1From the Department of Radiology, University of Texas Health Science Center, Houston. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received March 16, 1999; revision requested May 7 and received July 1; accepted July 8. Address reprint requests to J.E.B., Hermann Imaging and Breast Center, 6410 Fannin, Suite 170, Houston, TX 77030 (e-mail: [email protected]).
Congenital anomalies of the inferior vena cava (IVC) and its tributaries have become more commonly recognized in asymptomatic patients. The embryogenesis of the IVC is a complex process involving the formation of several anastomoses between three paired embryonic veins. The result is numerous variations in the basic venous plan of the abdomen and pelvis. A left IVC typically ends at the left renal vein, which crosses anterior to the aorta to form a normal right-sided prerenal IVC. In double IVC, the left IVC typically ends at the left renal vein, which crosses anterior to the aorta to join the right IVC. In azygos continuation of the IVC, the prerenal IVC passes posterior to the diaphragmatic crura to enter the thorax as the azygos vein. In circumaortic left renal vein, one left renal vein crosses anterior to the aorta and another crosses posterior to the aorta. In retroaortic left renal vein, the left renal vein passes posterior to the aorta. In circumcaval ureter, the proximal ureter courses posterior to the IVC. Other anomalies include absence of the infrarenal IVC or the entire IVC. These anomalies can have significant clinical implications. Awareness of these anomalies is necessary to avoid diagnostic pitfalls.
Anomalies of the inferior vena cava (IVC) and its tributaries have been known to anatomists since 1793, when Abernethy (,1) described a congenital mesocaval shunt and azygos continuation of the IVC in a 10-month-old infant with polysplenia and dextrocardia. Since the development of cross-sectional imaging, congenital anomalies of the IVC and its tributaries have become more frequently encountered in asymptomatic patients (,2). Vascular structures are usually readily identified on computed tomographic (CT) scans of the abdomen and pelvis obtained with intravenously administered contrast material. However, use of intravenous contrast material is occasionally contraindicated. In addition, with helical acquisition, the venous structures may be imaged during the arterial phase, when little or no contrast material is present in the veins. Therefore, familiarity with these variations is essential for correct interpretation of cross-sectional images, to avoid erroneous diagnosis of retroperitoneal and mediastinal masses or adenopathy, and to alert the surgeon and angiographer of potential sources of complications preoperatively.
This article reviews the embryogenesis of the IVC and describes the variations in IVC anatomy. The CT and magnetic resonance (MR) imaging appearances of the more frequently encountered anomalies and some unusual variants are presented. In addition, the clinical relevance of the variations is discussed.
Embryogenesis of the IVC
To better understand the etiology of IVC and renal vein anomalies, a concise review of the embryogenesis of the IVC is warranted. For the interested reader, Phillips (,3) has published a comprehensive review of the embryogenesis of the IVC. In brief, the infrahepatic IVC develops between the 6th and 8th weeks of embryonic life as a composite structure formed from the continuous appearance and regression of three paired embryonic veins. In order of appearance, they are the posterior cardinal, the subcardinal, and the supracardinal veins (,Fig 1).
Initially, all blood return from the body wall caudal to the heart proceeds through the posterior cardinal veins (dark violet in ,Fig 1). Blood return from the viscera is conveyed by the vitelline veins (green in ,Fig 1), which drain the yolk sac. Subsequently, the subcardinal veins (magenta in ,Fig 1) develop ventromedial to the posterior cardinal veins and ventrolateral to the aorta. The intersubcardinal anastomosis forms between the paired subcardinal veins anterior to the aorta and caudal to the superior mesenteric artery. Anastomoses between the posterior cardinal and subcardinal veins (light violet in ,Fig 1) develop on each side at approximately the level of the intersubcardinal anastomosis. At the same time, union occurs between the right subcardinal vein and the hepatic segment of the IVC, which forms from the vitelline vein. As the cranial portions of the posterior cardinal veins begin to atrophy, blood return from the lower extremities is shunted through the postsubcardinal anastomosis, then through the subcardinal-hepatic anastomosis to the hepatic segment of the IVC. This process establishes the prerenal division of the IVC.
