ultrasound diagnosis of fatal anomalies

1
2
3
4
5
ULTRASOUND SCREENING
Second Screening (18–21 Weeks)
In addition to confirming a viable pregnancy, the
aim of this screening is to evaluate the number of
fetuses, placental location, and—most importantly—disturbances
in fetal development and
anomalies, as well as assessment of the fetal
heart and amniotic fluid.
For biometric assessment, biparietal and
fronto-occipital diameters, abdominal circumference
and femur length are measured.
Measurements of other long bones may also
prove useful.
For an effective survey of fetal anatomy, ultrasonography
should be performed systematically.
This is reported as follows.
Head: Important structures here are the middle
echo, thalamus, lateral ventricles (normal width
of the posterior horn up to 8–10 mm), size and
form of the cerebellum, and the septum pellucidum.
Eyes, nose, and lips should also be visualized.
Neck: Measurement of nuchal translucency in
second-trimester screening is less predictive
and cannot be analyzed statistically (Snijders
and Nicolaides), but it should still be visualized
as an extra parameter.
Thorax: The axis and location of the heart and
lungs in the thoraxshould be identified. To exclude
cardiac situs abnormalities, it is important
to understand the fetal position within the
uterus as well as the orientation of fetal heart
within the thorax. Echocardiography is performed
as an extra examination. The diaphragm
is seen most easily in the longitudinal plane. For
diagnosis of diaphragmatic hernia, a knowledge
of the topography of the thoracic and abdominal
organs is essential (mediastinal shift, displacement
of the heart and stomach).
Abdomen: The stomach, liver, and gallbladder
are readily detected in the upper abdomen, the
kidneys lying caudally and dorsally. To visualize
the kidney lying furthest from the transducer
overshadowed by the vertebral column, the
position of the transducer has to be changed
often according to the fetal position. Doppler
flow evaluation of renal arteries can also be
useful in detecting renal anomalies such as
double kidney, renal agenesis, and pelvic kidney.
The insertion of the umbilical cord into the
fetal abdomen, the bladder, and the genitals
are localized more caudally. The umbilical
arteries can be readily detected lateral to the
bladder using Doppler flow, much more easily
than in an umbilical cord floating free in amniotic
fluid.
Spine: It is essential to examine the spine along
its entire length, starting from the cervical region,
and in a transverse plane. It is important to
note that the three hyperechoic centers seen on
ultrasonography represent the vertebral body
and the laminae, but that the vertebral spines
are not visualized. Neural tube defects are often
detected as widening of the ossification centers
of the laminae. In addition, soft tissue and skin
overlying the spine should be viewed carefully.
Examination of the head with regard to shape
and form, as well as the configuration of the
cerebellum and the ventricles, is important in
detecting neural tube defects secondary to “head
signs.” Most neural tube defects are located in
the lumbosacral region; lesions in the cervical
and thoracic areas are an exception.
Extremities: Evaluation of the extremities in second-trimester
screening can be difficult and
time-consuming depending on the fetal position
and maternal condition for sonography.
Nevertheless, to detect anomalies of the extremities,
all long bones, including bones of both the
lower arm and lower leg, as well as fingers and
toes, should be demonstrated. Measurement of
the middle phalanxof the fifth finger as a
method of screening for Down syndrome is
neither practicable nor relevant, as the differences
between normal and abnormal findings
are too small to provide reasonable sensitivity
and specificity.
Fetal echocardiography: A four-chamber view of
the heart is obtained, and the location of the
heart is confirmed (dextrocardia?). To measure
the atria and ventricles, biometric assessment in
B-mode is helpful. The four-chamber view
makes it possible to monitor the function of the
atrioventricular (AV) valves (stenosis or insufficiency)
using CFM. To exclude a ventricular septal
defect (VSD), the interventricular septum has to
be demonstrated, especially below the level of
the atrioventricular valves. For this, it is essential
to visualize the heart from the side, as the apical
8