Pseudo-arterio-arterial anastomoses in twin-twin transfusion syndrome

It has recently been claimed that fetoscopic recognition of a haemodynamic equator within an arterio-arterial anastomosis (AAA) suggests minimal net intertwin flow. This was based on blood from one fetus being dark and from the other bright red, the boundary between them reciprocating with the fetal heart beats. However, bright red indicates that the blood had passed through a cotyledon and been freshly oxygenated, which should be impossible in an AAA. We applied a computer model of chorionic vessels to determine a configuration that reproduced this phenomenon.

A previously published TTTS model was extended to provide placental detail in a segment containing four cotyledons of each placenta supplied by three generations of placental arteries and veins.

Reciprocating flow is not unique to AAAs. It also occurs in the chorionic arteries of any cotyledon deprived of its venous outflow, in a similar manner to that in which reverse end-diastolic flow occurs in umbilical arteries when whole placental resistance is high. If venous return from the common chorionic vein in the recipient (draining the venous end of an AVA) is blocked as might happen after laser, there can be bidirectional flow from one umbilical artery insertion, through two cotyledons to the other insertion. We define this phenomenon as a pseudo-AAA (PAAA). The inclusion of two cotyledons in this path means that its resistance cannot match the low flow resistance of a true AAA, and transmission of the contralateral pulsatile pattern is absorbed in the cotyledons. Thus, PAAA Doppler patterns differ from true AAA patterns in that two sets of systolic peaks, one forward and one reverse, can be discerned in true AAAs but only one in PAAAs.

We demonstrate how venous occlusion of an arterio-venous anastomosis may produce a pseudo-AAA colour equator at endoscopy. However, visual observation of reciprocating flow is not sufficient to define a vessel as a true AAA which instead requires ultrasonical identification of two systolic patterns.