What is TTTS?
Twin Twin Transfusion Syndrome (TTTS) is a condition unique to monochorionic twins. This type of twin gestation has two fetuses that share one placenta. Since the fetuses share one placenta, they have vascular connections through the placenta. In TTTS, the fetuses are not sharing the placenta equally. Instead, one fetus receives more of the placental blood supply than the other. Therefore, one fetus (the "recipient") receives too much blood and can become "volume overloaded". This increased volume results in increased fetal urination. While the other fetus (the "donor"), receives less of the placental blood volume and is "dehydrated" and does not urinate very much. The differences in fetal urination are what result in different levels of amniotic fluid volume. If this condition is left untreated, there is an estimated 80-100% mortality. Click here to see TTTS drawing.
There are stages of TTTS which range in severity. Stage I is an early stage in TTTS that reveals only a discrepancy in the amniotic fluid between the fetuses. One fetus can have "polyhydramnios" or too much amniotic fluid, while the other has "oligohydramnios" or too little amniotic fluid. Stage II is when the bladder can no longer be sonographically appreciated in the donor fetus. The bladder is present but is empty so on ultrasound we cannot visualize this structure. Stage III is when we note changes in blood flow through the arterial or venous systems of either fetus. The Fetal Care Center of Cincinnati has broken stage III into three groups depending on cardiac function of the recipient fetus based on mild (a), moderate (b), or severe (c) cardiac changes. Stage IV is when the recipient fetus becomes so volume overloaded that there is ultrasound evidence of severe cardiac compromise such that the fetus is hydropic and swollen. Stage V is when there is a demise of one fetus.
Depending on the stage and gestational age of the TTTS, there are a number of treatment modalities available. The least invasive are observation, septostomy, and amnioreduction. Most patients do not choose observation since we know the natural history of the condition and that if the condition is untreated, the fetuses will likely die. The benefits of a septostomy or amnioreduction are that they are not very invasive; therefore, pose little risk to the mother. However, they do not treat the underlying problem which as a result can many times recur. Some patients may need multiple procedures during the pregnancy. The fetoscopic laser ablation procedure is more invasive but provides a treatment directed at the underlying problem-the placental vascular connections. In this procedure a camera is placed in the uterus and the placenta is directly visualized. The vessels on the surface of the placenta are then cauterized by the laser energy and ablated. This corrects the underlying problem and separates the fetuses from each other.How common is TTTS?
How Common is TTS?
About one third of twins are monochorionic and approximately 15% will develop TTTS.
How is it diagnosed?
TTTS is diagnosed with ultrasound. The ultrasound will consist of a detailed evaluation of all the fetal structures that can be sonographically appreciated, Doppler studies of a number of vessels in both fetuses, and by echocardiogram to assess fetal cardiac function. If the diagnosis is made, then the stage would be accordingly assigned.
What are the available treatments and their outcomes?
If left untreated, the risk of the fetal demise is between 80-100%. With treatment, these risks can be dramatically reduced, but are never zero.
Septostomy - is when a small hole is placed in the intertwin membrane in attempt to equalize the pressure in each fetal sac. This also allows the "donor" fetus to have enough amniotic fluid for normal lung and limb growth. It can be performed with a needle or with the laser during fetoscopic surgery.
Amnioreduction - is performed when the amniotic fluid has increased to such a degree in the recipient sac that the patient is uncomfortable or having difficulty breathing. This technique may also be used as a first line of treatment in milder cases. The mechanism by which it is felt to improve the fetal condition is by relieving the intrauterine pressure created by the polyhydramnios in order to restore placental function and blood flow. This procedure is similar to an amniocentesis; however, more amniotic fluid is removed. Since it takes longer than an amniocentesis, local anesthesia can be given to numb the skin. The procedure is guided by ultrasound and the physician can visualize the location of both fetuses throughout the procedure. With this procedure, there is an increased risk for preterm delivery. Sometimes multiple amnioreductions are needed during a pregnancy to maintain the therapeutic effect.
Fetoscopic Laser Ablation Procedure - is a more invasive approach to the treatment of TTTS however it addresses the underlying problem and corrects the placental imbalance by using photocoagulation. The surface placental vessels are "mapped" from one fetal cord insertion to the other and the "abnormal" vessels are identified. Once they are identified, then they are photocoagulated with the laser. This eliminates the abnormal blood flow from one fetus to the other and treats the TTTS. Preliminary data have found this to improve the cardiac function of the recipient fetus. It is thought that the cardiovascular load on the recipient heart is relieved. There are deep connections which cannot be treated in 2% of cases because they will not be visible from the placental surface. Umbilical cord occlusion is reserved for severe cases in which it is not possible to save both fetuses and its purpose is to protect the co-twin from dying. This is considered a last resort since the outcome eliminates the possibility of survival for one twin.
There are cases which are so severe that the family may choose not to continue the pregnancy.
As with any procedure there are risks and benefits. These details should be discussed with your physician before you make a treatment plan.
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