Diagnostic ultrasound ( PDFDrive )

1170 PART IV Obstetric and Fetal Sonography
FIG. 34.3 Ventricular Measurement. Image at 20 weeks showing
the appropriate position of calipers to measure the lateral ventricle.
of a prenatal examination, MRI has been proven to be valuable
to further assess problems found with ultrasound. MRI provides
excellent anatomic images ater about 22 weeks’ gestation and
is superior in evaluating the character of brain tissue and the
periphery of the brain, where ultrasound visibility is limited
(Fig. 34.5). However, MRI has limitations in showing cerebral
calciications and small cysts. 18,54,55
Variants (Usually Normal)
Choroid Plexus Cysts
Choroid plexus cysts (CPCs) are cystlike spaces in the choroid
plexus and are due to entrapment of CSF within an infolding of
neuroepithelium (Fig. 34.6, Video 34.4). It is suggested that only
CPCs over 3 mm should be considered substantial enough to
be termed “choroid plexus cyst.” hey are common and seen in
1% to 6% of fetuses between 14 and 24 weeks’ gestation. Most
are small and disappear without consequence by about 28 weeks.
Rarely, large cysts may transiently dilate the lateral ventricles.
hey do not afect neurological development. 56
he great majority of CPCs are found incidentally in normal
fetuses, but they can be associated with trisomy 18. About 50%
of trisomy 18 fetuses have CPC, and they are the only obvious
initial inding in about 10% of trisomy 18 cases. Likelihood ratios
of trisomy 18 with isolated CPC are about 7 (range 4-12) times
the mother’s background risk. Size and bilaterality of the cysts
do not afect incidence of aneuploidy. Fetuses with trisomy 18
almost always have other detectable abnormalities. When CPCs
are found, there should be a detailed search for features of trisomy
18, especially the hands, heart, and CNS. Other tests of aneuploidy
risk should be reviewed, including maternal age, serum screening,
and cell free fetal DNA testing (noninvasive prenatal testing
[NIPT]) to ensure low aneuploidy risk. Many feel that patients
with isolated CPC and low aneuploidy risk, especially with normal
NIPT indings, do not warrant further counseling or testing. 20,57,58
Blake Pouch Cyst
Blake pouch cyst, described by Robert Blake over 100 years ago,
is a thin-walled cystic structure commonly seen in the midline
of the posterior fossa behind the lower portion of the cerebellar
vermis and brainstem. Because it contains CSF, this cyst is echo
free, unlike the adjacent subarachnoid luid, which is slightly
echogenic owing to ine strands in the subarachnoid space (Fig.
34.7). Of note, this diference in appearance of the luid and
Blake cyst walls are not visible on MRI. With careful scanning,
we have found that a Blake pouch cyst is visible in almost every
fetus varying in size from tiny to large and conspicuous. A large
Blake pouch can be associated with vermian rotation to 40 to 50
degrees with an intact vermis. Some feel this rotation results from
delayed fenestration of the foramina of Magendie and Luschka
and secondary cystic dilation of the inferior membranous area,
which is the pouch and elevates the vermis. If isolated, the
rotation typically decreases in later pregnancy and outcome is
good. 28,59 When isolated, it should not be mistaken for abnormality.
hree-dimensional ultrasound with midline reconstruction
is especially helpful in this regard and can conirm vermian
normality, as can MRI. We agree with those who feel that the
mega–cisterna magna (MCM) is simply a large Blake pouch cyst,
possibly resulting from delayed fenestration of the foramina;
however, a few with isolated MCM may show developmental
delay. 27,29,60
Cavum Veli Interpositi
Cavum veli interpositi (CVI) is a small cystic collection in the
midline, seen below the splenium of the corpus callosum, behind
the upper brainstem and above the region of the pineal gland
(Fig. 34.8). It represents luid in the potential space in the telea
choroidea above the third ventricle. Most CVIs are seen just
below the splenium, but they can extend anteriorly above the
third ventricle to the foramina of Monroe. Coronal, midsagittal,
and 3-D scans to help to conirm the diagnosis and MRI and
color Doppler exclude vascular abnormality such as vein of Galen
aneurysm 61 (see Fig. 34.8C). Although infrequently described
in the prenatal ultrasound literature, we have found small CVIs,
with maximal diameter less than 8 mm, to be very common on
axial scans at 18 to 20 weeks. he pediatric literature reports a
CVI incidence of 5% to 34%. 62 Most CVIs are of no clinical
consequence, and many recede spontaneously with normal
long-term neurologic outcome. 62-65 Occasionally, however, large
cysts can distort the brainstem and adjacent brain, causing
obstructive hydrocephalus (HC), and need treatment by
unrooing.
he physiologic CVI cysts tend to be isolated, unilocular, and
small (<10 mm) and remain stable or recede over time. here
is no known genetic association. he diferential diagnosis includes
cysts and cystlike conditions that occur in the midline, such as
dilated cavum Vergae, glioependymal cysts, arachnoid cysts,
cystic tumors (mainly cystic teratomas), vein of Galen aneurysm,
pineal cyst, and hemorrhage. Pathologic cystic collections are
generally larger, enlarge over time, and have associated abnormalities
such as corpus callosum dysgenesis, ventriculomegaly (VM),
or solid masses. 61,62,64,65