Center for Fetal Diagnosis and Treatment

Twin Reversed Arterial Perfusion Sequence (TRAP)

Overview

Twin reversed arterial perfusion (TRAP) 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.

Evaluation

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 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.

Acephalic Twin -- Case Studies

I. Radiofrequency ablation (RFA) of blood supply to acephalic twin The patient and her husband were of Brazilian origin and spoke only Portuguese, requiring the presence of a translator throughout their stay in our Center. She was a 24-year-old mother with two prior children who was referred at 17 1/2 weeks gestation because of a monochorionic, diamniotic twin gestation complicated by TRAP sequence.

Twin A was a typical acardiac/acephalic twin with massive diffuse soft tissue edema and an incompletely formed skeleton with no normal head structures, no normal stomach, kidneys or urinary bladder, and the absence of normal cardiac structures. An abnormal two-vessel umbilical cord with reverse arterial and venous flow was seen going to the abnormal twin. Twin B was structurally normal but had significantly increased amniotic fluid volume (polyhydramnios), and fetal echocardiography noted high-normal combined cardiac output due to the increased demands placed on the normal twin (pump twin) by the abnormal parabiotic twin.

When the volume of the acephalic twin was calculated, it was found to be 130 percent the size of the normal twin placing it in to a greater than 95 percent risk category for significant pregnancy complication. Pregnancy management options were discussed. They included termination of the pregnancy, continuation of the pregnancy without intervention allowing nature to follow its course with expectations of early delivery and possible cardiac complications in the pump twin, or interruption of the blood supply to the anomalous parabiotic twin using either bipolar cord cauterization or radiofrequency ablation of the blood supply. Selective bipolar cord cauterization is an accepted technique with a 95 percent pump twin survival and mean age of delivery of 36 weeks. However, RFA has emerged as a newer technique, which is significantly less invasive using only a 17-gauge (1.5 mm) needle devise and requiring only a 24-hour hospital stay. After careful consideration of their options, the family elected the RFA procedure.

Under epidural anesthesia a 3-mm skin incision was made, and with continuous ultrasound guidance, the RFA needle device was carefully directed to where the blood supply entered the abnormal parabiotic tissue mass. Radiofrequency ablation of the tissues and blood supply in this region was performed without complication. Color and power Doppler flow studies confirmed interruption of the blood supply to the abnormal fetus. The excess amniotic fluid volume was then removed (amnioreduction) from around the normal pump twin.

The patient was then transferred to labor and delivery at the Hospital of the University of Pennsylvania (HUP) where she was observed for four hours prior to being transferred to the high-risk obstetrics service. She received 24 hours of intravenous antibiotics and oral medication (indomethacin) to prevent preterm contractions. She was discharged from the hospital the following morning and remained at decreased activity at the Camden Ronald McDonald House for the next five days. Follow-up evaluation showed normal fluid around the pump twin, no blood flow to or within the parabiotic tissue mass, significant decreased fluid around the abnormal fetus and normal cardiac function in the surviving pump twin. She returned to her referring physician for ongoing pregnancy management. She is presently at 36 weeks gestation, has not yet delivered and her pregnancy is progressing normally.

II. Selective bipolar umbilical cord cautery and transection of anomalous parabiotic twin The patient was a 33-year-old, second pregnancy, referred at 21 weeks gestational age for pregnancy complicated by a monochorionic, monoamniotic (single shared placenta, single shared amniotic sac) twin pregnancy, with one twin reported to have multiple congenital anomalies.

High-resolution ultrasound confirmed monoamniotic twins. Twin A’s features were consistent with an anecephalic/acardiac parabiotic twin with an abnormal two-vessel umbilical cord and reverse arterial and venous perfusion. These features are consistent with TRAP sequence.

The co-twin was anatomically normal, and echocardiography showed normal heart structures and function with no significant elevation of cardiac output. However, as these fetuses shared a common amniotic sac, extensive entanglement of the umbilical cords throughout the mid-sections was seen placing both at high risk for intrauterine fetal demise.

