Explanation of Medical Terms
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
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
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.
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
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).
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.
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.
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
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).
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
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.
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.
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
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