Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1195
sporadic and of unknown cause. Typically there is macrocephaly
at birth. It may be isolated or associated with neurocutaneous
and somatic hemihypertrophy syndromes including neuroibromatosis,
tuberous sclerosis (TSC), epidermal nevus
syndrome, Proteus syndrome, Klippel-Trénaunay-Weber
syndrome, and others. On the afected side the hemisphere
and ventricle are enlarged and the brain has abnormal texture
and sulcation. he unafected hemisphere is generally normal
but can be distorted owing to compression and may also show
less severe migrational abnormalities. Functional deiciency
and seizures are common, the latter on occasion requiring
hemispherectomy. 204-207
FIG. 34.22 Microcephaly and micrencephaly at 25 weeks with
periventricular calciication (arrow) in fetus with pseudo-TORCH. Note
the underdeveloped brain parenchyma (arrowheads).
conditions with relative head enlargement such as asymmetrical
IUGR, HC, tumors, and triploidy. Megalencephaly refers to
cerebral gigantism (i.e., enlarged brain). It is typically categorized
as genetic (nonsyndromic or syndromic) or nongenetic (HC
following hemorrhage or infection, posttraumatic, arachnoid
cysts). Nonsyndromic implies isolated head or brain enlargement
without other signiicant abnormality. Syndromic implies association
with other abnormalities or syndromes such as overgrowth
syndromes (Beckwith-Wiedemann, Sotos, Weaver), skeletal
dysplasias (thanatophoric dysplasia, achondroplasia), neurocutaneous
syndromes (neuroibromatosis type 1), and metabolic
conditions such as leukodystrophy and organic acidurias. Head
enlargement can appear early by 20 weeks but typically does not
manifest until the third trimester or in early childhood. 200-203
Benign familial macrocephaly (external HC) accounts for
about 50% of cases of macrocephaly. It is an autosomal dominant
condition with increased subarachnoid luid. It typically manifests
late in pregnancy or even ater delivery but has been seen as
early as 18 weeks when there is a history of other family members
who had large heads. Most children are functionally normal. 201,202
When the head measurements are larger than expected
(macrocephaly), there should be a careful search for intracranial
abnormalities and non-CNS abnormalities. Family history of
large headedness can help. Multidisciplinary counseling and
investigation are important. Prognosis can be favorable to poor
depending on etiology.
Hemimegalencephaly
Hemimegalencephaly is a malformation of cortical development
with hamartomatous enlargement of one hemisphere
with defects in all aspects of neuronal migration. Most cases are
Lissencephaly
Lissencephaly describes a brain that has a smooth surface lacking
normal gyration. It is due to a global disturbance in neuronal
migration. he most severe manifestation is a completely smooth
cortex lacking gyri (agyria), but some cases show large, malformed
gyri (pachygyria). here are two general types that have fundamentally
diferent developmental mechanisms. In type 1
lissencephaly (classical lissencephaly), neurons fail to migrate
to the cortex. In type 2 lissencephaly (cobblestone lissencephaly),
the neurons migrate but do not stop at the cortical surface and
overmigrate into the subarachnoid membranes overgrowing
surface vessels, resulting in “cobblestone cortex.” Both types are
etiologically heterogeneous and related to many gene mutations
and are commonly associated with additional CNS and somatic
abnormalities. 21,91,192,208 Previously it was thought that lissencephaly
could not be diagnosed before 28 weeks, but familiarity with
the normal early sulcal development has allowed diagnosis of
type 1 (Miller-Dieker syndrome) by 23 weeks, 48 and type 2
(Walker-Warburg syndrome) at 14 weeks. 209
Type 1 or classical lissencephaly manifests as a smooth cortex
and hourglass shape on axial views associated with mild VM, a
primitive insula, delayed or absent sulcation, and abnormal
cortical vascularity but sparing of the cerebellar vermis (Fig.
34.23). here are several variants. he more common Miller-
Dieker syndrome has gene abnormality at 17p13.3 (LIS1) and
is associated with congenital heart disease, omphalocele,
genitourinary abnormalities, IUGR, and dystrophic facies.
Girls with X-linked type 1 lissencephaly may function normally
despite “double cortex” cerebral changes that can be seen with
MRI, but boys do poorly. 48
Type 2 or cobblestone lissencephaly is associated with
malfunction of genes that also function in muscle development
(e.g., POMT1/2, FKRP, FKTN). hese conditions are grouped as
congenital muscular dystrophy. hese infants typically have
additional CNS and somatic abnormalities and lack muscle tone
at birth (loppy baby). he most severe phenotype, Walker-
Warburg syndrome, is also called HARD-E for hydrocephalus,
agyria, retinal dysplasia, and/or encephalocele and includes a
kinked brainstem. Less severe phenotypes include Fukuyama
syndrome and muscle-eye-brain disease. Ultrasound changes
may be evident by 20 weeks and include VM and absent, delayed,
or abnormal sulcal development; cerebellar vermian dysplasia;
eye abnormalities; small encephalocele; and abnormal kinked
brainstem (Fig. 34.24). 48,208,209