A left-sided IVC is formed as a result of disappearance of the right supracardinal vein and persistence of the left supracardinal vein. The left-sided IVC typically terminates at the left renal vein. It then courses ventral to the aorta to the right side to form a normal right-sided suprarenal IVC (Figure 1). The reported incidence is 0.2% – 0.5%.^1,14,15 It is a mirror image variant without an increased risk of any abnormality. However, it can lead to confusion between venous and arterial access. Also, transjugular IVC filter placement and pulmonary thrombolysis is more challenging with a left-sided IVC. In addition, it can mimic left paraaortic adenopathy.^1,9 Rare spontaneous rupture of an abdominal aortic aneurysm into a left IVC has also been reported.^16,17

A double IVC has a left-sided IVC in addition to the normal right IVC. It results from persistence of both supracardinal veins. The left IVC crosses the midline anterior to the aorta to join the right IVC (Figure 2). There may be variations in calibre of the left and right IVC.^1,9 The incidence of double IVC has been reported between 1% and 3%.¹⁵ Recurrent pulmonary embolism despite IVC filter placement should raise suspicion of a double IVC. Review of prior cross-sectional images before IVC filter placement is useful to diagnose congenital IVC abnormalities such as double IVC. If no prior cross-sectional images are available for review, cavography through the left iliac vein should be performed prior to filter placement. A filter can be placed into each cava if a double IVC is present. The aberrant vessel may also mimic a lymph node.^1,7,9

Two left renal veins are present on the left side with one coursing anterior to the aorta and other posterior to the aorta. It results from persistence of the posteriorly located inter-supracardinal vein anastomoses as well as the anteriorly located inter-subcardinal vein anastomoses that form a venous ring around the aorta (Figure 3). The reported prevalence ranges from 2.4% to 8.7%.¹ The posterior renal vein is 1 cm – 2 cm inferior to the normal anterior vein. The left adrenal vein drains into the superior renal vein. The left gonadal vein drains into the inferior renal vein. Knowledge of this variation is significant in preoperative planning prior to nephrectomy and catheterisation for renal venous sampling. It can also mimic retroperitoneal adenopathy.^7,9 It is also important to recognise this variation prior to positioning of the IVC filter.¹⁸ These patients may uncommonly present with hypertension, haematuria and varicoceles.¹⁹

A single left renal vein with a retroaortic course as the dorsal arch of the renal collar persists, while the ventral arch (inter-subcardinal anastomosis) regresses. Reported prevalence is 1.7% – 3.4%.^1,7 Although mostly asymptomatic, occasionally patients may present with symptoms of haematuria, flank pain and varicocele.^19,20

The genitourinary system develops separately from the IVC. However, embryogenesis of the IVC determines the spatial relationship between the ureter and the IVC. In this anatomical variant, the infrarenal segment develops from the right posterior cardinal vein, which is anterolateral to the ureter, instead of the right supracardinal vein, which is posteromedial to the ureter. The anomaly almost always occurs on the right side with a handful of cases reported on left.^9,21 The reported prevalence of a retrocaval ureter is 0.13%.²² In intravenous urography, the proximal part of retrocaval ureter makes a characteristic course. It projects over or medial to the lumbar pedicles. This gives the characteristic fish hook or reverse J appearance (Figure 4).^7,9,19 Although most cases are asymptomatic, right flank pain is the most common symptom.²¹ Compression of the ureter between the IVC and vertebra can result in hydronephrosis. Ureteral obstruction or recurrent urinary tract infection (UTI) may necessitate surgical relocation of the ureter anterior to the cava.^9,19,21 Coexistence of other congenital genitourinary, cardiovascular and spine abnormalities have been reported.²¹

Absence of the hepatic segment of the IVC is thought to be because of failure of the right subcardinal-hepatic anastomosis with resultant right subcardinal vein atrophy and shunting of blood from the supra-subcardinal anastomosis to the azygos vein. The azygos vein joins the superior vena cava at the right paratracheal space in the expected normal location. The hepatic segment is not entirely absent. It drains directly into the right atrium. The right gonadal vein drains to the ipsilateral renal vein as the post-subcardinal anastomosis does not contribute to the formation of the IVC. It has a reported prevalence of 0.6%.^1,7,9 Azygos continuation is more common than hemiazygos continuation. Azygos continuation of the IVC can be seen in asymptomatic patients or in association with other congenital abnormalities such as severe congenital heart disease and asplenia or polysplenia syndromes (Figure 5).^1,8,23 An enlarged azygos vein can mimic a right paratracheal or retrocrural lymph node. Knowledge of this variation is useful for preoperative planning for cardiopulmonary bypass and vascular procedures.^1,9

