Normal Anatomy and Congenital Anomalies

The IVC extends from the confluence of the common iliac veins at the level of L5 vertebral body, to the right atrium of the heart in right prevertebral location, next to the abdominal aorta and is surrounded by a rich network of lymphatic vessels (Fig. 110-1). It is partially covered anteriorly by the peritoneal membrane. The retroperitoneal space where the IVC is located can communicate with the perirenal spaces and the anterior and posterior interfascial spaces.¹

FIGURE 110-1 Normal IVC. A, Contrast-enhanced CT of the abdomen depicts axial images at the level of the renal veins showing normal location and size of the IVC located to the right of the aorta. This image, obtained at portal-venous phase at the level of the left renal vein, shows heterogeneous luminal enhancement due to mixing of the renal vein blood with the inferior IVC blood. B, Sagittal reformat of the same study shows IVC segments with different luminal enhancement due to the normally observed different timing of the contrast return. Therefore we recommend imaging with longer delays (2 to 4 minutes) in order to obtain more homogeneous luminal enhancement.

The shape of the IVC varies from round to ovoid or even flat depending on a multitude of factors such as intrathoracic pressure, blood volume status, or the presence of congestive heart failure. The IVC receives a number of tributaries including common iliac, lumbar, renal, right adrenal, and hepatic veins. The IVC lies between the liver and the diaphragm and cephalad courses medially to enter the right atrium. At this level, a fat pad (continuous with the retroperitoneal fat) can be seen in many normal patients in an inferomedial location, sometimes bulging into the lumen of the IVC. This fat should not be considered pathologic and should not generate any further work-up studies.

Congenital anomalies of the IVC generally include abnormal position of the IVC or absence of IVC. The most common IVC anomalies are: (1) left IVC, (2) duplicated IVC, (3) azygos continuation of IVC, (4) circumaortic left renal vein, (5) retroaortic left renal vein, (6) circumcaval or retrocaval ureter, (7) duplicated right renal vein, (8) absence of infrarenal or entire IVC, (9) duplicated IVC with retroaortic right renal vein and hemiazygos continuation of the IVC, and (10) duplication of IVC with retroaortic left renal vein and azygos continuation of the IVC.

Left IVC: The infrarenal IVC is located to the left of the abdominal aorta, then it joins the left renal vein which then crosses anterior to the abdominal aorta and along with the right renal vein forms the normal right-sided prerenal IVC.

Duplicated IVC: There are two IVCs below the level of the renal veins—each connected to the ipsilateral common iliac vein. The left IVC joins the left renal vein, which then crosses anterior to the abdominal aorta and drains into the right IVC (Fig. 110-2). There may be variants in this anatomy and there may be significant discrepancy in the size of the two IVCs.

FIGURE 110-2 IVC duplication (infrarenal). Contrast-enhanced CT shows two IVC—one on each side of the aorta. The left sided IVC originates in the left iliac vein and drains into the left renal vein (arrows) to join the right IVC (I) and form a single vein superior to the renal vessels.

Azygos continuation of IVC: The infrarenal portion of IVC receives blood from the renal veins. It passes posterior to the diaphragmatic crura, enters the thorax as the azygos vein, and then joins the superior vena cava at the azygos arch. The hepatic and right adrenal veins drain directly into the right atrium. The gonadal veins drain into the ipsilateral renal veins. The right renal artery crosses abnormally anterior to the IVC (Fig. 110-3).

FIGURE 110-3 Azygos-hemiazygos continuation of the IVC with duplication of the infrarenal IVC. A, Contrast-enhanced CT axial images show infrarenal IVC (I) and a smaller duplicated left IVC (arrow) that arises from the left iliac vein. B, CECT at a higher level demonstrates the absence of the intrahepatic portion of the IVC, which continues through the azygos vein (A) in retrocrural location. The image also shows a large hemiazygos vein (arrow) that arises from the left renal vein as continuation of the left IVC. C, Thick coronal MIP reformat shows the duplicated infrarenal IVC with the left (arrow) draining into the left renal vein and crossing behind the aorta (black arrow). The right IVC (I) continues through the azygos vein (A) above the level of the renal veins. D, Sagittal MIP reformat shows the azygos (A) continuation of the right IVC and how it connects to the SVC (S) through the azygos arch. Note the absence of the intrahepatic IVC and how the hepatic veins drain directly into the right atrium (arrow).

Circumaortic left renal vein: There are two left renal veins. The superior-anterior renal vein receives the adrenal vein and crosses the aorta anteriorly to join the IVC. The second renal vein is approximately 1 to 2 cm more inferior and posterior. It receives the left gonadal vein and crosses posterior to the aorta to join the IVC (Fig. 110-4).

