ultrasound diagnosis of fatal anomalies

have fatal consequences, leading to the death of
one or both twins.
The chronic twin-to-twin transfusion syndrome,
if untreated, results in high morbidity
and mortality rates. The mortality can be as high
as 90%, and diagnosis and management of this
syndrome is therefore a challenge to modern
prenatal and pediatric medicine. The morbidity
associated with chronic TTTS is also high; onethird
of the affected children showneurological
damage at a later age.
The characteristic clinical features of chronic
TTTS are: 1, disparity in fetal size, the recipient
being larger than the donor; and 2, the amount
of amniotic fluid differs considerably between
the two amniotic cavities.
In the second half of pregnancy, fetal urine
production is mainly responsible for the formation
of amniotic fluid. In TTTS, the donor twin has
a lower blood volume and fluid load compared to
the recipient twin. Thus, there is a reduction or
complete absence of diuresis, leading to oligohydramnios
or in severe cases to anhydramnios. In
contrast to this, due to a higher volume load in
the recipient twin, there is increased diuresis and
thus development of hydramnios.
In addition, fetal hydrops develops in the recipient
twin due to cardiac overload, causing
ascites, skin edema, pleural effusion and pericardial
effusion. In contrast to this, it is often difficult
to detect the urinary bladder of the donor
twin, due to the absence of diuresis.
It is important to differentiate TTTS from
placental insufficiency in one of the twins. This
also results in disparity of fetal size; the affected
twin is smaller, and there may be oligohydramnios.
However, the larger twin, does not show
hydramnios, as long as other causative factors
for the development of it are absent.
To calculate the risk of developing TTTS, it is of
the utmost importance to knowwhether the
twin pregnancy is monochorionic or dichorionic.
TTTS is virtually unique to monochorionic
twin gestations and almost never occurs
in dichorionic gestations.
Early ultrasound examination in the first
trimester provides the best opportunity to identify
zygosity antenatally. This scan is included in
routine antenatal screening in Germany. At this
gestational age, the chorion is thick and surrounds
the amniotic cavity completely, while the
amnion is a very thin layer. The layer separating
the two cavities is easily recognizable in dichorionic
pregnancies, as it is at least 2–3 mm
thick. In monochorionic gestations, this layer is
detected as a very thin rim consisting of the two
amnion membranes. After the first trimester, it is
not easy to distinguish between the two forms of
pregnancy, as the chorionic layer degenerates
and does not appear as thick as in early pregnancy,
so that even in dichorionic gestations the
separation barely differs from that in monochorionic
twins. At this stage of pregnancy, a
search has to be made for the lambda sign (a
lambda-shaped connection between the fetal
membranes and placenta). However, the diagnosis
cannot be made with certainty.
Monochorionic gestation can be diagnosed
easily in the first part of the pregnancy; this is a
spot diagnosis and can be detected even by less
experienced sonographers if basic principles are
followed. This means that it is possible to assess
or exclude a risk of developing TTTS at an early
stage.
The prognosis in TTTS is mostly very poor, and
there are no standard treatment protocols. Premature
delivery is opted for if the gestational age
is adequate. After 28 weeks of gestation, this is
almost always the preferred choice of treatment.
For gestations below28 weeks, four different
therapeutic interventions are possible:
1. Repeated aspiration of amniotic fluid from
the recipient twin every fewdays, to relieve the
polyhydramnios. This reduces the risk of premature
labor and/or intrauterine death and also improves
placental perfusion by decreasing uterine
pressure. This procedure makes it possible to reduce
TTTS mortality by 50%. However, one-third
of the survivors will still suffer late neurological
damage.
2. Administration of digitalis to the mother
and thus indirectly through the placenta to the
fetus. The aim of this treatment is to prevent and
treat cardiac failure in the recipient twin, which
is struggling to cope with the volume overload.
3. As a last resort, selective fetocide of one
twin, usually the donor, has also been tried, to improve
the chances of survival of the second twin.
4. Endoscopic laser ablation of vascular shunts
is nowthe most important treatment option, if
available. In recent years, this method has been
used in the treatment of severe TTTS. Its main
disadvantage is the high rate of fetal demise, up
to 50%. However, recent studies show that if
successfully applied, late neurological complications
occur in only 5% of surviving infants.
It is possible to differentiate very early in
pregnancy between a high-risk monochorionic
pregnancy and low-risk dichorionic twin gestation.
Monochorionic pregnancies should be
monitored with serial ultrasound assessments
at short intervals.
DETERMINATION OF ZYGOSITY
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