Unveiling the Inferior Vena Cava!

Embryology

The development of the inferior vena cava (IVC) involves a complex sequence of vascular formations, anastomoses, and regressions that occur between the 4th and 8th weeks of gestation.

In the 4th week of embryological development, three distinct paired veins are present: the umbilical veins, which drain blood from the placenta; the vitelline veins, which drain from the yolk sac; and the common cardinal veins, which drain the rest of the embryo's general tissue.

By the 6th week, the posterior cardinal veins become the dominant vessels, carrying blood from the caudal portion of the embryo to the common cardinal veins. The umbilical and vitelline veins contribute to the formation of the hepatic sinusoidal network.

Between the 6th and 8th weeks, the subcardinal and supracardinal veins emerge as dominant tributaries, forming multiple channels that drain into the posterior cardinal veins. This period also sees the formation of significant anastomotic networks, including the intersubcardinal anastomosis between the paired subcardinal veins and the suprasubcardinal anastomosis between the supracardinal and subcardinal vessels.

Fig 1: Embriology of the IVC. Sequence of vascular formations, anastomoses, and regressions between the 4th and 8th weeks of gestation. These images were inspired by Insights Imaging 2021; 12(1):123.

Anatomy

The IVC is typically formed by the confluence of the right and left common iliac veins behind the right common iliac artery at the level of the 5th lumbar vertebra. It ascends, receiving many tributaries, along the right side of the vertebral column, behind the duodenum, portal vein, and liver, before piercing the diaphragmatic central tendon at the caval opening at the level of the 8th thoracic vertebra. This is followed by a short intra-thoracic course before terminating at the right atrium.

Fig 2: Embriology and anatomy of the IVC. Sequence of vascular formations, anastomoses, and regressions between the 4th and 8th weeks of gestation. These images were inspired by Insights Imaging 2021; 12(1):123.

Anomalies:

• Congenital absence
• Hypoplasia
• Pre-renal:
  • Azygos continuation of the IVC (absent vitelline veins).
• Renal:
  • Retroaortic left renal vein (persistent posterior supracardinal-subcardinal anastomisis).
  • Circumaortic venous collar (persistent anterior and posterior supracardinal-subcardinal anastomosis).
• Post-renal:
  • Transposition or left-sided IVC (persistent left supracardinal vein)
  • Duplication of the IVC (persistent both supracardinal veins).
  • Retrocaval or circumcaval ureter (persistent right posterior cardinal vein).
  • Absent infrarenal segment (absent posterior cardinal and supracardinal veins).
  • Preaortic iliac confluence (marsupial cava).

A) Congenital absence:

Congenital absence of the IVC has an unknown incidence and unclear cause, as it is difficult to identify a single embryonic event. However, complete failure of embryonic vein development and sequelae of intrauterine or perinatal venous thrombosis with atrophy have been suggested as possible causes. Patients may experience symptoms of lower-extremity venous insufficiency or idiopathic deep venous thrombosis.

Fig 3: Congenital absence of the IVC. 19-year-old male with recurrent bilateral lower extremity deep venous thrombosis and symptoms of lower-extremity venous insufficiency. Images from abdominal CT scan (A, C, volume-rendered three-dimensional reconstructions, B, coronal MIP [maximum intensity projection], D to F, axial) show absence of the entire IVC with pelvic (arrow), retroperitoneal (black arrowhead), and abdominal wall varicose veins. Note the prominent ascending vertebral venous plexus veins (white arrowhead), draining into the azygos-hemiazygos system in D, which may simulate paraspinal masses. Note the aorta (asterisk) without the inferior vena cava on its right side.

B) Hypoplasia

A rare congenital malformation that may be caused by embryological or acquired factors. It causes venous stasis, which increases the likelihood of venous thrombosis and collateral circulation.

Fig 4: Hepatic segment hypoplasia of the IVC. Images from abdominal CT scan (A, coronal, B to D, axial) show intra-hepatic segment hypoplasia of the IVC (arrow). Note the calibers difference between the vascular segments (arrowhead) before (B) and after (D) the hypoplastic segment.

C) Azygos continuation of the IVC

In most cases, it is found incidentally, but there is an association with congenital heart disease, asplenia and polysplenia syndromes, as well as with duplication of the IVC. The sequence of events begins with the failure to form the right subcardinal–hepatic anastomosis with atrophy of the right subcardinal vein. As a result, blood is shunted through the suprasubcardinal anastomosis into the retrocrural azygos vein, which is partially derived from the thoracic segment of the right supracardinal vein.

It can cause an enlarged azygos vein. Care should be taken to avoid misdiagnosing it as a right-sided paratracheal mass or retrocrural adenopathy.

Preoperative knowledge may be important for planning cardiopulmonary bypass and to avoid difficulties in catheterizing the heart.

Fig 5: Azygos continuation of the IVC (or absence of the hepatic segment of the IVC with azygos continuation): embryology.

