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Chapter 35 ■ The Fetal Spine 1247
TABLE 35-1. SPINE EMBRYOLOGY DURING THIRD AND FOURTH WEEKS
AFTER CONCEPTION
MENSTRUAL AGE
(DAYS)
31 17 0.5 2 Trilaminar disc
35 21 2 4 Notochord
42 28 5 10 Neural tube
In the fetus the notochordal remnant corresponds to
the nucleus pulposus of the intervertebral discs.
Abnormalities of neural tube closure not only affect
the spinal cord and brain, but also interfere with normal
development of surrounding vertebral arches, which are
derived from adjacent mesodermal somites. Disturbances
of neural tube closure underlie spina bifida and anen-
cephaly defects.
Caudal regression defects may be related to defective
development of the mesoderm layer in week 3 conceptual age, during transformation of the germ disc from
two layers (bilaminar) to three layers (trilaminar). Various
degrees of abnormal mesoderm development account for
the wide spectrum of abnormalities found in caudal
regression.
A failure of part of the neural tube to close, called
spinal dysraphism, disrupts development of the nervous
system and disrupts the induction of the overlying vertebral arches. The resulting open vertebral canal is called
spina bifida. If dura and arachnoid protrude from the
spinal canal, the result is a meningocele. If neural tissue
and meninges protrude, the result is a myelomeningo-
cele. In the most severe NTDs, the neural tube fails to
form and fails to separate from the overlying ectoderm.
In the spine the condition is called rachischisis or
myeloschisis; the opened spinal cord is exposed along
the dorsal surface of the fetus. If the defect involves the
cranial neural tube, the brain is represented by an exposed
dorsal mass of undifferentiated neural tissue, called exen-
cephaly, anencephaly, or craniorachischisis. Differentiated brain and meninges may bulge from a nonossified
gap in the skull (meningoencephalocele), but this is not
related to failure of neural tube closure.
In animals, certain teratogens can induce NTDs: reti-
noic acid, insulin, and high plasma glucose levels. In
humans, implicated factors include valproic acid (antiepileptic), maternal diabetes, and hyperthermia. Valproic acid may interfere with folate metabolism.
CONCEPTUAL AGE
(DAYS)
EMBRYO LENGTH
(MM)
10
SAC DIAMETER
(MM) LANDMARKS
Notochordal process
Paraxial mesoderm
(Fig. 35-1, A)
Neural plate
Somites
(Fig. 35-1, B)
Notochord
Sclerotome
(Fig. 35-1, C)
Ossification of the Fetal Spine
Prenatal sonography readily portrays the ossified portions of the fetal spine, whereas the nonossified cartilage
is more difficult to delineate. It is therefore important
for sonographers and sonologists to understand the temporal and spatial ossification patterns during fetal development in order to optimize spinal evaluation.
Each vertebra will develop three ossification centers:
the centrum, right neural process, and left neural
process.
11
The centrum will form the central part of the
vertebral body, and the neural process will form the
posterolateral parts of the vertebral body and pedicles,
the transverse processes, the laminae, and the articular
processes.
Ossification begins in the lower thoracic fetal spine at
approximately 10 weeks’ gestation (menstrual age).
Ossification of the centra progresses in cranial and
caudal directions simultaneously. Neural arch ossification proceeds caudally from the lower thoracic (T) spine
to the lumbar (L) spine. It proceeds sequentially from
L1 through L5 and then into the sacral (S) spine. By 13
weeks’ menstrual age, there are three ossification centers
in vertebrae C1 through L3
13
(Fig. 35-2). Neural arch
ossification begins as a small focus at the base of the
transverse process and extends simultaneously into
the pedicle anteriorly and into the lamina posteriorly
(Fig. 35-3).
Ultrasound evaluation for spina bifida usually occurs
between 16 and 22 weeks’ gestation. By 16 weeks, there
is enough ossification in the neural arches to assess for
spina bifida to level L5,
14
by 19 weeks to level S1, and
by 22 weeks to level S2 (Figs. 35-4 and 35-5). In some
fetuses, there may be enough neural arch ossification to
assess for spina bifida before these gestational ages.