This rare anomaly results from anomalies in multiple steps during the development of the IVC and renal vein. Similar to azygos continuation of the IVC, absence of the right subcardinal-hepatic anastomosis results in failure of development of the hepatic segment. Left lumbar and thoracic supracardinal veins persist giving rise to a left IVC and blood is shunted into the hemiazygos vein via the left supra-subcardinal anastomosis. Persistence of the dorsal limb and regression of the ventral limb of the renal collar leads to a right renal vein that meets the right IVC and courses posterior to the aorta to join the left IVC. The azygos vein is rudimentary. The hemiazygos vein drains via alternate collateral pathways. The hemiazygos vein can drain into the azygos vein, the coronary vein of the heart in patients with a persistent left superior vena cava or via the accessory hemiazygos vein into the left brachiocephalic vein. These aberrant vessels may simulate a left mediastinal mass or aortic dissection. Like azygos continuation of the IVC, the hepatic segment of the IVC drains into the right atrium independently.^1,24

This congenital variation occurs when the supracardinal vein and the dorsal limb of the renal collar persist while the ventral limb regresses and the subcardinal-hepatic anastomosis does not form.^1,25 These are extremely rare and detected incidentally. Very few cases of right-sided varicocele have been reported in patients with a double IVC and azygos or hemiazygos continuation.^25,26 The imaging findings are similar to azygos continuation of the IVC except there is a double IVC instead of the normal right-sided IVC and the left renal vein has a retroaortic course²⁵ (Figure 6). Clinical implications of this variation are to avoid misdiagnosis as lymphadenopathy and preoperative surgical planning.²⁵

The entire IVC or only the infrarenal portion of the IVC can be absent. Absence of the entire posthepatic IVC results from failure of development of all three paired venous systems. These are extremely rare. The absence of the infrarenal IVC implies failure of development of the posterior cardinal and supracardinal veins.⁹ As a single embryonic event cannot explain this abnormality, it is thought that it is not a true embryonic anomaly but the result of perinatal thrombosis and atrophy.^1,7,9 Imaging shows the absence of the IVC or a portion of the IVC with prominent venous channels providing collateral flow (Figure 7). These veins are prone to idiopathic deep vein thrombosis and lower extremity venous insufficiency. Prominent lumbar collateral vessels may develop, which can mimic paraspinal masses.⁹

The common iliac veins can be absent as well. The lower extremity blood drains to the azygos and hemiazygos veins via anterior paravertebral collateral veins. Anterior paravertebral collateral veins receive blood via enlarged ascending lumbar veins, which in turn are formed by the external and internal iliac veins.¹

The portal vein is formed cranially from a segment of the prehepatic right vitelline vein, the intervitelline anastomosis, and caudally from the left vitelline vein around the fifth week of life.¹⁹ The right vitelline vein thus serves as a common conduit during the development of both the IVC and the portal vein. The process of formation of hepatic tissue results in disruption of intervitelline vein anastomosis disconnecting the cranial and caudal segments of the portal vein. Eventually, the cranial segment of the portal vein disappears. Hence, there is no direct communication between the portal vein and the IVC.¹⁹ These extrahepatic portocaval shunts may result either from excessive involution of the vitelline vein or failure of the vitelline vein to establish an anastomosis with the hepatic sinusoids or hepatic veins.^9,27

Abernethy malformations are of two types. Type 1 is more common in females and is characterised by complete shunting of portal blood into the IVC and an absent portal vein (Figure 8). It is associated with congenital cardiac, gastrointestinal (e.g., biliary atresia) and genitourinary anomalies.^9,19 Type 2 is more often seen in males as an isolated abnormality with partial end-to side anastomosis between an intact portal vein and the IVC resulting in shunting of blood. The presence or absence of the portal vein is an important imaging finding because it helps to distinguish between the two types. The Abernethy malformation is associated with focal nodular hyperplasia and hepatocellular carcinoma (HCC).^7,9 Type I is managed with a liver transplant. The Type II shunt may require occlusion of the shunt if the patient develops hepatic encephalopathy or bleeding varices. It can be managed surgically or percutaneously using balloons or coils.¹⁹

These are uncommon IVC findings attributed to congenital vascular anomalies or the sequela of thrombus formation.⁹ North American and northern European populations are rarely affected. There is usually an underlying hypercoagulable state and they present acutely. Outcomes are usually fatal. Asian and South African populations are more commonly affected in whom it eventually leads to congestive cirrhosis.²⁸ The onset is insidious in these patients.^7,9 A complete or fenestrated membrane or a segment of fibrotic occlusion in the intrahepatic IVC with prominent intrahepatic and extrahepatic collaterals is seen on imaging (Figure 9). Inferior venacavography can be helpful in confirming the diagnosis. Endovascular treatment to relieve portal pressure can be performed depending on the severity of the associated liver disease.^7,9,29 The association between membranous obstruction of the intrahepatic IVC, Budd-Chiari syndrome and HCC is well established.⁷