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CHAPTER 34
The Fetal Brain
Ants Toi and Deborah Levine
Chapter Outline
DEVELOPMENTAL ANATOMY
Embryology
Sonographic Anatomy
Variants (Usually Normal)
Choroid Plexus Cysts
Blake’s Pouch Cyst
Cavum Veli Interpositi
VENTRICULOMEGALY AND
HYDROCEPHALUS
Pathogenesis
Ultrasound Examination
Ventricles
Ventriculomegaly
SPECIFIC ABNORMALITIES
Errors of Dorsal Induction
Acrania, Anencephaly, Exencephaly
Cephalocele and Encephalocele
Amniotic Band Sequence, Limb–Body
Wall Complex
Cranial Changes in Spina Bifida
Errors of Ventral Induction:
Holoprosencephaly
Posterior Fossa and Cerebellum
Dandy-Walker Malformation
Vermis Hypoplasia or Dysplasia
Rhombencephalosynapsis
Mega–Cisterna Magna
Other Posterior Fossa Abnormalities
Arachnoid Cysts
Malformations of Cortical
Development
Microcephaly
Macrocephaly and Megalencephaly
Hemimegalencephaly
Anomalies of the central nervous system (CNS) are
the most common cause of referral for prenatal diagnosis
and result in great anxiety for parents. CNS anomalies
occur with a frequency of about 1.4 to 1.6 per 1000 live
births but are seen in about 3% to 6% of stillbirths.
increased use of maternal serum alpha-fetoprotein (MSAFP) screening has resulted in increased numbers of
pregnancies being referred for evaluation of the CNS and
suspected anomalies. Fortunately, protocol-based ultrasound carefully performed by a knowledgeable and experienced examiner following established guidelines is very
sensitive in evaluating the CNS.
2-4
Routine scanning is
currently recommended at 18 to 20 weeks of gestation.
Although many cerebral anomalies are detectable in the
first semester and early in the second trimester, others
develop or only become apparent later in pregnancy.
Magnetic resonance imaging (MRI) is increasingly
used to supplement ultrasound evaluation. Currently, in
vivo MRI has less spatial resolution but higher contrast
resolution than ultrasound. MRI is multiplanar and can
evaluate many tissue properties beyond morphology
using techniques such as diffusion weighted imaging
(DWI), diffusion tensor imaging, and magnetic resonance spectroscopy (MRS).
6-8
This provides new insights
into ischemia, tumor characteristics, bleeding, and brain
metabolism and allows unprecedented clarification of
suspected disorders. There is debate regarding the role
1
The
5
Lissencephaly
Focal Cortical Changes
Other Malformations of Cortical
Development
Agenesis/Dysgenesis of Corpus
Callosum
Absence of Septi Pellucidi and
Septo-Optic Dysplasia
Intracranial Calcifications
Infections
Vascular Malformations
Thrombosis of Dural Sinuses
Hemorrhagic Lesions
Hydranencephaly
Tumors
CONCLUSION
of ultrasound versus MRI in evaluating the fetal CNS.9
We believe that ultrasound will continue to be the initial
screening modality and that MRI will increasingly be
used to clarify findings. The important issues for the
examiner are familiarity with the strengths and limitations of these imaging modalities, expertise in their use,
and collaboration with other specialties.
10,11
DEVELOPMENTAL ANATOMY
Embryology
Knowledge of fetal gestational age is particularly important when evaluating anatomy in early pregnancy. In
this chapter we use menstrual age and gestational age
to mean “age from last menses,” as typically used clinically and with ultrasound studies. We convert published
ages to menstrual age by adding 2 weeks to the conceptual age.
Central nervous system development starts at about
the fifth menstrual week, when cells destined to form the
notochord infiltrate into the embryonic disc. This notochord tissue induces overlying embryonic tissue to
thicken and ultimately fold over and fuse as the neural
tube. The fusion starts in the midtrunk of the embryo
and subsequently extends to the cranial and caudal ends
1197
1198 PART IV ■ Obstetric Sonography
TABLE 34-1. DIFFERENTIATION OF
BRAIN REGIONS FROM PRIMARY
VESICLES
PRIMARY
VESICLE
Forebrain Telencephalon Cerebral hemispheres
Midbrain Mesencephalon Midbrain
Hindbrain Metencephalon Pons
Modified from Moore KL. Essentials of human embryology. Toronto, 1988, BC
Decker.
