Diagnostic ultrasound ( PDFDrive )

1030 PART IV Obstetric and Fetal Sonography
Beneits of Routine Second-Trimester
Ultrasound Screening
• More accurate gestational age
• Detection of major malformations before birth
• Earlier detection of multiple pregnancy
• Fewer low-birth-weight singleton births
• Lower incidence of induction for postterm pregnancy
• Early detection of placenta previa
• Reassurance of a normal pregnancy
It can be diicult to interpret the results of studies designed
to assess the impact of routine screening. 3,16,21 Not only do
anomalies need to be detected by ultrasound, but to show the
beneit of ultrasound there must be a documented diference in
outcome, either in termination of pregnancies (potentially leading
to decreased perinatal mortality from loss of anomalous fetuses)
or in improved perinatal care. Because these studies do not
necessarily control for these outcomes, the beneits of screening—
in particular the importance of parental understanding of fetal
anomalies when pregnancies with fetal anomalies are continued—
are diicult to demonstrate.
he Helsinki Ultrasound Trial reported a signiicant decrease
in perinatal mortality among the ultrasound-screened group,
from 9 to 4.6 per 1000. 21 his was attributed to the relatively
high rate of detection of fetal anomalies in that study (58% of
major malformations were detected before 24 weeks) with
subsequent termination of fetuses with anomalies.
In the Routine Antenatal Diagnostic Imaging with Ultrasound
(RADIUS) trial, the investigators did not ind a signiicant difference
in “adverse perinatal outcome” (deined as fetal death,
neonatal death, or neonatal morbidity) in the screened versus
control groups. he explanation for the lack of improved outcome
was the limited sensitivity of routine sonography in the detection
of congenital abnormalities (16.6% before 24 weeks and 34.8%
before 40 weeks) coupled with a low rate of pregnancy termination
once the diagnosis had been made. 3 A subsequent meta-analysis
based on four randomized clinical trials with data on 15,935
women (7992 were allocated to routine sonography vs. 7943 to
selective scanning) found the perinatal mortality rate was signiicantly
lower in patients allocated to routine scanning, again
because of the early detection of fetal abnormalities that led to
induced abortions. 25 he authors concluded that routine ultrasound
scanning is efective and useful as a screening test for
malformations.
Fetal Malformations: Diagnostic Accuracy
he incidence of major congenital abnormalities at birth in the
general population is 2% to 3%, yet these abnormalities are
responsible for 20% to 25% of perinatal deaths and an even
higher percentage of perinatal morbidity. Prenatal detection of
an anomaly increases the options for pregnancy management,
and in select cases the disorder may be amenable to intrauterine
treatment. For these reasons, ofering routine ultrasound as a
screening test for congenital abnormalities is an attractive concept.
However, the performance of screening ultrasound in detecting
abnormalities in the low-risk population is variable, with sensitivity
and speciicity ranging from 14% to 85% and 93% to 99%,
respectively. 3,24,26-32 he wide range in sensitivity can be partially
explained by what authors used as the deinition of an “anomaly”
and the experience of the individuals performing and interpreting
the studies. 24,31 Another factor is the type of anomaly. In the
Eurofetus study, 33 the best detected abnormalities were of the
urinary system (88.5%) and central nervous system (88.3%).
Cardiac abnormalities were not well detected, whether major
(38.8%) or minor (20.8%), and the lowest rates of detection were
for minor abnormalities of the musculoskeletal system (18% vs.
73.6% for major defects) and clet lip and palate (18%). 33 Another
important issue is the gestational age when the study is performed.
In the Eurofetus Study, for example, 38.5% of the anomalies were
diagnosed ater 29 weeks’ gestation. Other factors inluencing
sensitivity of prenatal sonography include the quality of equipment,
prevalence of a particular defect, maternal body habitus,
and examination protocol. 24,33,34
Many of the beneits of ultrasound are nonquantiiable. Having
time to adjust prenatally to information about an anomaly can
improve both the clinician’s and the parents’ approach to the
pregnancy and birth, as well as their abilities to make decisions
about prenatal and postnatal treatment. 35 It is important for
patients and their physicians to understand the limitations of
ultrasound. Not all abnormalities can be detected. he accuracy
of prenatal ultrasound is variable and oten depends on where
and by whom it is being performed.
Three-Dimensional Ultrasound
In addition to two-dimensional (2D) images, 3D imaging allows
for reconstructed images in planes that were not previously
available. his allows for improved visualization of facial anomalies 36
and anomalies of the hands, feet, and spine. 37 In addition, 3D
images may be more comprehensible to the patient, promoting
better understanding of the abnormality. 36,37 4-dimensional imaging
is just viewing the 3D images over time and rarely adds additional
diagnostic value. Cervical assessment is also thought to be more
complete with volume imaging. 38 Volume imaging can be used
to assess the lungs, 39,40 which is used in fetuses with suspected
pulmonary hypoplasia. Reconstructed images can be helpful to
image portions of the brain. 41 Subsequent chapters integrate 3D
images as appropriate.
Prudent Use of Ultrasound
he AIUM, ACR, and ACOG collaborative guidelines on performance
of obstetric ultrasound state, “his diagnostic procedure
should be performed only when there is a valid medical indication,
and the lowest possible ultrasonic exposure setting should be
used to gain the necessary diagnostic information under the ‘as
low as reasonably achievable’ (ALARA) principle.” 5 Although
there is no reliable evidence of physical harm to human fetuses
from diagnostic ultrasound imaging using current technology,
public health experts, clinicians, and industry representatives
agree that casual use of sonography, especially during pregnancy,
should be avoided. he U.S. Food and Drug Administration
(FDA) views the promotion, sale, or lease of ultrasound equipment
for making “keepsake” fetal videos as an unapproved use of a