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The tronc réno-rachidièn is a  peculiar vein connecting the left renal vein and the hemi-azygos vein. It is not described in current anatomic textbooks.

This vessel develops as an adaptation to increasing pressure inside the left renal vein which is regularly a consequence of a lordogenetic compression of the left renal vein in front of the bulging origin of the right renal artery or front of the ventral circumference of the aorta. Such a constellation can be frequently found in patients with an accentuated lordosis, and is very frequently observed in patients with hypermobile Ehlers-Danlos syndrome or other connective-tissue disorders with hyperflexibility.

Since this vessel is not mentioned in conventional anatomic textbooks and not taught in the medical curriculum it is regularly overlooked in standard imaging examinations.

The only references are in French scientific papers:

It is the leading cause of back pain and neurological symptoms in patients with severe left renal vein compression especially in cases where a May Thurner syndrome occurs simultaneously.

In order to spread the knowledge of this important vascular connection I a publish here some instructive images: