Twins / Multiples | Twin-twin transfusion syndrome
In twins that share a placenta (monochorionic twins), the blood should flow equally to allow both twins to grow and thrive. If this balance is disrupted, one twin can receive too much blood and the other can receive too little. This condition, called twin-twin transfusion syndrome (TTTS), is serious, and, if left untreated, may result in one or both twins’ developing severe, possibly life-threatening, complications.
- Twin-twin transfusion syndrome (TTTS) occurs in 10-15 per cent of monochorionic twin pregnancies, where the twins share one placenta. In cases of TTTS, abnormal connections between arteries and veins in the placenta result in one twin (known as the “recipient” twin), receiving too much blood and the other twin (known as the “donor” twin) receiving too little.
- The recipient twin who receives excess blood becomes overloaded and may develop heart failure or pass away.
- The donor twin who receives too little blood may become anemic or develop kidney failure or other organ damage. This fetus may stop making urine, which will result in low levels of amniotic fluid and may lead to life-threatening developmental problems.
- Because monochorionic twins share placental circulations, if one passes away inside the womb, the other risks severe disability or even death.
- Fetoscopic laser ablation is a procedure whereby the abnormal placental connections are disrupted using a laser, while the twins are still inside the womb.
Twin-twin transfusion syndrome (TTTS) is a rare syndrome that only occurs in identical (monochorionic) twin pregnancies, where two fetuses share a single placenta. Both fetuses are connected by blood vessels (called “anastomoses”) on the surface of the placenta. As a consequence, these fetuses continuously exchange blood. TTTS is largely due to an imbalance in the exchange of blood through these vessels.
In 85-90 per cent of cases, the transfer of blood from one fetus to the other is balanced and each receives a similar quantity of blood as it gives away. In about 10-15 per cent of cases however, this exchange of blood is imbalanced and one fetus (the recipient, often called “Poly”) receives more blood than it gives away. The other fetus (the donor, often called “Oli”), gives away more blood than it receives.
The recipient fetus will become volume overloaded, and will try to get rid of this extra fluid by urinating. As a consequence, its bladder will become very full and this fetus will produce too much amniotic fluid. The volume overload will often lead to early signs of heart failure.
The donor fetus on the other hand will try to preserve fluid and will stop urinating. Its bladder will usually be empty and the fetus will have too little amniotic fluid. On ultrasound, the fetus is usually seen enclosed in its membranes like shrink wrap.
TTTS can be diagnosed on ultrasound, and the earliest sign is often a difference in amniotic fluid levels between the two fetuses. When TTTS is suspected, or significantly different amniotic fluid levels are seen, a detailed ultrasound is required to differentiate TTTS from other complications of identical twin pregnancies, as well as to evaluate the impact of TTTS on each fetus’ health and wellbeing.
The fetal therapy for TTTS is called fetoscopic laser ablation of the communicating placental vessels. This procedure involves making small incisions in the mother’s skin (called “laparoscopic” or “minimally invasive surgery”), introducing a small camera and then, using a laser beam to interrupt (or burn) the abnormal blood vessel connections in the placenta. The excess amniotic fluid is also drained from around the recipient twin, and genetic tests can be performed at the same time, if requested. After the procedure, the donor twin may resume making urine and will produce amniotic fluid again. The recipient’s heart, may, in time, recover.
In a large study, fetoscopic laser ablation resulted in higher survival rates, more advanced age at delivery (average gestational age at delivery: 33 weeks) and better infant outcomes than repeatedly draining amniotic fluid from the recipient twin (called “amniodrainage”). Despite these promising results, close surveillance of the twins after the procedure is still required. Overall, the chance of both fetuses surviving after a laser procedure is 50-60 per cent, and the chance of one fetus surviving is 80-90 per cent. Because the fetuses are likely to be born prematurely, they may suffer from complications of being born too early. These are difficult to predict in advance. Studies of long-term outcomes of survivors of TTTS suggest that the recipient’s heart function and the donor’s kidney function are normal, but that 8-12 per cent may suffer from developmental delay.
The procedure is typically done under light sedation, and a support person can accompany the mother in the operating room.
Referral Information (for physicians)
- Monochorionic/diamniotic multiple pregnancy.
- Severe polyhydramnios with deepest vertical fluid pocket in recipient twin >8cm prior to 20 weeks gestation and >10cm after 20 weeks.
- Oligohydramios in the donor twin with deepest vertical fluid pocket < 2cm
Twin-twin transfusion may develop acutely so please refer urgently. Inpatient admission is approximately 24 hours. Patients will usually be asked to return for one follow-up appointment with the OFC within 48-72 hours after their surgery, but will then be referred back to their local care provider for further follow-up and delivery.