Diagnostic ultrasound ( PDFDrive )

1526 PART V Pediatric Sonography
understood with major advances in fetal magnetic resonance
neuroimaging and with major advances in genetics. Examples
of reviews of cerebellar development and malformations are those
by Limperopoulos and du Plessis 49 and Patel and Barkovich. 50
DISORDERS OF NEURAL TUBE
CLOSURE
Chiari Malformations
here are three classic Chiari malformations. Chiari I malformation
is simply the downward displacement of the cerebellar tonsils,
without displacement of the fourth ventricle or medulla. Chiari
II malformation is the most common and of greatest clinical
importance because of its almost universal association with
myelomeningocele. Chiari III malformation is a high cervical
encephalomeningocele in which the medulla, fourth ventricle,
and virtually the entire cerebellum reside.
Classic sonographic indings of the Chiari II malformation
involve the entire brain. Present theories propose that a failure
in neural tube closure results in a small posterior fossa. 31,50 In
early brain development, abnormal neural tube closure may result
in a spinal defect such as a myelomeningocele. his decompresses
the ventricles and leads to underdevelopment of the posterior
fossa bony structures. he intracranial indings are the result of
the small posterior fossa. he tentorial attachment is low, and
the combined efect causes compression of the upper cerebellum
by the tentorium. he inferior cerebellum is compressed and
displaced into the foramen magnum. he cerebellar tonsils and
vermis herniate into the spinal canal through an enlarged foramen
magnum. he pons and medulla are inferiorly displaced, as is
the elongated fourth ventricle (Figs. 45.19 and 45.20; Video 45.4
and Video 45.5).
Chiari II Malformation: Sonographic Findings
Anterior and inferior pointed frontal horns
Batwing coniguration
Enlarged lateral ventricles
(colpocephaly if associated corpus callosum agenesis or
dysgenesis)
Enlarged massa intermedia ills third ventricle
Elongation and caudal displacement of fourth ventricle,
pons, medulla, and vermis
Nonvisualization of fourth ventricle due to compression
here is usually marked enlargement of the massa intermedia
on the coronal and midline sagittal ultrasound images. Although
the third ventricle is oten enlarged and the aqueduct is kinked,
the enlarged massa intermedia oten ills the third ventricle,
causing it to appear only slightly larger than normal. he fourth
ventricle is oten not visualized because it is thin, elongated,
compressed, and displaced into the upper spinal canal. he frontal
horns are typically small and pointed. he corpus callosum is
frequently either dysgenetic or absent. When callosal abnormalities
are present, the posterior horns of the lateral ventricles oten are
disproportionately large, called “colpocephaly.” he anterior and
inferior pointing of the frontal horns has oten been referred
to as a “bat wing” coniguration see (Figs. 45.19 and 45.20,
Video 45.6). he septum pellucidum may be completely or
partially absent. he interhemispheric issure usually appears to
be widened on coronal images, particularly ater shunting. here
may also be gyral interdigitation when the corpus callosum is
absent (see Fig. 45.20). he posterior fossa is usually small, and
the tentorium appears relatively low and hypoplastic. Hydrocephalus
resulting from the Chiari II malformation is frequently
mild in utero but may be progressive over time and associated
with a tethered spinal cord (Video 45.7). When CSF circulation
can no longer decompress into the myelomeningocele ater the
repair, hydrocephalus typically worsens. Neonatal screening for
hydrocephalus will be best done routinely about 2 days ater the
myelomeningocele repair, as the ventricles may appear undilated
before this. Ultrasound diagnosis of craniolacunia (lacuna skull,
lückenschädel) is possible. Lacunar skull is a dysplasia that is
oten present at birth in the Chiari II malformation. he lacunar
skull has a wavy, irregular appearance of the inner calvaria. 51
Craniolacunia will disappear over the irst year of life even without
shunting.
Routine prenatal screening for maternal serum alphafetoprotein
(MS-AFP), which is elevated with neural tube defects,
and the widespread use of sonography in pregnancy allow for
the prenatal diagnosis of most Chiari malformations. 52 Because
of its classic appearance, the Chiari II brain malformation is
oten recognized in utero and alerts the sonographer to look
closely for a myelomeningocele. In less than 2% of myelomeningoceles,
the neural tube defect is covered by skin, so there is
no elevation of MS-AFP and there may be no Chiari II malformation
(Fig. 45.21). Sonography or CT is reliable for follow-up of
shunting procedures. MRI may be necessary to evaluate infants
with symptoms suggesting brainstem compression because this
may require decompression of the foramen magnum. Fetal
myelomeningocele repair in selected patients may decrease the
severity of the Chiari II malformation ater birth. 53
Agenesis of Corpus Callosum
he corpus callosum forms broad bands of connecting ibers
between the cerebral hemispheres. Development of the corpus
callosum occurs between 8 and 20 weeks of gestation. Magnetic
resonance images from early fetuses show that the genu and
anterior body develop irst, and development proceeds both
anteriorly to the rostrum and posteriorly to develop the splenium.
31,54 herefore, depending on the timing of the intrauterine
insult, development may be partially arrested, or complete agenesis
may occur. If development is partial, the genu is usually present
and the dorsal splenium or the anterior rostrum is absent. Because
insults causing anomalies of the corpus callosum occur early
(8-20 weeks) in development, other CNS anomalies occur in up
to 80% of cases. 28,54 hese associated anomalies include Chiari
II and Dandy-Walker malformation, holoprosencephaly,
encephalocele, lipoma, arachnoid cyst, migrational abnormalities,
and Aicardi syndrome (female infants with agenesis of the
corpus callosum, ocular abnormalities, and infantile spasms). If
the corpus callosum is absent, MRI will be valuable either in