Twin-to-Twin Transfusion Syndrome (TTTS) is a serious prenatal condition affecting only monochorionic multiple pregnancies, where two or more fetuses share a single placenta. This shared placenta contains blood vessels connecting the twins’ circulations. TTTS occurs when an imbalanced flow of blood across these connections causes one twin to receive too much blood and the other too little, leading to significant complications for both. Early detection and intervention are important, as modern fetal therapy offers effective solutions.
How TTTS Develops
The root cause of TTTS is the presence of communicating blood vessels, known as anastomoses, on the surface of the shared placenta. The syndrome develops when the blood flow across these vessels becomes significantly unbalanced, creating a one-way transfusion of blood from one twin to the other. This imbalance causes a dramatic difference in blood volume between the two developing fetuses.
The twin who loses blood is the “donor twin,” experiencing hypovolemia, or low blood volume. The decreased blood supply reduces the donor twin’s kidney function, leading to decreased urine output. Since fetal urine is the primary component of amniotic fluid, this results in oligohydramnios (low amniotic fluid) and often restricts the twin’s growth.
Conversely, the twin who receives the excess blood is the “recipient twin” and develops hypervolemia, or high blood volume. The recipient twin’s kidneys work overtime to process the extra fluid, resulting in a large bladder and excessive urine production. This causes polyhydramnios (accumulation of amniotic fluid) in their sac, which can lead to significant uterine distension.
The recipient twin’s heart must pump the increased blood volume, which can cause strain and potentially lead to heart failure, known as hydrops. The donor twin’s insufficient blood supply can lead to organ failure and growth restriction. Both twins are at high risk of morbidity and mortality if the condition remains untreated.
Diagnosing and Staging Severity
Detection of TTTS relies primarily on specialized prenatal surveillance, beginning with frequent ultrasound examinations for monochorionic twin pregnancies. High-resolution ultrasound monitors the deepest pocket of amniotic fluid in each twin’s sac, noting the characteristic fluid imbalance (oligohydramnios and polyhydramnios). Doppler ultrasound assesses blood flow patterns in the umbilical cord and major fetal blood vessels, providing information about cardiovascular health.
The severity of the syndrome is classified using the Quintero Staging System, which determines the urgency and type of intervention needed.
Quintero Staging System
- Stage I is diagnosed when the fluid imbalance is present, but the donor twin’s bladder is still visible, and blood flow is normal in both twins.
- Stage II occurs when the donor twin’s bladder is no longer visible on ultrasound, indicating severely reduced urine output.
- Stage III is characterized by critically abnormal blood flow studies in either twin, such as absent or reversed end-diastolic flow in the umbilical artery, signaling significant cardiovascular compromise.
- Stage IV is reached if the recipient twin develops signs of heart failure and massive fluid retention, known as hydrops.
- Stage V is designated when one or both twins have died in utero due to complications of the syndrome.
Current Treatment Options
The primary and most effective intervention for TTTS is Fetoscopic Laser Ablation, a minimally invasive surgical procedure performed in utero. This procedure directly addresses the cause of the condition by sealing off the communicating blood vessels on the placental surface that are responsible for the unbalanced flow. A small instrument called a fetoscope is guided into the recipient twin’s amniotic sac under continuous ultrasound guidance.
A surgeon uses a fetoscope to identify the abnormal arteriovenous connections and uses laser energy to coagulate these vessels. By eliminating the vascular connections, the procedure separates the shared placenta into two distinct territories, allowing each twin an independent blood supply. This treatment is preferred for Quintero Stage II, III, and IV cases diagnosed between 16 and 26 weeks of gestation, as it cures the underlying placental disease.
For less severe cases, or as a temporary measure, amnioreduction may be performed. This involves using a needle to drain the excessive amniotic fluid from the recipient twin’s sac, which alleviates uterine pressure and reduces the risk of preterm labor. While amnioreduction can prolong the pregnancy, it does not correct the underlying abnormal blood vessel connections and may require repeated procedures.
Prognosis and Follow-Up Care
The prognosis for TTTS has significantly improved with the widespread adoption of Fetoscopic Laser Ablation. Following the procedure, the survival rate for at least one twin is typically between 80% and 90%, and the survival rate for both twins ranges from 60% to 70% in high-volume fetal centers. The success of the intervention depends on the stage of the syndrome at the time of treatment, with earlier-stage interventions yielding better outcomes.
Children who survive TTTS, particularly those born prematurely, require long-term monitoring after birth. Both the donor and recipient twins have an increased chance of neurodevelopmental impairment, which can manifest as issues with learning, movement, or behavior. A structured follow-up program is recommended, involving pediatric specialists like neurologists and cardiologists, to detect and address potential long-term effects stemming from the unequal blood flow experienced during fetal development.