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FIGURE 34-8. Ventriculomegaly at 30 weeks and
polymicrogyria. The ventricles are large. The shrunken
brain has fallen away from the skull, leaving a wide, CSF-filled
subarachnoid space (hydrocephalus ex vacuo). Note also the fine
nodularity of the surface of the brain (arrow), characteristic of
polymicrogyria.
or there may be failure of absorption (extraventricular
obstructive hydrocephalus, or communicating hydrocephalus). Less often, VM results from excess CSF secre-
tion with choroid plexus papillomas or follows cerebral
destruction and shrinkage (hydrocephalus ex vacuo) as
a result of diverse insults (Fig. 34-8).
Associated CNS and somatic malformations are
common and seen in about 70% to 83% of cases. Chromosomal abnormalities are more common with nonisolated (25%-36%) than isolated (3%-6%) VM and
include the trisomies and X-linked hydrocephalus in
males. D’Addario et al.37 reports that the main associations with VM are aqueductal stenosis (30%-40%),
Chiari II malformation with spina bifida (25%-30%),
Dandy-Walker complex (7%-10%), agenesis of the
corpus callosum, and other, less common conditions.
Ultrasound Examination
Ventricles
Determining the cause of VM can be difficult. Several
approaches to measuring ventricular size have been
described: atrial width, choroid separation, ventricle/
hemisphere ratio, combined anterior horn width, and
visual anatomic appearance. Of these, the universally
accepted method is the transverse measurement of
the atrium of the occipital horn (see Figs. 34-2, B; 34-9,
A; 34-10, B; and 34-11). The detection of ventricular
enlargement is the clue to the detection of most cerebral
Chapter 34 ■ The Fetal Brain 1207
ABNORMALITIES COMMONLY
ASSOCIATED WITH
VENTRICULOMEGALY
Obstructive hydrocephalus
Aqueductal stenosis (idiopathic, infections,
bleeding, X-linked, masses)
Spina bifida with Chiari II malformation
Excess cerebrospinal fluid (CSF) production
Choroid plexus cyst
Cerebral malformations
Agenesis of corpus callosum
Vermian dysgenesis and Dandy-Walker
malformation
Holoprosencephaly
Neuronal migration abnormalities
Microcephaly
Macrocephaly
Lissencephaly
Schizencephaly
Destructive processes (encephaloclasis)
Vascular insults, infections, porencephaly
Aneuploidy (trisomies 21, 18, 13)
Many syndromes
anomalies.2 The size of the head is not helpful in detecting
VM, and frequently the BPD remains normal even
with severe ventricular enlargement.
other ventricular dimensions are available but not in
common use.
46
45
Nomograms of
In the first trimester the choroid plexus should fill the
prominent-appearing lateral ventricle, except for the
conspicuous anterior frontal horn of the lateral ventricle,
which should not be mistaken for abnormality (see Fig.
34-1, B). VM manifests as a small-appearing choroid
plexus with excess surrounding fluid.
14
Atrial Width (Occipital Horn Width). This is the
most useful and accepted measurement of the ventricles.
The atrium of the lateral ventricles is the site of confluence of the bodies, occipital horns, and temporal horns.
This measurement is easily obtained during routine
obstetric scanning, but the measurement must be taken
on true axial views of the head, and calipers are placed
in the widest part of the ventricle, just touching inner
ventricle walls (see Figs. 34-2, B, and 34-11, A). Fortuitously, this is the part of the ventricle that undergoes
the earliest and most marked enlargement.
There are pitfalls to ventricular measurement. Errors
arise if the plane of view is not axial or if there is an
improper choice of ventricle boundary.
47
The insula, the
extreme capsule of the basal ganglia, the supraventricular
veins, and reverberation echoes of the proximal skull all
can appear as lines, which should not be mistaken for
ventricular walls (see Fig. 34-2, A). In the second trimes-
ter the normal white matter is so homogeneous that it
has been mistaken for abnormal intracranial fluid.
