Diagnostic ultrasound ( PDFDrive )

1196 PART IV Obstetric and Fetal Sonography
26 w
FIG. 34.23 Type 1 Lissencephaly (Miller-Dieker Syndrome) in
26-Week Fetus. Note the mild ventriculomegaly and lack of sulcal formation.
The insula is very smooth and shallow (arrow) and lacks the angular
plateau appearance expected at this age. Brain shows increased
echogenicity associated with migrational disturbance.
Fetal MRI and gene mutation analysis can help conirm the
diagnosis. Postnatal outcome is poor but varies with the speciic
condition. 192,208
Focal Cortical Changes
Polymicrogyria (PMG) is probably the most common malformation
of cortical development. It is a heterogeneous malformation
characterized by excessive numbers of small shallow, abnormal
gyri and irregularity of the cortex/white matter interface. he
distribution may be focal or global. PMG may be isolated or
associated with many genetic and acquired conditions; however,
in most cases the cause cannot be found. Mutations have been
found in over 30 genes including the tubulin family and include
metabolic conditions such as peroxisomal disorders and
DiGeorge syndrome (22q11.2). Nongenetic conditions include
infection with CMV and hypoxia and ischemia.
Focal cortical changes typically manifest in mid to late
pregnancy and are diicult to detect with ultrasound, where the
cortical contour can have a sawtooth appearance. hey are
typically found at MRI in fetuses referred for evaluation of other
abnormalities initially detected by ultrasound. PMG is the most
common cortical malformation in children with epilepsy and
can manifest with developmental delay. he heterogeneity of
causes and manifestations makes counseling diicult 192,195,205,210
(Fig. 34.25).
Heterotopia describes localized nodules of disorganized
neurons in abnormal locations anywhere in the brain. On MRI
three types are described: periventricular nodular heterotopia,
focal subcortical heterotopia, and band heterotopia. Only the
periventricular nodules are readily detectable on prenatal
ultrasound and visible as irregular lining of the ventricular surface
(Fig. 34.26). he nodules are more conspicuous on MRI, which
can also depict the intraparenchymal and cortical nodules, which
are diicult to recognize by ultrasound. Etiology is heterogeneous.
Cases can be isolated but other CNS and somatic abnormalities
are common, especially involving the corpus callosum. he
diferential diagnosis includes ventricular irregularity associated
with CMV, hemorrhage, and tuberous sclerosis. Prognosis varies
depending on underlying syndromes and associated abnormalities
and ranges from essentially normal to having variable deicits
and epilepsy, which are common; in some the disorder is
lethal. 192,195,205,211
Schizencephaly (split brain) is a rare structural malformation
of the cerebrum characterized by congenital clets or defects that
extend through the hemisphere from ventricle to outer cortex.
he clets are usually symmetrical and involve the parietal or
temporal regions. hey are lined by cortical gray matter oten
with PMG, unlike clastic injury resulting in porencephaly in
which white matter lines the clets. he tracts may be open (open
lip), allowing CSF communication between the ventricle and
subarachnoid space, or solid (closed lip, consisting of an abnormal,
solid gray matter tract between the ventricle and brain
surface). Etiology can be destructive (encephaloclastic) or
developmental. Destructive etiologies include vascular injury,
ischemia, teratogen exposure (e.g., cocaine), infections (especially
CMV), and trauma, and appearances can be similar to porencephaly
or hydranencephaly. Developmental cases are considered
to be related to PMG arising from disordered neuronal migration
and organization. Most cases are found in late pregnancy during
investigation of VM, but they have been seen in the middle
trimester. he diagnosis rests on detection of the cerebral clets
or abnormal gray matter traversing from ventricle to cortex (Fig.
34.27). here may be associated midline defects including SOD,
dysgenesis of the corpus callosum, absence of the septi pellucidi,
other malformations of cortical development, and optic nerve
abnormalities. Fetal MRI allows superior characterization of
cortical and gray matter changes.
Neurodevelopmental delay and seizures are common. Suspected
cases beneit from multidisciplinary consultation and
counseling. Prognosis relates to the size of the defect and associated
abnormalities. 19,195,205,212,213
Other Malformations of Cortical Development
Cortical malformations can accompany infections, inborn errors
of metabolism (e.g., peroxisomal disorders), mitochondrial
disease, and conditions with unknown etiology. 91,188 Metabolic
and neurodegenerative disorders can be associated with
abnormal brain development but are typically not detectable
prenatally by ultrasound. However, some are associated with
diverse changes such as IUGR, microcephaly or macrocephaly,
abnormal corpus callosum, migrational and cerebellar abnormality,
cerebral calciications, cerebral cysts, renal and hepatic changes,
congenital heart disease, and hydrops. 214,215 he most common
is Zellweger (cerebrohepatorenal) syndrome, a lethal peroxisomal
disorder. Afected fetuses can have increased NT in the
irst trimester and later show VM, subependymal germinolytic
cysts, migrational abnormality, hepatomegaly, cortical renal cysts,
and chondrodysplasia punctata. he diagnosis can be conirmed
by DNA and biochemical testing of fetal cells obtained by
CVS. 214-216