Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1201
20 weeks. Development proceeds in an anteroposterior manner,
beginning anteriorly with the genu, then body and splenium
posteriorly, and inally the anterior tip, the rostrum. 8,25,154 he
corpus callosum measures 4 mm at 16 weeks and grows to 45 mm
by term. here is no gender diference in length, but it is thicker
in girls. 221 Callosal development is associated with the development
of the septal lealets and the CSP. When septal lealets are present,
at least the anterior portion of the corpus callosum has formed. 25,40
ACC may be complete or partial, developmental or acquired.
ACC has a high incidence of associated malformations, suggesting
that it is part of a generalized widespread developmental disturbance.
Additional CNS abnormalities are reported in 45% to
80% of fetuses, especially involving the posterior fossa, including
DWM, inferior vermian hypoplasia, Chiari II, and abnormal
neuronal migration. Somatic and metabolic abnormalities are
seen in up to 60% and include facial abnormalities, congenital
heart disease, and skeletal and genitourinary abnormalities. ACC
is part of numerous syndromes including Aicardi and Joubert
syndrome and acquired conditions such as fetal alcohol syndrome
and infections. 25,154,220
Developmental disturbances ater the corpus callosum has
started to form usually interrupt the formation of the more
posterior parts of the corpus callosum, but insults ater callosal
development is complete can cause secondary atrophy of previously
developed central portions. Because callosal development
is not complete until 20 weeks, early diagnosis of ACC may be
diicult. 154,222 Diagnosis of abnormalities of the corpus callosum
requires it to be directly imaged in the midsagittal plane; this
view is not currently included in the recommended views for
the routine anatomy scan (see Figs. 34.2E and 34.17) but should
be used with detailed neurosonography whenever a fetal morphologic
abnormality is detected.
On routine scans reliance for screening for callosal and other
midline abnormalities is placed on indirect indings on the
standard views, especially the CSP, which should be sought on
all axial scans ater about 16 to 17 weeks. hese indirect indings
include absent or deformed CSP, VM with colpocephaly (teardrop
ventricles), ventricles parallel to the midline, interhemispheric
widening, and on occasion associated indings such as interhemispheric
cysts and lipomas (Fig. 34.29; see also Fig. 34.26;
Video 34.8, Video 34.9, Video 34.10, and Video 34.11). If the
CSP is not seen with certainty on standard scans, then additional
coronal and sagittal views should be taken to evaluate it and,
more important, the corpus callosum and to evaluate for other
midline conditions such as HPE. 2,25,223,224 Unless care is taken, it
is possible to overlook CSP abnormality because with partial
ACC a small CSP may be present and with complete ACC there
may be a high-riding third ventricle or an interhemispheric cyst
that mimics the CSP. Unfortunately, the indirect signs such as
VM, colpocephaly, and hemispheric separation are not always
present. 222
Once the diagnosis is suspected, coronal and median views,
especially with transvaginal scanning and 3-D ultrasound, help
to conirm the diagnosis. he diagnosis is made on midsagittal
views that show that the corpus callosum is absent or deicient
and can show corroborating indings. On coronal views the third
ventricle may be elevated and the crossing corpus callosum tracts
are absent. he medial walls of the anterior horns of the lateral
ventricles are commonly indented from their superomedial aspect
by the bundles of Probst (buildup of callosal ibers that fail to
cross midline) and change the ventricular coniguration from a
V to a U (Viking or steer horn appearance). Color low Doppler
scans can be used to demonstrate an abnormal course of the
pericallosal and cingulate arteries, which normally follow the
contour of the callosomarginal sulcus, but with ACC they assume
a more radial course. his vascular change may already be seen
in the irst trimester . 99 Occasionally, fetuses with callosal abnormalities
develop interhemispheric cysts or a midline lipoma. he
cysts may be herniations from the third ventricle or separated
from the ventricles when they are commonly associated with
migrational abnormality. In the third trimester the medial
hemispheric sulci assume a “sunburst” orientation radiating from
the thalamus instead of the normal arc of the cingulate sulcus.
On axial views this gives the appearance of a “hairy midline”
with sulci radiating out from the interhemispheric issure. 154
MRI is helpful to conirm the diagnosis and especially to search
for additional, subtle abnormalities such as migrational
disorders. 154,220
Pitfalls in sonographic interpretation include mistaking the
high position of the third ventricle, other luid spaces, and the
fornices for the CSP. On 3-D scans, care must be taken to avoid
mistaking the pericallosal sulcus for the corpus callosum. 225,226
Other conditions may also show CSP abnormality including HC
with septal fenestrations, SOD, HPE, and schizencephaly. Marked
HC may fenestrate the septal leaves, raising the suspicion of
ACC, but midsagittal scanning can show the displaced stretched
corpus callosum in its expected location.
Investigation of suspected cases includes detailed CNS and
somatic ultrasound, karyotype (chromosomal abnormalities occur
in about 18%), microarray, screening for TORCH infections,
and MRI. If pregnancy continues, follow-up examinations may
detect late-appearing abnormalities. Prognosis relates to the
associated anomalies, but unfortunately about 15% of associated
problems are not evident prenatally. he prognosis for truly
isolated ACC can be good in 65% to 75% of cases but guarded,
because unfortunately in some cases functional deicits do not
become apparent until later life. 220
Absence of Septi Pellucidi and
Septo-Optic Dysplasia
Absence of the septi pellucidi is rare and occurs in about 2 to
3 per 100,000 pregnancies. It can be primary, resulting from
developmental disturbances including HPE, SOD, schizencephaly,
ACC, and malformations of cortical development, or secondary
ater disruption as with HC or porencephaly. 25,154 Absence is
virtually always associated with other abnormalities, and absence
of the CSP warrants detailed neurosonography assessment. Rarely
it can be isolated with good outcome.
Septo-optic dysplasia (SOD, de Morsier syndrome) manifests
as heterogeneous triad of absence of septi pellucidi and CSP,
hypoplasia of optic nerves and chiasm, and pituitary dysfunction
(Fig. 34.30, Video 34.12). It is heterogeneous in etiology and
appearance. Most cases are sporadic, but some are familial. On
ultrasound, there is absence of the septi pellucidi. On coronal