Usually, only the ventricular atrium farther from the
transducer is measured, because the near ventricle is
4
1208 PART IV ■ Obstetric Sonography
obscured by artifact created by the skull bone. The
ventricles are presumed to be symmetrical. It is possible,
however, to measure the near ventricle directly by
exploiting access provided by the squamosal and lambdoid sutures and the posterolateral (mastoid) fontanelles
and on 3-D multiplanar reconstructions
48
(Fig. 34-9).
Visualization of the near ventricle should be performed
whenever VM is suspected.
Between 14 and 38 menstrual weeks, the transverse
atrial measurement is reportedly constant at 7.6 mm
(standard deviation [SD] 0.6 mm).2 Measurements of
10 mm or larger suggest VM with a low false-positive
2,49
rate.
Generally, 10 to 12 mm is termed mild or bor-
derline; 12 to 15 is moderate (although some authors
consider up to 15 mm in the mild range); and greater than
15 mm is marked VM.41 Although 10.0 mm has been
considered the upper limit of normal, there are reports of
normal outcomes with ventricles larger than 10 mm,49
and some have suggested raising the upper limit of normal
to 11 or 12 mm. Ten millimeters is already about 4 SD
above the mean, and we agree with others that 10 mm
should remain the criterion above which counseling and
investigation should occur
4,38,49
(Fig. 34-10).
Mild ventricular asymmetry is common, with the left
being larger than the right. The 2-SD range is 2.4 mm.48
Differences of more than 3 mm should be viewed with
caution.50 Further assessment of symmetry is possible by
evaluating the anterior frontal horns in a coronal direction through the anterior fontanelle, either transabdominally or transvaginally (Fig. 34-9, C ). Some suggest that
male fetuses have larger measurements than female
fetuses, 6.8 (±1.3) mm versus 6.4 (±1.3) mm, respectively.
51,52
Others have found no difference.
53
Choroid Separation from Medial Ventricle
Wall. Mahoney et al.54 reported that the distance
between the medial atrial wall and the choroid is 1
to 2 mm in normal fetuses after 15 weeks’ gestation.
Measurements of 3 mm and greater were associated with
abnormal outcomes when combined with other fetal
abnormalities, even if the ventricle measurement is
normal. Hertzberg et al.55 found that 20% of such fetuses
had abnormal outcomes. However, many believe that
this approach is too sensitive and unnecessarily creates
anxiety in parents.
Anatomic Appearance. Qualitative appearances suggesting ventricular enlargement include convexity
(outward bulge) to the lateral wall of the lateral ventricle
and asymmetry of the left and right choroids, which
fall with gravity when unsupported by ventricular
walls as the choroids are denser than CSF (“droopy’’ or
“dangling’’ choroids) (Fig. 34-11, B; Video 34-5). With
massive ventricular enlargement, the interhemispheric
structures, particularly the septum pellucidum, undulate
and can become disrupted (fenestrate) allowing the left
and right lateral ventricles to communicate. The upper
choroid can fall across the midline through this hole and
into the lower ventricle (Fig. 34-11, C).
Ventriculomegaly
Once ventricular enlargement is suspected, attention
turns to etiology and associated abnormalities. Investigations include history, detailed ultrasound, karyotype,
search for infections (TORCH screen: toxoplasmosis,
other/syphilis, rubella, cytomegalovirus, herpes), search
for causes of bleeding (e.g., platelet antibodies), and MRI.
Associated brain abnormalities may be subtle and not
readily appreciated on standard axial scanning. Coronal
and sagittal views both transabdominally and transvaginally and use of 3-D multiplanar scans can be especially
helpful to detect abnormalities of the cerebellar vermis
and corpus callosum.
37
MRI adds additional information
in 5% to 50% of cases, especially with respect to parenchymal injury, migrational disorders, ischemia, hemorrhage, and brainstem abnormalities.
