Introduction
Twin anemia-polycythemia sequence (TAPS) is a rare condition that is seen in monochorionic pregnancies. It occurs as a result of unequal sharing of red blood cells between twins. This condition is a form of twin-to-twin transfusion syndrome that complicates twin pregnancies. Twin anemia-polycythemia sequence occurs randomly or after an incomplete laser surgery that was performed to treat twin-to-twin transfusion syndrome (TTS).
What Is Twin Anemia-Polycythemia Sequence (TAPS)?
Twin anemia-polycythemia sequence is a condition that affects monochorionic pregnancies. Monochorionic pregnancy refers to a pregnancy with multiple fetuses (babies), such as twins or triplets. In a twin or triplet pregnancy, placentation can be dichorionic or monochorionic. In dichorionic placentation, each fetus has a separate placenta, whereas in monochorionic placentation, both fetuses share a common placenta. In monochorionic pregnancies, complications like twin-to-twin transfusion syndrome are usually seen. Twin-to-twin transfusion syndrome is a condition in which the blood supply between the fetuses is disproportionate, resulting in unequal amounts of amniotic fluid between each fetus. If this condition is left untreated, the unsupplied twin dies, and the surviving twin may have morphological abnormalities.
Twin anemia-polycythemia sequence is a type of twin-to-twin transfusion syndrome. However, it is significantly different from twin-to-twin transfusion syndrome. In the twin anemia-polycythemia sequence, there is an unequal sharing of red blood cells between the fetuses sharing a single placenta. As a result, an imbalance in hemoglobin and red blood cells occurs. Hence, one twin does not receive an adequate supply of oxygen and nutrients required for development. This unequal distribution of red blood cells between the monochorionic twins results in anemia (a condition in which the number of red blood cells is reduced) in one twin and polycythemia (a condition in which an increased number of red blood cells are present in the blood) in the other twin, causing twin anemia-polycythemia sequence.
What Are the Causes of TAPS?
The exact cause of the twin anemia-polycythemia sequence is not known. It can develop as a complication of monochorionic pregnancy or occur after fetoscopic laser surgery (a procedure that stops the abnormal fluid exchange between twins) to treat twin-twin transfusion syndrome. Also, there are known genetic causes for the twin anemia-polycythemia sequence.
How Does Twin Anemia-Polycythemia Sequence Occur?
Most of the monochorionic diamniotic (amniotic fluid) placentas are characterized by interwin anastomoses (connections) that are either bidirectional artery-to-artery anastomosis, unidirectional artery-to-vein anastomosis, or vein-to-vein anastomosis. This presence of artery-to-vein anastomosis causes unequal sharing. This unidirectional blood flow compensates for the vascular flow to one twin. This contributes to the development of twin-to-twin transfusion syndrome (TTTS). The difference in TAPS is that the twin with the compromised supply (anemic twin) responds with a compensatory release of erythropoietin (a hormone that stimulates the production of red blood cells). But, the polycythemic twin may suppress the production of erythropoietin, resulting in reticulocytosis (an increase in immature red blood cells) in the anemic twin. Reticulocytosis in an anemic twin is a classical sign that helps in distinguishing TAPS from TTTS.
What Are the Signs of a Twin Anemia-Polycythemia Sequence?
The imbalance in blood flow between the twins can cause the following signs:
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The recipient twin develops polycythemia as the twin receives too many red blood cells, causing thicker and sluggish blood flow.
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As there is a decrease in red blood cells, anemia and hydrops (fluid builds up in the baby’s tissues and organs resulting in extensive swelling) develop in the donor twin.
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In some conditions, the part of the placenta near the donor twin may appear thickened, and the liver of the recipient twin may show the presence of white spots, and is referred to as starry sky liver.
In addition, women carrying babies with this sequence do not experience any symptoms. Also, the level of amniotic fluid remains normal.
How Are TAPS Diagnosed?
It is essential to determine whether the twins share a common placenta. An ultrasound scan during the first trimester of pregnancy helps make this distinction. TAPS can have an impact on the cardiovascular system. Therefore, twins should be evaluated for cardiac problems in utero (inside the uterus). The diagnosis of TAPS depends on the following:
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High-Resolution Ultrasound Examination - An ultrasound examination is performed to rule out morphological abnormalities in twins. It also helps in detecting the placental location and umbilical cord insertion into the placenta, amniotic fluid levels, and blood flow patterns for each twin.
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Middle Cerebral Arterial Doppler Study - It is a type of ultrasound that is used to measure the blood flow through a blood vessel in the fetal brain. It also helps in detecting the difference in the speed of blood flow between the twins, if present. The blood will move faster than normal in the donor twin due to the presence of anemia as the blood is thinner, and the blood will move slower in the recipient twin due to the presence of polycythemia in which the blood is thicker.
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Fetal Echocardiogram - An ultrasound of each twin’s heart is performed to detect if any related cardiac conditions are present, as TAPS can cause cardiac strain in the donor twin, resulting in cardiac dysfunction.
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Genetic Amniocentesis - A small amount of amniotic fluid is removed from the amniotic sacs surrounding the fetuses and tested for genetic abnormalities.
After birth, TAPS can be diagnosed by testing the twins' red blood cell count and analyzing the placenta to determine blood vessel connections.
How Are TAPS Treated?
Treatment for TAPS involves the following:
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Expectant Management - In conditions where surgery is not indicated, close monitoring is performed with periodic Doppler examinations to evaluate the condition of the twins. Weekly Doppler studies are recommended between 16 and 18 weeks to monitor the progression.
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Fetoscopic Selective Laser Ablation - It is a minimally invasive surgical procedure that is performed on the placenta to disconnect the blood vessels that communicate. This procedure stops the sharing of blood between twins and prevents the progression of the transfusion of blood cells.
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Intrauterine Transfusion - In some cases where laser surgery is not indicated due to late gestational age, intrauterine transfusions are performed. In addition, diluting the blood of the polycythemic twin may also be performed.
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Preterm Delivery - If TAPS occurs in the later period of pregnancy, early delivery is recommended to prevent further complications.
Conclusion
Twin anemia-polycythemia sequence is a rare condition that is seen in monochorionic pregnancies. It occurs as a complication of monochorionic twin pregnancies or after a fetoscopy laser surgery that is performed to treat twin-to-twin transfusion syndrome. Early diagnosis, periodic monitoring, and timely intervention prevent the progression of the disease and the risk of complications.
