Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1191
A
B
C
FIG. 34.18 Dandy-Walker Malformation (DWM) at 22
Weeks. (A) Oblique axial view at 34 weeks shows enlarged
cisterna magna with keyhole deformity in this fetus with
DWM. (B) and (C) Corresponding magnetic resonance image
at 38 weeks. Note the elevated tentorium on the midsagittal
view.
Dandy-Walker variant; Dandy-Walker continuum; (inferior)
vermian hypoplasia, dysgenesis, or agenesis; and Blake pouch
cyst. 30 he currently accepted term is vermian hypoplasia or
dysplasia. Problems have arisen because on standard axial cerebellar
views the Blake pouch can mimic this appearance. 28
he vermis develops superiorly to inferiorly. Hypoplasia or
developmental arrest results in varying-size deicits of the inferior
portion, leaving a relatively square defect that communicates
with the fourth ventricle and separates the lower cerebellar
hemispheres. In general, the posterior fossa is not enlarged.
Midsagittal scans, 3-D imaging, and MRI are important to evaluate
the size and shape of the vermis and the shape of the fourth
ventricle and to determine if normal early issures have
developed. 30,59,169,171
here are diagnostic pitfalls. Hypoplasia or dysplasia should
not be diagnosed earlier than 18 weeks, before vermian development
is complete. On routine axial scans an excessively steep
scanning angle may give the appearance of a prominent clet
between the lower portions of the cerebellar hemispheres that
mimics vermian hypoplasia 30,178 (see Fig. 34.7B-C).
True cases of vermian hypoplasia are usually associated
with additional abnormalities similar to those seen with DWM,
and these help conirm the diagnosis (Fig. 34.19). Vermian
hypoplasia can be associated with several syndromes, including
Joubert and related syndromes, Walker-Warburg syndrome,
cerebro-oculo-muscular syndrome, and pontocerebellar
syndrome. 30,175
Counseling is diicult. In one series, up to 50% of fetuses
with the characteristic ultrasound indings of vermian dysplasia
were functionally normal ater delivery. 179 Ultrasound indings
more likely to predict true abnormality were trapezoidal vermian
defect, cisterna magna larger than 10 mm, and complete aplasia
of the vermis. In contrast, normal cases tended to have a keyholeshaped
defect. 174 Prenatal MRI is helpful but also has limitations.