Diagnostic ultrasound ( PDFDrive )

CHAPTER 40 The Fetal Musculoskeletal System 1381
A
B
FIG. 40.3 Foot Length Measurement. From the skin edge overlying the calcaneus to the distal end of the longest toe. (A) Sagittal measurement;
note the normal squared appearance of the heel. (B) Plantar measurement.
not have skeletal dysplasia. However, further deviation from the
mean by at least 1 SD suggests the presence of a skeletal dysplasia
or severe IUGR. When FL measures below 4 SD for gestational
age, there is a high likelihood of a skeletal dysplasia. Kurtz et al. 25
have shown that the number of millimeters below the −2 SD
line is a simple screening tool to evaluate femoral shortening
with the following guidelines:
• If FL is 1 to 4 mm below the −2 SD point, further serial
measurements are required to determine if a skeletal
dysplasia is present.
• If FL is more than 5 mm below the −2 SD point, there is
a high likelihood of a skeletal dysplasia.
he most common cause of a so-called short femur is either
inaccurate dating or a normal variant in a constitutionally small
fetus, which may be associated with a parental or family history
of less-than-average stature. In about 13% of cases, a remeasurement
will bring the FL into a normal range, likely representing
a false-positive diagnosis rather than a growth spurt. 27 Isolated,
symmetrical short femurs identiied at the second midtrimester
ultrasound evaluation are helpful in identifying a group of fetuses
at increased risk for low birth weight, small for gestational age,
or severe IUGR. 27-29 Typically, these fetuses will also have small
abdominal circumference measurements.
Occasionally, fetuses with severe IUGR will have greatly
shortened long bones. 30 Associated indings of normal or decreased
skin fold measurements, oligohydramnios, abnormal placental
morphology, and abnormal Doppler waveforms suggest the
diagnosis of IUGR, 31 whereas redundant, thickened skin
folds and polyhydramnios typically accompany short-limb
dysplasias.
Nonlethal skeletal dysplasias such as heterozygous achondroplasia
are oten not evident before 20 weeks’ gestation. he
indings of short long bones before 20 weeks indicate a more
serious and usually fatal skeletal dysplasia. As a rule, the earlier
the detection of limb shortening, the worse is the prognosis.
Virtually all cases diagnosed in irst trimester are considered
severe skeletal dysplasias, with the large majority representing
lethal conditions. 32 First-trimester skeletal biometry tables are
available. 33 Although mild, isolated shortening of the femur
indicates increased risk for trisomy 21 by 1.5-fold, other factors
are more important for assessing this risk. 34
he pattern of limb shortening should be assessed to determine
which long-bone segments are most severely afected 9,35
(Fig. 40.4). Rather than millimeters, we ind it useful to standardize
measurements to “weeks of size” to determine disproportion.
here are four main patterns of shortening of the long bones:
rhizomelia, shortening of the proximal segment; mesomelia,
shortening of the middle segment; acromelia, shortening of the
distal segment; and micromelia, shortening of the entire limb
(mild, mild/bowed, or severe).
Patterns of Limb Shortening
Rhizomelia: shortening of proximal segment (femur,
humerus)
Mesomelia: shortening of middle segment (radius, ulna/
tibia, ibula)
Acromelia: shortening of distal segment (hands, feet)
Micromelia: shortening of entire limb (mild, mild/bowed, or
severe)
he shape, contour, and density of the bones should be assessed
for the presence of bowing, angulation, fracture, or thickening.
Bowing is a nonspeciic inding, typically caused by underlying
osseous fragility. Although more than 40 distinct disorders are
associated with bowed, bent, or angulated femurs, most cases
can be accounted for by three relatively frequent disorders:
campomelic dysplasia, thanatophoric dysplasia, and osteogenesis
imperfecta (OI). 36 Patients with OI types I and IV can
present with apparently isolated in utero bowing of the long
bones (especially the femur) without frank fractures (see Fig.
40.2D). Anterior bowing of the tibia, femur, and humerus may
suggest the diagnosis of campomelic dysplasia; other more speciic
indings such as hypoplastic scapulae and cervical kyphosis
are typically present and help distinguish campomelic dysplasia