Twin Anemia Polycythemia Sequence (TAPS) is a rare complication affecting identical twins who share a single placenta (monochorionic pregnancy). It involves a slow, chronic imbalance in red blood cell counts between the twins. One twin becomes anemic, while the other becomes polycythemic. TAPS is a serious disorder requiring careful monitoring and specialized medical intervention.
The Underlying Cause of Twin Anemia Polycythemia Sequence
TAPS occurs in monochorionic pregnancies because the twins share a single placenta with connecting blood vessels (vascular anastomoses). While these connections usually balance blood flow, TAPS results from a small number of tiny, deep arteriovenous connections, typically less than one millimeter in diameter, that allow for a slow, one-way transfusion.
This chronic, unequal flow causes a progressive imbalance in the twins’ blood composition. The “donor” twin continuously loses red blood cells, resulting in chronic anemia (thin blood). Conversely, the “recipient” twin receives an excessive amount of red blood cells, leading to polycythemia (abnormally thick and sluggish blood).
The mechanism of TAPS differs from Twin-to-Twin Transfusion Syndrome (TTTS), which involves larger, rapid shifts of blood volume and extreme discordance in amniotic fluid. In TAPS, the slow transfer involves primarily red blood cells, meaning amniotic fluid levels often remain normal. The donor twin compensates for blood loss by producing new red blood cells, resulting in a high reticulocyte count, a hallmark of the condition.
Staging and Clinical Manifestations
The effects of TAPS depend on the severity of the red blood cell imbalance, which is categorized using a staging system. The anemic donor twin often appears pale and may develop heart strain as the heart works harder to pump thin blood. In severe stages, the donor twin may develop hydrops fetalis, a dangerous accumulation of fluid in multiple body areas due to heart failure.
The recipient twin has an overload of thick, polycythemic blood, which causes sluggish circulation. This strains the heart and increases the risk of blood clots. This thick blood may cause a “starry sky” appearance in the liver on ultrasound due to congested blood flow.
The progressive nature of TAPS means a twin pair may move from a mild stage, where only the blood flow velocities are abnormal, to a severe stage involving signs of cardiac compromise or fetal hydrops. This staging system helps doctors determine the necessary level of intervention, ranging from close observation to immediate treatment.
How Doctors Diagnose and Monitor TAPS
TAPS is diagnosed and monitored prenatally using a non-invasive technique called Doppler ultrasound. This specialized ultrasound measures the speed of blood flow through a specific vessel in the babies’ brains, the Middle Cerebral Artery (MCA). This measurement is referred to as the Middle Cerebral Artery-Peak Systolic Velocity (MCA-PSV).
The speed of blood flow is directly related to its thickness. Thin blood moves faster, so the anemic donor twin shows an increased MCA-PSV. Conversely, the polycythemic recipient twin shows a decreased MCA-PSV due to their thick, sluggish blood.
A diagnosis of TAPS is confirmed when the donor twin’s MCA-PSV is abnormally high, and the recipient twin’s MCA-PSV is abnormally low. Because TAPS can develop slowly and often without other obvious signs, monochorionic twin pregnancies are typically monitored with MCA-PSV Doppler measurements every two weeks, starting around 16 weeks of gestation. This frequent monitoring is essential to detect the condition early enough for successful intervention.
Available Treatment Strategies
Treatment for TAPS is guided by the stage of the disease and the gestational age of the pregnancy. For the mildest stages, expectant management involves intensive monitoring without immediate intervention, in the hope that the condition may remain stable or even resolve spontaneously. If TAPS progresses, active treatment becomes necessary to prevent serious harm to the babies.
Temporary Interventions
One intervention is intrauterine blood transfusion (IUT), where blood is injected directly into the anemic donor twin to temporarily correct the red blood cell deficiency. This procedure is often paired with a partial exchange transfusion for the recipient twin, where a small amount of the recipient’s thick blood is removed and replaced with saline solution to reduce viscosity. These transfusions improve the babies’ condition and prolong the pregnancy, but they are temporary measures that do not fix the underlying cause.
Definitive Treatment
The only treatment addressing the root cause of TAPS is fetoscopic laser coagulation. This procedure uses a thin scope and a laser to seal off the small, connecting blood vessels on the surface of the placenta that are causing the unequal blood flow. The goal is to separate the twins’ circulations completely, effectively curing the condition and preventing further transfusion imbalance.
Delivery Considerations and Postnatal Care
The timing of delivery is a critical decision, balancing the risks of preterm birth against the risks of continued imbalance in the womb. If the condition is managed successfully with laser surgery, delivery is often delayed until a safe gestational age, usually between 32 and 34 weeks, or later if the twins remain stable. When TAPS is severe or progresses rapidly, early delivery may be necessary for immediate treatment in the neonatal intensive care unit.
Postnatal Diagnosis and Treatment
After birth, TAPS is definitively diagnosed by immediate blood testing. Healthcare providers measure the twins’ hemoglobin levels. A difference greater than 8 g/dL, alongside a high reticulocyte count ratio in the donor twin, confirms the diagnosis. The recipient twin may require a partial exchange transfusion to thin their blood and reduce the risk of clotting. The donor twin may receive a standard blood transfusion for anemia.
Long-Term Outcomes
Long-term follow-up is important, as TAPS can have lasting effects, particularly on the donor twin. Spontaneous TAPS is associated with a higher risk of neurodevelopmental impairment, including cognitive delays. A significant minority of spontaneous TAPS donor twins are also at risk for bilateral deafness caused by Auditory Neuropathy Spectrum Disorder (ANSD). Specialized hearing tests are a necessary part of their postnatal care.