Fig 6: Azygos continuation of the IVC. Images from abdominal CT scan (A, B, D, axial, C, axial MIP, E, sagittal, F, volume-rendered three-dimensional reconstructions) highlight the absence of the hepatic segment of the inferior vena cava (IVC) and the orientation of the IVC to the retrocural space (black arrow), with its continuation with the azygos venous system (arrowhead) in a patient with polysplenia syndrome (asterisk in abdominal organs). The hepatic veins (white arrow) drain directly to the right atrium

D) Retroaortic left renal vein

Persistence of the dorsal arch of the renal collar occurs in this variation, where the ventral arch (intersubcardinal anastomosis) regresses. As a result, a single left renal vein passes posterior to the aorta.

Remember that in conventional anatomy, the left renal vein (longer than the right renal vein) courses anteriorly to the abdominal aorta.

Fig 7: Retroaortic left renal vein and circumaortic venous collar.

Fig 8: Retroaortic left renal vein. Images from abdominal CT scan (A, oblique axial, B, oblique axial MIP, C, sagittal, D, E, three-dimensional volume rendering in anterior [D] and posterior [E] views) show left renal vein (arrow) between abdominal aorta (AA) and vertebral body (VB). Note the white dashed path in posterior and anterior view (D and E). IVC, inferior vena cava, LK, left kidney, RK, right kidney.

E) Circumaortic venous collar

Persistence of the dorsal limb of the embryonic left renal vein and the dorsal arch of the renal collar (intersupracardinal anastomosis). The superior left renal vein receives the left adrenal vein and crosses the aorta anteriorly. The inferior left renal vein receives the left gonadal vein and crosses posterior to the aorta approximately 1–2 cm inferiorly to the anterior.

Fig 9: Circumaortic venous collar. Images from abdominal CT scan (A, oblique axial, B, oblique sagittal, C, D, three-dimensional volume rendering in anterior [C] and posterior [D] views) show one superior left renal vein (arrow) crossing the abdominal aorta (AA) anteriorly, and another inferior left renal vein (arrowhead) that crosses posterior to the aorta. Observe the posterior left renal vein narrowing between the aorta and the vertebral body (VB). IVC, inferior vena cava, LK, left kidney, RK, right kidney.

F) Transposition or left-sided IVC

It is the result of the regression of the right supracardinal vein and the persistence of the left supracardinal vein. The left-sided IVC crosses to the right side at the level of the renal veins.

Fig 10: Transposition of IVC or left-sided IVC: embryology.

Fig 11: Transposition of IVC or left-sided IVC. Images from abdominal CT scan (A to D, axial, E, F, coronal, G – three-dimensional volume rendering) show the inferior vena cava (IVC) ascending on the abdominal aorta (AA) left side after the iliac confluence, then crossing (arrow) to the right side at the level of renal veins (arrowhead). LK, left kidney, RK, right kidney.

Fig 12: Transposition of IVC with azygos continuation. Images from abdominal MRI (A to F, axial T1 fat-sat post contrast, G, H, three-dimensional reconstruction) show the iliac venous confluence (asterisk) posterolateral to the left common iliac artery with ipsilateral inferior vena cava (white arrow) ascending and draining to the retrocural azygos/hemiazygos system (black arrow). The azygos/ hemiazygos system drain in the right atrium (RA) with superior vena cava (SVC). Note the retroaortic right renal vein (black arrowhead) and that the hepatic veins draining directly to the RA (white arrowhead). Ao, aorta.

G) Duplication of the IVC

It results from the persistence of both supracardinal veins. The left IVC typically ends at the left renal vein, crossing anteriorly to the aorta to join the right-sided IVC.

Fig 13: Duplication of IVC : embryology.

Fig 14: Duplication of IVC. Images from abdominal CT scan (A to D, axial, E, coronal, F, three-dimensional volume rendering) show duplication of IVC (arrow), each side draining the respective common iliac veins (arrowhead), in a path adjacent to the abdominal aorta (AA). The left IVC crosses to the right side just after the confluence of the renal veins through a venous trunk (asterisk), draining shortly afterwards to the single hepatic segment of the IVC).

H) Absent infrarenal segment

It results from the failure of development of the posterior cardinal and supracardinal veins.

Fig 15: Absent infrarenal IVC: embryology.

Fig 16: Absent infrarenal IVC. Images from abdominal CT scan (A, B, axial, C, coronal MIP, D, E, three-dimensional volume rendering) show renal veins (black arrowhead) draining to the suprarenal and intrahepatic segments of the IVC (black arrow). Note medium and large caliber pelvic and retroperitoneal varices (white arrow) that drains to the suprarenal segment of the IVC, as well abdominal wall varices in E with recanalization of the umbilical vein (white arrowhead). Note the absence of IVC (yellow dotted circle) on the right side of the infrarenal segment of the abdominal aorta (Ao).

