Twin–twin transfusion syndrome in dichorionic twin pregnancy: rare but not impossible

Twin–twin transfusion syndrome (TTTS) is typically a complication of monochorionic (MC) twin pregnancy, related to the presence of vascular anastomoses on the surface of the shared placenta1. First-trimester determination of chorionicity is mandatory for risk stratification of twin pregnancy and guiding subsequent management, which is different for dichorionic (DC) and MC twins2, as early diagnosis of TTTS and treatment with fetoscopic laser surgery (FLS) can improve perinatal outcome. We describe a case of TTTS in a DC twin pregnancy. A 32-year-old woman, gravida 2, para 1, with a twin pregnancy underwent ultrasound examination at 11 weeks of gestation during which the lambda sign was identified, indicating dichorionicity (Figure 1). Monthly examination of the twins' biometry was carried out by the woman's obstetrician until symptomatic polyhydramnios developed in one twin sac at 25 + 5 weeks. The woman was referred immediately to our unit where, in addition to the signs of a DC pregnancy, we observed that the criteria for diagnosis of TTTS (Quintero stage III)2 were met: a twin with maximum vertical pocket of 11 cm, cardiac overload with moderate tricuspid valve regurgitation and reversed a-wave in the ductus venosus; the other twin had anhydramnios and an empty bladder, with evidence of both kidneys and without signs of premature rupture of membranes. FLS was performed as described previously3, with coagulation of two arteriovenous anastomoses. Resolution of TTTS was observed 48 h after the procedure, with normal a-wave in the ductus venosus in the recipient twin and evidence of a bladder in the donor twin. Weekly follow-up was performed until 29 weeks when preterm rupture of membranes occurred followed by uterine contractions, with delivery by Cesarean section of two female neonates with birth weights of 1120 g and 1015 g, respectively, and no discordance in hemoglobin values (18.1 and 16.1 g/dL, respectively; reticulocytes ratio, 1). Placental examination with color-dye injection1 was performed and two small residual arteriovenous anastomoses were found (Figure 2). Zygosity tests of the neonates confirmed monozygosity, while histological analysis of the placenta confirmed dichorionicity as there was trophoblastic residual tissue between the layers of amnion (Figure 3). The twin girls were discharged 7 weeks later with no major complications. Case reports of TTTS in DC twin pregnancies are rare4, 5, and the incidence has not been reported in published series of FLS. In our consecutive series from 2004 to 2018, out of 465 FLS treatments performed for TTTS, this was the first (1/465; 0.2%) case to occur in a proven DC pregnancy. The lambda sign has some limitations6; its sensitivity and specificity for the prediction of DC pregnancy are 99% and 95%, respectively, which means that it is present in 5% of MC pregnancies. In this case, the lambda sign identified correctly dichorionicity, but this could be confirmed only after delivery. Reporting such cases is useful because it reminds operators to bear in mind that, though rare, TTTS can also occur in DC twins. Therefore, regardless of chorionicity, polyhydramnios–oligohydramnios sequence in a twin pregnancy should always be considered as a possible sign of TTTS, and the case should be referred promptly to a fetal therapy center for appropriate treatment and management.