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 occurs in pregnancies where twins share one placenta (afterbirth) and a network of blood vessels that supply oxygen and nutrients essential for development in the womb. These pregnancies are known as monochorionic.
Sometimes the vessel connections within the placenta are not evenly dispensed and there is an imbalance in the blood exchange between the twins. One twin — the donor twin — gives away more blood than it receives in return and runs the risk of malnourishment and organ failure. The recipient twin receives too much blood and is susceptible to overwork of the heart and other cardiac complications.
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 affects development of the bladder and leads to low levels of amniotic fluid.
Amniotic fluid, which consists of fetal urine, provides a cushion in the womb. In addition, as a fetus grows, it swallows this fluid. This aids in the development of its respiratory, urinary and gastrointestinal systems. 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. This places the donor at risk for death.
The recipient twin
The recipient twin is at risk for successively increasing 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 persistent state of hypervolemia ultimately affects 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 even heart failure and death.
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What are the risk factors of Twin-to-Twin Transfusion Syndrome?
Being pregnant with identical twins or multiples puts a woman at risk for having a pregnancy complicated by TTTS. However, TTTS is an indiscriminate condition, occurring at random in monochorionic pregnancies.
Twin-to-Twin Transfusion Syndrome Diagnosis
A doctor may suspect 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 labor and shortening of the cervix. This may lead to preterm labor 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.
Twin-to-Twin Transfusion Syndrome Treatment
Fetoscopic laser ablation: This minimally invasive surgery is the treatment option that offers the best chance of two healthy survivors for pregnancies complicated by TTTS. The procedure involves the insertion of a small laparoscope (fetoscope) into the uterus. This allows the visual inspection of the placenta and enables the surgeon to identify the connecting vessels that are responsible for TTTS. The fetoscope has a second channel through which a laser fiber is inserted into the uterine cavity and the connecting vessels are coagulated until they are completely closed using laser energy. At the end of the procedure, the amniotic fluid in the recipient twin’s sac is drained to a near normal level and the mother is monitored and treated to prevent preterm labor. If there is evidence of cervical shortening prior to the surgery, doctors may perform another procedure to reinforce the cervix with a suture (cervical cerclage).
Conservative management: Continued observation of the pregnancy without a surgical intervention is an option for uncomplicated early-stage TTTS. Many women with stage I TTTS will not experience any worsening of symptoms. Such conservative management requires regular monitoring to evaluate risks of preterm labor, cervical shortening and/or a decline of health in one or both twins.