Twin to twin transfusion syndrome

Abstract

Twin to twin transfusion syndrome (TTTS) is a common complication that typically presents in the second trimester of pregnancy in 10-15% of monochorionic twins due to net transfer of volume and hormonal substances from one twin to the other across vascular anastomoses on the placenta. Without recognition and treatment, TTTS is the greatest contributor to fetal loss prior to viability in 90-100% of advanced cases. Ultrasound diagnosis of monochorionicity is most reliable in the first trimester and sets the monitoring strategy for this type of twins. The diagnosis of TTTS is made by ultrasound with the findings of polyhydramnios due to volume overload and polyuria in one twin and oligohydramnios due to oliguria of the co-twin. Assessment of bladder filling as well as arterial and venous Doppler patterns are required for staging disease severity. Assessment of fetal cardiac function also provides additional insight into the fetal cardiovascular impacts of the disease as well as help identify fetuses that may require postnatal follow up. Fetoscopic laser ablation of the communicating vascular anastomoses between the twins is the standard treatment for TTTS. It aims to cure the condition by interrupting the link between their circulations and making them independent of one another. Contemporary outcome data after laser surgery suggests survival for both fetuses can be anticipated in up to 65% of cases and survival of a single fetus in up to 88% of cases. However, preterm birth remains a significant contributor to postnatal morbidity and mortality. Long term outcomes of TTTS survivors indicate that up to 11% of children may show signs of neurologic impairment. Strategies to minimize preterm birth after treatment and standardized reporting by laser centers are important considerations to improve overall outcomes and understand the long-term impacts of TTTS.

Keywords: Fetoscopy; placental diseases, pregnancy, twin; twin to twin transfusion syndrome (TTTS).

Figure 1

Timeline for division of the monozygotic embryo and proportion of all monozygotic twin pairs. Earlier division of the monozygotic embryo results in more complete separation of the twin pair beginning from two separate placentas and amniotic sacs in dichorionic diamniotic twins when the division occurs in the first three days to conjoined twins when the division of the embryo occurs after 13 days.

Figure 2

First trimester ultrasound appearance of a monochorionic and dichorionic twin pregnancy. A monochorionic twin pregnancy is diagnosed when the membrane is thin and inserts directly on the placental surface (T-sign) as indicated by a thin arrow (A). A dichorionic twin pregnancy is diagnosed when the intertwin membrane is thick with intervening placental tissue at its base (λ-sign) as indicated by a thick arrow (B).

Figure 3

Features of the monochorionic placenta include separate placental cord insertions with presence vascular anastomoses (circles and placental dye injection image) that link the fetal circulation. The natural line along the placenta where the vessels from each twin meet is the vascular equator (dotted line). Volume and substance shift from the donor to the recipient twin is responsible for development of twin to twin transfusion syndrome. Fetoscopic laser surgery is performed by coagulating the individual anastomoses and the intervening chorionic plate to dichorionize the placenta (dotted line).