Braithwaite et al.15 assessed the fetal anatomy at 12 to 13
weeks’ gestation by a combination of transabdominal
and transvaginal sonography and reported successful
12
1248 PART IV ■ Obstetric Sonography
FIGURE 35-2. Spine ossification at 11
weeks + 4 days menstrual age. A, Ossifi-
cation of neural processes extends from C1 to L1
(arrow). The neural process ossification starts at the
base of the transverse process, which lies at the junction of the pedicle and the lamina. B, Spine ossi-
fication at 13 weeks’ menstrual age. Ossification
extends to level S1 neural process (S1 arrow) and
S3 vertebral body (S3 arrow). C, Spine ossification
at 15 weeks + 2 days’ menstrual age. Ossification
now extends into the laminae of the vertebral arches
in the thoracic and lumbar spine. Arrows demonstrate ossification of the laminae at levels L1 and L3.
Ossification of the laminae is negligible at S1.
L1
S3
S1
A B C
P
P
L
L
BA
FIGURE 35-3. Spine ossification on radiographs at 14 weeks’ gestation. A and B, Anteroposterior and lateral radio-
graphs. Well-developed ossification in the centra now extends down to level S3. Ossification in the lumbar neural processes extends into
the lamina (L) and pedicles (P). The neural process ossification starts to resemble the shape of the cartilaginous neural process rather than
the focal dotlike ossification at 13 weeks’ gestation.
Chapter 35 ■ The Fetal Spine 1249
S
S
T
T
A
FIGURE 35-4. Vertebral ossification of T9 at 16 weeks’ gestation. A, Specimen; B, radiograph. Ossification extends
quite far into the pedicles (P) and laminae (L). Note the early ossification in the base of the transverse processes (T). The width of the
vertebra is about 5 mm. Ossification within the centrum (C). S, Spinous process (cartilage); T, transverse process (cartilage).
L
P
C
B
L
P
C
5mm
L
L
P P
C
A
FIGURE 35-5. Vertebral ossification of L5 at 17 weeks’ gestation. A, Specimen; B, radiograph. There is usually enough
ossification at this gestation in the pedicles (P) and laminae (L) to determine the true course of these structures in radiographs and sonograms. The width of the vertebra is about 5 mm. C, Ossification within the centrum.
examination of the vertebrae and overlying skin in both
the transverse and the coronal plane in all cases. Others
have reported successful prenatal diagnosis of spina
bifida at 12 to 14 weeks’ gestation on the basis of abnormal cranial findings.
16-18
They caution that although the
characteristic cranial findings may be present at 11 to 14
weeks, the prevalence of these findings in the first trimester remains to be determined (Table 35-2).
B
Normal Position of the Spinal Cord
For fetuses at 19 to 33 weeks’ gestation, the conus
medullaris is normally situated at level L2-L3 or higher
(Fig 35-6). Level L3 is taken to be indeterminate and
L3-L4 or lower as abnormal.
tethered cord, the position of the conus medullaris is
usually lower than normal. For earlier pregnancy (13-18
C
19
For those fetuses with
1250 PART IV ■ Obstetric Sonography
weeks’ gestation), the conus medullaris may be normally
as low as L4. At term, the conus is normally above L2.
20
SCANNING TECHNIQUES
In clinical practice, the most useful scan planes to assess
the posterior neural arches are posterior transaxial (Fig.
35-7), lateral transaxial (Fig. 35-8), lateral longitudinal
(coronal) (Fig. 35-9), posterior longitudinal (sagittal)
(Fig. 35-10), and posterior angled transaxial (Fig. 35-11).
The posterior angled transaxial is useful to visualize
TABLE 35-2. TIMING AND PATTERN
OF FETAL SPINE OSSIFICATION
(10-22 WEEKS’ MENSTRUAL/
GESTATIONAL AGE)
AGE (WK) EVENTS
10 Ossification appears in lower thoracic spine
13 Some ossification is present from C1 to L5
13-22 Neural arch ossification simultaneously extends
16 Enough neural arch ossification appears to assess
19 Enough neural arch ossification appears to assess
22 Enough neural arch ossification appears to assess
vertebral bodies.
vertebral bodies and arches.
anteriorly into the pedicles and posteriorly into
the laminae.
for spina bifida to level L5.
for spina bifida to level S1.
for spina bifida to level S2.
the pedicles and laminae simultaneously. Because the
laminae course caudal to the transaxial plane, which
contains the centrum and pedicles, only the angled scan
plane can depict the pedicles and laminae simultaneously
in their entirety
The detection rate of spina bifida at 18 to 20 weeks’
gestation may be 80% or less during routine screening
ultrasound,
21
because the accuracy of ultrasound depends
on the skill and experience of the operator. The accuracy
of referral centers performing detailed targeted imaging
because of a suspected NTD or high maternal serum
alpha-fetoprotein (MS-AFP) is close to 100%.
