkes T et al: Peritoneal and retroperitoneal anatomy and its relevance for cross-sectional imaging. Radiographics. 32(2):437-51, 2012
- Lee SL et al: Comprehensive reviews of the interfascial plane of the retroperitoneum: normal anatomy and pathologic entities. Emerg Radiol. 17(1):3-11, 2010
- Sanyal R et al: Radiology of the retroperitoneum: case-based review. AJR Am J Roentgenol. 192(6 Suppl):S112-7 (Quiz S118-21), 2009
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