Our understanding of its causes and effects has expanded rapidly. We now know that a spectrum of disease can result when unequal placental sharing and/or unequal blood-volume sharing occurs in monochorionic pregnancies, for instance, and that a significant imbalance in blood-flow exchange between twins' circulations is the primary contributor to the development of TTTS. On the other hand, our knowledge is still quite simplistic: We have much to learn about the pathophysiology and the natural history and progression of the syndrome.
The observations we have made, however, are significant enough to justify the treatment of severe TTTS—especially given the advances in ultrasound assessment, which allow us to detect the syndrome early, as well as the dramatic improvements in technology for minimally invasive intrauterine therapy that have come about in recent years.
Endoscopic laser ablation (or laser coagulation) of placental anastomoses has been shown in numerous studies—including a multicenter, randomized trial comparing it with serial amnioreduction—to be an effective treatment for TTTS, and a preferable first-line approach for severe TTTS that is diagnosed before 26 weeks' gestation.
Because intrauterine procedures require a high level of expertise and infrastructure, it is likely that the management of these conditions will remain regionalized. Improved referral patterns and support for families, however, will promote the development of a nationwide network of designated centers, making such therapy more accessible.
Pathophysiology and Consequences
Identical twins are monochorionic, and these pregnancies present several potential risks: the risk that one baby will not get its fair share of the placenta, the risk that blood volume will be shared unequally, and an overall risk of vascular instability in each twin.
When the predominant issue in an identical twin pregnancy is unequal placental sharing, the growth of one baby becomes restricted and the other baby grows normally, resulting in a condition called selective intrauterine growth restriction (selective IUGR).
The other main issue—that of unequal blood-volume sharing—is what fuels TTTS. In uncomplicated pregnancies, blood is exchanged equally through the vascular anastomoses that characterize all monochorionic pregnancies. In complicated pregnancies, however, the exchange is unbalanced, and blood is shared in one direction without adequate return.
Arteries emanating from the placental cord insertion of one twin, for instance, can drain into a vein returning to the other twin. Such arteriovenous anastomoses are in the substance of the placenta and act as one-way valves for blood flow. If the amount of blood flow in one direction is not balanced by enough flow in the opposite direction—that is, if the magnitude of blood flow through unidirectional arteriovenous anastomoses is not compensated by vascular channels that permit flow in the opposite direction—then an imbalance develops that is potentially harmful to both babies.
In TTTS, which develops in about 15% of monochorionic pregnancies, the imbalance progresses to the extent that one twin becomes a “donor” of blood volume and the other becomes the “recipient” twin.
The donor twin moves blood across the anastomoses to the placenta and to the recipient twin, and does not receive an equal amount in return. A decline in blood volume leads to decreased urine output to the extent that, eventually, bladder filling in the donor twin virtually ceases. Under these circumstances, oligohydramnios may progress to anhydramnios, and the twin may become “stuck” in an essentially empty amniotic sac.
The recipient twin, in the meantime, receives an excess amount of venous blood volume. The increase in intravascular blood volume drives an increase of filtration in the kidneys, which results in excess urination. The increased urinary frequency, which may even result in constant bladder filling, leads to polyhydramnios.
When the sac of the recipient twin becomes distended by amniotic fluid, and the donor twin is no longer producing urine, the membrane between the twins may become wrapped so tightly around the donor twin that it is barely visible on ultrasound (See image below.) When the donor twin is “stuck” to the uterine wall in such a way, the ultrasound appearance resembles that of identical twins with one amniotic cavity (monoamniotic twins).
Untreated TTTS has serious consequences for each twin and for the whole pregnancy. First, the resultant polyhydramnios can stimulate preterm labor because of uterine distention. Second, abnormalities in blood volume can lead to cardiac problems and cardiovascular compromise for the babies, most often for the recipient twin. The excess blood cells and volume overload that this twin faces can lead to cardiac failure and hydrops.
The donor twin, meanwhile, is at risk for abnormalities and long-term effects resulting from compression, failing placental function, malnutrition, and hypovolemia.