Imaging of Type I Retroaortic Left Renal Vein & Nutcracker Syndrome

Retroaortic left renal veins are an often overlooked, but no so rare anatomic anomaly that might cause a few aspecific symptoms and signs, especially flank or inguinal pain, microscopic or even gross hematuria; in the vast majority of cases it is, thought, totally asymptomatic. The nutcracker syndrome that might arise in the case of RLRV is mainly due to the re- duced space between aorta and the vertebral body and the consequent compression on the venal rein; this might in turn cause upstream hypertension, hematuria, varicocele, pain. The compression of RLRV leads to hematuria because of renal venous hypertension, consequent left renal congestion. This upstream venous congestion might result, in some cases, in left-sided varicocele in men and pelvic congestion syndrome in women. From a radiologic point of view, the presence of RLRV and the precise reporting its subtypes is mandatory.


INTRODUCTION
Renal veins development derives from the articulate developmental process of the inferior vena cava (IVC), between the fourth and eight week of conception.
The IVC itself derives from a three pairs of veins, which are parallel and are called, in order of appearance, the posterior cardinal veins, the sub-cardinal and supra-cardinal veins [1,2]. Supra and sub-cardinal veins are, between the fourth and eight week of conception, in communication through anastomoses which basically encircle the aorta as a collar.
The normal left renal vein arises from the ventral arch of this circumaortic collar; a retroaortic left renal vein (RLRV) is then formed when the dorsal portion of the circumaortic collar persists and the ventral limb obliterates, creating a posterior vein.
The anomalies of the left renal vein can be divided into 4 different types [3,4]: RLRV originating from the obliteration of the ventral preaortic limb, joining the IVC in the orthotopic position -the incidence of type I RLRV is 0.3-1-9% [5,6]; II. RLRV originating from the obliteration of the ventral preaortic limb, joining the IVC at the level of L4-L5, after joining the gonadal and ascending lumbar veins -the incidence of type II RLRV is 0.4-0.9% [7,8]; III. RLRV originating from a circumaortic arch or collar, created by the persistence of subsupracardial and intersupracardial anastomoses and the dorsal limb of the left renal vein -the incidence of type III RLRV is 1.5-8.7% [5,9], but can reach 16% if all small retroaortic veins that drain into the IVC are considered [10,11,12,13]; IV. RLRV originating from the obliteration of the ventral preaortic limb, joining the left common iliac vein, after coursing caudally and obliquely behind the aorta -the incidence of type IV RLRV is 0.16% [12].
This often overlooked, but no so rare anatomic anomaly might cause a few aspecific symptoms and signs, especially flank or inguinal pain, microscopic or even gross hematuria; in the vast majority of cases it is, thought, totally asymptomatic.
The nutcracker syndrome that might arise in the case of RLRV is mainly due to the reduced space between aorta and the vertebral body and the consequent compression on the venal rein; this might in turn cause upstream hypertension, hematuria, varicocele, pain. This upstream venous congestion might result, in some cases, in left-sided varicocele in men and pelvic congestion syndrome in women [15].
In the case of kidney surgery, the presence of RLRV might influence the actual technical feasibility of the procedure; if not preventively recognised and know, the presence of RLRV might cause severe hemorrhage and/or renal damage [16].

IMAGING
From a radiologic point of view, considering that CECT has now become the main tool for a prompt diagnosis of the condition, which was once only observed at autopsy or, incidentally, during angiography, it is of utmost important to detect the anomaly, recognising the specific type and duly reporting it, both because of the symptoms and conditions that may arise from it and the possible complications and potential fatal haemorrhage in the case of left kidney surgery [4].
In our pictorial essay, we will discuss as an example the case of a 52-year old man with left flank pain and macroscopic hematuria, initially referred for a CT-KUB for a suspected acute renal colic. Images were acquired using a tri-phasic protocol after the infusion of 100 ml of iodinated contrast, at a speed of 3.5 ml/s.