SECONDARY
VESICLE
Diencephalon Thalamus
Myelencephalon Medulla
MATURE
STRUCTURE
Basal ganglia
Olfactory system
Hypothalamus
Cerebellum
(Table 34-1). The anterior end, the rostral neuropore,
1
5
closes by about
closes about
end enlarges and flexes to become the brain.
menstrual weeks, and the caudal end
2
1
week later. By the sixth week, the cephalic
2
12,13
By 12
to 15 menstrual weeks, almost all structures are in their
final form. Exceptions are the corpus callosum, cerebel-
lar vermis, neuronal migration from the periventricular germinal matrix, development of the sulci and gyri,
and myelination. These latter structures and processes
start developing from about 15 weeks onward. The
corpus callosum is formed by 20 weeks. As it develops,
the corpus callosum induces the formation of the two
septi pellucidi and the intervening space, which is the
cavum septi pellucidi and cavum vergae (after Andrea
Verga in 1851).
The cerebellum and vermis develop as proliferations
into the cephalic part of a thin dorsal membrane (area
membranacea) that forms the dorsal aspect of the
rhombencephalic neural tube. The enclosed part of the
hindbrain neural tube is the rhombencephalic cavity.
This cavity enlarges rapidly in early pregnancy, forming
a conspicuous dorsal cystlike space that should not be
mistaken for abnormality
hemispheres grow into this membrane from the sides,
and the vermis arises from its cephalic aspect.
14
(Fig. 34-1, A). The cerebellar
15
The
lower part of the rhombencephalic membrane below the
vermis eventually fenestrates to form the foramina of
Magendie and Luschka. This membranous part can
bulge to a variable extent, forming Blake’s pouch. With
high-resolution equipment, Blake’s pouch can be seen in
most fetuses, where it is often mistaken for arachnoid
strands.
15-17
The cerebellum and vermis are essentially
formed by 22 weeks. Care must be taken to avoid mistaking the incompletely developed vermis for vermian
dysplasia/hypoplasia.
18
Midsagittal views with ultrasound, especially 3D midsagittal scans, and MRI can
show the normal development of the vermis, the fourth
ventricle with pointed fastigial point (dorsal pointed
apex of fourth ventricle), and the vermian fissures, as well
as overall size of vermis and the normal brainstem-vermis
angle of less than 10 degrees. All these elements are used
to evaluate normal vermian development.
19,20
The cortex also undergoes complex development at
the neuronal cellular level. The cells that will become the
brain cells (neurons) at the outer surface of the cortex
undergo complex development in three overlapping
phases: proliferation, migration, and organization. In
general, neuron development starts at about 5 weeks and
is largely finished by 28 weeks. Neurons derive and proliferate from stem cells located in the germinal matrix by
the ependyma-lined ventricles. These stem cells proliferate and differentiate into glial cells and neurons. The glial
cells send processes to the cortical surface, creating a scaffold along which the neurons then migrate to the cortex.
To accommodate the accumulating neurons, the cortex
undergoes folding into gyri and sulci. Failure of normal
migration results in heterotopia (collections of neurons
in abnormal locations) and abnormal or absent cortical
convolutions (pachygyria or type 1 lissencephaly). A
normally functioning outermost layer of the cortex serves
to stop neuron migration and prevents overmigration of
neurons into the meninges. Failure of this stopping function results in neurons migrating beyond the normal
limits of the cortex into the meninges and subarachnoid
space. This gives the brain surface a finely granular texture
called cobblestone lissencephaly. Once the neurons
arrive at the cortex, they organize local connections and
send axons remotely, thereby forming large tracts or
commissures such as the corpus callosum to connect the
hemispheres. All these elements require the normal function of many genes working together, and the process is
easily disrupted by intrinsic and extrinsic insults, such as
fetal and maternal metabolic disorders, hypoxia, infections, and teratogens.