The next major development is the appearance of the paired supracardinal veins (goldenrod in ,Fig 1), which lie dorsomedial to the posterior cardinal veins and dorsolateral to the aorta. Initially, multiple anastomoses form between the posterior and supracardinal veins. On each side, a suprasubcardinal anastomosis (yellow in ,Fig 1) develops from union of the postsupracardinal and the postsubcardinal anastomoses. In addition, intersupracardinal anastomoses develop dorsal to the aorta. The supracardinal veins then separate into cranial (azygos) and caudal (lumbar) ends. Meanwhile, inferiorly, anastomoses develop between the two posterior cardinal veins and between the posterior and lumbar supracardinal veins. With further atrophy of the posterior cardinal veins, blood return from the lower extremities is shunted through the supracardinal system to the suprasubcardinal anastomosis, then to the prerenal division of the IVC. In addition, blood return from the left side of the body is shunted to the right across the intersupracardinal and interpostcardinal anastomoses. Finally, the left supracardinal vein is one of the last veins to disappear, although Huntington and McLure (,4) state that the vessel does not so much atrophy as become incorporated into the right supracardinal vein by coalescence of the multiple anastomoses.
In summary, the normal IVC is composed of four segments: hepatic, suprarenal, renal, and infrarenal. The hepatic segment is derived from the vitelline vein. The right subcardinal vein develops into the suprarenal segment by formation of the subcardinal-hepatic anastomosis. The renal segment develops from the right suprasubcardinal and postsubcardinal anastomoses. It is generally accepted that the infrarenal segment derives from the right supracardinal vein, although this idea is somewhat controversial (,3). In the thoracic region, the supracardinal veins give rise to the azygos and hemiazygos veins. In the abdomen, the postcardinal veins are progressively replaced by the subcardinal and supracardinal veins but persist in the pelvis as the common iliac veins.
Since the embryonic ureter passes posterior to the posterior cardinal veins and anterolateral to the supracardinal vein, formation of the postsupracardinal anastomosis inferiorly and the suprasubcardinal anastomosis at the level of the kidney allows development of a potential periureteric venous ring. The renal collar is formed from the intersupracardinal anastomosis dorsally, the intersubcardinal anastomosis and postsubcardinal anastomoses ventrally, and the suprasubcardinal anastomosis laterally. The embryonic kidneys are initially drained by paired ventral and dorsal limbs. Ordinarily, both dorsal limbs regress. On the right side, the ventral limb is incorporated into the lateral wall of the renal segment of the IVC. On the left side, the ventral limb and the anterior limb of the renal collar form the normal adult left renal vein.
Variations in IVC Anatomy
In a study of the development of the IVC in the domestic cat (Felis domestica), Huntington and McLure (,4) proposed a classification system for IVC anomalies based on abnormal regression or abnormal persistence of various embryonic veins. These investigators suggested that there could be up to 14 theoretical variations in the anatomy of the infrarenal IVC. They noted that 11 of the 14 variants had been observed in the domestic cat or in humans. In addition, these authors observed that other anomalies seen in humans, such as abnormal development of the prerenal division of the IVC and persistence of the renal collar in the adult, could be explained on a similar basis. In the remainder of this article, we present nine cases that illustrate these and other variations of IVC and renal vein anatomy using CT and MR images.
A left IVC results from regression of the right supracardinal vein with persistence of the left supracardinal vein. The prevalence is 0.2%–0.5% (,3). Typically, the left IVC joins the left renal vein, which crosses anterior to the aorta in the normal fashion, uniting with the right renal vein to form a normal right-sided prerenal IVC (,,,,,,Fig 2). The major clinical significance of this anomaly is the potential for misdiagnosis as left-sided paraaortic adenopathy (,5). In addition, spontaneous rupture of an abdominal aortic aneurysm into a left IVC has been reported (,6). Transjugular access to the infrarenal IVC for placement of an IVC filter may be difficult.
