The presence of persistent left superior vena cava (PLSCV) is a congenital venous anomaly, due to regression failure of left anterior cardinal vein
at eighth gestational week.
During normal development, the anterior cardinal veins
, which drain the head, neck, and arm, unite with the posterior cardinal vein
in the very early embryonic stage and enter the heart as the right and left horns of the sinus venosus.
The lowest part of right posterior cardinal vein
The left superior intercostal vein and accessory hemiazygos vein are derived from left posterior cardinal vein
and this vein simultaneously forms upper part of azygos vein.
This anastomosis shunts blood from the left to the right anterior cardinal vein
and ultimately becomes innominate (or brachiocephalic) vein when the caudal part of the left anterior cardinal vein
regresses to become "ligament of Marshall".
the primitive maxillary vein and the ventral pharyngeal vein which drain the rapidly growing mandibular and the hyoid arches into the common cardinal vein