Twin reversed arterial perfusion sequence (TRAP sequence) occurs only in the setting of a monochorionic pregnancy and complicates approximately 1 percent of monochorionic twin gestations, with an incidence of 1 in 35,000 births. In the TRAP sequence, the acardiac/acephalic twin receives all of its blood supply from the normal, or so-called pump twin. The term “reversed perfusion” is used to describe this scenario because blood enters the acardiac/acephalic twin through reversed flow through its umbilical artery and exits through the umbilical vein, which is opposite to the normal blood supply of the fetus.
Due to the abnormal circulation and the increased demand that the abnormal twin places on the heart of the pump twin, cardiac failure is of primary concern in TRAP sequence. If heart failure is left untreated, the pump twin dies in 50 to 75 percent of cases. This is especially true when the acardiac/acephalic twin is greater than 50 percent of the size of the pump twin by calculated estimated weight.
Increased cardiac demands on the pump twin results in increased cardiac output and blood flow to the kidneys, leading to the overproduction of fetal urine (the primary source of amniotic fluid) and eventual polyhydramnios (excess amniotic fluid), which leads to preterm labor and premature delivery.
It is important to exclude a chromosomal abnormality prior to offering a fetoscopic procedure in TRAP sequence since the incidence of chromosomal abnormality in the pump twin may be as high as 9 percent. As noted above, over half of these pregnancies are complicated by polyhydramnios and 75 percent by preterm labor.
The difference in fetal weight between the twins is predictive of outcome. When the acardiac/acephalic twin outweighs the pump twin by more than 50 percent, death occurs in 64 percent of cases. If the acardiac/acephalic twin outweighs the pump twin by more than 75 percent, death occurs in 95 percent of cases. Because of the strain on the pump twin's heart, every case of Twin Reversed Arterial Perfusion Sequence should be evaluated by fetal echocardiography. The reversed arterial flow in the acardiac/acephalic twin should be documented using color flow Doppler. Twin Reversed Arterial Perfusion Sequence (TRAP sequence) in monoamniotic (single, shared amniotic sac) gestations requires occlusion and division of the cord due to the risk of cord entanglement. Fetal echocardiography is essential for monitoring the pump twin for signs of decompensation and possible intervention.
With Twin Reversed Arterial Perfusion Sequence (TRAP sequence), one twin, lacking a functioning cardiac system, receives blood from the normally developing twin called the “pump twin.” This places an enormous demand on the heart, essentially representing a “parasite” tissue mass, putting the pump twin at risk for cardiac failure.
Left untreated, the pump twin will die in 50 to 75 percent of cases. The risk to the pump twin rises proportionately to the increasing size of the abnormally developing twin. Twin Reversed Arterial Perfusion Sequence (TRAP) is also associated with excess amniotic fluid, which may lead to premature rupture of the fetal membranes and preterm labor.
“Reversed arterial perfusion” is used to describe this condition because blood enters the abnormal twin through the umbilical artery (which usually carries blood away from the fetus back to the placenta) and exits through the umbilical vein, which normally carries blood from the placenta to the fetus.
Twin Reversed Arterial Perfusion Sequence (TRAP sequence) occurs in approximately 1 percent of identical twin pregnancies with an incidence of one 1 in 35,000 births.
Twin Reversed Arterial Perfusion Sequence (TRAP sequence) is a rare complication of identical twinning. Although the initiating events are unclear, Twin Reversed Arterial Perfusion Sequence (TRAP) sequence is thought to result from communication between the blood vessels in the two fetal circulations as well as a cardiac problem in the recipient twin.
There are no published reports of recurrence in the same family.
Risk of pregnancy complication is related to the size of the abnormal fetus. Due to significant cardiac risk to the pump twin, all Twin Reversed Arterial Perfusion Sequence (TRAP) sequence cases need to be evaluated with fetal echocardiography.
The goal of fetal surgery is to interrupt blood supply to the non-viable twin. This reduces cardiac strain on the pump twin and increases his or her chance of survival.
The entire selective bipolar cord coagulation procedure is performed using small, hollow needles called trochars. These are inserted through the mother’s abdomen. Mom typically arrives at the hospital on the morning of the procedure, which is performed in the operating room under epidural anesthesia.
Once the epidural has taken effect, her abdomen is prepped with an antibacterial solution and a trochar is placed into the amniotic space to allow the introduction of instruments, such as a small camera (fetoscope) and a coagulation device into the amniotic cavity. The coagulation device will be used to block blood flow to the non-viable twin.
After the surgery, the mother is transferred to the Special Delivery Unit for a typical post-operative stay of two to three days. During this time, you and your baby are carefully monitored for complications. Mothers are typically on bed rest and receive medications to help prevent preterm labor.
After discharge from the hospital, the mother usually returns to her referring doctor for ongoing care and delivery.
In the last six years, we have evaluated more than 35 pregnancies with Twin Reversed Arterial Perfusion Sequence (TRAP sequence) of which 20 underwent selective bipolar umbilical cord cauterization procedures. We have had a 95 percent survival for the normal pump twin with an average age at delivery between 36 and 37 weeks. No significant neurologic abnormalities have been identified in these children.
In addition, we have more recently performed five radiofrequency ablation (RFA) procedures for Twin Reversed Arterial Perfusion Sequence (TRAP sequence), with one unexpected post-operative pump twin demise, but the remaining four pregnancies are ongoing and undelivered but reported to be doing well. Our colleagues at the University of California-San Francisco have reported a 92 percent survival with an average age of delivery of 38 weeks for 13 cases of TRAP sequence treated with RFA.
Updated: June 2011