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a Chapter 20 Doppler Velocimetry and Multiple Gestation 323
stolic flow in the donor umbilical artery; abnormal
pulsatility in the venous system of the recipient; and
absence of an arterioarterial anastomosis [80].
Unbalanced arteriovenous shunting is probably the
major pathophysiologic event in TTS, while the variety
of potential vascular arrangements does not allow
for uniformity of the fetal Doppler findings. In most
cases the donor twin becomes progressively hypovolemic
and sometimes, but not always, relatively anemic
[57, 58]. The donor twin is not infrequently oliguric
following delivery [76]. In the donor these
events most likely result in increased umbilical vascular
resistance, as reflected in an abnormal umbilical
artery waveform. Because of the hypovolemia, urine
output diminishes in the donor fetus, resulting in significant
oligohydramnios. Mari et al. [81] found that
the PI of the renal artery in the twin with oligohydramnios
was higher than that in the renal artery of
the twin with hydramnios. The hypervolemia in the
larger twin results in right-sided cardiac failure, abnormal
venous waveforms, and fetal hydrops. With
increased urine output and possibly decreased fetal
swallowing, hydramnios usually occurs in the recipient's
sac [82]. Occasionally, the shunt reverses with
high resistance, which is then reflected in the umbilical
circulation of the larger (recipient) twin, with the
smaller (donor) twin consequently developing hydrops.
We have observed spontaneous reversal of
these events, indicating the likely development of a
compensatory anastomosis. Other authors have reported
resolution of hydrops following the death of
one of the twins [83].
The rigid neonatal standards (e.g., hemoglobin
and birth-weight differences) for the diagnosis of
TTS seem no longer tenable. Any number of weight
and hemoglobin differences are possible (reflecting
varied hemodynamic arrangements) in monochorionic
twin pregnancies [84, 85].
Quintero et al. [86] proposed a staging system for
TTS which considers a sequence of events in progressive
TTS. A negative correlation was noted between
survival of at least one fetus and stage and TTS was defined
as polyhydramnios (maximum vertical pocket
> 8 cm) in the recipient and oligohydramnios (maximum
vertical pocket of < 2 cm) in the donor. All cases
also had a single placenta, absent twin-peaks sign, and
same-sex fetuses. The individual stages are as follows:
1. Stage I: polyhydramnios in the recipient, severe
oligohydramnios in donor but visible bladder in
the donor (BDT)
2. Stage II: polyhydramnios in the recipient, a stuck
donor, BDT not visible, diastolic flow present in
the umbilical artery and forward flow in the ductus
venosus
3. Stage III: polyhydramnios and oligohydramnios,
BDT not visible, critically abnormal Doppler (at
least one of absent or reverse end-diastolic flow in
the umbilical artery, reverse flow in the ductus venosus,
or pulsatile umbilical venous flow)
4. Stage IV: presence of ascites or frank hydrops
(fluid collection in two or more cavities) in either
donor or recipient
5. Stage V: demise of either fetus
In Quintero et al.'s report [86] on staging, a number
of patients were treated with laser or umbilical
cord ligation. Taylor et al. [87] applied this staging
methodology on a population treated with serial amnioreduction,
septostomy, and selective reduction
alone or in combination. They found no significant
influence of staging at presentation with survival in
their conservatively treated group. Survival was significantly
poorer where stage increased rather than
decreased. These authors concluded that the Quintero
et al. [86] staging system should be used with caution
for determining prognosis at the time of diagnosis,
but may be better suited for monitoring disease progression.
Treatment
Options to improve prognosis in TTS include serial
amnioreduction for hydramnios (Fig. 20.8), laser ablation
of anastomoses between twins, septostomy, and
selective feticide.
In a study of eight pregnancies with severe polyhydramnios
secondary to TTS, uterine artery mean
blood velocity, and volume of flow was significantly
increased after amnioreduction [88]. The RI and PI
in the same vessel were decreased after amnioreduction
compared with Doppler values prior to the procedure,
although the difference was not significant.
Four of the eight RI measurements were > 97.5th percentile
of published reference for twins before the
Fig. 20.8. Massive hydramnios in recipient sac of a fetus
with TTS