Fig. 1

Embryologic developmental stages of the primitive venous system (simplified and adapted from Azizova et al.) [4]. a) the double horned sinus venosus (SV) is yet a separate part of the human heart in the 4th week of intrauterine development. The right and left superior and inferior cardinal veins (RSCV, LSCV, RICV, LICV) drain into the SV via the common cardinal veins (CCV). The RSCV and the LSCV are connected via an anastomosis. b) up to the 12th week the RSCV and the LSCV form the jugluar veins (JV) and the innominate vein (IV). The right CCV and part of the RSCV form the superior vena cava (SVC). The azygos vein (AzV) is formed by the RICV, and the LICV obliterates. The right horn of the SV becomes part of the right atrium (RA) whereas the left horn forms the coronary sinus (CS). c) if obliteration of the LSCV does not occur, the persistens left superior vena cava (PLSVC) remains. It can drain into the left atrium directly or indirectly through the CS or, as seen with Patient A, via pulmonary veins. The SVC on the right side is then labeled the right superior vena cava (RSVC)