**2.1 Left IVC**

A left IVC results from regression of the right supra-cardinal vein with persistence of the left supra-cardinal vein. The prevalence is 0.2%–0.5%(i). 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).

#### **2.2 Double IVC**

Duplication of the IVC results from persistence of both supracardinal veins. The prevalence is 0.2%–3%(i). 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).

### **2.3 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(k). The embryonic event is theorized to be failure to form the right sub-cardinal–hepatic anastomosis, with resulting atrophy of the right sub-cardinal vein. Consequently, blood is shunted from the supra-sub-cardinal anastomosis through the retro-crural azygos vein, which is partially derived from the thoracic segment of the right supra-cardinal vein. The prevalence is 0.6%(k). 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).

Finally, the left supracardinal vein is one of the last veins to disappear, although Huntington and McLure(j) state that the vessel does not so much atrophy as become incorporated into the right supracardinal vein by coalescence of the multiple anastomosis. 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 anastomosis. It is generally accepted that the infra-renal segment derives from the right supracardinal vein, although this idea is somewhat controversial(i). 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

In a study of the development of the IVC in the domestic cat (Felis domestica), Huntington and McLure(j) 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 pre-renal division of the IVC and persistence of the renal

A left IVC results from regression of the right supra-cardinal vein with persistence of the left supra-cardinal vein. The prevalence is 0.2%–0.5%(i). Typically, the left IVC joins the left renal vein, which crosses anterior to the aorta in the normal fashion, uniting with the right renal

Duplication of the IVC results from persistence of both supracardinal veins. The prevalence is 0.2%–3%(i). The left IVC typically ends at the left renal vein, which crosses anterior to the

Azygos continuation of the IVC has also been termed absence of the hepatic segment of the IVC with azygos continuation(k). The embryonic event is theorized to be failure to form the right sub-cardinal–hepatic anastomosis, with resulting atrophy of the right sub-cardinal vein. Consequently, blood is shunted from the supra-sub-cardinal anastomosis through the retro-crural azygos vein, which is partially derived from the thoracic segment of the right supra-cardinal vein. The prevalence is 0.6%(k). The renal portion of the IVC receives blood return from both kidneys and passes posterior to the diaphragmatic crura to enter the

pelvis as the common iliac veins.

**2. Variations in IVC anatomy** 

**2.1 Left IVC** 

**2.2 Double IVC** 

collar in the adult, could be explained on a similar basis.

vein to form a normal right-sided prerenal IVC (Fig 2).

aorta in the normal fashion to join the right IVC (Fig 3).

**2.3 Azygos continuation of the IVC** 

thorax as the azygos vein (Fig 4).

Fig. 2. Partial malrotation and left IVC in a 49-year-old man. (a) Schematic shows a left IVC terminating at the left renal vein. (b-e) CT scans presented from caudal to cranial show the anomaly. (b) Note the left IVC (arrow) inferior to the renal veins. (c) The left IVC joins the left renal vein (arrow). (d) The left renal vein (arrow) crosses anterior to the aorta in the normal fashion. (e) A normal right-sided prerenal IVC is formed from the confluence of the left (straight arrow) and right (curved arrow) renal veins. Note the increased attenuation of the right renal vein relative to that of the left due to absence of dilution from relatively unenhanced lower-extremity venous return. The major clinical significance of this anomaly is the potential for misdiagnosis as left-sided paraaortic adenopathy(k).

Vena Cava Malformations as an

arrow in c).

avoid difficulties in catheterizing the heart(m).

