Diagnostic ultrasound ( PDFDrive )

1228 PART IV Obstetric and Fetal Sonography
detected by routine sonography during the second trimester
without knowledge of MS-AFP values. Sixty-nine (37%) were
diagnosed by targeted sonography ater MS-AFP screening
indicated a higher risk for NTD. Two (1%) were diagnosed by
pathology examination ater miscarriage.
MS-AFP is also elevated in multifetal pregnancy, fetal death,
fetomaternal transfusion, and in other fetal anomalies associated
with a defect in the skin, such as omphalocele and gastroschisis
(50%-60% of cases), congenital nephrosis (Finnish type, 100%
of cases), and infrequently in esophageal or duodenal atresia,
polycystic kidney disease, renal agenesis, urinary obstruction,
epidermolysis bullosa, sacrococcygeal teratoma, cystic hygroma,
osteogenesis imperfecta, cloacal exstrophy, and cyclopia.
Because of the high sensitivity of the cerebellar signs associated
with open spina biida, some centers rely almost exclusively on
ultrasound to diagnose NTDs. For women with elevated MS-AFP
and no sonographic explanation for the abnormal test result
(e.g., wrong dates, multiple fetuses, dead fetus, anencephaly, spina
biida, abdominal wall defect, other fetal abnormality causing
elevated AFP), or when there is poor visualization of the spine,
amniocentesis may be ofered. If the amniotic luid AFP is normal
and there is no acetylcholinesterase (AChE) present, the likelihood
of an open NTD is very low. If the amniotic luid AFP is
elevated and AChE is present, an open NTD or abdominal wall
defect may be present but undetected by sonography.
Between 1989 and 1990, 1.1 million women in California had
MS-AFP tests in early pregnancy. 63 From these tests, 1390 fetal
abnormalities were found (1.3 fetal anomalies per 1000 pregnancies),
consisting of 710 NTDs (417 cases of anencephaly, 247
cases of spina biida, and 46 cases of encephalocele) and 680
nonneural abnormalities (286 anterior abdominal wall defects,
163 cases of trisomy 21, and 231 other chromosomal abnormalities).
With the declining use of MS-AFP testing in favor of genetic
screening, sonography is increasingly important in screening
for NTDs.
Sonographic Findings in the Spine
Spina biida may occur anywhere in the fetal spine but is most
common in the lumbosacral area. 64 Ultrasound indings in the
spine consist of abnormalities of the ossiied posterior elements
and related sot tissues.
In spina biida the laminae fail to converge toward midline,
and this is best visualized with the posterior transaxial scan
plane (Fig. 35.13A and B, Video 35.3). If the pedicles are normally
positioned and there is no myelomeningocele, the posterior
transaxial scan plane is the only view that will depict the abnormality
with reliability. When the pedicles are displaced more laterally
than usual, the lateral transaxial and lateral longitudinal scan
planes will also demonstrate the bony abnormalities of spina
biida (Fig. 35.13C and D). All these scan planes will usually
demonstrate the meningocele or myelomeningocele if it is present
(Figs. 35.14, 35.15, and 35.16; Videos 35.4 and 35.5). he posterior
longitudinal scan best demonstrates a myelomeningocele and
the sot tissue defect when no cystic mass is present.
In most cases of spina biida, there is abnormal divergence
or splaying of the pedicles over several vertebral levels. his is
best appreciated in 3D images and in lateral longitudinal views,
where multiple interpedicular distances can be evaluated simultaneously
(see Fig. 35.13). However, there is normally mild
divergence of the pedicles in the cervical spine compared with
the thoracic spine, and there may be slight divergence (by 1 to
2 mm) in the lumbar spine compared with the thoracic spine
in normal fetuses (see Fig. 35.2).
Sonography can also be used to determine the level and extent
of the spinal abnormality. he level of the defect is taken to be
the top or most cephalic end of the bone malformation. 65 Fetal
MRI and fetal ultrasound are equally efective in determining
the lesion level in a fetus with myelomeningocele, 66 although
each modality may misdiagnose the upper level by two or more
segments in 20% of cases. Fetal MRI is more sensitive in evaluation
of the spinal cord itself, yielding additional information in about
10% of cases. 67
he prognosis is inluenced by the defect level, the NTD type,
the presence or absence of associated anomalies, the presence
or absence of chromosomal abnormalities, and the presence or
absence of cranial abnormalities, such as Chiari II malformation
and ventriculomegaly. Biggio et al. 68 described the outcome in
33 infants with isolated open spina biida. Lower lesion levels
and smaller ventricular size were associated with ambulatory
status. All infants with L4-sacral NTD were ambulatory. Of
patients with L1-L3 NTD, 50% were ambulatory. No infant with
thoracic NTD was ambulatory. No infant with myeloschisis was
ambulatory.
Associated Cranial Abnormalities
he biparietal diameter may be less than expected (even when
the lateral ventricles are enlarged). 69,70 Four other cranial indings
are particularly useful in raising suspicion of an NTD: nonvisualization
of the cisterna magna, deformation of the cerebellar
shape (banana sign), concave frontal bones (lemon sign), and
dilation of the lateral cerebral ventricles.
Chiari II malformation is highly associated with open spina
biida (>97% of cases). his cranial lesion consists of variable
degrees of displacement of the cerebellar vermis, fourth ventricle,
and medulla oblongata through the foramen magnum into the
upper cervical canal and is usually easier to identify than the
spinal lesion between 16 and 24 weeks’ gestation. In transaxial
scans through the posterior fossa, Chiari II malformation is
manifest as a deformation of the cerebellar shape (banana sign)
and nonvisualization of the cisterna magna. Cranial malformations
may signal the sonographer that a detailed study of the spine is
required to search for spina biida.
Anatomic Landmarks Used to Establish Level
of Bony Defect
• T12 corresponds to the medial ends of the most caudal
ribs.
• L5-S1 lies at the superior margin of the iliac wing. 28
• S4 is the most caudal vertebral body ossiication center
in the second trimester. 65
• S5 is the most caudal vertebral body ossiication center
in the third trimester. 65
Thoracic (T), lumbar (L), and sacral (S) vertebrae.