FIGURE 110-4 Circumaortic left renal vein. A, CECT of the abdomen shows a left renal vein crossing anterior to the aorta (arrow). The origin of a second, posterior left renal vein is visualized (arrowhead). B, CECT of the abdomen shows the posterior left renal vein (arrow) crossing behind the abdominal aorta 1 cm caudad to the anterior left renal vein. C, Axial oblique MIP shows both renal veins (R) surrounding the aorta.

Retroaortic left renal vein: The renal vein crosses posterior to the aorta to join the IVC.

Circumcaval ureter (also known as retrocaval ureter): The anomaly always occurs on the right side. The proximal right ureter courses posterior to the IVC, emerges to the right of the aorta, and lies anterior to the right iliac vessel.

Duplicated right renal vein: There is presence of two right renal veins, one anterior and one posterior, usually at the same level.

Partial or complete absence of IVC: The variants of this anomaly include complete absence of the entire IVC which may include the iliac veins as well and partial absence of IVC with preservation of the suprarenal segment. In either case, the iliac veins join to form enlarged ascending lumbar veins. If the entire IVC is absent, the anterior paravertebral collateral vessels convey the blood return to the azygos and hemiazygos veins. If the suprarenal IVC is present, it receives blood from the renal veins. With partial or complete absence of the IVC, large gonadal and parauterine veins can be seen.

Duplication of IVC with retroaortic right renal vein and hemiazygos continuation of the IVC: There are two IVCs below the level of the renal veins. The right IVC joins the right renal vein, which crosses posterior to the aorta to drain in the left IVC. The left IVC then passes posterior to the diaphragmatic crura and continues into the thorax as the hemiazygos vein. There may be a significant size difference between the two vessels. In the thorax, the hemiazygos vein may have any of these different drainage pathways: (1) it crosses posterior to the aorta at about T8 to T9 to join the rudimentary azygos vein; (2) it joins a persistent left SVC and drains into the coronary vein; (3) an accessory hemiazygos continues to join the left brachiocephalic vein.

Duplication of IVC with retroaortic left renal vein and azygos continuation of IVC: There are two infrarenal IVCs. The left IVC joins the left renal vein which then crosses posterior to the aorta to join the right IVC. It passes posterior to the diaphragmatic crura and enters the thorax as azygos vein. It then joins the superior vena cava at its normal location in the right paratracheal space. The hepatic segment may not be truly absent. It drains directly into the right atrium.

Embryogenesis

Knowledge of the IVC embryogenesis is necessary for a better understanding of the IVC anatomic aberrations. The IVC is composed of four segments which form during the 6 to 8 weeks postconception.^2,3 This is due to continuous appearance and regression of three paired embryonic veins, which include the posterior cardinal veins, the subcardinal veins, and the supracardinal veins. The first step in this complex process is the formation of the posterior supracardinal and more anterior subcardinal veins. Then, the most caudal segment of the right supracardinal vein becomes the infrarenal vena cava. The hepatic segment of IVC is derived from the vitelline vein, which conveys blood from the viscera. The suprarenal segment is formed via a subcardinal-hepatic anastomosis. The renal segment forms via anastomosis of the right supra-subcardinal and post-subcardinal veins. The infrarenal segment arises from the right supracardinal vein.

Congenital anomalies of IVC are due to interruption of normal regression or lack of development of the different segments. The circumaortic venous ring and retroaortic left renal vein are related to aberrant development of the renal segment. Azygos continuation of the IVC results when there is a developmental anomaly involving the suprarenal segment.

Early in embryogenesis, there are two renal veins for each kidney: ventral and dorsal. Normally, the dorsal renal vein involutes as the anterior persists as the main renal vein in adult patients. Anatomic anomalies can occur if the involution of the dorsal veins does not occur, including retrocaval and circumaortic left renal vein and duplication of right renal vein.

Clinical Presentation

Patients with IVC anatomic aberrations are most commonly asymptomatic and the anomaly is discovered fortuitously during an imaging study ordered to assess other problems. Nevertheless, complications sometimes occur directly related to the presence of anatomic aberrations.

With retroaortic left renal vein, an increased incidence of testicular varicoceles has been reported, presumably due to compression of the left renal vein by the abdominal aorta. With the circumcaval ureter type, there could be partial right ureteral obstruction or recurrent urinary tract infection. With the absence of infrarenal IVC or entire IVC, patients may present with venous insufficiency of the lower extremities or idiopathic deep venous thrombosis. Approximately 5% of patients younger than 30 years with idiopathic deep venous thrombosis show IVC absence on CT. Almost 10% of these patients with a coexisting thrombophilia have congenital absence of the IVC.⁴

In female patients, enlarged gonadal and pelvis veins can simulate pelvic congestion syndrome. Anatomic variants of the IVC can be seen in association with other anomalies. Azygos continuation, in particular, can be associated with significant congenital heart disease.

