Diagnostic ultrasound ( PDFDrive )

CHAPTER 32 Multifetal Pregnancy 1127
A
B
FIG. 32.14 Twin Reversed Arterial Perfusion Sequence. (A) Acardiac twin at 16 weeks. There is marked soft tissue edema around the trunk
of the twin (arrowheads) and a club foot (arrows). Despite the absence of a heart or upper body, there is active movement of the fetus. See also
Video 32.8. (B) Same patient as A. Reversed low in the acardiac twin umbilical artery, with pulsatile arterial low entering at the umbilicus.
discordance. 84 TAPS occurs when there is suicient unequal
passage of red cells via the placental anastomoses such that one
twin becomes anemic (the donor) and one becomes polycythemic
(the recipient). he transfusion occurs extremely slowly via tiny
unidirectional AV anastomoses, over a long period of time. 33,72
TAPS occurs in approximately 5% of monochorionic twins, 85
although this rate is higher following laser treatment for TTTS
when all of the anastomoses have not been ablated. 86 Although
amniotic luid discordance is oten seen as part of the TAPS
sequence, the discordance is not as great as seen in TTTS. he
diagnosis of TAPS is made prenatally with Doppler interrogation
of the middle cerebral artery in each fetus. Doppler imaging of
the middle cerebral artery has been shown to be an accurate
noninvasive method for determining the presence of anemia in
utero, nearly replacing percutaneous umbilical blood sampling
in many centers. 87 Optimal technique is critical because incorrect
technique can lead to false-positive diagnoses of anemia. 88 he
anemic fetus will demonstrate elevated peak systolic velocity in
the middle cerebral artery (>1.5 multiples of the median), while
the polycythemic fetus demonstrates decreased velocities (<1.0
multiples of the median) 72,89 (see Chapter 41, Table 41.2). An
additional sonographic inding occasionally seen in TAPS is two
diferent placental echogenicities, with each region corresponding
to a diferent fetal circulation. 90,91 Because TAPS typically occurs
at a later gestational age than TTTS, the morbidity and mortality
rates are much lower than those for TTTS. 92 However, late-onset
TAPS can be associated with growth discordance and even
intrauterine demise, and assessments of middle cerebral artery
velocities are recommended every 2 weeks in the third trimester
in monochorionic twins. 72,90
Twin Reversed Arterial Perfusion Sequence
he twin reversed arterial perfusion (TRAP) sequence is a rare
condition that occurs when one twin has an absent or severely
malfunctioning heart and there is a large unbalanced AA
anastomosis within a monochorionic placenta. 72 Blood low is
directed from the umbilical artery of one twin (pump twin with
a normal heart) through the placenta and into the umbilical
artery in the second twin, thus reversing low in that umbilical
artery. Flow is then directed inferiorly down the iliac arteries to
the lower extremities of the second fetus and then out of the
umbilical vein in a reversed direction back to the placenta. VV
anastomoses allow that blood to then return to the pump twin.
Flow in the recipient twin is deoxygenated, and its circulation
exists only inferior to the diaphragm. he recipient is oten grossly
malformed above the diaphragm with an absent or malformed
heart, head, and upper extremities, but can be remarkably intact
inferior to the diaphragm, and has been termed an “acardiac
twin” or “acardiac monster.” his fetus retains the ability to move
independently, as the spinal cord and lower extremities can be
normally developed (Fig. 32.14, Video 32.8). he exact mechanism
that creates the TRAP process is unclear, but it may be related
to demise of the recipient twin with continued growth and
development in the absence of a beating heart from the reversed
circulation. Alternatively, the recipient fetus could be anomalous
from conception. he TRAP sequence can be missed at irst- or
early second-trimester sonography when it is mistaken for a
fetal demise because of the lack of cardiac activity. Color and
pulsed Doppler interrogation will identify reversal of low in the
umbilical arteries and vein of the acardiac twin (Fig. 32.15).
here is a high risk of IUFD of the donor twin related to high