21
Sonographic Anatomy
The early embryo is best examined transvaginally. The
cephalic end is identifiable by about 8 weeks (see Fig.
34-1). By 10 to 11 weeks, bones of the vault show
minera lization. At this age, the brain mantle is very thin.
The ventricles are large and filled with choroid, which
provides nourishment for the developing brain.
22
A
large, echo-free space behind the hindbrain represents
the rhombencephalic cavity, which decreases in size as
the cerebellum begins to form. This normal echo-free
space appears especially large and prominent in firsttrimester scanning and should not be mistaken for
abnormality.
14
After about 13 to 14 weeks, most of the cerebral
structures can be identified ultrasonographically. Three
standard transaxial planes or views (thalamic, ventricular, cerebellar) can lead to the detection of more than
95% of sonographically detectable cerebral anomalies.
2,4
These three views form a useful starting point, but the
examination should not be limited to these views alone.
The entire brain should be examined, using whatever
Chapter 34 ■ The Fetal Brain 1199
*
C
C
A B
FIGURE 34-1. Early normal fetal head images obtained with transvaginal probe. A, At 9 menstrual weeks the
head is clearly differentiated from the trunk and limb buds. The intracranial cystic structure is the fetal rhombencephalic cavity (arrow),
a normal space that eventually becomes the fourth ventricle. B, Scan at
arrow). The choroid plexuses (C) are very large and fill the ventricles (*) from side to side. Ossification is already visible in the skull bones.
projections are needed to show all the structures (Fig.
34-2; Videos 34-1, 34-2, and 34-3). The transvaginal
approach can be employed when the head is deep in the
pelvis and allows a better view of the brain.
4,23
The thalamic view used to measure the biparietal and
occipitofrontal diameters (BPD and OFD) (see Fig.
34-2). It displays the thalamus, third ventricle, forni-
ces, basal ganglia, insula, and ambient cistern. The
ventricular view is slightly higher than the thalamic
view and shows the bodies and, more importantly, the
atrium of the lateral ventricle as well as the interhemi-
spheric fissure. The atrium of the lateral ventricle is at
1
12
weeks. Note that the cerebral cortex is very thin (at tip of
2
CRANIAL STRUCTURES TO NOTE AT
ROUTINE ANATOMIC SCAN
Measurement of biparietal diameter and head
circumference
Head shape
Bone density
Ventricle size and appearance
Cavum septi pellucidi
Thalamus
Cerebellum and vermis
Cisterna magna
Nuchal fold
the base of the occipital horn, where it joins with the
temporal horn and the body of the ventricle. The atrium
is an important landmark at which ventricular size is
measured. The cerebellar view is obtained by rotating
the transducer into a suboccipitobregmatic plane centered on the thalamus to show the cerebellar hemi-
spheres. This view shows the cerebellum, cisterna
magna, cavum septi pellucidi (CSP), and frequently the
anterior horns of the lateral ventricles. Cerebellar measurements may be used to determine gestational age if
the head has undergone molding.
24
The cisterna magna
is the cerebrospinal fluid space between the cerebellum
and the occipital bone. It is the distance between the
cerebellar vermis and inner surface of the occipital bone
measured on an axial plane that includes the anterior end
of the CSP and the midplane of the cerebellum posteriorly. The cisterna magna should be noted at every study
and normally measures 2 to 10 mm.4 Its obliteration
suggests a Chiari II malformation, a common finding
in spina bifida. Its excessive enlargement is termed
mega–cisterna magna, which may be normal if found
in isolation,
such as trisomy 18 and cerebral dysfunction.
25
but also increases risks of abnormalities
26
Additional sonographic views and projections that
exploit the normal windows provided by the fontanelles
and sutures can be helpful in clarifying brain anatomy
and development. The median (midsagittal) view
through the metopic suture–anterior fontanelle–sagittal
suture shows midline structures such as the corpus callosum and occasionally the cerebellar vermis and brainstem. The posterolateral mastoid fontanelles provide
effective access to the cerebellum and occipital lobes and
ventricles. The sulci and gyri undergo predictable development patterns that can be assessed as early as 18 weeks.