7,39,40,56-59
Prognosis. Prognosis relates to the degree of VM and
especially the presence of associated abnormalities. In
cases of VM associated with chromosomal or CNS and
somatic abnormalities, the prognosis and recurrence risk
relate to underlying conditions.
40
When VM is truly
isolated, neurodevelopmental outcome relates to the
degree of enlargement. Normal functional outcome is
reported in about 85-96% of mild (10-12 mm), 76% of
moderate (12-15 mm) and 28% of severe (>15 mm)
cases. Outcome is better if VM is stable or decreases, and
worse with enlarging ventricles.
behave similarly.
61,62
39,60,61
Unilateral cases
Counseling of parents remains difficult. Postpartum
abnormalities were found in 10% to 20% of fetuses
where VM appeared isolated and all prenatal testing
was negative. Long-term outcome remains guarded.
Laskin et al.60 reported that initially, 85% of children
had a favorable outcome, but this decreased to 79% by
20 months.
SPECIFIC ABNORMALITIES
Congenital CNS abnormalities often reflect the time of
insult in prenatal life rather than the specific cause. Etiologic classification is not as useful for counseling as are
specific malformation patterns. Many brain structures
form at the same time, and thus the timing of the insult
may affect final outcome more than its nature. Classification can be difficult. Even identical twins who sustain
the same insult to development may be born with differing phenotypes.
Increasingly, CNS abnormalities are being associated
with gene abnormalities at the molecular level. The
abnormal genes give rise to abnormal proteins that
prevent normal cortical development and neuron migration. Understanding the molecular mechanisms of
disease and maldevelopment will change the understanding of developmental disorders. Specific malformations
are starting to be categorized based on their underlying
63
Chapter 34 ■ The Fetal Brain 1209
LT
1
A
RT
1
C
X
X
B
C
FIGURE 34-9. Assessment of ventricles at 23 weeks shows normal, mild asymmetry. A, Axial view at left shows
the usually measured lower ventricle (calipers +). On the right image, by viewing through the posterior squamous (squamosal) suture, one
can visualize the upper ventricle in the oblique axial plane (calipers x). Oblique image planes can increase the ventricular measurement;
thus attempts should be made to obtain a true axial view for measurement if the upper ventricle appears enlarged. B, Coronal “owl’s eye”
view through the lambdoid suture shows asymmetry of the occipital horns (calipers).
and normal; c, cerebellum. C, Views taken through the anterior fontanelle analogous to neonatal head ultrasound. Left image shows the
occipital horns, and right image shows the anterior horns and confirms slight ventricular asymmetry.
Mild asymmetry less than 2 to 3 mm is common
1210 PART IV ■ Obstetric Sonography
10
v
5
A B C
FIGURE 34-10. Supraventricular marginal venous echoes at 26 menstrual weeks versus ventricular walls.
A, Transverse view above the level of the ventricles shows echoes from the marginal veins, which form a finely dotted line (arrowheads)
parallel to the interhemispheric fissure (long arrows). B, Transverse view at the ventricular plane shows mild ventricular enlargement, with
the occipital horn measuring over 10 mm (short white arrows) and the choroid separated from the medial wall of the occipital horn by
5 mm (5). The margin of the lateral ventricle (long white arrows) is curved and diverges from the midline, unlike the venous “line,” which
is straight and parallels the midline. C, Coronal view through the region of the thalamus. The lateral ventricle wall echo (white arrow) is
lateral to the supraventricular venous echo (black arrow), which goes from the top of the ventricle (v) to the surface of the hemisphere.
genetic and molecular abnormalities rather than on morphologic appearances.
63-65
Even if the genes are normal,
their functions can be variably disturbed by external
influences, such as anoxia, infections, and teratogens.
Again, the final outcomes will typically reflect the timing
of the insults rather than their specific nature.
