Diagnostic ultrasound ( PDFDrive )

1226 PART IV Obstetric and Fetal Sonography
Pathogenesis and Pathology
Most cases of spina biida result from failure of closure of the
embryologic neural tube, although some may be caused by rupture
of the neural tube ater primary closure. Most NTDs occur as
isolated malformations in chromosomally normal individuals,
although 9% to 17% of fetuses with spina biida have chromosomal
abnormalities (mostly trisomy 18 and trisomy 13). 58,59 Typically,
chromosomally abnormal fetuses have other abnormalities
detected by sonography in addition to the spinal abnormality.
Some NTDs are part of a genetic condition. Autosomal dominant
conditions include Lehman syndrome. Autosomal recessive
conditions include Meckel-Gruber syndrome and VATER
syndrome (vertebral defect, imperforate anus, tracheoesophageal
istula, radial and renal dysplasia). Two X-linked conditions are
the Mathias laterality sequence and X-linked neural tube
defects. 2
A number of studies have found the incidence of NTD to be
about 10 times higher in spontaneously aborted pregnancies
than in term births, indicating an in utero selection against
embryos with such defects. 60
In the most severe form of NTD, the embryologic neural tube
(the precursor to the spinal cord) remains open in addition to
the overlying mesodermal structures, which include the neural
arch, muscles, and skin. he resultant pathology is myeloschisis;
the open, lattened spinal cord is exposed posteriorly through a
wide defect in the posterior neural arch and associated musculature
and skin.
In less severe cases of NTD, the major anatomic defect is in
the structures derived from the mesodermal tissues overlying
the embryologic neural tube. Although the spinal cord oten is
anatomically intact, the embryologic neural tube has failed to
induce closure of the overlying neural arches, muscles, and skin.
he result is a myelomeningocele, a cystic mass protruding
from the spinal canal. he cystic mass wall is composed of thin
arachnoid membrane without skin covering, and the contents
are cerebrospinal luid and neural elements. Occasionally, a
myelomeningocele is covered with skin. A skin-covered myelomeningocele
is considered a closed defect, and a myelomeningocele
without skin covering is considered an open defect. An
open defect allows AFP to escape into the surrounding amniotic
luid; a closed defect does not. hus a closed or skin-covered
defect is not usually associated with raised levels of AFP in the
amniotic luid or maternal serum. Infrequently, the protruding
cystic mass contains only cerebrospinal luid and no neural
elements, a meningocele.
Spina biida occulta is restricted to involvement of the
mesoderm of the posterior vertebral arch and rarely exhibits
intrinsic maldevelopment of the spinal cord. his may result
from an insult occurring at the end of the fourth embryologic
week (sixth menstrual week), causing failure of complete formation
of the posterior midline structures. he prevalence of spina
biida occulta, excluding cases that later disappear (i.e., delayed
ossiication of preexisting intact cartilage), is approximately 17%. 61
he lumbosacral spine is most oten involved. About 66% of
spina biida occulta cases have skin manifestations: nevi, lumbosacral
lipomas, dermal sinus, hypertrichosis (tut of hair, “horse’s
tail or fawn’s tail”), or scarred area. A sacral pit or dimple is not
highly correlated with spina biida occulta. Although infrequently
associated with other abnormalities, spina biida occulta may
be associated with urologic dysfunction and tethered cord
syndrome, foot deformity, increased incidence of spondylolisthesis,
and intervertebral disc herniation. Spina biida occulta is diicult
to detect with prenatal ultrasound unless it is associated with a
lipoma, a simple meningocele, or tethered cord. A history of
familial spina biida occulta is not known to be a risk factor for
an open NTD. 2
Alpha-Fetoprotein and
Ultrasound Screening
Because most NTDs occur in families with no history of such
abnormalities, prenatal detection relies on routine screening
measures, including ultrasound and MS-AFP measurement.
Alpha-fetoprotein is a glycoprotein (molecular weight 70,000)
produced by fetal liver. Some of it enters the amniotic luid
through fetal urine, and a small amount crosses the placenta to
maternal serum. Normal AFP levels in amniotic luid and maternal
serum vary with gestational age. MS-AFP and amniotic luid
AFP are elevated in NTDs that are not skin covered. If the upper
limit of normal MS-AFP is taken to be 2.5 multiples of the median
(MOM) for a given gestational age, MS-AFP will be elevated in
approximately 90% of open NTDs. About 2% of normal pregnancies
have an elevated MS-AFP; that is, of all the elevated test
results for MS-AFP, most fetuses will be normal (Fig. 35.12). At
this stage, a detailed ultrasound examination is required to
determine which fetuses actually have an NTD.
Causes of Elevated Maternal Serum
Alpha-Fetoprotein
Multifetal pregnancy
Fetal death
Fetomaternal transfusion
Omphalocele and gastroschisis
Congenital nephrosis
Esophageal or duodenal atresia
Polycystic kidney disease
Renal agenesis
Urinary obstruction
Epidermolysis bullosa
Sacrococcygeal teratoma
Cystic hygroma
Osteogenesis imperfect
Cloacal exstrophy
Cyclopia
Normal (2% of pregnancies)
Norem at al. 62 found that MS-AFP testing was normal in 25%
of NTDs (25 of 102 cases). hese included 15 of the 40 (38%)
spina biida cases screened, 6 of the 9 (67%) encephalocele cases
screened, and 4 of the 53 (8%) anencephaly cases screened. Of
the 186 NTD cases diagnosed prenatally, 115 (62%) were initially