Case report: twin-to-twin transfusion syndrome resulting from placental collateral artery development

BACKGROUND: The twin-to-twin transfusion syndrome (TTTS) is a severe complication of monochorionic twin pregnancies, caused by a net inter-twin transfusion of blood from one fetus (the donor) towards the other fetus (the recipient) through placental anastomoses. TTTS is driven by unidirectional arterio-venous anastomoses, and mitigated by bidirectional arterio-arterial or veno-venous anastomoses which reduce the net inter-twin transfusion. In contrast to these accepted concepts, cases have been described paradoxically devoid of arterio-venous anastomoses but including arterio-arterial anastomoses. We hypothesized that TTTS may develop in such cases as a consequence of a stenosed chorionic artery in the recipient placenta that connects with the arterio-arterial anastomosis. CLINICAL CASES: We describe two cases of monochorionic twin placentae without arterio-venous anastomoses but with only an arterio-arterial and veno-venous anastomosis. In one case severe TTTS developed. There, the arterio-arterial anastomosis connected to a stenosed chorionic artery in the recipient placenta and showed a tortuous appearance. The other case developed uneventful. It lacked a stenosed chorionic artery and the arterio-arterial anastomosis was non-tortuous. CONCLUSION: We present evidence that the arterio-arterial anastomosis represented a functional collateral artery whose outgrowth was driven by an increased shear-stress caused by an increased flow to a lower pressure vascular bed in the placenta of the recipient. The lower arterial pressure occurred from the moment that a chorionic artery which was connected to the anastomosis developed a significant stenosis. The resulting collateral flow through the anastomosis maintained blood supply to the lower pressure placental bed, the beneficial function of collaterals, but also resulted in an increasing net inter-twin transfusion which triggered onset of severe TTTS