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The double retro-aortic left renal vein

The renal veins drain the kidney into the inferior vena cava and unite in a variable fashion to form the renal vein. The left renal vein is normally located in front of the aorta. However, the retro-aortic renal vein may course posterior to the aorta due to embryological developmental anomalies. During educational dissection, a rare variation of the left renal vein was found in a 66-year old male cadaver. The double retro-aortic renal veins coursed behind the aorta to drain into the inferior vena cava. The superior retro-aortic renal vein drained into the inferior vena cava at the lower border of the L2 vertebra, and the inferior retro-aortic renal vein drained into the inferior vena cava at the upper border of the L4 vertebra. Such a variant is rare, and is a clinically important observation which should be noted by vascular surgeons, oncologists, and traumatologists.


Introduction
The renal veins connect the kidney to the inferior vena cava (IVC). They are usually singular to each kidney and located anterior to the aorta. However, because of its embryological development, the anatomy of the renal veins is variable. Various positioning of renal vein have been reported and the retro-aortic renal vein, located posterior to aorta, was also studied by many authors [1][2][3][4][5][6]. According to previous studies, the incidence of retro-aortic renal vein was reported to be 0.6-3.7% [1][2][3][4][5][6]. Even though, retro-aortic vein is the common venous variation, it is usually clinically silent. Therefore, it usually remains unnoticed unless discovered accidently during operation or diagnosis.
Many diagnostic and therapeutic procedures have been carried out in the renal vein and kidney, including venography

Discussion
We have described the case of two RA-LRV which drained into the IVC at the L2 vertebra and L4 vertebra, respectively. The retro-aortic left renal vein has been studied by many authors [1][2][3][4][5][6], and its prevalence is presented in Table 1. Among various authors, Hoeltl et al. [2] classified retro-aortic renal veins as two main types, according to the obliteration type of embryonic renal venous collar. Retro-aortic left renal vein type I was developed by the persistence of the left sub-supracardinal anastomosis, the intersupracardinal anastomosis, and the left dorsal renal vein. Type II was developed by the persistence of the left subsupracardinal anastomosis and the supracardinal vein. Hoeltl et al. [2] reported that the frequency of type 1 and type 2 was 0.56% (2/354) on autopsy. On computed tomography (CT), type 1 and type 2 were found in 0.26% (12/4,520) and 0.38% (17/4,520), respectively [2]. The present case showed type 1 and type 2 retro-aortic renal veins together and its frequency was extremely rare.
Two occurrences of retro-aortic renal veins are rare, however, similar cases have been observed by many authors. Koc et al. [9] observed this anomaly, not on cadaver, but on CT, and it was suggested that this condition may be associated with left-sided pelvic congestion syndrome. In the case observed by Anjamrooz et al. [10], the patterns of the two retro-aortic renal veins were similar, however the left testicular vein joined the main renal vein. The left testicular vein in the present case drained into the bifurcation of the left main renal vein. Senecail et al. [7] also reported a similar case with a partially bifid and retro-aortic left renal vein by dissection. However, this variation was a bifid form of the renal vein only, which appearance was not considered as the persistence of circumaortic venous ring, as the embryogenesis of renal vein system. Therefore, this is the first report with double retro-aortic renal vein connected to the testicular vein.
To understand the development of this variation, the  mbryogenesis of the renal vein system should be considered. At 8 weeks of embryogenesis, a circumaortic venous ring (so-called 'renal collar') is developed completely (Fig.  2A). As development procedes, certain predestined parts of this primitive venous drainage obliterate, and normal development of renal vein in adult is completed (Fig. 2B). In the present report, a rare case with double retro-aortic left renal vein was described. Our report may contribute to the expansion of anatomic knowledge in reno-vascular structure because of its rarity. Awareness of venous variations in kidney may be useful because its variation could enhance iatrogenic injuries or complications during several invasive procedures and influence hemodynamic problems with unexplained symptoms [1,3,8].