Twin-twin transfusion syndrome (TTTS) is a fascinating condition in which fetuses of identical genotype adopt discordant cardiovascular phenotypes, secondary to unbalanced placental inter-
twin transfusion. Flow along the primary units of inter-
twin transfusion, unidirectional arteriovenous anastomoses, can be as high as litres/day each, and TTTS develops when the placenta has insufficient compensatory counter-transfusional anastomoses. The initial phenotype reflects dysvolaemia, with added contributions from uteroplacental insufficiency in the donor, and raised afterload with diastolic dysfunction secondary to discordant
endothelin and placentally derived renin-angiotensin system effectors in the recipient. Endoscopic
laser ablation of placental anastomoses has become the primary treatment modality, supported by a randomised trial showing improved survival and short but not long-term neurological morbidity. Its uptake has facilitated comparative pre- and post-
laser studies, which provide considerable insight into the pathophysiology. Despite the therapeutic advance, placental
laser remains associated with a 25% incidence of
fetal death within a week, and
a 10% risk each of recurrence and twin anaemia/polycythaemia sequence due to residual anastomoses. In Stage I, high rates of non-progression with more
conservative management have resulted in therapeutic equipoise as to whether
laser is indicated primarily or only for progressive disease. The challenge ahead lies in improving double intact survival rates, which in addition to randomised trials will require technical advances, better understanding of the circulatory pathophysiology and more sophisticated surveillance tools.