Duplication of the IVC results from persistence of both supracardinal veins. The prevalence is 0.2%–3% (,3). The left IVC typically ends at the left renal vein, which crosses anterior to the aorta in the normal fashion to join the right IVC (,,,,,,Fig 3). However, there may be variations in this arrangement (see the section entitled “Double IVC with Retroaortic Right Renal Vein and Hemiazygos Continuation of the IVC”). There may be significant asymmetry in the sizes of the left and right veins. Double IVC should be suspected in cases of recurrent pulmonary embolism following placement of an IVC filter (,3). As with left IVC, misdiagnosis of the aberrant vessel as lymphadenopathy should be avoided.
Azygos Continuation of the IVC
Azygos continuation of the IVC has also been termed absence of the hepatic segment of the IVC with azygos continuation (,7). The embryonic event is theorized to be failure to form the right subcardinal–hepatic anastomosis, with resulting atrophy of the right subcardinal vein. Consequently, blood is shunted from the suprasubcardinal anastomosis through the retrocrural azygos vein, which is partially derived from the thoracic segment of the right supracardinal vein. The prevalence is 0.6% (,7).
The renal portion of the IVC receives blood return from both kidneys and passes posterior to the diaphragmatic crura to enter the thorax as the azygos vein (,,,,Fig 4). The azygos vein joins the superior vena cava at the normal location in the right paratracheal space. The hepatic segment (often termed the posthepatic segment) is ordinarily not truly absent; rather, it drains directly into the right atrium. Since the postsubcardinal anastomosis does not contribute to formation of the IVC, each gonadal vein drains to the ipsilateral renal vein (,4).
Formerly thought to be predominantly associated with severe congenital heart disease and asplenia or polysplenia syndromes, azygos continuation of the IVC has become increasingly recognized in otherwise asymptomatic patients since the advent of cross-sectional imaging (,2). It is important to recognize the enlarged azygos vein at the confluence with the superior vena cava and in the retrocrural space to avoid misdiagnosis as a right-sided paratracheal mass or retrocrural adenopathy (,2,,7). Preoperative knowledge of the anatomy may be important in planning cardiopulmonary bypass and to avoid difficulties in catheterizing the heart (,8).
Circumaortic Left Renal Vein
A circumaortic left renal vein results from persistence of the dorsal limb of the embryonic left renal vein and of the dorsal arch of the renal collar (intersupracardinal anastomosis). The prevalence may be as high as 8.7% (,3). Two left renal veins are present. The superior renal vein receives the left adrenal vein and crosses the aorta anteriorly. The inferior renal vein receives the left gonadal vein and crosses posterior to the aorta approximately 1–2 cm inferior to the normal anterior vein (,,,,,,Fig 5). The major clinical significance is in preoperative planning prior to nephrectomy and in renal vein catheterization for venous sampling. Misdiagnosis as retroperitoneal adenopathy should be avoided.
Retroaortic Left Renal Vein
As with circumaortic left renal vein, a retroaortic left renal vein results from persistence of the dorsal arch of the renal collar. However, in this variation the ventral arch (intersubcardinal anastomosis) regresses so that a single renal vein passes posterior to the aorta (,,,,Fig 6). The prevalence is 2.1% (,3). The clinical significance is preoperative recognition of the anomaly.
Double IVC with Retroaortic Right Renal Vein and Hemiazygos Continuation of the IVC
More than one anomaly can coexist in a patient. In the case of a double IVC with a retroaortic right renal vein and hemiazygos continuation of the IVC, the embryologic basis is persistence of the left lumbar and thoracic supracardinal vein and the left suprasubcardinal anastomosis, together with failure of formation of the right sub-cardinal–hepatic anastomosis. In addition, the right renal vein and right IVC meet and cross posterior to the aorta to join the left IVC and continue cephalad as the hemiazygos vein (,,,,,,Fig 7). Thus, there is also persistence of the dorsal limb of the renal collar and regression of the ventral limb. In the thorax, the hemiazygos vein crosses posterior to the aorta at approximately T8 or T9 to join the rudimentary azygos vein. Alternate collateral pathways for the hemiazygos vein include (a) cephalad continuation to join the coronary vein of the heart via a persistent left superior vena cava and (b) accessory hemiazygos continuation to the left brachiocephalic vein (,9).