A detailed sonogram of the fetal spine may be requested
for several reasons: previous suspicious ultrasound;
family history of NTD; and raised serum or amniotic
fluid AFP. To enhance detection of spina bifida, a
detailed protocol should be consistently followed. The
first step in assessing for spina bifida is scanning the head,
because most fetuses with spina bifida have signs of a
Chiari II malformation in the brain at 16 to 22 weeks’
gestation. These signs include obliterated cisterna magna
(banana sign), concave frontal bones (lemon sign), and
dilated lateral cerebral ventricles.
22,23
The sensitivity of
the banana sign for open spina bifida is close to 99%,
and false-positive diagnoses are rare, although the lemon
sign may occur in 1% to 2% of normal fetuses.
The next step is to determine the position of the fetal
spine. The scan plane is placed perpendicular to the long
axis of the fetal spine, either posterior transaxial or lateral
transaxial (see Figs. 35-7 and 35-8). The sonographer
should scan from one end of the spine to the other while
maintaining the scan plane perpendicular to the spine.
This is repeated several times. In the process, one builds
F
T
F
T
A
FIGURE 35-6. Normal spinal cord. A, Posterior longitudinal, and B, posterior transaxial (transverse), sonograms of a normal
spinal cord. Note the normal position of the cord (arrows) and filum terminale (T) in the dependent portion of the spinal canal. Cerebrospinal fluid (F) between the anterior aspect of the spinal cord and the anterior wall of the spinal canal.
B
Chapter 35 ■ The Fetal Spine 1251
A
L
L
C
L
C
L
B C D
FIGURE 35-7. Posterior transaxial scan plane. A, Diagram shows the incident sound beam (arrows) reflecting off the posterior
surfaces, clearly demonstrating the laminae and centrum, but not the pedicles. The red structures represent the ossified portions of the
vertebra. B, The L3 vertebra at 17 weeks’ gestation demonstrates the ossified laminae (L) and the ossified centrum (bottom arrow) but not
the pedicles. C, Scan of S1 vertebra at 17 weeks shows early ossification at the lamina-pedicle junction on each side (long thin arrows).
With this amount of ossification, determine the course of the laminae is not possible, and thus exclude spina bifida is difficult. C, Ossified
centrum; short arrows, iliac wing. D, Scan of T10 at 24 weeks’ gestation shows advanced ossification in the laminae (L, arrows) almost
reaching midline. Despite their advanced ossification, the pedicles are not visualized in this scan plane. C, Ossified centrum.
P
C
P
BA
FIGURE 35-8. Lateral transaxial (transverse) scan plane. A, Diagram shows the incident sound beam (arrows) reflecting
off the lateral surfaces of the centrum and the near pedicle and off the medial surface of the far pedicle, demonstrating the centrum and
pedicles, but not the laminae. The laminae course toward midline (thus sound beam is not perpendicular to laminar surface) and caudally
(thus out of the plane of sound beam). The red structures are ossified portions of the vertebra. B, Scan of vertebra L3 at 17 weeks’ gestation shows the ossified pedicles (P) and the ossified centrum (C), but not the laminae.
1252 PART IV ■ Obstetric Sonography
A
FIGURE 35-9. Lateral longitudinal scan plane. A, Diagram
shows the incident sound beam (arrows) reflecting off the lateral surface of
the near pedicle and the medial surface of the far pedicle. Therefore this
scan plane will show the cross section of the pedicles of each vertebra, but
not the centrum and laminae. The red structures are ossified portions of
the vertebra. B, Lateral longitudinal scan of the lumbar spine at 16 weeks
shows the ossified pedicles (small arrows). The lumbar pedicles usually
form a series of parallel echogenic foci, although they may normally diverge
by 1 to 2 mm. Note the faint echogenic structures between the pedicles;
these represent echoes from the centra that intercept the edge of the
insonating beam. (large arrow, iliac wing). C, When the tomographic scan
plane is thick or is placed closer to the centrum, the pedicles and centra
may be visualized simultaneously. The centra will appear as an extra set of
echogenic dots (arrows) between the series of pedicles. D, 3-D scan of a
19-week fetus shows the ossified spinal elements from the cervical area to
the lumbosacral level, as viewed from the posterior aspect of the fetus. The
12 ribs are visualized. L1 vertebra is immediately caudal to the 12th rib
level (arrows). (D courtesy Siemens Ultrasound.)