Special views to optimize sulcal development can be
helpful in detecting abnormal development such as lissencephaly
27,28
(Fig. 34-3).
1200 PART IV ■ Obstetric Sonography
v
t
c
t
v
c
m
A
D
B
c
C
E
FIGURE 34-2. Standard planes for viewing cerebral structures. A, Thalamic view at 20 menstrual weeks. This trans-
verse view at the level of the diamond-shaped thalamus-hypothalamus complex (t) contains the slitlike midline third ventricle. The
echogenic triangular area behind the thalamus and between the occipital lobes is the ambient cistern (arrow), which contains cerebrospinal
fluid (CSF) but is rendered echogenic because of strands of meninges supporting the brain structures. The insula is a short, brightly
echogenic line (open arrow) containing the pulsating middle cerebral artery branches. It is surrounded by normal white matter that is very
hypoechoic and should not be mistaken for fluid. The echogenic band between thalamus and insula is the basal ganglia. Anteriorly are
the tips of the anterior frontal horns of the lateral ventricles (v) and between them is the boxlike cavum septi pellucidi (c). B, Ventricular
view at 18 weeks. The atrium of the occipital horn is filled with echogenic choroid, and the ventricle measurement is indicated (arrow-
heads). Note that the choroid fills more than 60% of atrium width. The measurement between the medial ventricle wall and the choroid
is less than 3 mm. The tips of the anterior frontal horns are visible (arrows); c, cavum septi pellucidi. C, Cerebellar view at 18 menstrual
weeks is obtained by rotating the transducer from the thalamic view so that the cerebellar hemispheres (arrows) in the posterior fossa come
into view, connected in the midline by the slightly more echogenic vermis. The cisterna magna (m) is visible between the cerebellum and
the occipital bone. Also visible in this view are the thalamus, third ventricle, anterior horns, and cavum septi pellucidi. D, Coronal view
at 19 weeks through the coronal suture shows anterior frontal horns (black arrows) and large nerve trunks; the fornices (white arrows)
are clearly visible below the cavum septi pellucidi (c). E, Midsagittal view through metopic suture at 19 weeks shows normal corpus
callosum (arrows) containing the cavum septi pellucidi in its arc below the corpus callosum. The echogenic cerebellar vermis is visible
posteriorly.
Cingulate
PO
Calcarine
A B
CB
V
C D
21 wk
21 wk
csp
25 wk
Insula
In ST
E F
ST
FIGURE 34-3. Scan planes (dark lines) used to assess early-appearing sulci. Note that the scan planes are perpendicular
to the direction of the sulcus being evaluated. A, Medial hemispheric surface at 26 weeks. Coronal scan plane (CP) is best for cingulate
sulcus (yellow arrow) and calcarine sulcus. Semi-axial plane is best for parieto-occipital sulcus (PO). B, Axial view at 21 weeks shows the
diamond shape formed by the normal parieto-occipital sulci (arrow). C, Coronal view through the occipital lobes at 21 weeks shows
the calcarine sulcus (arrow) of the upper occipital lobe; the lower calcarine sulcus has not developed as far yet. This slight side-to-side
variation is normal. CB, Cerebellum. D, Coronal view shows the notch of the cingulate sulcus above the bodies of the lateral ventricles
and cavum septi pellucidi (csp). E, Lateral view of brain surface at 26 weeks shows the scan plane used to evaluate the insula (In) and
superior temporal sulcus (ST) behind it. F, Axial view shows the angular plateau of the insula and behind it the subtle indentation of the
superior temporal sulcus (arrow with ST). (Anatomic images modified from Dorovini-Zis K, Dolman CL. Gestational development of brain.
Arch Pathol Lab Med 1977;101:192-195. Ultrasound images from Toi A, Chitayat D, Blaser S. Abnormalities of the foetal cerebral cortex.
Prenat Diagn 2009;29:355-371.)
1202 PART IV ■ Obstetric Sonography
Multiplanar three-dimensional (3-D) imaging can be
utilized to reconstruct axial and median views to assess
the brain from any perspective. Midsagittal reconstructions are especially helpful in evaluating abnormalities
of the corpus callosum and cerebellum.
