Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1189
lobar holoprosencephaly forms cerebral hemispheres, it may
be diicult if not impossible to diagnose prenatally. Features that
suggest lobar HPE include absence of septi pellucidi, fusion and
squaring of the frontal horns, and an abnormal appearance of
two large nerve trunks, the fornices, which are rudimentary
and appear fused into a single tract above the third ventricle.
With color Doppler sonography, the anterior cerebral artery may
have an azygous (wandering) course, “crawling” under the skull.
Fusion of the fornices in HPE may help diferentiate HPE from
septo-optic dysplasia (SOD). 157
Many authors emphasize detection of associated facial
abnormalities in diagnosing HPE (see Fig. 34.16B). Facial changes
are seen more frequently with more severe HPE and have been
categorized into four main groups, with severity approximating
degree of brain abnormality. 151-155
Facial Changes Associated With
Holoprosencephaly 155
Cyclopia with single eye, with or without proboscis
Ethmocephaly (hypotelorism and proboscis between the
eyes)
Cebocephaly (hypotelorism, nose with single nostril)
Median cleft lip and palate and hypotelorism
Single central incisor
he diferential diagnosis includes severe hydrocephalus,
septo-optic dysplasia (SOD), absence of septi pellucidi,
schizencephaly, hydranencephaly, and porencephaly. 153,158 Milder
forms can be diicult to diagnose even ater delivery and may
need specialized experienced neuroradiologic consultation. 153
For counseling purposes, it is important to determine the degree
of cerebral malformation and whether it is isolated or part of a
syndrome or associated with additional abnormalities. MRI and
chromosome and genetic analysis can be helpful. 151,154
Prognosis is variable and depends on severity of HPE, associated
abnormalities, and medical and neurologic complications.
Severely afected fetuses do poorly or die in utero. Mildly afected
children may exhibit few symptoms and may live a normal life.
About 75% need shunting. 151 In general, the severity of clinical
problems and neurologic dysfunction correlates with the degree
of hemispheric and hypothalamic failure of separation and can
include endocrinopathy (especially diabetes insipidus), temperature
dysregulation, motor and movement abnormalities, and
developmental delay. 154,159 Pregnancy termination is generally
considered ater prenatal diagnosis. Survivors are managed
symptomatically.
he fourth variant of HPE, the middle interhemispheric
form of HPE, also called syntelencephaly, has a separation of
the anterior and posterior parts of the hemispheres, but fusion
is present in the central region of the brain between parietal
hemispheres, and the face is typically normal. In general, cases
involve mild VM, absence of parts of the septi pellucidi, abnormality
of the midpart of the corpus callosum, and dorsal cysts. he
condition can resemble mild VM with septal fenestration. Coronal
scanning may show fusion of the central parts of the hemisphere.
MRI helps conirm this appearance and typically shows sylvian
issures connecting abnormally across the midline 153,154,158,160 (see
Fig. 34.16E).
he middle interhemispheric form tends to have better
outcomes with functional disabilities similar to those with lobar
HPE, but endocrine functions are normal because the hypothalamus
tends to be spared.
Posterior Fossa and Cerebellum
Cerebellar development starts at about 6 to 7 weeks’ gestation,
and the inal gross form is achieved by about 18 to 20 weeks. It
develops as thickenings of lateral rhombic lips, which enlarge
posteriorly and are joined in the midline by the vermis, which
develops from the rostral aspect. hese thickenings grow into
the thin membranous dorsal aspect of the neural tube, the area
membranacea, which is the rhombencephalic “cyst” that is
prominent in early pregnancy and later becomes the fourth
ventricle and fenestrates, forming the foramina of Magendie and
Luschka. 27,29 here is a rapid acceleration of growth starting about
24 weeks, ater which the rule that “cerebellar diameter = weeks
gestation” no longer holds. Cerebellar components continue to
develop to about 7 months ater birth, and inal neuronal organization
continues to about 20 months ater delivery. As the posterior
fossa develops, the brainstem initially becomes lexed or kinked
(mesencephalic, pontine, and cervical lexures) but again
straightens out by about 14 to 16 weeks as the spinal cord expands
with developing spinal nerve tracts. 161 here are many genetic
and molecular mechanisms involved in cerebellar development,
and these have additional roles in CNS and somatic development.
Consequently, abnormal cerebellar development is oten accompanied
by developmental and functional changes in the cerebral
cortex and other parts of the body. Speciic malformations may
suggest genetic syndromes, but any malformation can also result
from disruptive processes including infection or teratogenic,
metabolic, or traumatic insults. 162,163 Functionally, the cerebellum
not only controls voluntary movements but is also involved in
nonmotor and cognitive functions. Many children with cerebellar
malformations come to medical attention because of developmental
and behavioral issues. 162,164-166
he cerebellum is routinely examined with standard axial
views, focusing on transverse diameter, the intactness and size
of the vermis, and the depth of the cisterna magna, which should
measure less than 10 mm. 5 When an abnormality is suspected,
midsagittal views are important to evaluate the integrity, size,
rotation, and issures of the vermis as well as the appearance of
the fourth ventricle and brainstem. Transvaginal scanning, 3-D
ultrasound, and MRI including sagittal views are especially useful
(Fig. 34.17). Care should be taken before 18 weeks to not mistake
the incompletely formed vermis for a defect. 30,86,167-171
With careful technique, as discussed earlier, a small physiologic
Blake pouch is visible in over 80% of fetuses, as a small cyst in
the midline just below the valleculae at the lower border of the
cerebellum that does not afect the cerebellum in the midline
(see Fig. 34.7A). When this structure enlarges, it can rotate the
vermis or it may expand behind the cerebellum as a mega–cisterna
magna. 29
Additional anatomic features of the vermis are visible on
midsagittal views (see Fig. 34.17). he fourth ventricle is a small