Diagnostic ultrasound ( PDFDrive )

1382 PART IV Obstetric and Fetal Sonography
FIG. 40.4 Patterns of Limb Shortening. Left to right, Normal, mesomelia, rhizomelia, mild micromelia, mild and curved micromelia, and severe
micromelia. (Courtesy of J. Tomash, MD, University of Toronto.)
from OI and thanatophoric dysplasia, which are both more
common. Bone fractures may appear as angulations or interruptions
in the bone contour or as thick, wrinkled contours
corresponding to repetitive cycles of fracture and callus formation
(see Fig. 40.2F). Decreased or absent acoustic shadowing is a
marker for decreased mineralization of the long bones. When
evident, this is helpful in narrowing the diferential diagnosis,
but in its absence the bone mineralization may still be
abnormal.
he spine is assessed for segmentation anomalies, kyphoscoliosis,
platyspondyly (lattened vertebral bodies), demineralization,
myelodysplasia, and caudal regression syndromes.
Although platyspondyly is the most common spine abnormality,
it is a challenging prenatal diagnosis. 37 Demineralization of
the spine can result in the appearance of ghost vertebrae or
nonvisualization of one or all of the three ossiication centers
(Fig. 40.5). A progressively narrowed lumbar interpedicular
distance is associated with homozygous achondroplasia; a
widened interpedicular distance is associated with myelodysplasia.
hree-dimensional imaging may provide a large ield of
view and additional details that may be helpful in achieving a
diagnosis (Fig. 40.6).
he most reliable prognostic indicator of the lethality of a
given skeletal dysplasia is pulmonary hypoplasia. Ultrasound
is 85% to 95% accurate in the diagnosis of a lethal skeletal dysplasia
on the basis of pulmonary hypoplasia. 8,9 he thoracic circumference
is measured at the level of the four-chamber heart and
compared to nomograms (see Chapter 36, Table 36.2). A thoracicto-abdominal
circumference ratio of less than 0.8 is considered
abnormal. he thoracic length (from the neck to the diaphragm)
is also measured, and the ribs are assessed to determine if they
are short. At the level of the four-chamber cardiac view, the
ribs should normally encircle at least 70% of the thoracic
circumference. 38 he ribs remain in a relatively horizontal plane,
as does the cardiac axis, facilitating this evaluation. In a sagittal
view, a markedly narrowed anteroposterior diameter of the thorax
is associated with pulmonary hypoplasia. In the coronal view, a
concave or bell-shaped contour is associated with pulmonary
hypoplasia (Fig. 40.7).
he hands and feet are examined for deformities such as
clubfoot or clubhand. A hitchhiker thumb, or abducted thumb,
is associated with diastrophic dwarism. Fixed postural deformities
may suggest the diagnosis of arthrogryposis multiplex
congenita. Polydactyly is associated with short-rib polydactyly
syndromes, Ellis–van Creveld syndrome, asphyxiating thoracic
dystrophy, and some chromosomal abnormalities.
he fetal cranium is assessed for the presence of macrocranium,
frontal bossing, cloverleaf skull deformity, underlying
brain abnormalities, and facial abnormalities, such as saddle
nose, hypertelorism, and clet lip and palate. An abnormal
cranial contour may indicate craniosynostosis or premature
fusion of the sutures. he most reliable sonographic sign of
demineralization is increased compressibility of the calvarium.
his inding is typically present in fetuses with osteogenesis
imperfecta type II and hypophosphatasia. he falx may appear
abnormally bright or echogenic compared to the demineralized
calvarium.
he ribs are assessed to ensure an adequate length, thus
minimizing the risk of pulmonary hypoplasia, and are examined
for abnormal number or appearance. 39 he inding of an abnormal
number of fetal ribs is an isolated inding of no clinical importance
in the majority of cases, 39 associated with minor anomalies in a
smaller group (29%) and only occasionally associated with severe
malformations. Associated syndromes include Poland syndrome,
VACTERL association (vertebral abnormalities, anal atresia,
cardiac abnormalities, tracheoesophageal istula, renal agenesis