o_1977r8vv9vk1ts2ms0kd8pksa.pdf

320 E.P. Gaziano, U. F. Harkness
ing at < 28 weeks, a survival rate of only 21% was reported
for TTS and neither decompression amniocentesis
nor tocolysis altered this outcome [65].
Pathology of Monochorionic Placentation
When placentas from monochorionic twins are compared
with the presence or absence of TTS, placentas
from pregnancies with TTS had fewer anastomoses
overall, and fewer anastomoses for each specific type,
such as arterioarterial (AA), venovenous (VV) and
arteriovenous (AV) [66, 67]. In addition, anastomoses
in the TTS group tended to be indirect or AV, rather
than direct AA or VV when compared to monochorionic
twins without TTS. Hemodynamic models of
TTS in monochorionic twins support the pathologic
findings that fewer anastomoses effect greater discordance
between monochorionic twins and that the discordance
increases beyond fetal compensatory capacity
[68]. Color Doppler insonation of placental vasculature
in monochorionic twins also demonstrates an
absence of functional AA anastomoses in monochorionic
twins with TTS [69]. In assessment of clinical
outcomes, AV anastomoses in the absence of AA and
VV, especially if present with placental asymmetry,
carry the worse prognosis [67]. Unidirectional AV
flow in the absence of compensatory bidirectional
anastomoses results in adverse fetal effects and poor
outcomes for some monochorionic gestations.
The placenta of the donor twin may have a small,
inconspicuous vascular supply and may demonstrate
a large number of villi per unit volume of placenta,
whereas the recipient twin demonstrates an increase
in the volume of fetal capillaries [70]. Donor twins
may have a deficiency of maternally transferred immunoglobulin
G (IgG), indicating impaired intervillous
maternal-fetal transfer [71].
Placental Symmetry and TTS
Abnormalities of monochorionic placental symmetry
have received less attention than the anastomoses but
are of importance. Monochorionic twin placental
asymmetry has been variously described as unequal
sharing of venous return zones, unequal allocation of
parenchyma, and discordant vascular perfusion zones,
but a precise quantitative definition is lacking [6].
The morphogenesis of MC placental asymmetry is
also unknown, but the monochorionic monozygotic
(monochorionic MZ) twin blastocyst has an intrinsic
polarity defect at implantation [6]. The portions of
the monochorionic twin placenta in TTS are often
asymmetrical with the donor twin's placenta typically
the smaller [5, 67, 72]. The threshold for significant
asymmetry (e.g., 60 : 40, 70 : 30, 80:20, 90 : 10, etc.)
and clinical placental insufficiency in one twin may
vary in each case and depend on gestational age and
type and number of anastomotic vessels. In the absence
of anastomoses, unequal sharing is an important
cause of growth discordance in monochorionic
twins [73]. The vascular anastomoses place an monochorionic
twin with an adequate placental share at
risk for pathophysiologic processes of the twin with
placental insufficiency. Conversely, the anastomoses
may sustain a twin with a small share by supplementing
nutrients, which would otherwise be deficient. In
addition to quantitative differences, the asymmetric
monochorionic portions may differ qualitatively in
placental circulation relative to umbilical cord insertions,
chorion surface vessel pattern, and villous capillaries.
In monochorionic twin gestations, the relative
roles of reduced placental mass, abnormal cord insertions,
and intertwin vascular anastomoses are difficult
to ascertain and often occur together.
Umbilical Artery Doppler Velocimetry ±TTS
A summary of fetal Doppler evaluation of TTS is presented
in Table 20.3, including the method of assessment,
principal findings, and conclusions.
Umbilical artery velocimetry was first reported in
5 of 76 twin pregnancies with TTS [24]. Using CW
Doppler technology and a cutoff for A/B (S/D) ratio
difference of 1.57 (greater than the 75th percentile),
TTS pregnancies showed normal A/B ratios that were
concordant. Growth restriction was present in 12 of
18 monochorionic pairs, but an elevated A/B ratio
was present only in 7 pairs. The authors concluded
that the A/B ratios in TTS are normal and concordant,
and that differences in size on ultrasonography
in the presence of no S/D difference suggest TTS.
Another report described TTS in two twin pregnancies
± one at 20 weeks and one during labor [25].
The amniotic fluid volume assay indicated hydramnios/oligohydramnios
in the first case and a normal
state in the second case. Using CW Doppler technology,
the S/D ratio of the umbilical artery was abnormal
in the SGA donor twin and normal in the recipient,
and the S/D ratio difference was 1.9 (an S/D ratio
of > 0.4 was the cutoff for twin abnormality).
Serial Doppler and ultrasound examinations were
reported in a twin pregnancy with TTS [74]. The donor
twin showed abnormal velocimetry with cyclic
variations thought to be due to an arterial anastomosis.
This donor fetus demonstrated abnormal flow
(zero and reversal) and died in utero. The recipient
survived, demonstrating normal velocimetry.
Pretorius et al. [75] reported fetal Doppler ultrasound
findings in eight cases of TTS. Pulsed-wave
duplex Doppler sonography was employed with highresolution
ultrasound, and A/B ratios were determined
for the umbilical artery. Significant A/B ratio
differences were noted in seven of the eight cases,