40,63,66
To reach an accurate diagnosis and help with investigation, counseling, and treatment, the clinician should
evaluate not only the obvious changes, but also look for
additional, subtle findings. Clues can be found in pregnancy history, family history, course of current pregnancy (including maternal conditions such as diabetes),
medications, illnesses, occupations and exposures, and
evaluation of parents and close relatives. If there is a
known risk of specific conditions, online resources such
as Online Mendelian Inheritance in Man (OMIM,
http://www.ncbi.nlm.nih.gov/omim) and other sites
can be helpful in providing a list of findings to target
at ultrasound.
Errors of Dorsal Induction
Errors of induction and development of the dorsal neural
plate and canal (neurulation) result in defects of closure,
including anencephaly, encephaloceles, spinal dysraphism, and Chiari malformations.
Acrania, Anencephaly, Exencephaly
Acrania, or absence of the cranial vault bones of the
calvarium, is common to all these lesions. However, it
should be understood that acrania can occur with a
normal underlying brain, and therefore this term should
not be used when the appropriate diagnosis is anencephaly or exencephaly. Anencephaly occurs in about 1 in
1000 births and is characterized by the absence of the
cranial vault, cerebral hemispheres, and diencephalic
structures. These are replaced by a flattened, amorphous
vascular-neural mass (area cerebrovasculosa) (Fig. 34-12,
A). The amorphous mass may resemble brain structures
uncovered by bone (exencephaly), but in all cases, there
is absence of normally formed skin, cranial bones, and
brain superior to the orbits.
orbits are present. Associated spinal and non-CNS
abnormalities and polyhydramnios are common.
67,68
Facial structures and
69
On
occasion, the dysraphic abnormality involves the head
and entire spine (craniorachischisis). The outcome of
anencephaly is invariably fatal, and pregnancy termination is offered at any gestational age.
Detection of anencephaly prior to 14 weeks can be
difficult, although the diagnosis has been suggested as
1
early as
weeks. Before 10 weeks, apparently normal-
2
appearing brain structures are present. Unless the examiner specifically looks for ossified cranial bones, the
diagnosis can be missed.
70,71
Using transvaginal probes,
ultrasonically visible ossification of frontal bones may
not be apparent until 10 weeks, and anencephaly should
not be diagnosed before this gestational age. It has been
suggested that exencephaly may be an early phase of
anencephaly. In early pregnancy, the area cerebrovas-
culosa can be prominent and resemble brain structures
without overlying ossified cranium (exencephaly), sometimes forming an appearance resembling “Mickey Mouse
72
It is postulated that the brain becomes destroyed
ears.”
as pregnancy continues and assumes the characteristic
flattened, disrupted appearance of anencephaly.
68,71
An
Chapter 34 ■ The Fetal Brain 1211
C
C
12 mm
A
C
B
FIGURE 34-11. Ventriculomegaly. A, Mild ven-
triculomegaly of 12
in the CSF (black fluid) touching the inner surface of the
ventricle wall. There is prominent space (>
the choroid and ventricle wall. The cavum septi pellucidi
has been compressed but is still visible (arrow). B, Marked
ventriculomegaly at 27 weeks shows the convex margin
of the ventricle. The choroids (C) are drooping (toward
the dependent ventricle), and the cavum septi pellucidi
has fenestrated, allowing free communication between
the ventricles. C, Massive ventriculomegaly at 35 weeks
from aqueductal stenosis. The septal leaflets have fenestrated
(open arrow), allowing the upper choroid (arrowhead) to
fall across the midline. The cerebral cortex is greatly thinned
but present (small arrows), allowing differentiation from
hydranencephaly.
mm. Note placement of calipers (+)
3 mm) between
1212 PART IV ■ Obstetric Sonography
A B
F
C
additional clue to diagnosis is the identification of echogenic amniotic fluid caused by debris shed from the
uncovered brain or other unprotected fetal parts
73
(Fig.
34-12, B).
Chromosomal abnormality is seen in 2% of fetuses
with anencephaly and additional abnormalities seen in
4% to 8%.