I) Retrocaval or circumcaval ureter

It results from the failure of the right supracardinal system to develop, while the right posterior cardinal vein persists. Patients may develop partial right ureteral obstruction or recurrent urinary tract infections. Surgical relocation of the ureter anterior to the IVC is the treatment of choice.

Fig 17: Retrocaval or circumcaval ureter: embryology.

Fig 18: Retrocaval or circumcaval ureter. Images from abdominal CT scan (A and B, axial MIP, C, coronal MIP, D, E – three-dimensional volume rendering in anterior [D] e posterior [E] views) show the right ureter (arrow) posterior to the IVC with marked posterior displacement of its proximal segment (“fish hook” or “reverse J” appearance). RK, right kidney.

J) Preaortic iliac confluence (marsupial cava)

Between the 6th and 10th weeks of gestation, channels form circumumbilical venous rings around the primitive common iliac arteries on each side. Normally, the dorsal portion of the ring persists, and the ventral limb regresses, resulting in an iliac confluence behind the right common iliac artery.

Marsupial IVC results from the anomalous persistence of the ventral limb and regression of the dorsal limb. Several types of marsupial cava have been described, with variations in the location of the common iliac veins and their tributaries:

• The left common iliac vein passes anterior to the aortic bifurcation, and the right common iliac vein follows a posterior course along the right common iliac artery;
• The right common iliac vein follows an anterior course along the right common iliac artery, forming a confluence with the left common iliac vein anterior to the aortic bifurcation;
• Both common iliac veins are posterior to their respective arteries and then cross over the right common iliac artery to form a preaortic confluence.

Fig 19: Marsupial cava: embryology.

Fig 20: Marsupial cava: anatomy.

Fig 21: Marsupial cava. Images from abdominal CT (A, B, axial, C, sagittal, D, coronal oblique, E, three-dimensional volume rendering) show both common iliac veins (arrow) posterior to their respective arteries (arrowhead) and then cross over the right common iliac artery to form a preaortic confluence (asterisk).

Pitfalls

• Paracaval lipoma or pseudolipoma: volume-averaging artifact due to prominent pericaval fat above the caudate lobe rather than a true intraluminal lesion, more frequent in patients with chronic liver disease. It can mimic  bland thrombus, spread of tumors, fat embolism and intracaval spread of renal angiomyolipoma. Multiplanar reconstructions confirm the true nature of this finding.
  

  Fig 22: Paracaval lipoma or pseudolipoma. Images from cardiac CT angiography (A, axial, B, coronal, C, sagittal) shows a band of adipose tissue (arrow) next to the IVC (arrowhead).
• Contrast flow phenomenon or pseudothrombosis.
  

  Fig 23: Contrast flow phenomenon or pseudothrombosis. Images from chest CTA (A-C, axial, D, coronal, E, sagittal), an early phase dynamic scanning, show uniformly contrast enhanced blood in the renal veins (arrowhead) entering the IVC (arrows) when it is diluted by normal blood (without contrast) coming into the IVC from the pelvis and lower limbs (asterisk). This causes a swirl-like appearance. Later scans times allow contrast-enhanced blood to enter the lower limbs thus mixing evenly with the renal vein blood.
• Congenital malformations that drain into the IVC (increasing its flow and caliber) or vessels with similar pathways to the IVC may be potential confounding factors, such as anomalous pulmonary venous return and portosystemic shunts.
  

  Fig 23: Contrast flow phenomenon or pseudothrombosis. Images from chest CTA (A-C, axial, D, coronal, E, sagittal), an early phase dynamic scanning, show uniformly contrast enhanced blood in the renal veins (arrowhead) entering the IVC (arrows) when it is diluted by normal blood (without contrast) coming into the IVC from the pelvis and lower limbs (asterisk). This causes a swirl-like appearance. Later scans times allow contrast-enhanced blood to enter the lower limbs thus mixing evenly with the renal vein blood.
  

  Fig 25: Infracardiac total anomalous pulmonary venous return. 11-day-old male neonate with cyanosis and tachypnea. Imagens from cardiac CT (A, coronal, B, C, axial maximum intensity projections, E, F, anterior and posterior three-dimensional volume-rendered reconstructions show the pulmonary veins draining through a single pulmonary venous trunk (SPVT). The pulmonary veins do not communicate with the hypoplastic left atrium (LA), descending to the infradiaphragmatic region and connecting to the portal vein (PV), resulting in a portal venous system aneurysm. An ostium secundum atrial septal defect (asterisk) and dilated right ventricle (RV) are observed. A = aorta, IVC = inferior vena cava, LIPV = left inferior pulmonary vein, LSPV = left superior pulmonary vein, LV = left ventricle, PV = portal vein, RA = right atrium, RIPV = right inferior pulmonary vein, RMPV = right middle pulmonary vein, RSPV = right superior pulmonary vein.