Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1185
FIG. 34.15 Closed, Skin-Covered Spina Biida at 21 Weeks. The right image shows an obvious meningomyelocele (arrow). On the left image,
note the normal ventricles, cerebellum, and cisterna magna. Fetuses with closed, skin-covered spina biida do not have a Chiari II malformation
and lack the typical intracranial signs. The maternal serum alpha-fetoprotein (MS-AFP) is normal because the skin covers the defect.
TABLE 34.3 Second-Trimester Cranial Signs Associated With Open Spina Biida 122,130,133,135
Finding Frequency Comments
Effacement of cisterna magna 95%-100% From about 14 weeks onward
Bifrontal indentation (“lemon sign”) 53%-100% 14-24 weeks, disappears later.
Also seen in 1% of normal fetuses and in other
conditions such as skeletal dysplasia
Cerebellar “banana” sign 62%-97% Cerebellum wrapped around brainstem
Ventriculomegaly > 10 mm 46%-86% More common after 24 weeks
Small BPD < 5th percentile 26%-70%
Small head circumference < 5% 26%-70%
Small cerebellum < 10% 96%
Funneling of posterior fossa 96% Clivus-supraocciput angle < 72 degrees
Pointed dilated occipital horns 70% Seen on axial view
or efacement of the cisterna magna, the “lemon sign” (bifrontal
indentation), the “banana sign” (cerebellum wrapped around
brainstem) and VM exceeding 10 mm. Less frequently used
second-trimester signs include small BPD and head circumference
(even if VM is present), funneling of the posterior fossa as
measured by a small clivus-supraocciput angle, and pointing of
the occipital horn. Some caveats to these signs bear attention.
he level or extent of the spina biida does not relate to the
cranial signs. he lemon sign is seen more commonly before 24
weeks (5%-100%) but in only 13% ater 24 weeks. It is less
pronounced in fetuses with abnormal chromosomes and can
also be seen in about 1% of normal fetuses. 133-135 he banana
sign and VM can worsen over the course of gestation.
here are increasing eforts to screen for spina biida with
ultrasound at time of NT scan. 99,136-145 Interestingly, the spinal
defect has been detected as a dorsal abnormality as early as 9
weeks before head changes become apparent. 38,137 Ater about
12 weeks, cranial changes may begin to be observed (see Fig.
34.14G-H, Table 34.4). hese include decrease in head size, altered
head shape, and alterations in the posterior fossa including
posterior displacement of the midbrain and cerebral peduncles,
thickening of the brainstem, and loss of luid spaces in the cisterna
magna and fourth ventricle. A recent prospective study of markers
in the irst trimester by highly trained operators using IT and
cisterna magna appearances found positive or suspicious indings
in 11 of 11 cases (100%). 140 Management in irst-trimester cases