74,75
Differential diagnosis includes other conditions in which the cranial bones are absent or lack
mineralization, such as amniotic band sequence, large
encephaloceles, osteogenesis imperfecta and hypophosphatasia (Fig. 34-4, D). With amniotic band
sequence (early amnion rupture sequence) the fetuses
generally have an asymmetrical defect accompanied by
body wall defects and/or amputation of body parts and
occasional oligohydramnios. Membranes may be visible
in amniotic fluid, or the fetus may be stuck to the side
of the uterus or placenta. Unlike anencephaly and open
spina bifida, early amnion rupture sequence is sporadic
and without increased recurrence risk.
69,76
Large encephaloceles generally have more calvarial development than
seen with anencephaly, but occasionally the two can
FIGURE 34-12. Anencephaly. A, Coronal view of fetus
at 14 menstrual weeks shows the spine ending in a clump
of basal skull bones without a formed cranial vault (arrow).
B, Anencephaly at 12 weeks. Note that there is an amorphous
mass of tissue above the face and orbits (F), resembling “Mickey
Mouse” ears (arrow). Also, the normally echo-free amniotic
fluid surrounding the fetus has become more echogenic than
chorionic fluid (arrowheads, amniotic sac surface). C, Amniotic
band sequence at 15 weeks mimics anencephaly, but this fetal
head and brain are stuck to the uterine wall by obvious bands
(arrow). Unlike anencephaly, this condition is sporadic and
unlikely to recur.
appear similar. In either case, the prognosis remains
hopeless.
Cephalocele and Encephalocele
A cephalocele is a herniation of intracranial structures
through a defect in the cranium. A cranial meningocele
contains only meninges and CSF, whereas when the
defect contains brain tissue, it is termed an encephalo-
cele. Most encephaloceles occur in the midline in the
occipital (75%) or frontal (13%) region, although some
are parietal (12%) (Fig. 34-13). Encephaloceles can
extend into the mouth, nasal, and sphenoid areas, where
their identification can be difficult.
77-81
They can occur
as isolated lesions or may be associated with other anomalies or syndromes, involving the head, spine, face, skeleton, or kidneys. Chromosomal abnormality is seen in
about 14% to 18%, especially trisomies 18 and 13.
75
Ultrasonographically, an encephalocele manifests as a
cystic mass at the surface of the skull, typically in the
midline. Brain tissue with a visible bony defect confirms
Chapter 34 ■ The Fetal Brain 1213
A
C
B
cb
D
FIGURE 34-13. Encephalocele. A, Transverse view through
occiput at 22 menstrual weeks shows a large midline encephalocele
with considerable brain tissue (arrow) herniating through the occipital bony skull defect. The head is microcephalic. B, Asymmetrical
parietal encephalocele (arrow). Coronal view at 21 menstrual
weeks. C, Anterior encephalocele (arrow) at 18 weeks herniated
through a defect between the orbits. D, Atretic encephalocele seen
as a small blister (curved arrow) in the scalp on midsagittal color
Doppler image. Typically, these do not contain brain tissue but are
associated with an abnormal falcine venous sinus of Markowski
(arrow), which courses from the cerebral vein to the superior sagittal
sinus. The vein of Galen can be absent; cb, cerebellum. E, Small
E
encephalocele (arrow) associated with severe ventriculomegaly on
MR image.
1214 PART IV ■ Obstetric Sonography
the diagnosis, but the bony defect may be small and
difficult to detect. Other lesions that should be considered in the differential diagnosis when a scalp lesion is
visualized include cystic hygroma, hemangioma, scalp
edema or cephalohematoma, epidermal scalp cyst,
branchial cleft cyst, dermoid cyst, dacryocystocele,
epignathus, and cervical teratoma.
80
A variant that is infrequently recognized prenatally is
the atretic encephalocele (Fig. 34-13, D), which usually
manifests as small, blisterlike subcutaneous collection in
the skin in the midline near the vertex, external to a
seemingly intact skull. Diagnostic clues are abnormalities
of the superior sagittal sinus, which can have multiple
channels, and the persistence of the prosencephalic vein
of Markowski, which runs in the falx (falcine sinus) from
the region of the start of the vein of Galen to the sagittal
sinus underlying the encephalocele. These children generally do well.
