Introduction:
Acardiac twinning, acardiac, or craniopagus parasiticus are alternative names for the rare congenital anomaly known as twin reversed arterial perfusion sequence. Monochorionic twin pregnancies may result in the rare complication known as TRAP sequence. It occurs in one percent of monochorionic twin pregnancies and one in 35,000 pregnancies. It is a condition in which one twin lacks partial or complete cardiac development, and the morphologically normal twin supplies both circulations.
What Is Twin Reversed Arterial Perfusion (TRAP) Sequence?
TRAP sequence is a rare congenital anomaly that occurs in monochorionic twin pregnancies. Monochorionic twins are referred to identical twins (monozygotic twins). The identical twins share a placenta and the blood vessels that supply oxygen and nutrients.
TRAP sequence occurs in one in every 35,000 monochorionic twin pregnancies. It is a condition in which the cardiac system of one twin supplies blood for both twins.
TRAP sequence is a variant of twin-to-twin transfusion syndrome (a condition in which there is a disproportionate blood supply between the fetuses).
What Causes the TRAP Sequence?
The exact etiology remains unknown, but two pathways have been established to describe the etiology of the TRAP sequence.
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Abnormal Vascular Pattern in the Early Stages of Monochorionic Placentation - A disproportionate blood flow between the twins occurs due to the lack of development of a functioning heart in one twin. Hence, the acardiac twin relies on the other twin for blood supply.
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Primary Defect in Cardiac Embryogenesis - Due to the failure in heart development caused by chromosomal abnormality or environmental factors, the acardiac twin receives circulation support from the normal twin through anastomoses between the umbilical vessels.
What Happens In TRAP Sequence?
In the TRAP sequence, one twin lacks complete or partial cardiac development, and the cardiac system of the other twin supplies blood for both the twins. The networks of blood vessels in the twins are connected instead of being independent. The heart is completely absent or deformed, and the upper structures of the body malformed in one twin, which is referred to as an acardiac twin (as the heart is partially or completely absent).
In addition, the internal structures of the trunk are malformed, and the legs missing or partially present. The other twin is morphologically normal, and is referred to as the pump twin, as it supplies blood for both fetuses.
The term “reversed arterial perfusion” denotes that the blood flow in the acardiac twin occurs in a retrograde direction. TRAP sequence occurs as a result of artery-to-artery placental shunt, and is also accompanied by a vein-to-vein shunt. The arterial blood from the pump twin is transported to the acardiac twin in a retrograde pattern, rather pumping it into the placenta. This poorly oxygenated blood supplies the lower structures of the acardiac twin preferably, as it flows in a reversed direction.
What Are the Abnormalities Associated With the Acardiac Twin?
The acardiac twin is also called a parasitic twin because it lacks cardiac development and relies on the other twin for blood supply. The upper structures of the body are malformed due to the lack of cardiac development. The acardiac twin receives blood from the pump twin via an umbilical cord-like structure. Though the acardiac twin grows, it does not develop the structures due to inadequate oxygenation.
Every cardiac twin is different. However, acardiac twins are grouped into four groups depending on the abnormalities.
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Acephalus - The most common type in which the acardiac twin lacks a head, though it may have arms. Thoracic organs are absent, malformed, and unidentifiable.
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Anceps - In this type, the acardiac twin has most body parts, including the head. However, the brain formation is incomplete, and the organs present are malformed.
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Acormus - In this type, only a head without a neck is present. The body is entirely absent, and the umbilical cord remains attached to the head.
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Amorphous - The amorphous type lacks the head and limbs, the internal organs may or may not be present, and consists of tissue with blood vessels that branch from the umbilical cord. Sometimes, it may be a tumor-like growth attached to the umbilical cord without blood vessels from the umbilical cord and the tumor then dies.
What Are the Abnormalities Associated With the Pump Twin?
The pump twin is morphologically normal. However, it is smaller than normal. In addition, the mortality rate is high for the pump twin due to the associated problems, such as the rapid growth of the acardiac twin, umbilical cord entanglement, polyhydramnios (excess amniotic fluid in the amniotic sac), and congestive heart failure due to a high cardiac output.
The mortality rate depends on the size of the acardiac twin. If the size of the acardiac twin is 50 percent greater than the pump twin, the survival rate of the pump twin is only ten percent.
How Is TRAP Sequence Diagnosed?
TRAP sequence is diagnosed using an ultrasound scan. An ultrasound examination shows the presence of features, such as gross differences in the measurements of the twins, the absence of a heart in one twin, malformations in the head, trunk, and lower extremities, and fluid collection in the anomalous twin.
A doppler ultrasound (a test to determine the blood flow through the vessels using high frequency sound waves) demonstrates retrograde circulation in the acardiac twin, during the first trimester.
How Is TRAP Sequence Treated?
The pregnancy continues normally unless the condition of the pumping twin is not affected. Early delivery may be advised if the condition of the pump twin deteriorates. Preserving the survival of the pump twin is the primary objective of the treatment. Hence, the acardiac twin is removed.
The acardiac twin is removed surgically by cutting the umbilical cord. It is separated from the pump twin. The minimally invasive procedure such as radiofrequency ablation with a needle or fetoscopic cord occlusion may be performed to coagulate the blood in the umbilical cord of the acardiac twin.
The survival rate of the pump twin increases by 80 percent, after the removal of the acardiac twin. Although the best time for treatment is not known, recent studies suggest intervention in the first trimester of pregnancy.
Conclusion:
Twin reversed arterial perfusion sequence is an uncommon congenital condition. Recent advancements in interventions have increased the survival of the normal twin in TRAP sequence. Of all the techniques, ultrasound-guided laser coagulation and radiofrequency ablation are the preferred methods to disrupt the vascular supply to the acardiac twin. Despite the unresolved challenges, the survival rate of the normal twin increases by 80 percent after the removal of the acardiac twin.