Radiography

Plain radiographs of the abdomen and pelvis have no role in the anatomic assessment of the IVC because they are unable to differentiate the veins from other retroperitoneal soft tissues. They may provide gross information though, of the relative location of foreign bodies such as filters, and catheters installed within the IVC.

Ultrasound Imaging

Ultrasound imaging with color flow Doppler imaging can be diagnostic for a variety of IVC anomalies. The key is to properly identify the abdominal venous structures (i.e., IVC), its location, its number, and its connections. With azygous continuation of the IVC, the infrahepatic IVC can be seen draining into the azygos vein with direct hepatic venous drainage into the right atrium. With left IVC, the IVC is positioned to the left of the abdominal aorta. In duplicated IVC, two vertical venous vascular structures can be seen adjacent to and paralleling the abdominal aorta. On Doppler, one IVC will drain the left renal vein.

Circumaortic left renal vein: A circumaortic left renal vein may be difficult to see on Doppler imaging because the two veins do not join the IVC at the same level.

Retroaortic left renal vein: A retroaortic left renal vein may be difficult to see on Doppler imaging. One could see a renal vein crossing behind the abdominal aorta to join the IVC. In more complex IVC anomalies, such as duplication of IVC with retroaortic right renal vein and hemiazygos continuation of the IVC or complete absence of the IVC, ultrasonography may be unable to fully delineate all venous connections and CT or MRI may be required. A circumcaval ureter is also typically not seen on ultrasound images unless there is hydroureter.

CT Scan

CT is a very useful modality to evaluate IVC anomalies because of its ability to generate volumetric data quickly for multiplanar reformation.

Technique: To study the anatomy of the IVC, the CT protocol should include imaging of the chest, abdomen, and pelvis with the use of an intravenous iodinated contrast agent. It is recommended to ensure that there is at least a 2-minute delay between intravenous contrast administration and CT scanning because this will improve the likelihood for homogeneous enhancement of the IVC. If CT is begun at the traditional portal venous phase (65 to 70 sec delay), the infrarenal IVC may have poor luminal enhancement owing to the relatively longer delay necessary for the venous return from the pelvis and inferior extremities to the IVC. The suprarenal IVC, moreover, may show heterogeneous enhancement because of mixing of the contrast bolus returning from the renal veins.

MR

Similar anatomic detail can be seen on MRI as is seen on CT. Steady-state free precession (SSFP; also termed b-FFE, Philips Medical Systems; FIESTA, General Electric Healthcare; True-FISP, Siemens Medical Solutions) pulse sequences are “bright blood” techniques that are particularly good for illustrating abdominal veins. SSFP, especially performed in cine mode, is useful for identification of central venous thrombosis. This technique does not require the intravenous administration of gadolinium-chelate contrast agents.

“Black blood” MRI pulse sequences, in which the vessel lumen is dark secondary to the washout phenomenon (also termed flow void) associated with moving spins (i.e., flowing blood that washes out of the imaging slice prior to sampling of the echo). Examples of this technique include T4-weighted spin echo and single shot T2-weighted imaging (e.g., SSFSE, HASTE), which can provide excellent anatomic assessment almost free of motion artifacts.

Gadolinium-enhanced MRI is arguably the most reliable method to assess vascular patency. We recommend postcontrast imaging using a 3D T1-weighted gradient sequence (e.g., LAVA, THRIVE, or VIBE). Because MRI does not expose the patient to ionizing radiation, it is possible to acquire multiple series of images postcontrast injection, including axial, coronal, and sagittal planes with different timing for more homogeneous luminal enhancement.

Angiography

X-ray catheter angiography studies provide limited anatomic information. In our institution, they are used mostly for therapeutic procedures. Anatomic variants should be diagnosed prior to angiographic procedures in the IVC or otherwise may cause confusion and prolong fluoroscopic time at the time of the intervention, therefore for therapy planning purposes, we recommend the use of CT or MRI.

Reporting: Information for the Referring Physician

IVC and renal vein anatomic variants have minimal increased risk for medical complications such as thrombus or embolism, but are particularly important to identify in patients planning to undergo surgical or percutaneous interventions because their identification can aid in procedure planning and reduce likelihood of complications. Recommendations for clinicians are

Clinical Presentation

Best Imaging Modality