23
Head shape
and ossification should be noted at all these views (see
Chapter 33).
Although ultrasound is the mainstay of a prenatal
examination, MRI is useful as a problem-solving technique when questions remain after the ultrasound scan.
Currently, MRI provides excellent anatomic images after
about 22 weeks’ gestation and is superior in evaluating
the character of brain tissue and the periphery of the
brain, where ultrasound visibility is limited (Fig. 34-4).
However, MRI has limitations in showing cerebral calcifications and small cysts.
10,29,30
Variants (Usually Normal)
Choroid Plexus Cysts
Choroid plexus cysts (CPCs) are cystlike spaces in the
choroid plexus (Fig. 34-5; Video 34-4). They are
common, seen in 1% to 6% of fetuses between 14 and
24 weeks’ gestation. Most are small and disappear without
consequence by about 28 weeks. CPCs are thought to
represent entrapment of cerebrospinal fluid within an
infolding of neuroepithelium. Many suggest that only
CPCs over 3 mm should be considered substantial
enough to be termed “choroid plexus cyst.” Although
frequently found in normal fetuses, CPC are associated
with trisomy 18.
The incidence of CPCs in trisomy 18 is about 50%,
and in about 10% of trisomy 18 fetuses, CPC is the only
ultrasonographic findings. Likelihood ratios of trisomy
18 with isolated CPC are about 7 (range 4-12) times the
mother’s background risk.
31
Size and bilaterality of the
cysts do not impact incidence of aneuploidy.
Fetuses with trisomy 18 almost always have other
detectable abnormalities. Therefore, when a CPC is
found, there should be a detailed search for features of
trisomy 18, especially the hands, heart, and CNS. Maternal age should be taken into account because the risk of
trisomy 18 increases with age. Maternal serum and firsttrimester nuchal translucency can be helpful as independent screens for trisomy 18.
31,32
Blake’s Pouch Cyst
Blake’s pouch cyst describes a thin-walled cystic structure normally seen in the posterior fossa behind the
lower portion of the cerebellar vermis (Fig. 34-6, A). It
should not be mistaken for abnormality. Its walls appear
as lines or strands that were previously thought to represent “suspensory ligaments of the cerebellum.” In fact,
these are most likely remnants of the extremely thinwalled posterior roof of the rhombencephalon. The
cephalad part of the rhombencephalic roof forms the
cerebellum and vermis; the inferior part remains thin
and eventually fenestrates to form the foramina of
Magendie and Luschka. The lower part of this membrane often bulges into the posterior fossa as a “cyst”
described by Robert Blake over 100 years ago. Because
it contains cerebrospinal fluid, this cyst is echo free,
unlike the adjacent subarachnoid fluid, which is rendered slightly echogenic due to fine strands in the subarachnoid space. With careful scanning, the Blake’s
pouch cyst is visible in almost every fetus, varying in size
from tiny to large and conspicuous. If large, the cyst can
elevate and rotate the vermis, creating an appearance that
mimics vermian dysplasia, but in these cases, midsagittal scans show that the vermis is intact (Fig. 34-6, B and
C). Some think that mega–cisterna magna is simply a
large Blake’s pouch, possibly resulting from delayed fenestration of the foramina.
15,17
Cavum Veli Interpositi
Cavum veli interpositi describes a small cystic collection
in the midline, usually seen below the splenium of the
corpus callosum, behind the upper brainstem and above
the region of the pineal gland (Fig. 34-7). It represents
fluid in the potential space in the telea choroidea above
the third ventricle. Most collections are seen below the
splenium, but can extend anteriorly above the third ventricle to the foramina of Monroe.
33
Although infrequently described in the prenatal ultrasound literature,
we have found small cysts, less than 8 mm, to be very
common on scans at 18 to 20 weeks. A cavum veli interpositi incidence of 5.5% to 34% is reported in the pediatric literature.
many recede spontaneously, and long-term neurologic
outcome is normal.
34
Most cysts are of no clinical consequence;
34-36
Occasionally, however, the
cyst is large and can distort the brainstem and adjacent
brain, causing obstructive hydrocephalus, and requires
treatment by unroofing. There is no known genetic
association.