B
C
up an impression of the three-dimensional structures of
the spine. The scan plane should then be repositioned
parallel to the long axis of the fetal spine to obtain posterior longitudinal and lateral longitudinal views. The
sonographer then examines all levels of the spine in
posterior transaxial, lateral transaxial, lateral longitudinal, and posterior longitudinal scan planes. This may not
be possible in a short time because of fetal position, but
this usually changes enough in 30 to 45 minutes at 16
to 22 weeks to obtain all scan planes. If the spine cannot
be visualized optimally, a repeat scan can be performed
at a later gestational age.
Three-Dimensional Ultrasound
Three-dimensional ultrasound imaging has shown
promise in evaluating normal fetal structures and in
providing additional information in abnormalities of
many fetal structures including the spine, hand, foot,
and face.
24-33
Bony structures can be visualized with
12
12
D
maximum-intensity projection methods (see Fig. 35-9,
D). In evaluation of spinal abnormalities, 3-D ultra-
sound is most helpful in localizing spinal defects accurately by using simultaneous multiplanar imaging and
referencing to the volume-rendered image.
24,27
For determination of spinal level, T12 is taken to be the most
caudal vertebra with a corresponding rib.
SPINA BIFIDA
Spina bifida implies a physical defect in the structure
of the spinal canal that may result in a protrusion of
its contents (meninges, cerebrospinal fluid, and neural
tissue) (Table 35-3). These defects usually occur along
the dorsal midline (most often in the lumbosacral area)
but rarely may occur anteriorly.
Open NTDs occur in 0.5 to 2 per 1000 births
in North America and with higher frequencies in
other geographic areas. In one area of China, the overall
Chapter 35 ■ The Fetal Spine 1253
A
S
D
L5
B
FIGURE 35-10. Posterior longitudinal scan plane.
A, Diagram shows the incident sound beam (arrows) reflecting
off the posterior surface of the centrum. If there is no ossification
in the laminae near the midline, the laminae will not be visible
on the scan; only the centra will be seen. If the laminar ossification
is present near the midline, the centra and laminae will be seen as
echogenic foci. The red structures are ossified portions of the
vertebra. B, Posterior longitudinal scan of the lumbosacral spine
at 15 weeks shows ossification in the centra of the lower thoracic,
lumbar, and sacral spine (L5, centrum of vertebra). In this midline
scan, no ossification is present posterior to the posterior surface of
the dural sac (D). S, Skin surface.
A
B
FIGURE 35-11. Posterior angled transaxial (trans-
verse) scan plane. A, Diagram shows the incident sound
beam (arrows) reflecting off the posterior aspects of the laminae
and portions of the pedicles. The beam may also reflect off the
ossified centrum. This scan plane can depict the entire ring of
ossification of the spinal canal. The red structures are ossified
portions of the vertebra. B, Endovaginal scan at 18 weeks in the
midlumbar spine outlines the curvilinear structure of each neural
arch (short arrows, lamina plus pedicle) and the ossified centrum
(long arrow). Together these structures form the ossified ring of
the spinal canal.
PROTOCOL TO
EVALUATE SPINA BIFIDA WITH
3-D VOLUME DATA
Volume data are acquired from sagittal and
transverse sweeps through the spine.
Volume data are reformatted to display
standardized multiplanar views of the fetal spine.
3-D reconstruction of the fetal spine (with
maximum-intensity projection filter) visualizes the
ossified spinal elements.
To determine spinal level, the 12th thoracic (T12) is
taken to be the most caudal vertebra with a
corresponding rib.
prevalence of NTDs in 2003 was 13.9 per 1000 live
34
births.
