Twin Anemia Polycythemia Sequence (TAPS) is a rare complication affecting identical twins who share a single placenta (monochorionic pregnancy). The condition involves a chronic, unbalanced transfer of blood between the fetuses, leading to unequal blood counts. One twin, the donor, develops anemia due to a lack of red blood cells, while the other, the recipient, develops polycythemia (an excess of red blood cells and thick blood). TAPS can occur spontaneously or follow an incomplete fetoscopic laser procedure for the related condition, Twin-to-Twin Transfusion Syndrome (TTTS). Early detection and expert management are essential for a positive outcome.
Understanding the Unequal Blood Flow
TAPS originates from the anatomy of a shared placenta, which contains vascular anastomoses (connections between the twins’ circulatory systems). In TAPS, a few minuscule artery-to-vein connections allow a slow, one-way transfer of blood. This persistent flow causes the donor twin to continuously lose red blood cells to the recipient. Since the donor’s body cannot compensate quickly enough for this chronic loss, they develop progressive anemia.
The recipient twin constantly receives the donor’s red blood cells, resulting in polycythemia, where the blood becomes abnormally thick or viscous. This slow, chronic process fundamentally distinguishes TAPS from TTTS, which involves a rapid, large-volume fluid shift. Because TAPS is chronic, amniotic fluid levels often remain normal, unlike the volume differences seen in TTTS, making TAPS harder to detect during routine ultrasound screening. The disparity in blood viscosity poses different physiological stresses on each fetus, demanding specific diagnostic and treatment approaches.
Methods for Identifying and Staging
The primary method for diagnosing TAPS during pregnancy involves a specialized ultrasound technique called Doppler velocimetry. This technique focuses on measuring the speed of blood flow in the Middle Cerebral Artery (MCA), a major blood vessel in the fetal brain. Anemic blood is thinner and flows faster than normal, meaning the donor twin will have an elevated MCA Peak Systolic Velocity (MCA-PSV). Conversely, the polycythemic recipient twin’s blood is thicker and flows slower, resulting in a lower-than-normal MCA-PSV reading.
Diagnosis is confirmed by comparing the MCA-PSV values of both twins, typically expressed as Multiples of the Median (MoM) for gestational age. The severity of TAPS is then classified using the standardized Leiden staging system, which ranges from Stage 1 to Stage 5. Stage 1 represents the mildest form, indicated solely by the Doppler flow difference. Stages 3 and 4 include signs of cardiovascular compromise, such as abnormal blood flow patterns or hydrops fetalis, while Stage 5 indicates the death of one or both twins.
Treatment Options During Pregnancy
Management of TAPS is individualized and depends on the disease stage and the gestational age of the pregnancy. For the mildest cases, physicians may recommend expectant management, which involves close monitoring with frequent Doppler ultrasounds. This approach tracks the condition’s stability and avoids intervention unless TAPS progresses to a higher stage.
For more advanced stages, direct fetal intervention may become necessary to correct the blood imbalance. One option is an intrauterine transfusion (IUT), where blood is transfused into the anemic donor twin to temporarily raise their red blood cell count. This procedure is often combined with a partial exchange transfusion for the polycythemic recipient, where a small amount of the recipient’s thick blood is removed and replaced with saline to reduce its viscosity. These transfusions are temporary measures that treat the symptoms but not the underlying cause.
The only curative treatment option is fetoscopic laser surgery, which permanently stops the flow imbalance by coagulating the vascular anastomoses on the placenta. This procedure is typically considered for spontaneous TAPS diagnosed before 28 weeks’ gestation or for higher stages of the disease. In cases diagnosed later in pregnancy, or when intervention is not feasible, early delivery may be necessary to prevent further progression and allow for immediate postnatal care.
Post-Delivery Monitoring and Long-Term Outlook
Following delivery, a postnatal diagnosis of TAPS is confirmed by analyzing the twins’ blood for differences in hemoglobin levels and reticulocyte counts. Immediate care focuses on correcting the blood count disparity; the anemic donor twin often requires a blood transfusion. The polycythemic recipient may need a partial exchange transfusion to thin their blood and reduce the risk of clotting. Inspection of the placenta for the characteristic vascular connections also helps to confirm the diagnosis.
TAPS survivors, especially those with spontaneous TAPS, face a risk of long-term neurodevelopmental impairment (NDI) for both the donor and recipient. The anemic donor twin is particularly vulnerable, with studies indicating a higher risk for cognitive impairment and developmental delays. An elevated risk of bilateral deafness has been observed in approximately 15% of donor infants in spontaneous TAPS cases. Consistent follow-up with specialized pediatric and developmental care is recommended for several years to monitor and address potential neurological or developmental issues.