What is twin anemia polycythemia sequence?

Twin anemia polycythemia sequence (TAPS) is a form of twin-to-twin transfusion syndrome (TTTS) that can complicate monochorionic twin pregnancies. In a monochorionic pregnancy, identical twins share one placenta (afterbirth) and a network of blood vessels that supply oxygen and nutrients essential for development in the womb. These pregnancies are susceptible to complications like TTTS and twin anemia polycythemia sequence (TAPS).  

TAPS is a rare condition that occurs when there are unequal blood counts between the twins in the womb. Unequal blood counts cause the twins to suffer from an imbalance in red blood cells and hemoglobin. This means one twin is not receiving the appropriate amount of oxygen and nutrients it needs to develop properly. TAPS can occur randomly or after an incomplete laser surgery to treat TTTS.

In contrast to classical TTTS, TAPS is caused by the joining of a few small caliber artery-to-vein vessel connections (AV anastomoses). Arteries are the blood vessels that carry oxygen-rich blood from the heart to the rest of the body. Veins are the vessels through which blood returns to the heart. These AV anastomoses are typically near the edge of the placenta and are less than 1 mm in diameter.

The twin that loses blood is called the donor twin, while the one that receives the blood is called the recipient twin. When TAPS occurs, the recipient twin is at risk for successively increasing blood count, called polycythemia, and the donor twin for progressive blood loss, or anemia.

TAPS also differs from classical TTTS in that amniotic fluid levels remain normal during the pregnancy.

What are the symptoms of twin anemia polycythemia sequence?

Signs and symptoms related to TAPS include thicker than normal blood in the recipient twin and anemia in the donor twin. If suffering from polycythemia, the recipient twin may have a sluggish blood flow in small vessels and may even suffer a spontaneous blockage of blood flow (thrombosis).

What are the risk factors of twin anemia polycythemia sequence?

• Monochorionic pregnancy: Being pregnant with identical twins or multiples

• Prior laser surgery for TTTS

Twin Anemia Polycythemia Sequence Diagnosis

For a physician to diagnose TAPS, two events must be present:

• Slower than normal blood flow in the recipient twin accompanied by polycythemia

• Faster than normal blood flow in the donor twin accompanied by anemia

These events are detected most reliably by Doppler — a type of ultrasound that examines blood circulation. If there is abnormally decreased blood flow in the middle cerebral artery of one fetus and concurrently increased blood flow in the other fetus, TAPS is confirmed.

Leiden staging system

Many times TAPS is not suspected until one of the twins is very sick. This is because the classic signs of TTTS are missing. Likewise, the diagnostic test for TAPS is not part of the standard staging examination performed for TTTS. To counteract this, the Leiden staging system was developed. The Leiden staging system is similar to the Quintero staging system, but incorporates monitoring of the maximum speed of blood flow through an artery supplying the brain.

Leiden stages I and II represent progressive stages of imbalance in the blood flow, while stages III and IV indicate progressive stages of cardiovascular illness. Stage V refers to the death of one or both twins.