Brown University

Fetal Medicine @ Brown

The Warren Alpert Medical School of Brown University

Twin to Twin Transfusion Syndrome
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Explanation of Medical Terms

Absent end-diastolic flow: Normally, the blood flow through the umbilical artery (and in all other arteries) is forward; as the heart pumps the blood forward, the flow through the arteries is more rapid right after each heartbeat, and slows down a little in between heartbeats. In some disease states (such as severe TTTS), the blood flow in the umbilical artery comes to a stop in between heartbeats: there is absent flow at the end of diastole (the phase in between heartbeats, as opposed to systole, or heartbeat). In very severe cases, the blood flow in diastole is even reversed (blood flows backward, toward the heart): this is termed reversed end diastolic flow in the umbilical artery.

Amnioreduction: refers to the removal of excess amniotic fluid. Under ultrasound guidance, and after numbing of the mother's abdominal wall, a long, very fine needle is introduced into the uterus and into the amniotic cavity. While the fetus is being monitored with ultrasound, excess fluid from the amniotic cavity is aspirated. It is similar to amniocentesis, where a small amount of amniotic fluid is aspirated for diagnostic reasons. With amnioreduction, the purpose is to aspirate large amounts of fluid, in order to correct polyhydramnios.

Amniotic membrane: the fetus is surrounded by amniotic fluid, and both fetus and fluid are surrounded by a number of layers. Closest to the fetus is the amniotic membrane. The amniotic membrane itself is surrounded by the chorionic membrane, which is contained within the muscle layer of the uterus (the myometrium). In twin pregnancies, fetuses can share a common amniotic membrane (monoamniotic twins), or each fetus and its amniotic sac can be contained by a common chorionic membrane (monochorionic, diamniotic twins). In this case, they still share a common placenta. Non-identical twins have separate amniotic and chorionic membranes: they are dichorionic twins.

Amniotic space: in the womb, a fetus 'floats around' in amniotic fluid, which is contained within an amniotic sac, or amniotic membranes. This defines the fetus's amniotic space.

Anemia: condition where there are too few red blood cells in the blood. Because of that, the heart has to work faster, to be able to pump the same amount of red blood cells. This may lead to heart failure and hydrops.

Ascites: the presence of large amoounts of fluid in the abdomen. It can be due to a number of causes, depending on the type of fluid, associated conditions, etc. In TTTS, ascites is most often a sign of hydrops, as fluid accumulates outside the blood vessels. Other spaces where fluid can collect: around the heart (pericardial effusion), around the lungs (pleural effusion) and under the skin (edema).

Coagulation: closing a blood vessel through heat, such as with an electric current (electrocoagulation) or a laser. Using a laser in twin-to-twin transfusion syndrome allows reliable coagulation of the vessel only, without damage to surrounding tissues, all this within the amniotic fluid.

Doppler ultrasound: the Doppler phenomenon refers to the cange in wave length that occurs when a sound travels toward, or away from, the observer (think of the siren's pitch that changes when an ambulance drives past). The same occurs with ultrasounds, and a doppler allows one to measure the amount and direction of flow inside a blood vessel, by measuring the wave length reflected off the blood. In TTTS, "critical dopplers" refers to abnormal flow in the blood vessels or the heart: blood flow in the umbilical vein can be pulsatile, it can be absent in diastole in the umbilical artery; or blood can be seen to leak back through a heart valve (tricuspid regurgitation).

Edema: the accumulation of fluid under the skin, giving it a puffy appearance. In the fetus, it is often measured in the skin of the neck: the skin fold in the back of the neck (the nuchal fold) is then thickened.

Endoscope: a long, thin instrument that contains lenses, and can be introduced in a cavity. Through the lenses (and with adequate lighting through the instrument), one can look at the inside of that body cavity without the need for an incision. Endoscopes are used a lot in medicine, and can be utilized to look inside a joint (arthroscopy), the stomach (gastroscopy), the abdomen (laparoscopy) or the uterus (fetoscopy or amnioscopy).

Effusion: abnormal collection of fluid. a pericardial effusion (fluid around the heart) or pleural effusions (fluid around one or both lungs) can be signs of heart failure of the fetus (hydrops). Other signs of abnormal fluid accumulation are ascites and edema.

Hydrops: heart failure or fluid overload of the fetus, a condition that, if not rapidly corrected, will lead to fetal death. It is characterized by swelling (edema), thickened skin, fluid accumulation around the lungs and in the abdomen of the fetus.

Intrauterine growth retardation: is said of a fetus who does not grow as fast as he should, often because not enough nutrients or not enough blood (and oxygen) arrive to him, or because the fetus has to 'work' harder than usual. In twin-to-twin transfusion, it is often seen in the donor twin, who has to pump blood to himself and to the recipient twin. Intrauterine growth retardation can be seen by ultrasound, when the fetus appears smaller than expected for a given gestational age. At birth, this will lead to a baby who is 'small for gestational age.'

Monoamniotic twins: identical twins who not only share a common placenta, but a common amniotic space as well. This is a much rarer situation than identical twins who each have their own amniotic sac (so-called monochorionic, diamniotic twins).

Monochorionic twins: identical twins who share a common placenta. In addition, they can either have their own amniotic sac (they are then called monochorionic, diamniotic twins) or share a common amniotic sac (which is much less common, and is called a monochorionic, monamniotic twin pregnancy).

Nuchal fold: the skin fold in the back of the neck. Measuring its thickness can be used to decide whether there is edema, in which case that fold will be thicker.

Oligohydramnios: there should always be a sufficient amount of amniotic fluid around the fetus, to allow him to develop and move around. When there is too little fluid, it is called oligohydramnios. Anhydramnios refers to a situation where there is virtually no fluid left.

Polyhydramnios: too much amniotic fluid. Although the fetus can now freely move around, polyhydramnios can represent a danger because of an increased risk of premature rupture of the membranes (PPROM). It can also cause too much pressure on the umbilical cord. Polyhydramnios is often noted because the mother's abdomen is much larger than expected for the time of pregnancy.

Preterm Premature Rupture Of Membranes (PPROM): around the time of labor, the membranes rupture (or are ruptured by the obstetrician, if labor has to be induced). If membranes rupture before the fetus has reached term, it is called preterm premature rupture. This will often lead to the onset of labor, causing the baby to be born prematurely.

Pulsatile flow: blood flow in arteries is pulsatile: it is fastest with each heartbeat, and slows down a little in between beats. In contrast, blood flow in a vein is generally uniform: always at the same speed. In severe TTTS, the fetus may be so stressed that flow in the umbilical vein becomes pulsatile. This can be seen with a doppler ultrasound examination of the vein.

Randomized Controlled Trial (RCT): A clinical research trial set up to evaluate a new form of treatment or a new drug, by comparing it to an existing one (the control group). In this, the most scientifically correct form of research, patients agree to be "randomized," or assigned to one or the other form of treatment in a random fashion: neither the patient nor the treating physician can choose to which group the patient will belong. Thus, any possible bias is eliminated. In the case of Twin-to-twin Transfusion Syndrome, a randomized controlled trial is currently in progress. Our Fetal Treatment Center participates in the first such trial, sponsored by Eurofoetus

Tricuspid regurgitation: the tricuspid valve is the heart valve between the right atrium and the right ventricles (two of the heart chambers). Valves keep the blood flowing in one direction only (from the veins into the atrium, from the atrium into the ventricle, and from the ventricle into the arteries. If the heart is stressed and enlarged, the tricuscpid valve may be stretched too, and may not close properly anymore: blood can then be seen to leak backward, or regurgitate. This is best seen with doppler ultrasound.



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