AJR 2003; 181:1676-1678
© American Roentgen Ray Society
Prenatal MRI of Heteropagus Twins
Pei-Lin Chen1 and
Kyuran Ann Choe
1 Both authors: Department of Radiology, University of Cincinnati, 234 Goodman
St., Cincinnati, OH 45267-0761.
Received February 24, 2003;
accepted after revision May 28, 2003.
Address correspondence to P.-L. Chen.
Introduction
Conjoined twins are a rare occurrence without genetic predisposition. The
least common form is heteropagus twinning, in which one fetus is not
completely formed or developed
[1]. The prenatal diagnosis of
conjoined twins is usually suggested at prenatal sonography, which has the
advantage of offering a safe, accurate, and reliable method of detecting
anomalies of fetal growth and structure
[2,
3]. However, because of the
intrinsic limitation of sonography with regard to tissue contrast, MRI has
been explored as a safe alternative with superior image contrast
[4]. In the case of conjoined
twins, improved anatomic definition of the anomalies encountered may allow
improved obstetric planning and postnatal care.
We present a case of antenatal diagnosis of heteropagus twinning based on
prenatal MRI findings.
Case Report
The patient was an 18-year-old gravida 1, para 0 woman who received minimal
prenatal care: her first prenatal sonography was performed at 32 weeks'
gestation. Sonographic findings were suspicious for fetal anomalies. The
patient was then referred for prenatal MRI to better delineate the anatomic
features and establish the diagnosis.
Single-shot fast spin-echo sequences with a torso-array coil were performed
with a 1.5-T MRI scanner (Echospeed, General Electric Medical Systems,
Milwaukee, WI). A single anterior placenta with polyhydramnios was identified.
A single identifiable fetus was seen with an exophytic protuberance that was
joined anteriorly to the fetus from the level of the chin to the inferior
margin of the heart (Fig. 1A).
This exophytic protuberance contained multiple small cystic areas with no
identifiable solid organs. However, two lower extremities extended from the
protuberance (Figs. 1B and
1C).
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Fig. 1A. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. T2-weighted coronal
(A) and axial (B) images show exophytic tissue with mixed signal
intensity attached to anterior chest wall of fetus (arrow). Note
polydydraminos and maternal hydronephrosis bilaterally.
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Fig. 1B. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. T2-weighted coronal
(A) and axial (B) images show exophytic tissue with mixed signal
intensity attached to anterior chest wall of fetus (arrow). Note
polydydraminos and maternal hydronephrosis bilaterally.
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Fig. 1C. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. T2-weighted axial image
shows partially formed extremity anterior to anterior chest wall of fetus
(arrow).
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The primary anatomy of the fetus was further evaluated and showed ambiguous
cardiac situs with the descending thoracic aorta on the left of the fetus. A
normal-appearing liver and gallbladder were seen. The anterior abdominal wall
was slightly protuberant at the level of the liver. We saw no fluid-filled
stomach. A three-vessel umbilical cord was noted extending from the primary
fetus with a slightly prominent cord insertion site at the abdomen. The
attachment site of the partially formed conjoined twin in combination with
polyhydramnios and the absence of a fluid-filled stomach was suggestive of
upper alimentary tract compromise in the primary fetus, as well as additional
airway compromise.
At 35 weeks' gestation, the patient began active labor. Given the prenatal
diagnosis of conjoined twins, cesarean section was performed, in the presence
of obstetric, pediatric surgical, otolaryngologic, and neonatal intensive care
unit teams. A viable girl, twin A, was delivered without assigned Apgar score
or weight. Twin B was found to be an incomplete fetus attached at the level of
twin A's mandible, neck, and anterior chest with two lower extremities and
underdeveloped malformed upper extremities (Figs.
1D and
1E). Twin A was immediately
intubated before the cord was clamped. The neonates were then taken to the
neonatal intensive care unit for further resuscitation and surgical
separation.
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Fig. 1D. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. E, Photographs of
neonate A at birth show partially well-formed extremities of parasitic twin
joined at neck and upper thorax wall.
