Diagnostic ultrasound ( PDFDrive )

1192 PART IV Obstetric and Fetal Sonography
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csp
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B
FIG. 34.19 Vermis Dysplasia or Hypoplasia. (A) Axial view shows fetus at 20 weeks with cleft (*) separating the cerebellar hemispheres
(arrowheads). (B) Sagittal view shows the cerebellar vermis to be deicient in its inferior portion (arrow), with a luid space in the expected region
of the lower vermis (*). csp, Corpus callosum and cavum septi pellucidi.
In studies from Children’s Hospital in Boston, postnatal MRI
failed to conirm prenatal MRI indings in 6 of 42 cases (5 vermian
hypoplasia, 1 mega–cisterna magna) but found additional, mainly
cerebral abnormalities in 10 of 42 cases (heterotopias, brainstem
hypoplasia, lissencephaly, hemorrhage). 164,180 Because of the
diagnostic uncertainties, when vermian abnormality is suspected,
additional anomalies should be sought. Additional investigation
includes history, assessment for maternal TORCH infection,
MRI, chromosome analysis, and molecular DNA analysis if the
chromosomes are normal. 30
Mega–Cisterna Magna
Mega–cisterna magna (MCM) refers to an enlargement of the
cisterna magna beyond 10 mm with intact vermis (Fig. 34.20).
On careful scanning, it can be noted that the contained luid is
anechoic and cyst walls can be seen in the far periphery, similar
to a large Blake cyst. Many believe that MCM is just enlargement
of a Blake cyst behind the cerebellum and without vermis rotation.
When the condition is isolated, most children are normal.
However, if not isolated, only 11% have normal outcome. he
majority of nonisolated cases have VM, congenital infection, or
karyotype abnormalities, especially trisomy 18. Diferential
diagnosis includes arachnoid cyst compressing the posterior fossa
and DWM. Arachnoid cysts displace an otherwise normally
formed cerebellum and frequently lie behind or above the cerebellum,
may be asymmetrical, and do not communicate with the
fourth ventricle. 46,177
Rhombencephalosynapsis
Rhombencephalosynapsis is a rare hypoplasia of the cerebellum
characterized by complete or partial absence of the vermis and
fusion of the cerebellar hemispheres and dentate nuclei. Most
cases are sporadic, although familial cases have been described.
Rhombencephalosynapsis may manifest with VM as early as
14 weeks. Cerebellar indings may not be initially conspicuous
on the usual axial views. he deining indings at ultrasound are
a small, bean-shaped cerebellum that lacks the typical echogenic
waistlike narrowing at the vermis and cerebellar hemispheric
issures that are continuous from side to side without midline
interruption. Midsagittal views show absence of the typical
vermian issures and may show an abnormally shaped fourth
ventricle. hese indings are more readily seen on MRI. 181,182
Additional cerebral and somatic abnormalities are common.
Cerebral indings primarily involve midline structures and include
aqueduct stenosis, agenesis of the corpus callosum, absent septum
pellucidum, and SOD. Somatic abnormalities include segmentation
errors of the spine, phalangeal and radial ray defects,
and occasional defects of the cardiovascular, respiratory, and
urinary tract.
Prognosis is poor and most children die in childhood, but
some may survive into adulthood. Most survivors are neurologically
delayed and have movement disorders. 182
Other Posterior Fossa Abnormalities
he vermis receives a disproportionate degree of discussion
because vermian abnormalities are relatively conspicuous on
routine second-trimester ultrasound. Numerous other, diicultto-detect
or late-appearing abnormalities also involve the posterior
fossa. hese include hypoplasia, clets, changes related to ischemia,
hemorrhage and infarct, cortical migration disorders (type 2
cobblestone lissencephaly), metabolic disorders, and other
neurodegenerative disorders.
When abnormalities of the cerebellum and posterior fossa
are suspected, then detailed neurosonography and somatic
sonography should be performed and consideration given to
further investigations including MRI and karyotype, assessment
for infections, and expert counseling. 30,166,172,175,183
Arachnoid Cysts
Arachnoid cysts are benign, noncommunicating luid collections
within arachnoid membranes. Most appear stable and
require no surgical treatment. hey can occur intracranially
and in the spinal canal and on occasion they can be detected in
the irst trimester. Locations by order of frequency are the
sylvian issure or temporal fossa, posterior fossa, over the
cerebral convexity, and midline supratentorial, including
suprasellar (Fig. 34.21). Even if very large, arachnoid cysts
rarely cause symptoms. Occasionally they interfere with CSF