Cavum veli interpositi cysts are readily seen at axial
scanning when specifically sought and can be confirmed
with coronal and midsagittal views and multiplanar 3-D
images (Fig. 34-7). The remaining brain should be
scanned to confirm normal anatomy, especially with
respect to corpus callosum and ventricle size. Color
Doppler ultrasound is used to exclude vascular dilations,
such as vein of Galen aneurysms. Cavum veli interpositi
cysts usually are seen at 18 weeks, but the larger cysts
may present in the first trimester.
34
The physiologic cavum veli interpositi cysts tend
to be isolated, unilocular, and small (<
10 mm) and
remain stable or recede over time. The differential diagnosis includes cysts and cystlike conditions that occur in
the midline, such as dilated cavum vergae, glioepen-
dymal cysts, arachnoid cysts, cystic tumors (mainly
cystic teratomas), vein of Galen aneurysm, pineal
Chapter 34 ■ The Fetal Brain 1203
SCC
BCC
FV
CV
MO
A B
CC
M
CV
Cx
WM
SF
TV
TL
SF
TS
P
MO
C D
FV
CaS
VOG
SS
ToH
FIGURE 34-4. Normal brain appearance on MRI. T2-weighted images at 20 weeks (A and B), 22 weeks (C), 26 weeks
(D and E), 27 weeks (F, G, and H), and 34 weeks (I); in sagittal midline (A, C, G, I), parasagittal (F), coronal (B, H), and axial (D, E)
planes. Note developing body (BCC) and splenium of the corpus callosum (SCC), fourth ventricle (FV), cerebellar vermis (CV), medulla
oblongata (MO), cerebral cortex (Cx), white matter (WM), sylvian fissure (SF), temporal lobe (TL), third ventricle (TV), pons (P), midbrain
(M), ethmoid bone (E), extra-axial space (EAS), sphenoid bone (S), middle cerebellar peduncle (MCP), cerebellar hemispheres (CH), vein
of Galen (VOG), straight sinus (SS), calcarine sulcus (CaS), torcular Herophili (ToH), temporal sulcus (TS), precentral sulcus (PreCS),
central sulcus (CeS), post–central sulcus (PostCS), cingulate sulcus (CiS), cingulate gyrus (CG), parieto-occipital sulcus (POS), tectum (Te),
interhemispheric fissure (IHF), superior sagittal sinus (SSS), and choroid plexus (CP). (From Levine D, Robson C. MR imaging of normal
brain in the second and third trimesters. In Levine D, editor. Atlas of fetal MRI. Bristol, Pa, Taylor & Francis, 2005.)
Continued
1204 PART IV ■ Obstetric Sonography
CC
CiS
PreCS
E
EAS
S
P
PB
TL
MCP
E
CG
CH
POS
CaS
F
SSS
IHF
CP
CeS
PostCS
Te
G
CV
SS
CH
CV
H
I
FIGURE 34-4, cont’d. Normal brain appearance on MRI. T2-weighted images at 20 weeks (A and B), 22 weeks (C), 26
weeks (D and E), 27 weeks (F, G, and H), and 34 weeks (I); in sagittal midline (A, C, G, I), parasagittal (F), coronal (B, H), and axial
(D, E) planes. Note developing body (BCC) and splenium of the corpus callosum (SCC), fourth ventricle (FV), cerebellar vermis (CV),
medulla oblongata (MO), cerebral cortex (Cx), white matter (WM), sylvian fissure (SF), temporal lobe (TL), third ventricle (TV), pons
(P), midbrain (M), ethmoid bone (E), extra-axial space (EAS), sphenoid bone (S), middle cerebellar peduncle (MCP), cerebellar hemispheres
(CH), vein of Galen (VOG), straight sinus (SS), calcarine sulcus (CaS), torcular Herophili (ToH), temporal sulcus (TS), precentral sulcus
(PreCS), central sulcus (CeS), post–central sulcus (PostCS), cingulate sulcus (CiS), cingulate gyrus (CG), parieto-occipital sulcus (POS),
tectum (Te), interhemispheric fissure (IHF), superior sagittal sinus (SSS), and choroid plexus (CP). (From Levine D, Robson C. MR imaging
of normal brain in the second and third trimesters. In Levine D, editor. Atlas of fetal MRI. Bristol, Pa, Taylor & Francis, 2005.)
