Circumaortic left renal vein
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Circumaortic left renal vein, also known as circumaortic renal collar is an anomaly of left renal vein when a supernumerary or accessory left renal vein passes posterior to the aorta, apart from the normal renal vein passing anterior to the aorta. This anomaly is potentially hazardous, if unrecognised during retroperitoneal surgery (see Complications below) 1.
By the 8th week of life, the posterior cardinal system (which represents iliac bifurcation in adults) is replaced by four longitudinal channels - a paired supracardinal veins posterior to the aorta and paired subcardinal veins anterior to the aorta. Connections between these veins form a circumaortic plexus. Usually, the ventral portion of this plexus forms the normal left renal vein. If both the dorsal and ventral venous plexus persists, they form a circumaortic renal venous collar.
The incidence of circumaortic renal vein 2 is reported to be ~10% (range 2-17%). A more common variant is a retro-aortic single left renal vein.
CT or conventional angiography may accurately delineate this abnormality, where an accessory renal vein is seen to pass posterior to the aorta.
During retroperitoneal surgery or nephrectomy, the surgeon may be falsely assured by the normal anterior renal vein, confidently clamp the aorta and mobilize the kidney. In this process, the retroaortic renal vein may shear off, leading to fatal hemorrhage.