The Retroperitoneum – Anatomy, Structure, Functions

The retroperitoneal space (retroperitoneum) is the anatomical space (sometimes a potential space) behind (retro) the peritoneum. It has no specific delineating anatomical structures. Organs are retroperitoneal if they have peritoneum on their anterior side only. Structures that are not suspended by mesentery in the abdominal cavity and that lie between the parietal peritoneum and abdominal wall are classified as retroperitoneal.^[rx]

The retroperitoneum can be further subdivided into the following:^[rx]

• Perirenal (or perinephric) space
• Anterior pararenal (or paranephric) space
• Posterior pararenal (or paranephric) space

Retroperitoneal structures

Structures that lie behind the peritoneum are termed “retroperitoneal”. Organs that were once suspended within the abdominal cavity by mesentery but migrated posterior to the peritoneum during the course of embryogenesis to become retroperitoneal are considered to be secondarily retroperitoneal organs.

• Primarily retroperitoneal, meaning the structures were retroperitoneal during the entirety of development:
  • urinary
    • adrenal glands
    • kidneys
    • ureter
  • circulatory
    • aorta
    • inferior vena cava
  • anal canal
• Secondarily retroperitoneal, meaning the structures initially were suspended in mesentery and later migrated behind the peritoneum during development^[3]
  • the duodenum, except for the proximal first segment, which is intraperitoneal^[4]
  • ascending and descending portions of the colon (but not the transverse colon, sigmoid and the cecum)
  • pancreas, except for the tail, which is intraperitoneal

Subdivisions

Perirenal space

It is also called the perinephric space. Bounded by the anterior and posterior leaves of the renal? fascia. It contains the following structures:

• Adrenal gland
• Kidney
• Renal? vessels
• Perirenal fat, which is also called the “adipose capsule of the kidney” and may be regarded as being part of the renal? capsule^[5]

Anterior pararenal space

Bounded by the posterior layer of the peritoneum and the anterior leaf of the renal? fascia. It contains the following structures:

• Pancreas
• Ascending and descending colon
• Duodenum

Posterior pararenal space

Bounded by the posterior leaf of the renal? fascia and the muscles of the posterior abdominal wall. It contains only fat (“pararenal fat”), and is also called the “perinephric body”, or “pararenal fat body”.

Gross anatomy

There are five compartments:

• lateral compartments divided into three spaces by renal? fascia
  • anterior pararenal space
  • perirenal space
  • posterior pararenal space
• great vessel compartment from T12 to L4/5 between perirenal spaces (defined in the recent literature ^{2, 3} )
• posterior compartments containing psoas muscles which join iliacus (note that this is behind transversalis fascia)

Perirenal space

• inverted cone shape with base resting on diaphragm and point directing towards the pelvis
• boundaries
  • anterior and posterior renal? fascia continuous with each other (fascia of Gerota and Zuckerkandl respectively)
    • anterior renal? fascia blends anteriorly with connective tissue around aorta and inferior vena cava
    • posterior renal? fascia (thicker, two lamina) blends with quadratus lumborum fascia
      • superficial lamina is made up of lateroconal fascia which extends anteriorly and attaches to the peritoneum
• contents
  • kidney, adrenal gland, renal? pelvis, proximal ureter, renal hilar vessels, lymph nodes, fat
• relations
  • upper-right perirenal space communicates with retrohepatic space at the bare area of the liver
  • posterior pararenal space-related posteriorly
  • anterior pararenal space-related anteriorly
  • theoretical communication between both perirenal spaces known as the Kneeland channel in front of the aorta and inferior vena cava

Anterior pararenal space

• boundaries
  • posterior parietal peritoneum anteriorly, anterior renal? fascia posteriorly and lateroconal fascia (continuation of posterior renal fascia) laterally
• contents
  • duodenum (D2-D4), ascending and descending colon, pancreas, lymph nodes, fat
• relations
  • potentially continuous across mid-line, intraperitoneal space anteriorly, perirenal space posteriorly

