Management of TTTS is one of the most challenging clinical problems concerning multiple gestations. Approximately 20 percent of all twin pregnancies are monochorionic, and the incidence of Twin-Twin Transfusion Syndrome in monochorionic diamniotic gestations is approximately 10 to 20 percent. Twin-Twin Transfusion Syndrome is a phenomenon almost exclusive to monochorionic twin pregnancies. The natural history of severe Twin-Twin Transfusion Syndrome is well established. Mortality rates approach 80 to 100 percent if left untreated, especially when it presents at less than 20 weeks gestation.

Pathophysiology

Twin-Twin Transfusion Syndrome is a complication of monochorionic multiple gestations mediated through vascular communications.

The cause of TTTS is unknown but vascular connections within the placenta between the twins are necessary for it to occur. The majority of monochorionic twin gestations have vascular anastomoses (connections) between the twins, although only around 15% develop TTTS. The communications can be artery to artery (AA), vein to vein (VV), or artery to vein (AV). Depending on the number and type of anastomoses present, the exchange of blood may be balanced or unbalanced.

AV anastomoses consist of a single unpaired artery carrying blood from one twin connecting with a single unpaired vein carrying blood to the other twin. AV anastomoses are thought to be responsible for exchange of blood from the donor to the recipient twin. AA anastomoses, on the other hand, are thought to be protective. They may be able to compensate for the AV anastomoses to some extent. The role of VV anastomoses is less certain.

Severe TTTS will cause cardiovascular compromise in most recipient twins and is a major cause of death for these fetuses. Recipient twins can develop a progressive cardiac impairment with the right ventricle (RV) usually becoming compromised first and to a more significant degree than the left ventricle (LV). The function of the heart valves can also become impaired, with the pulmonary valve being particularly critical in this regard. Cardiovascular changes in the donor twin are usually less dramatic.

If treatment of the TTTS is successful, the separation of the vessels connecting the twins may result in an arrest in the progression of TTTS related cardiac impairment; in fact an improvement in heart function is often seen.