Diagnostic ultrasound ( PDFDrive )

CHAPTER
35
The Fetal Spine
Elizabeth Asch and Eric Sauerbrei
SUMMARY OF KEY POINTS
• Neural tube defects (NTDs) are a common congenital
anomaly with clinically important associated morbidity and
mortality rates.
• Most open NTDs will have associated intracranial
abnormalities, most commonly Chiari II malformation,
detected by sonography.
• Although much less common, closed NTDs are less
likely to have associated intracranial abnormalities
and require careful examination of the spine
for detection.
• Other spinal abnormalities such as scoliosis, sacral
agenesis, caudal regression, sirenomelia, and
sacrococcygeal teratoma have distinct sonographic
appearances that aid in prenatal diagnosis.
CHAPTER OUTLINE
DEVELOPMENTAL ANATOMY
Embryology of the Spine
Ossiication of the Fetal Spine
Normal Position of the Spinal Cord
SCANNING TECHNIQUES
Three-Dimensional Ultrasound
SPINA BIFIDA
Folic Acid Fortiication
Pathogenesis and Pathology
Alpha-Fetoprotein and Ultrasound
Screening
Sonographic Findings in the Spine
Associated Cranial Abnormalities
Associated Noncranial Abnormalities
Prognosis
Fetal Surgery for Myelomeningocele
MYELOCYSTOCELE
DIASTEMATOMYELIA
SCOLIOSIS AND KYPHOSIS
SACRAL AGENESIS
CAUDAL REGRESSION
SIRENOMELIA
SACROCOCCYGEAL TERATOMA
PRESACRAL FETAL MASS
Abnormalities of the spine are some of the most common
congenital abnormalities. In the United States, overall
incidence of neural tube defects (NTDs) was approximately 1
to 2 in 1000 births 1 before 2000, but by 2007 it was less than 0.5
in 1000 pregnancies primarily as a result of the widespread use
of folic acid before conception 2,3 and the addition of folic acid
to enriched grain products. 4 Since the institution of folic acid
supplementation, an estimated 1300 cases of NTDs each year
have been prevented in the United States. 5 At least 42 nations
practice mandatory folic acid fortiication to combat NTDs. 6-10
NTDs are associated with substantial morbidity and mortality
rates. Many survivors have severe long-term morbidity that has
a profound emotional, physical, and inancial impact on their
families. Fortunately, the birth incidence of spina biida and
anencephaly is decreasing in many areas of the world as a result
of maternal screening programs (maternal serum tests and
antenatal ultrasound) and, more recently, the administration of
folic acid to women of child-bearing age.
In prenatal imaging, three-dimensional (3D) ultrasound and
fetal magnetic resonance imaging (MRI) are making a positive
impact, especially for precise localization of spina biida and
complete delineation of associated abnormalities. his precise
information is useful for prognosis and possibly for prenatal
surgery. Prenatal surgery for closure of myelomeningoceles is
practiced in only a few centers at this time.
DEVELOPMENTAL ANATOMY
Embryology of the Spine
he precursors of the spinal cord and surrounding spinal column
develop in the third and fourth weeks ater conception (ith
and sixth menstrual weeks). During the third conceptual week,
the bilaminar germ disc evolves into the trilaminar germ disc,
which consists of the ectoderm layer (part of the amniotic cavity),
the middle mesoderm layer, and the endoderm layer (part of
the yolk sac cavity) (Fig. 35.1A). he mesoderm layer develops
a midline central tube, the notochordal process, which runs
along the long axis of the embryonic disc. he mesoderm lateral
to the notochordal process has three components: paraxial
mesoderm, intermediate mesoderm, and lateral plate mesoderm.
By day 21 conceptual age, the hollow-tube notochordal process
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