Placental Vascular Anastomoses Visualized During Fetoscopic Laser Surgery in Severe Mid-trimester Twin–twin Transfusion Syndrome

The aim of this study was to describe the type and number of placental vascular anastomoses identified during fetoscopic laser coagulation in severe mid-trimester twin–twin transfusion syndrome (TTS). In 126 patients with severe TTS, undergoing fetoscopic laser coagulation between 16 and 25 weeks of gestation, the different types of placental anastomoses [arterio-venous (AV), arterio-arterial (AA) and veno-venous (VV)] were counted. In cases of AV anastomoses their direction was identified and four groups were defined: in group 1 there were only anastomoses shunting from donor to recipient. Group 2 consisted of placentae with more anastomoses shunting from donor to recipient than in the opposite direction, group 3 showed an equal number in both directions and in group 4 there were more anastomoses shunting from recipient to donor than in the opposite direction. In 9 cases (7 per cent) the anastomoses could not be clearly identified due to impaired visualization, leaving 117 cases for analysis. The median number of anastomoses found was 5 with a range from 1–14. In all cases AV anastomoses from donor to recipient were present, 36 cases (31 per cent) had also AA anastomoses and 14 cases (12 per cent) showed VV anastomoses. Regarding the direction of AV anastomoses, the results were as follows: there were 35 cases (30 per cent) in group 1, 52 cases (44 per cent) in group 2, 14 cases (12 per cent) in group 3 and 16 cases (14 per cent) in group 4. In 54 (46 per cent) placentae there was at least one thin anastomosis whose type was unclassifiable. There were no placentae showing AV anastomoses only from the recipient to the donor fetus. This study demonstrates that AV anastomoses are the prerequisite for the development of TTS in monochorionic placentae. The majority (74 per cent) of cases showed a higher number of AV anastomoses from donor to recipient than in the opposite direction. In approximately one third of placentae there are also AA anastomoses and the overall number of anastomoses seen and coagulated during fetoscopy is higher than reported in postnatal studies.