Double SVC is a rare anatomical variant that results from a persistent left superior vena cava, which is due to the embryological failure of regression of the anterior cardinal vein.

The left sided SVC drains into the right atrium in 90% of cases via a dilated coronary sinus, alternative sites include the inferior vena cava, a hepatic vein and the left atrium. Atypical left SVC drainage into the left atrium results in a right to left shunt, which can cause cyanosis and has been linked in cases of septic embolism. 

A right sided SVC is seen in 82-90% of cases of double SVC. Absence of right SVC or a left SVC draining into the left atrium has been associated with increased incidence of congenital heart diseases such as ASD,VSD and TOF.

It is important to be aware of this normal variant especially during upper limb central venous catheterization on the left side.