81,82
Meckel-Gruber syndrome is a rare, lethal autosomal
recessive condition characterized by encephalocele, cystic
renal dysplasia, and polydactyly.
83
The detection of
either cystic kidneys or an encephalocele should lead to
a search for the other components of this syndrome,
which has been detected as early as 14 weeks.
84
The prognosis of encephalocele depends on the location of the lesion, the amount of brain herniation, the
formation of the underlying brain, and associated
anomalies. Mortality is up to 44%, and in survivors,
intellectual impairment ranges from 40% to 91%. If
discovered before viability, pregnancy termination is
considered. Later in pregnancy, management depends
on the size and location of the encephalocele and associated anomalies.
85
Amniotic Band Sequence, Limb–Body
Wall Complex
This variable collection of disruptive abnormalities is
associated with anomalies of the amnion, often loosely
referred to as “amniotic band sequence.” Fetuses with
these conditions have variable and complex abnormalities, such as asymmetrical encephaloceles and facial disruptions. Three overlapping types of processes are
described: constrictive amniotic bands, amniotic adhesions, and limb–body wall complex. The literature on
the etiology of these sporadic conditions is confusing and
controversial regarding etiology and pathogenesis. There
is a common theme of combinations of amniotic abnormality and asymmetrical fetal disruptions. Constrictive
amniotic bands result in annular constrictions or amputations of limb parts. Amniotic adhesions result in
severe defects that are covered by amnion. These often
affect the craniofacial region and present as asymmetrical
encephaloceles and facial clefts. They may be the result
of disrupted fetal parts that are initially disrupted and
then secondarily adhere to the amnion. Limb–body wall
complex terminology is used if there is a more severe
malformation consisting of encephalocele with facial
clefts, thoraco/abdominoschisis, and limb defects and
generally associated with internal abnormalities that
cannot be explained by amniotic bands, such as congenital heart disease, renal agenesis, and intestinal atresias.
Some suggest that limb–body wall complex follows early
vascular disruptions, with secondary involvement of
amniotic membranes, or possibly represents general disordered morphogenesis.
86,87
The findings at ultrasound vary with affected areas.
With adhesions and limb–body wall complex, fetal
anatomy can be disrupted and almost unrecognizable.
This massive disruption is often the clue to the diagnosis,
because the associated amniotic bands and adhesions
may not be visible
88
(Fig. 34-12, C).
The incidence ranges from 1 in 1200 to 15,000
births, but is about five times more common in stillbirths. The major disruptions are lethal. 3-D ultrasound
and MRI can be helpful in clarifying the extent of the
lesions. Prognosis, counseling, and management depend
on the nature and degree of disruptions. Fortunately,
these anomalies are sporadic, with negligible recurrence
88,89
risk.
Cranial Changes in Spina Bifida
Spina bifida is classified as open or closed, depending on
whether the spinal lesion is skin covered (closed) or not
(open). Virtually all fetuses with open spina bifida have
cerebral changes typical of a Chiari II malformation
(Fig. 34-14; Video 34-6). However, those with closed,
skin-covered lesions can have normal-appearing heads
and brains (Fig. 34-15). The open spinal lesions are
more readily detected prenatally and account for about
80% of spina bifida. Maternal serum alpha-fetoprotein
(MS-AFP) is elevated in about 80% of fetuses with open
spina bifida. In contrast, most fetuses with closed lesions,
even if large, have no intracranial changes and normal
MS-AFP levels.
90
It is important to examine the spine
even if the head appears normal, to avoid missing an
obvious deformity of closed spina bifida.
