Diagnostic ultrasound ( PDFDrive )

1392 PART IV Obstetric and Fetal Sonography
FIG. 40.17 Osteogenesis Imperfecta Type IIA at 32 Weeks.
Postmortem radiograph shows severe micromelia, thickened bones
with wavy contours caused by innumerable fractures and exuberant
callus formation, shortened ribs with multiple fractures, and
platyspondyly.
of type 1 collagen (Table 40.5). Most of these forms it within
the Sillence OI phenotypic classiication and can still be categorized
as OI type I, II, III, or IV despite several having a recessive
mode of inheritance. Type V OI is phenotypically distinguishable
with hyperplastic callus formation dense metaphyseal bands and
ossiication of the interosseous membranes of the forearm, and
has an autosomal dominant form of inheritance. 70,71 A modiication
of the Sillence classiication, based on skeletal radiographic
indings, is still used most oten to distinguish the subtypes
of OI. 71,72
Prenatal diagnosis is possible based on DNA obtained from
chorionic villus sampling 70 or amniocentesis. Osteogenesis types
I, III, and IV are further described in the section on nonlethal
skeletal dysplasias.
OI type II is the classic neonatal lethal form and usually results
from a new, dominant null mutation in the COL1A1 gene. 72 he
empiric recurrence risk is 6%, most of which are caused by
parental germline and somatic mosaicism but can also be the
result of one of the autosomal recessive forms of OI. Multiple
repetitive in utero fractures occur secondary to defective collagen
formation, which results in osseous fragility. Prevalence of OI
type II is 0.18 in 10,000 births. he key ultrasound features are
severe micromelia, decreased thoracic circumference and trunk
length, decreased mineralization, and multiple bone fractures.
he cranial vault remains normal in size, but as a result of
decreased mineralization there is increased visibility of the
intracranial contents and the skull is compressible by the ultrasound
transducer (Fig. 40.19). he falx may appear bright or
more echogenic than the demineralized cranial vault, with unusual
clarity of detail in the near ield cerebral hemisphere. Large
fontanelles and wormian bones may be noted. Micrognathia is
commonly present (Fig. 40.20).
he generalized demineralization results in innumerable
fractures. he tubular bones exhibit a classic “accordion” or
wrinkled contour caused by multiple in utero fractures with
repetitive callus formation. Angulation and bowing are common
in association with severe micromelia. On ultrasound, the bones
may appear thickened because demineralized bone relects sound
waves less than a normally ossiied bone. Acoustic shadowing
may be present, absent, or diminished and thus is an unreliable
sign. Multiple rib fractures cause the lateral chest contour to be
concave rather than convex. he concavity is oten most evident
at the lateral thorax, and it is speculated that the elbows “bash”
in the fragile rib cage. he ribs are hypoplastic, thus appearing
shortened. he ribs may have a continuous, beaded, or wavy
appearance secondary to repetitive fractures and callus formation.
Platyspondyly secondary to multiple compression fractures may
be present.
he three criteria for a speciic diagnosis of OI type II are (1)
FL greater than 3 SD below the mean, (2) demineralization of
the calvarium, and (3) multiple fractures within a single bone. 72
he diagnosis may be made as early as 13 to 15 weeks’ gestational
age. A normal ultrasound examination ater 17 weeks excludes
this diagnosis.
Hypophosphatasia
Hypophosphatasia congenita, the lethal neonatal form of
hypophosphatasia, is an autosomal recessive skeletal dysplasia
caused by a deiciency of tissue-nonspeciic alkaline phosphatase. 73
Frequency of hypophosphatasia congenita is 1 in 100,000 births.
he key features are severe micromelia, decreased thoracic
circumference with normal trunk length, and decreased mineralization
with occasional fractures. Cranial vault size remains
normal.
he demineralized long bones may be bowed with occasional
angulations caused by fractures. he bones appear thin and
delicate and may be diicult to visualize. he cranial vault fails
to mineralize and may be compressible under locally applied
transducer pressure. In contrast to OI, the demineralization in
hypophosphatasia congenita varies from a patchy distribution
to a difuse form with severe involvement of the spine and
calvarium. he ribs are short, resulting in a decreased thoracic
circumference, but the trunk length is normal. Polyhydramnios
is a common inding.
he main diferential diagnosis is OI type II. Both hypophosphatasia
and OI type II display a severe form of micromelia,
demineralization, decreased thoracic circumference, and a
normal-sized cranial vault that is compressible due to demineralization.
In OI type II, the greater degree of osseous fragility
results in innumerable fractures and a thickened, wavy appearance
of the bones, in contrast to the thin, delicate appearance of the