Twin Anemia-Polycythemia Sequence

Overview

Twin Anemia-Polycythemia Sequence (TAPS) is a rare and serious complication that occurs in monochorionic (identical) twin pregnancies, where both fetuses share a single placenta. TAPS involves an imbalance in red blood cell distribution between the twins due to minuscule vascular connections in the placenta. As a result, one twin becomes anemic (the donor twin) while the other develops polycythemia (the recipient twin), meaning an excess of red blood cells. Unlike Twin-to-Twin Transfusion Syndrome (TTTS), TAPS does not involve significant differences in amniotic fluid volumes and is often more difficult to detect. TAPS can occur spontaneously or after treatment for TTTS, such as laser ablation therapy. Early diagnosis and appropriate intervention are crucial to improve outcomes for both twins.

Causes

TAPS is caused by tiny arteriovenous anastomoses (blood vessel connections) in the placenta of monochorionic twins. These connections are too small to allow large-volume blood flow but permit the slow transfer of red blood cells from one twin to the other. Over time, this imbalance leads to:

• Anemia in the donor twin: The loss of red blood cells reduces the ability to carry oxygen.

• Polycythemia in the recipient twin: The accumulation of excess red blood cells makes the blood thicker and can impair circulation.

TAPS can be classified as:

• Spontaneous TAPS: Occurs naturally in about 3–5% of monochorionic twin pregnancies without prior intervention.

• Post-laser TAPS: Occurs in approximately 13% of cases following laser surgery for TTTS, due to residual minuscule vascular connections.

Symptoms

TAPS may not present obvious external symptoms, particularly in the mother. The condition is often detected during routine prenatal monitoring. Symptoms or signs that may be observed in the fetuses include:

Donor Twin (Anemic):

• Pale appearance on ultrasound

• Reduced fetal movements

• Cardiac strain or hydrops fetalis (severe cases)

Recipient Twin (Polycythemic):

• Reddish or dark appearance on ultrasound (due to thickened blood)

• Increased risk of blood clots or poor perfusion

• Organ dysfunction due to sluggish circulation

At Birth:

• Significant differences in hemoglobin levels between the twins

• Donor twin may require immediate transfusion

• Recipient twin may require partial exchange transfusion to reduce blood viscosity

Diagnosis

Early and accurate diagnosis of TAPS is essential. It is typically identified through specialized Doppler ultrasound and postnatal laboratory findings. Diagnostic steps include:

• Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV):

  • MCA Doppler ultrasound detects anemia and polycythemia by measuring blood flow velocity in the fetal brain.

  • High velocity (MCA-PSV > 1.5 MoM) suggests anemia in the donor twin.

  • Low velocity (MCA-PSV < 1.0 MoM) indicates polycythemia in the recipient twin.

• Absence of amniotic fluid discordance: Helps differentiate TAPS from TTTS.

• Postnatal blood tests:

  • Hemoglobin difference of > 8 g/dL between twins confirms TAPS diagnosis.

  • Reticulocyte count ratio > 1.7 between twins also supports the diagnosis.

• Placental pathology: Examination of the placenta may reveal the tiny vascular anastomoses characteristic of TAPS.

Treatment

The treatment of TAPS depends on gestational age, severity of the condition, and overall fetal health. Management options include:

1. Intrauterine Treatments:

• Intrauterine transfusion (IUT): Administered to the anemic twin to correct low hemoglobin levels.

• Partial exchange transfusion: Performed on the polycythemic twin to reduce red blood cell concentration.

• Laser coagulation surgery: Targeted laser ablation of the small placental vascular connections to stop further blood exchange. This is a definitive treatment in ongoing pregnancies.

2. Delivery:

• Timing: If TAPS is diagnosed later in pregnancy and the risks of prematurity are low, early delivery may be recommended.

• Neonatal care: Intensive care may be needed for both twins, including transfusions and close monitoring of organ function.

3. Supportive Care:

• Frequent ultrasound monitoring to track disease progression.

• Maternal counseling and emotional support.

• Referral to a maternal-fetal medicine specialist or fetal therapy center.

Prognosis

The prognosis of Twin Anemia-Polycythemia Sequence varies based on the severity of the condition and the timeliness of intervention. Key points include:

• Early detection and treatment: Greatly improve outcomes for both twins.

• Complications: Without treatment, TAPS can lead to severe anemia, hydrops, neurological injury, or death in one or both twins.

• Neurological risks: The anemic twin may be at risk of brain injury due to low oxygen supply; the polycythemic twin may face complications from poor blood circulation.

• Survival rate: Varies depending on gestational age at diagnosis and intervention. Combined survival with treatment approaches 80–90% in specialized centers.

Long-term neurodevelopmental outcomes depend on the severity of anemia and polycythemia, gestational age at delivery, and quality of neonatal care. Regular follow-up is essential to assess and manage any developmental delays or health issues.