Posterior pararenal space

• boundaries
  • posterior renal? fascia anteriorly, transversalis fascia posteriorly
• contents
  • fat
• relations
  • communicates with properitoneal space in the anterior abdomen between peritoneum and transversalis fascia
  • communicates potentially with contralateral side
  • communicates with flank laterally (external to lateroconal fascia and deep to transversalis fascia); visualized as flank strip radiologically
  • opens inferiorly towards pelvis following length of the ureter

Great vessel compartment

• not well-defined by fascial planes
• boundaries
  • laterally: perirenal space and ureter
  • posteriorly: vertebral body, psoas major muscle
  • superiorly: continuous with the posterior mediastinum
• contents
  • aorta, inferior vena cava, fat, lymph nodes

Structure and Function

The retroperitoneum divides into three main anatomical spaces:  the anterior pararenal space, perirenal space, and posterior pararenal space. The anterior pararenal space contains the head, neck, and body of the pancreas (the tail of the pancreas is within the splenorenal ligament?), ascending and descending colon, and the duodenum (except for the proximal first segment). Structures contained within the perirenal space include the adrenal gland, kidney, ureters, and renal? vessels. The posterior pararenal space, which is surrounded by the posterior leaf of the renal fascia and muscles of the posterior abdominal wall, contains no major organs and is composed primarily of fat, blood vessels, and lymphatics.[rx] There is also a fourth, less well-defined space known as the great vessel space. It lies anterior to the vertebral bodies and psoas muscles and contains the aorta, inferior vena cava, and surrounding fat.[rx]

Blood Supply and Lymphatics

The retroperitoneum contains the large vessels of the abdomen and pelvis. Arterial blood supply is from the abdominal aorta and all of its branches. The inferior vena cava and its tributaries provide for venous drainage of the retroperitoneum and its structures.[rx] The lymphatic chain of the retroperitoneum is rich and extensive. The lymphatics generally follow arteries, with named lymph nodes typically found near the root of the arteries. Lymph nodes lying in the retroperitoneal space of the abdomen are the inferior diaphragmatic nodes, and the lumbar? nodes. Lumbar nodes surround the inferior vena cava and aorta and further classify as left lumbar, intermediate, and right lumbar.  Surrounding the great vessels lie, three groups of lymph nodes, with their names corresponding to each vessel. Around the aorta are the pre-aortic, para-aortic, and retro-aortic nodes. Similarly, around the inferior vena cava exist the pre-caval, para-caval, and retro-caval nodes. The retroperitoneal lymphatic chain of the pelvis is made up of the common iliac, external and internal iliac, obturator, and sacral lymph nodes.[rx]

Nerves

There is an extensive network of nerves that both pass through and supply the retroperitoneum and its associated structures. Six named pairs of parietal nerves branch from the lumbar? plexus bilaterally. They are the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, obturator, and femoral nerves. The lumbosacral trunk and ventral rami of S1-S3 and part of S4 come together to form the sacral plexus, which gives rise to many of the nerves found within the retroperitoneum. The superior and inferior gluteal nerves form from the sacral plexus bilaterally. Also arising from the sacral plexus are right and left lumbar sympathetic trunks, as well as the greater, lesser, and least thoracic? splanchnic nerves and four lumbar splanchnic nerves. All of these provide sympathetic innervation to the abdominal and pelvic viscera. The pelvic splanchnic nerves provide parasympathetic innervation to structures of the peritoneal cavity. The vagus and splanchnic nerves, along with celiac, superior mesenteric, and aortic renal? ganglia form the autonomic plexuses. These carry sympathetic, parasympathetic, and sensory (predominantly pain?) fibers.[rx]

Muscles

Muscles within the retroperitoneum can be organized based on their location. Muscles contributing to the posterior margin of the retroperitoneal space consist largely of the transverse abdominal, psoas, quadratus lumborum, and iliacus. The paraspinous muscles contribute to the medial boundary on either side of the spine, and the abdominal musculature forms the lateral margin. The superior border is formed in part by the diaphragm, while the iliopsoas muscle is the primary muscle contributing to the inferior border.[rx]