In recent years, however, there has been a
decline in the incidence of NTD. Some of this decline
may be attributed to screening programs, which include
measurement of MS-AFP and performance of secondtrimester ultrasound.
35,36
Folic Acid Fortification
Another major factor in the decline of the incidence of
NTDs is the use of folic acid to prevent NTDs. Several
1254 PART IV ■ Obstetric Sonography
TABLE 35-3. DEFINITION OF TERMS FOR SPINAL ABNORMALITIES
TERM DEFINITION COMMENT
Spinal dysraphism (neural tube
defect, NTD)
Spina bifida Defect in posterior midline neural arch. Arches fail to fuse along dorsal midline and fail
Spina bifida occulta Vertebral arches of a single vertebra fail to fuse. Underlying neural tube differentiates normally;
Meningocele Dura and arachnoid protrude from vertebral canal
Myelomeningocele Dura, arachnoid, and neural tissue protrude from
Rachischisis (e.g., myeloschisis) Neural folds corresponding to future spinal cord fail
Cranioschisis (e.g., exencephaly,
anencephaly)
Inionschisis Failure of neural tube to differentiate properly and
clinical trials have demonstrated a decreased risk of NTD
by at least 60% with the use of periconceptual folic acid
supplements.
37-41
The reduction occurs in mothers with
previously affected pregnancies and in mothers without
this risk. In 1992 the U.S. Department of Health and
Human Services
United Kingdom
42
and the Expert Advisory Group in the
43
recommended supplementation of
0.4 mg of folic acid for women in the general population
while trying to conceive. Women who are at high risk
because of a previously affected fetus should take 4 mg of
folic acid daily.1 Routine folic acid supplementation
around the time of conception confers a 72% to 87%
decrease in probability of NTD.
edge and these recommendations have not translated into
a reduction of the incidence of NTD in the general popu-
44,45
lation,
largely because only a minority take folic acid
routinely in the reproductive years, and in those who do,
supplementation may not be taken at the proper time.
Studies in the 1990s demonstrated that less than 45% of
pregnant women took folic acid before conception.
2007, 40% of all U.S. women of reproductive age (15-45
years) took daily supplements of folic acid. Daily supplements could be one serving of breakfast cereal fortified
with 100% of the recommended daily value of folic acid
or a supplement with 0.4 mg of folic acid daily.
Another strategy to increase folic acid levels is the
systematic fortification of food stuffs with folic acid. In
March 1996 the U.S. Food and Drug Administration
(FDA) ordered that fortification with folate of all
enriched grain products be started no later than January
1, 1998 (0.14 mg per 100 grams of grain). Honein
et al.49 demonstrated a 19% reduction in NTDs in the
Failure of part of neural tube to close. This disrupts both differentiation of central
through spina bifida defect in posterior midline
neural arches
vertebral canal through spina bifida defect in
posterior midline neural arches.
to fuse and fail to differentiate (myeloschisis),
invaginate, and separate from surface ectoderm.
Neural folds corresponding to future brain fail to fuse
and fail to differentiate, invaginate (exencephaly,
anencephaly), and separate from surface ectoderm.
close in occipital and upper spinal region.
nervous system and induction of vertebral
arches.
to enclose vertebral canal.
does not protrude from vertebral canal.
The deformed underdeveloped spinal cord is
exposed dorsally.
This is the most severe form of spinal neural
tube defect.
The brain is represented by an exposed dorsal
mass of undifferentiated neural tissue.
United States as an effect of folic acid fortification of
grains. This study did not take into account the large
percentage of NTDs that are prenatally diagnosed and
electively terminated. A study in Nova Scotia demonstrated a decrease of annual incidence of NTDs by 54%
after implementation of folic acid fortification, from
2.58:1000 births from 1991 to 1997 to 1.17:1000 from
1998 to 2000.
50
This study included terminated pregnancies, which is important because more than 50% of
all pregnancies affected with NTD in Nova Scotia result
in elective termination. A failure to include these termi-
3,38
However, this knowl-
nated pregnancies may underestimate the benefit of folic
acid–fortified grains. In Canada, folic acid fortification
of grain products was legislated to begin in November
1998, at levels similar to U.S. levels. Since then in
Canada, the prevalence of NTDs nationally has decreased
from 1.58:1000 before fortification to 0.86:1000 births
during the full-fortification period, a 46% reduction.