Emerging Etiologic Factor for Deep Vein Thrombosis in Young Patients 49

(a)

Fig. 4. CT images of azygos continuation of the IVC in a 48-year-old man. (a) Schematic shows lack of contiguity between the pre-renal segment of the IVC (arrow) and the hepatic segment. The vessel parallel to the aorta under the crus is the azygos vein. (b, c) CT scans obtained at the level of the diaphragmatic crus (b) and the level of the azygos vein arch (c) show the enlarged azygos vein (straight arrow) draining into the superior vena cava (curved

The azygos vein joins the superior vena cava at the normal location in the right para-tracheal space. The hepatic segment (often termed the post-hepatic segment) is ordinarily not truly absent; rather, it drains directly into the right atrium. Since the post-sub-cardinal anastomosis does not contribute to formation of the IVC, each gonadal vein drains to the ipsi-lateral renal vein(j). Formerly thought to be predominantly associated with severe congenital heart disease and a-splenia or poly-splenia syndromes, azygos continuation of the IVC has become increasingly recognized in otherwise asymptomatic patients since the advent of cross-sectional imaging. 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 para-tracheal mass or retro-crural adenopathy(k,l). Preoperative knowledge of the anatomy may be important in planning cardiopulmonary bypass and to

(a)

Fig. 3. Double IVC in a 53-year-old woman with lymphoma. (a) Schematic shows left and right infrarenal IVCs. The left IVC terminates at the left renal vein. (b) CT scan obtained inferior to the renal veins shows left (straight arrow) and right (curved arrow) IVCs. (c-e) CT scans show the left IVC ending at the confluence with the left renal vein (arrow in c), which crosses anterior to the aorta in the normal fashion (arrow in d) to join a normal pre-renal IVC (arrow in e). There may be morphological variation and asymmetry of the left and right veins. Double IVC should be suspected in cases of recurrent pulmonary embolism following

placement of an IVC filter(i).

Fig. 4. CT images of azygos continuation of the IVC in a 48-year-old man. (a) Schematic shows lack of contiguity between the pre-renal segment of the IVC (arrow) and the hepatic segment. The vessel parallel to the aorta under the crus is the azygos vein. (b, c) CT scans obtained at the level of the diaphragmatic crus (b) and the level of the azygos vein arch (c) show the enlarged azygos vein (straight arrow) draining into the superior vena cava (curved arrow in c).

The azygos vein joins the superior vena cava at the normal location in the right para-tracheal space. The hepatic segment (often termed the post-hepatic segment) is ordinarily not truly absent; rather, it drains directly into the right atrium. Since the post-sub-cardinal anastomosis does not contribute to formation of the IVC, each gonadal vein drains to the ipsi-lateral renal vein(j). Formerly thought to be predominantly associated with severe congenital heart disease and a-splenia or poly-splenia syndromes, azygos continuation of the IVC has become increasingly recognized in otherwise asymptomatic patients since the advent of cross-sectional imaging. 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 para-tracheal mass or retro-crural adenopathy(k,l). Preoperative knowledge of the anatomy may be important in planning cardiopulmonary bypass and to avoid difficulties in catheterizing the heart(m).

Vena Cava Malformations as an

**2.4 Circum-aortic left renal vein** 

**2.5 Retro-aortic left renal vein** 

anomaly.

inferior to the normal anterior vein (Fig 5).

descending to cross posterior to the aorta.

Emerging Etiologic Factor for Deep Vein Thrombosis in Young Patients 51

A circum-aortic 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 (inter-supra-cardinal anastomosis). The prevalence may be as high as 8.7%(i). 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

As with circum-aortic left renal vein, a retro-aortic 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%(i). The clinical significance is preoperative recognition of the

(a)

Fig. 6. Retroaortic left renal vein in a 27-year-old man. (a) Schematic shows a single left renal vein, which crosses posterior to the aorta. (b, c) CT scans show the left renal vein (arrow)

Fig. 5. Circumaortic left renal vein in a 73-year-old woman. (a) Schematic shows two left renal veins, with the inferior vein crossing posterior to the aorta. (b-e) Contiguous 5-mmthick CT sections presented from cranial to caudal show the anomaly. (b) The superior left renal vein (arrow) crosses anterior to the aorta. (c-e) The inferior vein (curved arrow) descends approximately 2 cm and receives the left gonadal vein (straight arrow in d) before crossing posterior to the aorta. 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.
