Diagnostic ultrasound ( PDFDrive )

1190 PART IV Obstetric and Fetal Sonography
Increased luid and torcular elevation suggest Dandy-Walker
malformation, whereas normally sited torcular position and
normal cerebellar morphology suggest mega–cisterna magna,
Blake pouch cyst, or arachnoid cyst. If the cerebellum is globally
small, then the brainstem should be evaluated. A normal brainstem
suggests cerebellar hypoplasia, and a small brainstem suggests
pontocerebellar hypoplasia. Focal reductions suggest ischemia
or hemorrhage. Global reductions suggest syndromes, aneuploidy,
and infections. If there is anatomic abnormality, then considerations
include vermian agenesis as with Joubert syndrome and
spina biida with Chiari II.
Counseling regarding posterior fossa abnormalities is diicult
because fetuses with diferent conditions are variously grouped
together. 172,173 Frustratingly, prenatal appearances oten do not
correlate with indings at pathology. 174 Genetic abnormalities
have been described with many posterior fossa disorders, and
many have autosomal recessive inheritance. 162,175 If posterior
abnormalities are suspected, expert imaging opinion and interdisciplinary
consultation should be considered. 30
FIG. 34.17 Three-Dimensional (3-D) Midsagittal View of Normal
Brain. Three-dimensional midsagittal view of normal brain at 21 weeks
using volume contrast imaging (VCI). This is basically a thick-slice scan
that increases contrast and decreases noise. Note the triangular shape
of the cerebellar vermis behind the brainstem. This is the best view to
evaluate the corpus callosum (cc) and vermis. Line indicates apex of
the fourth ventricle and fastigial point. Normally as here, the upper and
lower parts of the cerebellum touch the brainstem. A line connecting
the fastigium and declive (the most posterior bulging part of the vermis)
normally divides the vermis into approximately equal upper and lower
portions. After about 20 weeks, three vermian issures can usually be
identiied as white septa invaginating into the darker cerebellum. They
are the primary (1), the prepyramidal (pp), and the secondary (2) issures.
More issures become visible with advancing age as are seen in this
fetus.
triangular space that normally has a pointed apex, the fastigial
point. he upper and lower parts usually touch the brainstem,
but the lower part may rotate outward (tegmento-vermian angle)
to about 10 degrees. A line drawn between the fastigial point
and the most posterior bulge of the vermis bisects the vermis
into approximately equal halves. Nomograms are available for
vermian dimensions. Vermian issures become visible ater about
18 weeks and can be used to help determine normal vermis
development. he primary issure between the culmen and declive
is seen at about the 2 o’clock position by 24 weeks, and the secondary
between the pyramis and uvula at the 5 o’clock position by
30 weeks. Commonly a third issure is visible horizontally between
these two.
Practically, it is easiest to approach posterior fossa abnormalities
anatomically and consider three elements as suggested by
Guibaud: luid space in the posterior fossa, cerebellar size,
and cerebellar morphology. 169,170 Suspected abnormalities
should be further evaluated with detailed examination of the
brain and body including midsagittal views and 3-D ultrasound
along with MRI to evaluate the brainstem and cerebellar
vermis. 30,59,169-171
Dandy-Walker Malformation
Classic DWM is a triad of partial or complete vermian agenesis,
cystic enlargement of the fourth ventricle, and elevated torcular
or tentorium (Fig. 34.18). It occurs in about 1 per 30,000 pregnancies
and is felt to arise from abnormal development of the
membranous roof of the fourth ventricle at about 7 to 10 weeks.
Midsagittal imaging is important because prognosis is associated
with the degree of vermian abnormality as well as the presence
of associated abnormalities. Additional CNS anomalies are
common, especially hydrocephalus (70%-90%), dysgenesis of
the corpus callosum (30%), encephalocele (16%), migrational
disorders, brainstem dysgenesis, and spina biida. Somatic
abnormalities occur in 20% to 30%, including cystic kidneys,
congenital heart disease, and facial clets. DWM has a multifactorial
etiology, and most cases are seen in association with genetic
and nongenetic syndromes. At the 11- to 14-week scan, a large
posterior fossa may be the irst sign of DWM but can also be
seen with chromosomal abnormality and other malformations
and should be reassessed later in the pregnancy. 99,176
he diferential diagnosis includes vermian dysplasia, Blake
pouch cyst, and posterior fossa subarachnoid cysts. Investigations
include fetal MRI, assessment for maternal TORCH
infection, chromosome analysis, microarray analysis, and consideration
of syndromic forms, including Walker-Warburg
syndrome.
he prognosis of DWM depends on associated abnormalities.
hose with isolated DWM and relatively normal vermis do better,
but those with associated CNS or somatic abnormalities do poorly.
Neonatal mortality ranges from 12% to 55%. In live-born children,
intelligence is normal in about 40%, borderline in 20%, and
subnormal in 40%. 170,177
Vermis Hypoplasia or Dysplasia
Vermis hypoplasia or dysplasia describes a conspicuous clet
between the inferior parts of the cerebellar hemispheres due to
deiciency or absence of the lower part of the vermis. Controversy
surrounds this appearance, which has been variously termed