46-48
In
Geographic differences almost disappeared after fortification began. The observed reduction rate was greater
for spina bifida (53%) than for anencephaly and encephalocele (38% and 31%, respectively).
Lipomyelomeningocele (LMMC) is a type of NTD
4
similar to myelomeningocele, with a prevalence of
about 0.5 per 10,000 births. However, studies in
Hawaii and Canada have shown that LMMC rates
are not affected by folic acid fortification, unlike the
significant rate reduction in myelomeningoceles.
LMMC seems to be pathogenetically distinct from
myelomengocele.
51,52
The risk of NTD rises to 20 to 30 per 1000 live births
for women with a previous infant with NTD. This
6
Chapter 35 ■ The Fetal Spine 1255
RISK FACTORS FOR NEURAL TUBE
DEFECT (NTD)
Folic acid deficiency
Previous sibling with NTD
Maternal anticonvulsants
Valproic acid
Carbamazepine
Maternal warfarin
Maternal vitamin A
Pregestational diabetes
Obesity
Parent with spina bifida
Low maternal vitamin B
constitutes about a tenfold increase in risk over the
general population.
12
53
A meta-analysis of randomized
trials of folic acid for the prevention of recurrent NTDs
demonstrated an 87% reduction in NTDs in women
who took supplements before the start of pregnancy.
Other factors that increase the risk of NTD include
anticonvulsant therapy with valproic acid or carbamazepine (10-20:1000), warfarin and vitamin A use, pregestational diabetes, obesity, parent with spina bifida
(11:1000), and sibling of fetus with multiple vertebral
defects and scoliosis (15-30:1000).
vitamin B
status may also be a risk factor for NTDs.
12
54
Low maternal
In Ontario, Ray et al.55 demonstrated a tripling of the
risk for NTD in the presence of low maternal B
status,
12
as measured by serum holotranscobalamin at 15 to 20
weeks’ gestation.
Pathogenesis and Pathology
Most cases of spina bifida result from failure of closure
of the embryologic neural tube, although some may be
caused by rupture of the neural tube after primary
closure. Most NTDs occur as isolated malformations in
chromosomally normal individuals, although 9% to
17% of fetuses with spina bifida have chromosomal
abnormalities (mostly trisomy 18 and trisomy 13).
56,57
Typically, chromosomally abnormal fetuses have other
sonographic abnormalities detected 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 syn-
drome (vertebral defect, imperforate anus, tracheoesophageal fistula, 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.
58
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. The resultant
pathology is myeloschisis; the open, flattened 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 often is anatomically intact, the embryologic
neural tube has failed to induce closure of the overlying
neural arches, muscles, and skin. The result is a myelo-
meningocele, a cystic mass protruding from the spinal
canal. The cystic mass wall is composed of thin arachnoid membrane without skin covering, and the contents
are cerebrospinal fluid (CSF) 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
3
to escape into the surrounding amniotic fluid; a closed
defect does not. Thus, a closed or skin-covered defect is
not usually associated with raised levels of AFP in the
amniotic fluid or maternal serum. Infrequently, the protruding cystic mass contains only CSF and no neural
elements, a meningocele.
Spina bifida occulta is restricted to involvement of
the mesoderm of the posterior vertebral arch and rarely
exhibits intrinsic maldevelopment of the spinal cord.
This 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. The prevalence of spina bifida
occulta, excluding cases that later disappear (i.e., delayed
ossification of preexisting intact cartilage), is approximately 17%.
59
The lumbosacral spine is most often
involved. About 66% of spina bifida occulta cases have
skin manifestations: nevi, lumbosacral lipomas, dermal
sinus, hypertrichosis (tuft of hair, “horse’s tail or fawn’s
tail”), or scarred area. A sacral pit or dimple is not highly
correlated with spina bifida occulta. Although infrequently associated with other abnormalities, spina bifida
occulta may be associated with urologic dysfunction and
tethered cord syndrome, foot deformity, increased incidence of spondylolisthesis, and intervertebral disc herniation. Spina bifida occulta is difficult to detect with
prenatal ultrasound unless it is associated with a lipoma,
a simple meningocele, or tethered cord. A history of
familial spina bifida occulta is not known to be a risk
factor for an open neural tube defect.
