TTTS, which complicates 10-15% of MCDA twin pregnancies, is characterized by a net imbalance of volume between twins, mediated through abnormal placental blood vessel anastomoses that connect the two placental circulations. Clinically, the "donor" twin develops features of anemia and hypovolemia, while the "recipient" twin shows signs of hypervolemia and hypertensive fluid overload. If untreated, the syndrome has a perinatal mortality rate as high as 80-100%. Although modern intrauterine therapies have improved the rates of fetal death, significant risks of morbidity and mortality remain even after treatment.
Twin-twin transfusion syndrome is most commonly classified according to a staging system developed by Quintero et al in 1999, which is based on discrete, categorical ultrasound findings (amniotic fluid volume, presence/absence of a fetal bladder, umbilical artery Doppler studies, fetal hydrops, and death). The system includes 5 stages ranging from mild disease with isolated discordant amniotic fluid volumes, to severe disease with demise of one or both twins. Although this system has some prognostic value, it also has significant limitations due to the highly complex physiologic conditions that are involved in the disease. For example, some criteria in the staging system are not consistently representative of fetal physiology. Additionally, the stages do not correlate well with overall perinatal survival or with outcomes following intrauterine therapies.
Recent work has demonstrated that the complex pathophysiology of twin-twin transfusion syndrome involves a discordant activation of the renin angiotensin system (RAS). RAS is normally important in fluid and salt regulation in both the adult and the fetus, and TTTS in marked hypovolemia and hypervolemia in monozygous fetuses within the same maternal environment. The renal RAS in the donor is up-regulated, presumably as a consequence of hypovolemia. The recipient is also exposed to high levels or RAS components, either due to the transfusion of these components from the donor via anastomoses, or via discordant placental RAS activation, resulting in a hypertensive, hypervolemic state.
Multiple studies have identified a correlation between Doppler assessment of the fetal renal artery and the development of oligohydramnios, a hypovolemic state, in singleton pregnancies. However, the use of renal artery Doppler studies has not yet been fully evaluated in twin gestations. In particular, it has not been evaluated in MCDA twin gestations complicated by TTTS, the pathophysiology of which involves significant alterations in fetal volume and fluid status.
This project is intended to serve as a single-center study to determine if there is indeed a difference in renal artery Doppler parameters in sets of MCDA twins with TTTS compared to sets of MCDA twins without TTTS. The identification of a significant difference would potentially provide support for further investigation into this measurement as a screening tool or prognostic indicator when applied to MCDA twin pregnancies.
Secondary goals of this study include: comparing donor to recipient renal artery Doppler findings among pregnancies with TTTS, evaluating serial renal artery Doppler findings over time per pregnancy, and evaluating pre- and post-therapy renal artery Doppler findings in those pregnancies undergoing therapy for TTTS.
