Doppler Waveform Analysis of the Intertwin Blood Flow in Acardiac Pregnancy: Implications for Pathogenesis

OBJECTIVE: To investigate interfetal hemodynamics in acardiac twins and the implications for pathogenesis. DESIGN : A retrospective study. SUBJECTS: All acardiac twins involved in this study were identified by B- mode ultrasound at a teaching hospital in Taiwan. METHODS: Color Doppler imaging and, in one case, color power angiography were used to assess umbilical blood flow between acardiac twins and their normal co-twins. Placental anastomoses were examined by pathologists after delivery. RESULTS: In total five sets of acardiac twins and their normal co-twins were enrolled. Autonomous cardiac activity in the form of a contractile pocket was detected in two cases. Analysis of the pattern of the Doppler waveforms in each case enabled us to classify the type of hemodynamics into one of three categories: 'collision-summation', typified by a pattern of cyclic alternations of bidirectional flow; 'twin-pulse', which described the simultaneous recordings of two opposite constant flows with different pulsating rates; and the 'pump in' pattern, which indicated pulsatile flow in the reversed direction towards the acardiac mass. In two cases we detected, we believe for the first time, an artery-to-vein placental anastomosis between the acardiac twin and its co- twin. The nature of the vascular connections were confirmed on pathological follow- up. CONCLUSIONS: Our observations suggest that acardiac twins may be not only the result of but also the cause of placental vascular anastomoses, which may involve either artery-to-artery or artery-to-vein anastomoses