2
Alpha-Fetoprotein and
Ultrasound Screening
Because most NTDs occur in families with no history of
such abnormalities, prenatal detection relies on routine
1256 PART IV ■ Obstetric Sonography
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 fluid through fetal urine, and a small amount
crosses the placenta to maternal serum. Normal AFP
levels in amniotic fluid and maternal serum vary with
gestational age. MS-AFP and amniotic fluid 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.
Norem at al.
60
found that MS-AFP testing was normal
in 25% of NTDs (25 of 102 cases). These included 15
of the 40 (38%) spina bifida 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 detected by
routine sonography during the second trimester without
knowledge of MS-AFP values. Sixty-nine (37%) were
diagnosed by targeted sonography after MS-AFP screening indicated a higher risk for NTD. Two (1%) were
diagnosed by pathology examination after miscarriage.
Maternal serum AFP is also elevated in multifetal
pregnancy, fetal death, fetomaternal transfusion, and
in other fetal anomalies associated with a defect in the
skin, such as omphalocele and gastroschisis (50%-60%
of cases), congenital nephrosis (Finnish type, 100%
of cases), and infrequently in esophageal or duodenal
atresia, polycystic kidney disease, renal agenesis, urinary obstruction, epidermolysis bullosa, sacrococcygeal
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 imperfecta
Cloacal exstrophy
Cyclopia
Normal (2% of pregnancies)
teratoma, cystic hygroma, osteogenesis imperfecta,
cloacal exstrophy, and cyclopia.
Because of the high sensitivity of the cerebellar signs
associated with open spina bifida, some centers rely
almost exclusively on ultrasound to diagnose NTDs. For
women with elevated MS-AFP and no sonographic
explanation for the abnormal test result (e.g., wrong
dates, multiple fetuses, dead fetus, anencephaly, spina
bifida, abdominal wall defect, other fetal abnormality
causing elevated AFP), or when there is poor visualization of the spine, amniocentesis may be offered. If the
amniotic fluid AFP is normal and there is no acetylcho-
linesterase (AChE) present, the likelihood of an open
NTD is very low. If the amniotic fluid AFP is elevated
and AChE is present, an open NTD or abdominal wall
defect may be present but undetected by sonography.
Between 1989 and 1990, 1.1 million women in California had MS-AFP tests in early pregnancy.
61
From
these tests, 1390 fetal abnormalities were found (1.3 fetal
anomalies per 1000 pregnancies), consisting of 710
NTDs (417 cases of anencephaly, 247 cases of spina
bifida, and 46 cases of encephalocele) and 680 nonneural
abnormalities (286 anterior abdominal wall defects,
163 cases of trisomy 21, and 231 other chromosomal
abnormalities).
Sonographic Findings in the Spine
Spina bifida may occur anywhere in the fetal spine but
is most common in the lumbosacral area.
62
Ultrasound
findings in the spine consist of abnormalities of the ossified posterior elements and related soft tissues.
In spina bifida the laminae fail to converge toward
midline, and this is best visualized with the posterior
transaxial scan plane (Fig. 35-13, A and B; Video 35-1).
If the pedicles are normally positioned and there is no
myelomeningocele, the posterior transaxial scan plane is
the only view that will depict the abnormality with reliability. When the pedicles are displaced more laterally
than usual, the lateral transaxial and lateral longitudinal
scan planes will also demonstrate the bony abnormalities
of spina bifida (Fig. 35-13, C and D). All these scan
planes will usually demonstrate the meningocele or
myelomeningocele if it is present (Figs. 35-14, 35-15,
and 35-16). The posterior longitudinal scan best dem-
onstrates a myelomeningocele and the soft tissue defect
when no cystic mass is present.
In most cases of spina bifida, there is abnormal divergence or splaying of the pedicles over several vertebral
levels. This is best appreciated in 3-D images and in
lateral longitudinal views, where multiple interpedicular
distances can be evaluated simultaneously (Fig. 35-13).
However, there is normally mild divergence of the pedicles in the cervical spine compared to the thoracic spine
(Fig. 35-14), and there may be slight divergence (by 1
to 2 mm) in the lumbar spine compared with the thoracic spine in normal fetuses (see Fig. 35-2).
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