Twin Reversed Arterial Perfusion (TRAP sequence/Acardiac)

What is Twin Reversed Arterial Perfusion (TRAP) sequence?

Twin reversed arterial perfusion sequence is a complication specific to monochorionic twins (identical twins who share a single placenta). In this disease, one of the fetuses does not have a normal functioning heart. This fetuses is also called an ‘acardiac’ fetus. Due to the absence of a functioning heart, the acardiac fetus cannot live after birth. During pregnancy however, it receives its blood supply from the normal, ‘pump’ twin through blood vessel connections (anastomoses) at the level of the placenta and through backward flow in the umbilical cord. As such, the acardiac fetus may grow before birth.

The acardiac fetus places an increased demand on the heart of the pump twin. If the acardiac twin grows very large, this can cause heart failure in the pump twin.

Twin reversed arterial perfusion can be diagnosed by ultrasound. The typical image is that of a very abnormal, underdeveloped fetus with a nonfunctional heart. If heart failure is present in the pump twin, this may result in hydrops (fluid accumulation under the skin, in the chest and in the abdomen) and excess amniotic fluid (polyhydramnios).

What is the outcome for a pregnancy complicated with twin reversed arterial perfusion?

In cases where the acardiac twin remains small, and no heart failure develops in the pump twin, the outcomes for the pump twin are excellent. However, when the acardiac fetus becomes large or polyhydramnios develops, preterm delivery is very common. Overall, about 75% of pregnancies complicated by twin reversed arterial perfusion will deliver preterm. If hydrops develops in the pump twin, the risk of fetal death is high.

What other tests should we consider?

All pregnancies with suspected twin reversed arterial perfusion should be referred to a specialized perinatal center, such as the Fetal Medicine Unit at Mount Sinai Hospital, for a detailed ultrasound. Often, a detailed heart ultrasound (echocardiogram) will be requested to document the heart function in the pump twin. Ongoing ultrasounds are used to monitor the growth of the acardiac mass and to look for early signs of heart failure or excess amniotic fluid in the pump twin.

What therapy is available?

If the acardiac mass remains small and cardiac failure does not develops in the pump twin, no interventions are needed before birth. Delivery will likely occur close to term and the long-term outcomes for the pump twin are excellent. Delivery can take place in the local hospital. No therapy is required after birth.

If a large acardiac fetus is diagnosed later in pregnancy and polyhydramnios is present, drainage of the excess amniotic fluid through a needle inserted into the uterus, such as for amniocentesis, may be required to decrease the risk of preterm birth. If hydrops is present or impending, delivery is indicated. When hydrops happens before viability, or very preterm, prenatal therapy aimed at stopping the blood flow towards the acardiac fetus may be attempted. This can be done by closing off the umbilical cord of the acardiac mass (cord occlusion) or by closing off the large blood vessels within the acardiac fetus using radiofrequency energy (radiofrequency ablation, RFA). Outcomes are similar with both techniques, with survival of the pump twin in about 85% of cases. Mean gestational age at delivery after such procedures is 34 to 35 weeks gestation. Developmental delay in survivors is rare (2% of cases). If developmental delay occurs, it is most often the consequence of preterm delivery.

If TRAP is diagnosed early in pregnancy, parents are typically offered the choice between a preventative RFA intervention or close surveillance with intervention reserved for cases where the acardiac mass would grow rapidly. Unfortunately, doctors are unable to predict in early pregnancy whether the acardiac fetus will grow rapidly or not.


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