Chapter 34 ■ The Fetal Brain 1205
FIGURE 34-5. Bilateral choroid plexus cysts (arrows) on transverse ventricular view.
*
A B
C
FIGURE 34-6. Blake’s pouch “cyst.” A, Note the clear,
echo-free space (*) in the midline behind the normal cerebellum
and vermis. This is the Blake’s pouch cyst, which contains clear
CSF. It has thin, lateral cyst walls (arrows), which separate it from
the adjacent, mildly echogenic subarachnoid space of the cisterna
magna that is visible on either side. Such “cysts” are common and
with careful scanning can be found in most fetuses (see also Figure
34-3, C). B and C, Pseudo–vermis dysgenesis/hypoplasia
caused by rotation of cerebellum and vermis by a Blake’s pouch
cyst. B, Axial view shows apparent cleft or defect in the vermis
(arrow) that could easily be called vermian dysgenesis. C, Midsag-
ittal view shows an intact, symmetrical normal-size vermis with
three fissures visible. It is rotated such that its lower part is elevated
away from the brainstem (arrow). Many now believe that this
represents a normal fetus, with Blake’s pouch cyst elevating the
lower part of the vermis and giving it the appearance of a cleft.
1206 PART IV ■ Obstetric Sonography
csp
CC
Vermis
cyst, and hemorrhage. Pathologic cystic collections are
generally larger, enlarge over time, and have associated
abnormalities, such as corpus callosum dysgenesis, ventriculomegaly, or solid masses.
33,34,36
Fetuses with suspected cavum veli interpositi cysts
should undergo detailed neurosonography and anatomic
scan. If there are any atypical features or associated findings, MRI can be helpful in further investigation. We no
longer monitor incidentally discovered small (<
8 mm)
isolated cysts but perform follow-up scans if the cysts are
especially conspicuous or if there is parental concern.
VENTRICULOMEGALY AND
HYDROCEPHALUS
The term ventriculomegaly (VM) describes large ventricles. The head itself may be normal, large, or even
smaller than expected for menstrual age. Hydrocepha-
lus (HC) refers to enlarged ventricles associated with
increased intracranial pressure and thus is typically associated with head enlargement. Ventriculomegaly is the
most commonly encountered cranial abnormality at prenatal ultrasound, with incidence ranging from 0.3 to 1.5
in 1000 births.
37-40
FIGURE 34-7. Cavum veli interpositi (CVI). Multi-
planar 3-D scan shows CVI as the small cystic space (arrowheads)
seen in all three views in the midline between the hemispheres and
just behind/below the splenium of the corpus callosum (cc); csp,
cavum septi pellucidi. CVIs of this size are common and normal.
Enlargement of the lateral cerebral ventricles is not
the primary problem. Although VM may be an isolated
finding, it is usually the sonographically conspicuous
finding of numerous disorders and syndromes.
37,39
The underlying changes in the brain are clinically
important, not only the size and appearance of the ventricles. Cerebral functional alterations are only variably
predicted by ventricular size, cortical thinning, and
appearance.
41-43
Pathogenesis
Cerebrospinal fluid (CSF) is secreted by the choroid
plexus of the lateral, third, and fourth ventricles, as well
as by the cerebral capillaries.
44
CSF flows from the lateral
ventricles through the foramina of Monro, third ventricle, aqueduct of Sylvius, and fourth ventricle and out
the foramina of Magendie and Luschka, into the subarachnoid space of the posterior fossa. CSF then courses
over the surface of the brain to the pacchionian or
arachnoid granulations, which are distributed at the
top of the head adjacent to the superior sagittal sinus and
absorb CSF. Ventricular enlargement generally results
from obstruction of CSF flow in the brain (intraven-
tricular obstructive hydrocephalus). Alternatively, the
site of blockage may be outside the ventricular system,
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