The characteristic head changes of open spina bifida
include VM,
indentation)
(Chiari II malformation)
magna effacement,
tal ventricles,
91
the “lemon” sign (bifrontal scalloping or
91-93
(see Fig. 34-4, A), the “banana” sign
97,98
and BPD and trunk measurements that
94,95
96
“pointing” of the tips of the occipi-
(see Fig. 34-14), cisterna
are typically slightly small for gestational age. MRI often
shows additional brain abnormalities, including cerebral
hypoplasia, polymicrogyria, heterotopia, and callosal
abnormality.
99
Ventriculomegaly is common with open spina bifida
but usually occurs in later pregnancy. Babcook et al.91
found VM in 44% of fetuses under 24 weeks’ gestation
and in 94% after 24 weeks. It was often associated
with severe posterior fossa deformities. After delivery
and after repair of the skin lesion, virtually all infants
A B
Chapter 34 ■ The Fetal Brain 1215
C D
E F
FIGURE 34-14. Cranial findings of Chiari II malformation typical (98%) with open spina bifida. A, Lemon
sign with indentation of frontal bones. B, Banana sign of compressed cerebellum (arrows) viewed from the occipital direction. The cerebel-
lum is tightly compressed against the occipital bone and obliterates the cisterna magna. C, Axial view of head shows the pointed appearance
of the ventricles. D, Sagittal T2-weighted MR image shows inferior displacement of the cerebellum typical of the Chiari II malformation
(arrowhead) and inferiorly, the spinal defect (arrow). E, Axial MR image shows the pointed appearance of the ventricles. F, Coronal MR
image shows ventriculomegaly and obliteration of the cisterna magna.
1216 PART IV ■ Obstetric Sonography
21 w
FIGURE 34-15. Closed, skin-covered spina bifida at 21 weeks. The right image shows an obvious meningomyelocele
(arrow). On the left image, note the normal ventricles, cerebellum, and cisterna magna. Fetuses with closed, skin-covered spina bifida do
not have a Chiari II malformation and thus lack the typical intracranial signs. The MS-AFP is normal because the skin covers the defect.
FINDINGS ASSOCIATED WITH OPEN
SPINA BIFIDA
Chiari II malformation
Lemon sign
Ventriculomegaly
Banana sign: cerebellar tonsillar herniation into
foramen magnum
Effacement of cisterna magna
Slightly small fetal measurements
Spinal defects
Clubfoot (talipes)
Family history
Elevated maternal serum alpha-fetoprotein
(MS-AFP)
develop progressive enlargement of ventricles and head
(hydrocephalus) and require shunting.
The lemon sign or bifrontal indentation is seen in
89% to 98% of spina bifida fetuses under 24 weeks’
gestation but becomes less obvious later.
92,93
It can also
be seen in normal fetuses and others with diverse abnormalities, including encephalocele, Dandy-Walker malformation, and thanatophoric dysplasia.
100
The banana sign and effacement of the cisterna
magna are the result of fluid leakage from the open
spinal defect and hypoplasia of the posterior fossa (see
Fig. 34-14). This allows the cerebellum to be compressed
into the lower posterior fossa and where it conforms to
the remaining available space. The fluid in the cisterna
magna is displaced (cisterna magna effacement) by the
cerebellum. The cerebellar tonsils and vermis typically
herniate inferiorly through the foramen magnum. The
cerebellar hemispheres wrap around the lateral aspect of
the brainstem, and the cerebellum assumes a C shape,
the banana sign. These findings constitute the sonographic Chiari II malformation. On occasion, the cerebellum may be displaced so far inferiorly into the bony
base of the skull that it cannot be imaged with ultrasound. This absence is generally artifactual due to shadowing by bone. It is rare for the cerebellar compression
in Chiari II actually to result in cerebellar atrophy, but
it can occur and may be associated with neurologic disturbances.
101
Fluid leak as the cause of cerebellar changes
is supported by observations that after in utero repair of
the open spinal defect, the cerebellum may migrate
upward, and the cerebellum and posterior fossa may
assume a more normal appearance.
102
Remember, even large, closed, skin-covered spina
bifida cases typically have no cranial findings, so a
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