Twin-to-Twin Transfusion Syndrome

Overview

Twin-to-Twin Transfusion Syndrome (TTTS) is a rare but potentially life-threatening condition that affects monochorionic (identical) twin pregnancies, where both fetuses share a single placenta. TTTS occurs when abnormal blood vessel connections form in the shared placenta, allowing blood to flow unevenly between the twins. As a result, one twin (the donor) transfers more blood to the other (the recipient), leading to serious imbalances in blood volume and organ function. If untreated, TTTS can lead to complications for both twins, including heart failure, neurological damage, or death. TTTS typically develops between 16 and 26 weeks of gestation and requires timely diagnosis and intervention to improve outcomes.

Causes

TTTS is caused by abnormal vascular connections in the placenta of monochorionic twins. These twins share a single chorionic sac and placenta, though each has their own amniotic sac in most cases (monochorionic diamniotic). Within the shared placenta, there can be three types of vascular connections:

• Arteriovenous anastomoses (AVAs): One artery from the donor twin feeds into a vein connected to the recipient twin, creating an unbalanced flow of blood.

• Arterio-arterial (AA) anastomoses: Arteries from both twins connect; these are often protective by allowing bidirectional flow.

• Veno-venous (VV) anastomoses: Veins from both twins connect; these are less common and may contribute to instability.

The net result of these imbalanced connections is that the donor twin loses blood volume (leading to anemia and growth restriction), while the recipient twin receives too much blood (leading to polycythemia and heart strain).

Symptoms

Symptoms of TTTS vary depending on the severity and stage of the condition. It is usually detected during routine prenatal ultrasounds. TTTS is staged using the Quintero staging system:

Quintero Staging:

• Stage I: Discrepancy in amniotic fluid (polyhydramnios in the recipient twin, oligohydramnios in the donor) with a visible bladder in the donor twin.

• Stage II: Bladder in the donor twin is not visible on ultrasound, indicating reduced urine output.

• Stage III: Abnormal Doppler studies in either twin (e.g., absent or reversed blood flow in umbilical artery or ductus venosus).

• Stage IV: Hydrops fetalis (fluid accumulation) in one or both twins.

• Stage V: Death of one or both twins.

Donor Twin Symptoms:

• Poor growth (intrauterine growth restriction)

• Oligohydramnios (low amniotic fluid)

• Pale appearance and anemia

• Bladder may appear small or non-visible on ultrasound

Recipient Twin Symptoms:

• Polyhydramnios (excess amniotic fluid)

• Enlarged heart (cardiomegaly)

• Signs of heart failure or hydrops fetalis

• Over-perfused and at risk for high-output cardiac failure

Diagnosis

TTTS is usually diagnosed during routine second-trimester ultrasounds in monochorionic twin pregnancies. Key diagnostic steps include:

• Ultrasound: The primary diagnostic tool. Findings may include:

  • Discordant amniotic fluid levels (maximum vertical pocket > 8 cm in the recipient and < 2 cm in the donor)

  • Growth restriction in the donor twin

  • Visualization of a small or absent bladder in the donor twin

  • Cardiac enlargement or hydrops in the recipient twin

• Doppler studies: Assess umbilical artery and ductus venosus blood flow for signs of compromised circulation.

• Fetal echocardiography: May be used to assess cardiac function, particularly in the recipient twin.

Early and accurate diagnosis is crucial for initiating timely treatment and improving outcomes.

Treatment

The management of TTTS depends on gestational age, severity of the disease (Quintero stage), and the overall health of the twins. Treatment options include:

1. Fetoscopic Laser Photocoagulation (FLP):

• Considered the gold standard for moderate to severe TTTS (Stages II-IV).

• Involves endoscopic surgery to locate and coagulate the abnormal vascular connections in the placenta, stopping the transfusion imbalance.

• Improves survival rates and reduces neurological complications.

2. Amnioreduction:

• Used to drain excess amniotic fluid from the recipient twin’s sac.

• Temporarily relieves symptoms such as maternal discomfort or preterm labor.

• Does not address the underlying vascular issue and is usually used when FLP is not available or feasible.

3. Septostomy:

• Creates a hole in the membrane between the two amniotic sacs to equalize fluid levels.

• Less commonly performed and has fallen out of favor due to associated risks.

4. Selective reduction (rare cases):

• May be considered in extreme cases where one twin’s survival threatens the life of the other.

5. Close Monitoring:

• For mild cases (Stage I), frequent ultrasounds and Doppler studies may be recommended to watch for progression before intervening.

Prognosis

The prognosis for TTTS varies depending on the stage at diagnosis, treatment received, and gestational age. Key points include:

• Without treatment: The survival rate of both twins is less than 10–15% in severe cases.

• With laser surgery: Survival of at least one twin is achieved in over 85% of cases, and both twins survive in 60–70% of cases.

• Neurological outcomes: Risk of brain injury due to anemia, hypoxia, or premature delivery; however, the risk is significantly reduced with early treatment.

• Preterm birth: Common in TTTS cases, often occurring before 34 weeks, necessitating NICU support.

With modern advances in fetal therapy and high-risk obstetric care, the outlook for TTTS has greatly improved. Early detection, expert fetal monitoring, and timely surgical intervention are critical to optimizing outcomes for both twins.