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Fig. 1E. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. Photographs of neonate A
at birth show partially well-formed extremities of parasitic twin joined at
neck and upper thorax wall.
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Contrast-enhanced CT of the neonates was performed for further anatomic
evaluation to assist surgical planning. CT showed a maldeveloped conjoined
twin attached at the right anterior chest wall and neck of the primary
neonate. The sternum and the associated subcutaneous tissues and musculature
were absent. The aortic arch was left-sided and the heart was midline. The
outward protuberance of the heart extended beyond the normal contour of the
anterior chest wall with no measurable covering of the heart. The parasitic
twin consisted of one nearly complete lower extremity connected to a mass of
mixed attenuation, including some cysts, some solid unrecognizable tissue, and
bones of partial extremities (Fig.
1F). Excision of the parasitic twin and reconstructive surgery was
performed at day 4 of life. The neonate died of cardiac and renal failure on
day 31. Complex congenital heart disease was found at autopsy.
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Fig. 1F. —18-year-old gravida 1, para 0 woman in whom previous
sonography revealed possibility of conjoined twins. Contrast-enhanced CT scan
shows well-ossified lower extremity (arrow) adhering to right
thoracic wall of neonate A. a = aortic arch, s = superior vena cava. (Courtesy
of Cincinnati Children's Hospital Medical Center, Cincinnati, OH)
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Discussion
Conjoined twins are a rare condition, with an estimated occurrence of
1:50,000 to 1:100,000 births
[2]. Most conjoined twins are
female, and most are born prematurely with an extremely high mortality rate.
Conjoined twins occur sporadically, and all pathologic events are related to
monozygosity with incomplete division of the embryonic disk 13 days after
fertilization [2,
3]. Conjoined twins may be
classified as symmetric or asymmetric. The asymmetric type, heteropagus, is
the rarest form and accounts for 1–2% of conjoined twins. Heteropagus
twinning is characterized by an incomplete and parasitic portion, usually
smaller than the autosite (the well-formed fetus). The parasite (the dependent
fetus) may be attached to or included in the autosite. The exact pathogenesis
of heteropagus twinning is uncertain and may be due to an ischemic event at
early gestation or incomplete cleavage of the inner cell mass of the
blastocyte [1].
The lack of ionizing radiation in both sonography and MRI make them ideal
prenatal imaging modalities. Because of its low cost, abundant availability,
and real-time imaging, sonography has been the method of choice for in utero
imaging and prenatal detection of fetal structural and growth anomalies
[4]. However, in late
pregnancy, especially in the presence of maternal obesity or oligohydramnios,
and because of limited sonographic ability to differentiate soft tissues, MRI
provides an excellent and accurate alternative technique. Also, MRI offers
superior tissue contrast and greater clinical information during the third
trimester of the pregnancy, when the fetus is larger and less mobile
[5].
The recent popularity of prenatal MRI has been attributed to the
development of ultrafast MRI techniques such as the single-shot fast spin-echo
sequence, in which high-resolution heavy T2-weighted images can be obtained in
2 sec
[6–8].
This ultrafast imaging method allows minimal image degradation by fetal motion
and high-quality visualization of fetal organs without the need for fetal or
maternal sedation. In addition, the larger field of view in MRI allows better
evaluation of the spatial relationships of anatomic anomalies or between large
lesions and the adjacent structures
[6]. There are no known or
reported adverse biologic effects caused by MRI during pregnancy; therefore,
it is considered a safe imaging modality in pregnancy
[7].
To our knowledge, this is the first reported case of antenatal MRI
diagnosis of heteropagus twinning. Despite the unfavorable outcome of the
neonates in our report, the prognosis for the autosite generally is good after
surgical excision of the parasitic portion and surgical correction of the
anomalies [4]. The antenatal
MRI diagnosis of heteropagus twinning may allow better prenatal care,
monitoring, and treatment. Given its superior tissue differentiation, larger
field of view and no known adverse biologic effect on the fetus, ultrafast MRI
will likely gain an important role in antenatal diagnosis of fetal anomalies
in the future, especially when sonographic findings of the fetus are ambiguous
or uncertain.
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Introduction
Case Report
