Twin-to-Twin Transfusion Syndrome, also known as TTTS, Feto-Fetal Transfusion Syndrome (FFTS), or Twin Oligohydramnios-Polyhydramnios Sequence (TOPS), is a rare condition occurring only in pregnancies involving identical twins or multiples that share a placenta. In cases of TTTS, the circulatory systems of a pair of identical twins become connected through the placenta, and one twin—the “donor” begins to lose blood to the other, the “recipient” twin. The result is insufficient blood supply foe the donor twin and excessive blood volume in the body of the recipient. This condition occurs in approximately two out of every 10,000 pregnancies.
Causes and Signs of Twin-to-Twin Transfusion Syndrome
Twins are either identical or fraternal. Fraternal twin pregnancies occur when two different sperm cells fertilize two separate eggs, resulting in the separate formation of two distinct zygotes (embryos). Identical twins, however, are the result of a single fertilized egg splitting into two identical zygotes.
In over 99 percent of pregnancies involving identical twins, each twin floats in its own amniotic sac. Approximately a third of these twins have their own placentas, but the remaining 66.6 percent share a placenta, and these twins are at risk for twin-to-twin transfusion syndrome due to abnormal blood-vessel connections that may form between them.
In 15 to 20 percent of these pregnancies, an imbalance develops in blood flow, resulting in TTTS. The reasons why this happens are not well understood, but it is not caused by genetics, nor is it because of anything the mother has done or failed to do. Rather, it is a result of the random occurrence of identical twinning, the timing of the twinning event (the splitting of the single embryo into two embryos), the particular way in which the placenta is shared by this particular set of twins, and the unique formation of their shared blood vessel connections.
Progression and Complications of Twin-to-Twin Transfusion Syndrome
If TTTS is not properly treated, its effects will be apparent after the twins are born, if they both survive. The donor twin is usually smaller than the recipient twin, and often anemic, while the recipient will have high blood pressure (which may result in heart failure) and a reddish appearance to his or her skin. In many cases, one or both twins may be stillborn. The likelihood of stillbirth depends in part on how the placenta is shared; in some cases one twin has too small a share of the placenta and cannot obtain sufficient nutrients to survive.
As TTTS progresses, the twins will develop complementary conditions known as polyhydramnios and oligohydramnios. The recipient twin will increase its urine production in an attempt to handle its excessive blood volume. This will result in polyhydramnios, an excess of amniotic fluid in the amniotic sac. This can cause a number of pregnancy complications, including an increased risk of placental abruption (separation of the placenta from the uterine wall), movement of the fetus into breech position, or preterm labor. In the later stages of TTTS, hydrops fetalis may also develop in the recipient twin. This is a serious condition characterized by tissue swelling throughout the body of the fetus.
The donor twin, on the other hand, will reduce its urine output in an attempt to cope with the loss of fluid, resulting in oligohydramnios—an amniotic fluid deficiency. Oligohydramnios can lead to skeletal deformities and other birth defects, in addition to growth problems. When combined with the anemia that may result from TTTS, these conditions place the donor twin at grave risk.
Diagnosing Twin-to-Twin Transfusion Syndrome
Twin-to-twin transfusion syndrome is generally diagnosed with ultrasound examination, which will show a shared placenta combined with oligohydramnios in one twin and polyhydramnios in the other. Ultrasound may also reveal that the recipient twin has an abnormally large bladder.
An Unusual Form of TTTS
In about one out of every 15,000 pregnancies an unusual and especially nightmarish form of TTTS occurs, known as acardiac twin syndrome. When this condition occurs, one twin develops almost normally, but the other fails to develop a heart, and other organs may also fail to develop properly. Because this acardiac twin lacks a heart, its blood flow comes entirely from its normal sibling, known in this case as the pump twin. This condition is also known as TRAP, an acronym for twin reversed arterial perfusion syndrome.
In some cases of TRAP, blood flow from the pump twin to the acardiac twin eventually stops, and the acardiac twin stops growing and dies. If the acardiac twin does not stop growing, however, its demands can eventually become too much for the pump twin, which will develop polyhydramnios and eventually experience heart failure. In the absence of treatment, more than half of all cases of TRAP end in the death of the pump twin.
Treatment of twin-to-twin transfusion syndrome generally involves repeated amniocentesis for the recipient twin, in order to keep the level of excess amniotic fluid down. Fetal laser surgery may be considered in some cases to sever the blood-vessel connection between the twins. In 70 percent of cases laser surgery ensures the survival of at least one twin, and in 33 percent of cases it saves both of them. In cases in which one twin dies, the likelihood of complications for the surviving twin drops from 35 percent to 7 percent.
In some tragic cases, a procedure known as selective cord coagulation may be undertaken. This is done if laser ablation of the connecting blood vessels is impossible for some reason, or if one twin is already dying. With selective cord coagulation, the flow of blood and nutrients to the dying fetus is stopped at the umbilical cord. This ensures the death of one fetus while providing the other with a reasonable shot at survival.
The prognosis for twin-to-twin transfusion syndrome depends on the severity of the condition. If the TTTS is mild and the blood imbalance is not too great, both babies are likely to recover completely. In more severe cases, however, one fetus is likely not to survive.