The calculated weight ratio of the abnormal to normal twin was 90 percent, placing this pregnancy in the highest risk category for Twin Reversed Arterial Perfusion Sequence (TRAP) in addition to the risks associated with the entangled umbilical cords. After multidisciplinary consultation, the family requested selective bipolar umbilical cord cauterization of the abnormal parabiotic twin with transection (cutting) of the cord to reduce the risks associated with co-twin cord entanglement.

Under epidural anesthesia, a 3-mm skin incision was made under continuous ultrasound guidance, and a 3-mm trocar sheath passed into the common amniotic cavity. Approximately 4 inches lateral to the trocar sheath, a small 1.2 mm needle sheath was also introduced into the amniotic cavity under ultrasound direction. The needle stylet was removed and a 1-mm fiber-optic micro endoscope was introduced into the needle sheath to allow direct visualization of the bipolar cord cauterization procedure.

A segment of the abnormal two-vessel cord was identified adjacent to the beginning of the cord entanglements. The two-vessel cord was cauterized in three locations approximately 1inch apart. Using color and power Doppler on ultrasound, complete occlusion of blood flow to the acardiac/acephalic twin was confirmed. Direct visualization using the micro endoscope also showed complete occlusion of the abnormal umbilical cord. The bipolar instrument was removed from the amniotic cavity, and a tiny endoscopic scissors introduced and guided to the operative site with ultrasound.

Under direct micro-endoscopic visualization, the middle of the three cauterization sites was cut to free the anomalous twin from the entangled cords. No bleeding was visualized from the cut ends of the abnormal cord. Antibiotics were then placed into the amniotic sac, and all of the instruments were removed.

The co-twin’s heart rate remained normal throughout the surgery. The patient was transferred to the Labor and Delivery ward at the Hospital University of Pennsylvania where she remained for 24 hours on intravenous medication (magnesium sulfate) to prevent preterm labor, as well as 24 hours of antibiotics to reduce the risk of infection. The following morning the intravenous medications were discontinued and she was started on an oral medication (nifedipine) to be taken four times daily to reduce the risk of pre-term labor. The following morning she was discharged from the hospital and remained at the Philadelphia Ronald McDonald House at bed rest.

Five days later she underwent sonographic evaluation that confirmed no secondary operative complications and significant decrease in the degree of umbilical cord entanglement. She returned home to her referring physician and had a normal prenatal course until 36 ½ weeks at which time she experienced spontaneous rupture of membranes and onset of labor. A 6 lb. 4 oz. infant male with APGAR scores of 8 and 9 at one and five minutes respectively, was delivered vaginally without complications and was discharged from the hospital ten days after birth. The child is now 5 years of age and is developmentally normal. Following delivery of the normal twin, a small, and shrunken tissue mass, which had previously been the parabiotic twin, was passed without difficulty just prior to delivery of the placenta.

Frequently asked questions

What is Twin Reversed Arterial Perfusion Sequence (TRAP)?

With Twin Reversed Arterial Perfusion Sequence (TRAP), 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.

How common is Twin Reversed Arterial Perfusion Sequence (TRAP)?

Twin Reversed Arterial Perfusion Sequence (TRAP) occurs in approximately 1 percent of identical twin pregnancies with an incidence of one 1 in 35,000 births.

What causes this condition?

Twin Reversed Arterial Perfusion Sequence (TRAP) 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.

Does Twin Reversed Arterial Perfusion Sequence run in families?

No.

If I become pregnant again, will I be at risk for a recurrence of Twin Reversed Arterial Perfusion Sequence?

There are no published reports of recurrence in the same family.

How can I know if my viable twin might be helped by fetal surgery?

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.

What are the goals of fetal surgery?

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.

How is the fetal surgery performed?

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.

What can I expect after fetal surgery?

After the surgery, the mother is transferred to the Hospital of the University of Pennsylvania (HUP) 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.

What is known about pregnancy outcomes?

In the last six years, we have evaluated more than 35 pregnancies with Twin Reversed Arterial Perfusion Sequence (TRAP) 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.