In both of these alternate pathways, the aberrant vessel may simulate a left mediastinal mass (,9). In addition, with accessory hemiazygos continuation, the aberrant vessel may mimic an aortic dissection (,10). More important, perioperative death following inadvertent ligation of hemiazygos-to-azygos continuation of a left IVC during thoracic surgery has been reported (,11). As discussed in the section entitled “Azygos Continuation of the IVC,” when the subcardinal-hepatic anastomosis fails to form, the hepatic segment of the IVC ordinarily drains independently directly into the right atrium. However, there has been one case report of a patient with Budd-Chiari syndrome in whom the bulk of systemic venous drainage from the liver was via the right renal vein to a hemiazygos continuation of a left IVC (,12). With MR imaging, misdiagnosis of vessels as masses may be less likely due to the presence of flow voids or flow-related enhancement in the aberrant venous structures.
Double IVC with Retroaortic Left Renal Vein and Azygos Continuation of the IVC
A double IVC with a retroaortic left renal vein and azygos continuation of the IVC is an interesting combination. It results from persistence of the left supracardinal vein and the dorsal limb of the renal collar with regression of the ventral limb. In addition, the subcardinal-hepatic anastomosis fails to form (,,,,,,Fig 8). A recent study (,13) demonstrated that azygos continuation of the IVC can be predicted with ultrasonography by identifying the right renal artery crossing abnormally anterior to the IVC.
A circumcaval ureter is also termed a retrocaval ureter. The right supracardinal system fails to develop, whereas the right posterior cardinal vein persists. With one reported exception (,14), the anomaly always occurs on the right side. The proximal ureter courses posterior to the IVC, then emerges to the right of the aorta, coming to lie anterior to the right iliac vessels (,,,,,Fig 9). Patients with this anomaly may develop partial right ureteral obstruction or recurrent urinary tract infections. Therapeutic options include surgical relocation of the ureter anterior to the cava (,3).
Absent Infrarenal IVC with Preservation of the Suprarenal Segment
Several reports have described absence of the entire IVC (,15–,17) or absence of the infrarenal IVC with preservation of the suprarenal segment (,,,,,,,,Fig 10) (,18,,19). Absence of the entire posthepatic IVC suggests that all three paired venous systems failed to develop properly. Absence of the infrarenal IVC implies failure of development of the posterior cardinal and supracardinal veins. Since it is difficult to identify a single embryonic event that can lead to either of these scenarios, there is controversy as to whether these conditions are true embryonic anomalies or the result of perinatal IVC thrombosis (,15,,18,,19). In the case presented herein (,,,,,,,,Fig 10), the common iliac veins are also absent. The external and internal iliac veins join to form enlarged ascending lumbar veins, which convey blood return from the lower extremities to the azygos and hemiazygos veins via anterior paravertebral collateral veins. A normal suprarenal IVC is formed by confluence of the renal veins. Patients with absent IVC may present with symptoms of lower-extremity venous insufficiency (,16–,18) or idiopathic deep venous thrombosis (,19,,20). The collateral circulation may simulate a paraspinal mass (,15).
The complexity of the ontogeny of the IVC, with numerous anastomoses formed between the three primitive paired veins, can lead to a wide array of variations in the basic plan of venous return from the abdomen and lower extremity. Some of these anomalies have significant clinical implications. Although vascular structures can usually be readily identified on contrast-enhanced CT scans, identification of unusual venous arrangements may be difficult in those cases in which intravenous contrast material is contraindicated. In such patients, MR imaging may be used to distinguish aberrant vessels from masses by demonstrating flow voids or flow-related enhancement. A working knowledge of IVC and renal vein anomalies is essential to avoid diagnostic pitfalls.
Abbreviation: IVC = inferior vena cava
LEARNING OBJECTIVES FOR TEST 2 After reading this article and taking the test, the reader will be able to:⋅ Name the four segments of the IVC and describe their embryonic origins.⋅ List at least three anomalous courses of the IVC and the left renal vein and describe the aberrant embryonic vessels and the embryologic errors involved. ⋅ Recognize and describe the appearance of aberrant IVCs and renal veins on cross-sectional images.
The authors express their gratitude to Maxine Davis, Dan Klepac, and David Reyna for their help in the preparation of the manuscript and the illustrations.
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