TTTS Twin to Twin Transfusion Syndrome  



Vyvian Borse; Anthony L. Shanks. 

Twin-Twin Transfusion Syndrome (TTTS) is a condition that can affect twin gestations that share one placenta. This disorder highlights the importance of determining the chorionicity (number of placentas) and amnionicity (number of amniotic sacs) for all twin gestations, which will influence management. Twin gestations can be dichorionic diamniotic (DCDA, two placentas, and two amniotic sacs), monochorionic diamniotic (MCDA, one placenta, and two amniotic sacs), or monochorionic monoamniotic (MCMA, one placenta, and one amniotic sac). 

In the most accepted model of monozygotic twinning, the number of placentas and amniotic sacs depends upon when the splitting of the zygote occurs; DCDA twins result when splitting occurs between days 1 to 3, MCDA twins result when splitting occurs between days 3 to 8, MCMA twins result when splitting occurs between days 8 to 13, and conjoined twins result when splitting occurs on or after day 13. Ultrasound findings that help distinguish dichorionic and monochorionic twin gestations are the findings of a “lambda sign” in DC and a “T sign” in MC twin gestations. The “T sign” is created by the thin dividing membrane between two amniotic sacs when there is only one placenta supporting both gestations (monochorionic). This requires an absence of the “lambda” or “twin peak” sign, which is seen when each gestation is supported by its own placenta (dichorionic). The greatest risk of developing TTTS is in monochorionic twins, and it is more common in MCDA twins than MCMA twin gestations. 

To diagnose TTTS prenatally, an ultrasound must show a single placenta, one twin with oligohydramnios, and one twin with polyhydramnios. Oligohydramnios is typically defined as a maximal vertical pocket (MVP) of < 2 cm, while polyhydramnios is typically defined as an MVP of > 8 cm. Growth discordance and intrauterine growth restriction can occur but are not required for the diagnosis of TTTS. In monoamniotic twin gestations, findings consist of polyhydramnios and fetal bladder differences.


What is Twin-to-Twin Transfusion Syndrome? 

Twin-to-twin transfusion syndrome (TTTS) is a rare pregnancy condition affecting identical twins or other multiples. TTTS happens in pregnancies where twins share one placenta and a network of blood vessels that supply oxygen and nutrients that is essential for development of the babies. These types of pregnancies are also known as monochorionic (MoDi) 

What happens in this situation is that the vessels within the placenta are not evenly distributed and there is an imbalance in the blood exchange between the twins. One twin is called the Donor Twin and the other is called the Recipient Twin.  

The Donor Twin 

The donor twin experiences progressive loss of blood volume (hypovolemia). Therefore, its kidneys don’t have to filter as much fluid from the blood, and as a result, urination decreases. This leads to low levels of amniotic fluid. Amniotic fluid, which consists of fetal urine, provides a cushion in the womb. An abnormal reduction in amniotic fluid is known as oligohydramnios. If amniotic fluid ceases to exist this is known as anhydramnios. If the blood volume in the donor twin’s circulatory system is not effectively reaching the body, cardiovascular dysfunction can occur placing the donor in a life-threatening situation.  

The Recipient Twin 

The recipient twin is at risk for increased blood volume (hypervolemia). Hypervolemia leads to increased urination, more frequent bladder filling and the production of larger amounts of urine every time the bladder is emptied. This results in polyhydramnios — an abnormal increase in amniotic fluid. A continuous state of hypervolemia will affect the function of the heart muscle in the recipient, which is working hard to pump the increased amount of blood. Hypervolemia can exceed the capacity of the cardiovascular system (the heart and circulatory system of blood vessels) and can then lead to cardiovascular dysfunction and heart failure creating a life-threatening situation. 

Twin-to-Twin Transfusion Syndrome Diagnosis  

If a doctor suspects Twin-to-Twin Transfusion Syndrome based on the results of a routine prenatal ultrasound, a maternal-fetal medicine specialist can confirm the diagnosis by conducting more detailed testing to measure amniotic fluid volume, bladder filling and blood flow in the recipient and donor twins. When there is a rapid increase of amniotic fluid volume, the uterine cavity also expands at an accelerated pace, placing the mother at risk for preterm labour and shortening of the cervix. This may lead to preterm labour or preterm rupture of membranes followed by delivery. For this reason, maternal assessment of the cervical length and uterine activity is essential in all women presenting with suspected TTTS. An important factor in determining the prognosis of TTTS is the state of cardiovascular dysfunction in the twins. This is why a diagnosis of TTTS will include a detailed examination of the fetal heart (fetal echocardiography) in both the recipient and donor. 

Quintero staging system 

Doctors can assess the severity of each case of TTTS using the Quintero staging system. Quintero stages I and II represent progressive stages of blood volume imbalance, while stages III and IV indicate progressive stages of cardiovascular dysfunction. Stage V refers to the death of one or both twins. Quintero staging is important because it provides a standardized prenatal estimate on disease severity and the likelihood the condition will worsen. 


What are the treatment options for TTTS? 

Expectant management or observation 

Expectant management involves monitoring the pregnancy by regular ultrasound scans but without any specific treatment. Expectant management is usually recommended for Stage one TTTS, as in some cases it will remain stable or resolve without treatment.  

Fetal Laser Surgery 

Fetal laser surgery is the primary mode of treatment for TTTS. This procedure involves passing a small camera (called a fetoscope) into the amniotic sac of the recipient twin. A laser fibre is then used to block the connecting vessels on the surface of the placenta and stop the flow of blood from one twin to the other. Fetal laser surgery is performed in an operating theatre under local anaesthetic and sedation. An overnight hospital stay is required after this surgery. The main advantage of laser therapy over repeated amnioreduction is that it treats the underlying cause of TTTS, and usually requires only one intervention. It also means that the co-twin is better protected if one twin passes away. 

Amnioreduction (amniodrainage or amniocentesis) 

Amnioreduction is also called amniodrainage or amniocentesis. It is an ultrasound guided procedure that removes amniotic fluid from around the recipient twin with the aim to reduce the risk of the waters breaking and/or early birth. A needle is inserted into the amniotic sac of the recipient twin and one to four litres of amniotic fluid may be removed. An amnioreduction may need to be repeated on more than one occasion during the pregnancy as the fluid can return within a week or two. There is a small risk of the waters breaking and/or early delivery associated with the procedure. Talk to your Doctor for the latest statistics on this procedure. 

Useful links 

Australian Multiple Birth Association's%20TTTS%20support%20contact%20can, 

The Twin to Twin Transfusion Syndrome Foundation  

Maters Mother Hospital 

Preterm Infants Parents Association 

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