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Twin-Reverse-Arterial Perfusion Sequence (TRAP)

The incidence of birth defects in monochorionic twins is approximately four times higher than in singletons or dichorionic twins.

TRAP sequence is a particular form of discordant monochorionic twins. The term TRAP is an acronym that stems from “twin-reverse-arterial-perfusion sequence.”

What are discordant anomalous monochorionic twins?

The incidence of birth defects in monochorionic twins is approximately four times higher than in singletons or dichorionic twins. In most cases, only one twin is affected. In 20% of cases, both twins have the same congenital anomaly. In some cases, the anomaly, the affected twin may compromise the pregnancy or the health of the unaffected twin. In other cases, the anomaly of one twin does not affect the health of the other twin or the other twin, but the parents may choose to treat the birth defect antenatally or request selective feticide of that twin.

What is TRAP sequence?

TRAP sequence is a particular form of discordant monochorionic twins. The term TRAP is an acronym that stems from “twin-reverse-arterial-perfusion sequence.” In these pregnancies, a normal twin (PUMP) sends blood backwards through a vessel into the anomalous (TRAP) fetus. The blood vessel that allows this to happen is an arterio-arterial anastomosis (AA anastomosis). Blood from the PUMP twin is low in oxygen and thus may cause a number of malformations in the TRAP fetus, including lack of development of the head and upper extremities. The TRAP twin may or may not have a functioning heart, and thus, in the past, was called “acardiac twin.” Complications to the PUMP twin from this condition include polyhydramnios, abnormal Doppler studies, overt heart failure, and death. The pregnancy may also be lost from polyhydramnios and miscarriage or extreme premature delivery.

How is TRAP sequence diagnosed?

SIUGR is diagnosed antenatally with ultrasound by showing a reverse flow in the umbilical artery of the TRAP fetus. This requires the combined use of color and pulsed Doppler, to show that blood entering the TRAP twin is arterial. Once the diagnosis is confirmed, additional ultrasound measurements are obtained, including transvaginal cervical length, amniotic fluid volume of the PUMP twin, Doppler studies in the PUMP twin, and a ratio of the estimated fetal weight (EFW) of the TRAP twin and the PUMP twin (TRAP/PUMP ratio). If the TRAP/PUMP ratio is >50%, such pregnancies are at an increased risk for adverse outcome.

Are there different degrees of TRAP sequence?

TRAP sequence patients can be classified as having a good or a poor prognosis. Patients with a poor prognosis are those in which expectant management is more likely to result in loss of the pregnancy. Poor prognosis findings include:

  • A TRAP/PUMP ratio >100 %. The original data suggested that patients with a TRAP/PUMP ratio >50% were associated with a poor prognosis. However, in our experience, most cases with a TRAP/PUMP ratio <100% do well with expectant management. Therefore, we do not offer antenatal treatment unless the TRAP/PUMP ratio is at least 100%.
  • Polyhydramnios. This is defined as a maximum vertical pocket (MVP) > 8 cm in the sac of the PUMP twin.
  • Abnormal Dopplers in the PUMP twin. Dopplers in the PUMP twin may show evidence of impending heart failure in this twin. Typically, the ductus venosus or the umbilical vein are most reflective of these changes. The function of the heart of the PUMP twin may also be affected, and can be measured using the myocardial performance index (MPI), also known as the Tei index.
  • Hydrops of the PUMP twin. This is reflective of overt heart failure in the PUMP twin.
  • Short cervical length or signs/symptoms of preterm labor. A short cervical length is defined as a cervical length of <2.5 cm via transvaginal ultrasound. Patients may have a short cervix in the absence of polyhydramnios or other poor prognostic factors. However, a short cervical length is associated with an increased risk for pregnancy loss or extreme premature delivery.

 

How can TRAP be treated?

Depending on the gestational age at which TRAP is diagnosed, delivery may be an option. If the diagnosis is made far from viability, different treatment options have been proposed, including:

  • Expectant management. This may be associated with an increased risk of pregnancy loss or prematurity. Classically, approximately 50% of TRAP pregnancies are lost, 25 % may deliver at an extremely early gestational age. The remaining 25% have an unremarkable course and deliver at a viable and adequate gestational age with no significant sequelae. The prognosis depends on the presence or absence of poor risk factors.
  • Umbilical cord occlusion of the TRAP fetus. Originally described by Quintero and collaborators in 1994, this option is associated with approximately 75% chance that the PUMP fetus would survive, with a 5% risk of cerebral palsy. Criteria for intervention include polyhydramnios, abnormal Doppler studies or hydrops in the PUMP fetus, large TRAP/PUMP ratio (>100%), shortened cervix, or signs of preterm labor. Patient choice, regardless of surgical criteria, is also an option. Surgery is typically offered after 16 weeks, allowing for the physiologic attachment of the chorioamniotic membranes to the uterine wall to occur. The mean gestational age at intervention is approximately 20 weeks.

Can other discordant anomalous monochorionic twins be treated?

Depending on the nature of the anomaly, antenatal treatment may be considered. For example, if the anomaly of one twin compromises the healthy co-twin or the pregnancy as a whole, antenatal treatment may be considered. The anomalous twin may be at an increased risk for spontaneous demise, for example, as in the case of fetuses with anencephaly. Death of the anencephalic fetus may result in death or harm to the healthy co-twin. In such cases, cord occlusion of the anomalous twin may be indicated. In other instances, the anomalous cotwin does not have an increased risk for spontaneous demise but may nonetheless compromise the entire pregnancy. An example of this situation is a fetus with duodenal atresia, in which massive polyhydramnios in the sac of this twin may result in miscarriage or extreme premature delivery. In these cases, cord occlusion of the anomalous twin may also be considered, unless a palliative measure to treat the polyhydramnios can be offered. In other cases, the birth defect of one of the twin’s does not affect the pregnancy or the healthy co-twin. Nonetheless, the anomaly may be very severe and be associated with neonatal demise. Such is the case of fetuses with bladder outlet obstruction, congenital diaphragmatic hernia, or congenital heart disease.

If antenatal treatment is available to improve the prognosis for the anomalous twin, a consideration can be given to treating this fetus antenatally. Treatment of an anomalous co-twin to improve its prognosis may compromise the outcome for the healthy cotwin. Therefore, a significant degree of clinical judgment must be exercised before offering such treatments. Our group has treated twins with discordant anomalies, including fetal bladder outlet obstruction, with good success. In other instances, the birth defect of one twin does not warrant antenatal treatment either because it does not compromise the health of the co-twin, the pregnancy, or in which antenatal treatment does not change its own prognosis. Nonetheless, in some cases, the parents may choose umbilical cord occlusion of this twin.

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