Diagnostic ultrasound ( PDFDrive )

CHAPTER 34 The Fetal Brain 1183
asymmetrical abnormalities that range from minor to massively
disruptive. Cranial manifestations may initially mimic anencephaly,
encephalocele, and other cerebral abnormalities and are oten
associated with asymmetrical facial disruptions. he literature
on the etiology and pathogenesis of these sporadic conditions
is confusing and controversial and includes amniotic rupture,
vascular disruption, and abnormal development of embryonic
body folds. here is a common theme of combinations of amniotic
abnormality and asymmetrical fetal disruptions. Bands may be
seen attached to abnormal body parts, or the fetus is partly ixed
to the uterine wall. Limb–body wall complex terminology is
oten used if there is a more severe malformation consisting of
encephalocele with facial clets, thoracoschisis or abdominoschisis,
short umbilical cord, and limb defects. Oten there are internal
abnormalities that cannot be explained by amniotic bands such
as congenital heart disease, renal agenesis, and intestinal atresias.
Minor variants may have only constrictive amniotic bands
resulting in annular constrictions or focal malformations of limb
parts.
he indings at ultrasound vary with afected areas. Clues to
diagnosis are asymmetrical abnormalities and amniotic bands
attaching to the fetus or fetal parts that are stuck to the uterine
wall (as with limb–body wall complex). he bands can be hard
to see, and technique needs to be optimized and gain increased
to demonstrate them. In some cases fetal anatomy is so disrupted
that parts become unrecognizable and this massive disruption
is the clue to the diagnosis. he condition can mimic anencephaly
or encephalocele, and oten additional intracranial abnormities
are present (see Fig. 34.12C).
he major disruptions are lethal. hree-dimensional ultrasound
and MRI can be helpful in clarifying the extent of the lesions.
Prognosis, counseling, and management depend on the nature
and degree of disruptions. Fortunately, these anomalies are
sporadic, with negligible recurrence risk. 96,119-121
Cranial Changes in Spina Biida
Spina biida or neural tube defect (NTD) is the second most
common birth defect ater congenital heart disease. Two general
groups are recognized: open (not skin covered) and closed (skin
covered). Both result from abnormalities associated with neural
tube closure. Open lesions result from failure of closure of the
neural tube, leaving a defect that leaks CSF and exposes neural
tissue to the damaging efects of amniotic luid. About 80% to
90% are open, appearing as dorsal cyst (myelocele or meningomyelocele
depending on neural content) or rachischisis (uncovered
open defect). Open lesions are commonly associated with elevated
MS-AFP above 2.50 multiples of the median (MoM). he remaining
10% to 20% are closed (occult) lesions that are skin covered
and do not leak AFP and in which MS-AFP levels are normal.
Closed defects result from failure of separation of the neural
tube from the skin and abnormal adjacent mesenchymal tissue
development. hese cause variable spinal, vertebral, and cutaneous
lesions including lipoma, lipomeningocele, hypertrichosis, tags,
dorsal pits, and others. 122
he incidence is about 0.2 to 10 per 1000 pregnancies and
has decreased by about 70% following folate supplementation.
he etiology is multifactorial and includes syndromes, genetic
abnormalities, and maternal and fetal factors including diabetes
and teratogens such as anticonvulsants. 108,123,124
Open spina biida is virtually always associated with the Chiari
II malformation which is a complex deformity involving calvaria,
dura, cerebrum, and hindbrain. In Chiari II malformation the
striking abnormalities are evident in the small posterior fossa
and include herniations of the vermis, pons, and medulla;
brainstem kinking; small fourth ventricle; aqueduct stenosis;
and tectal beaking. here are, however, additional pancranial
changes that involve the skull bones (beaten silver appearance
or Lükenschädel), small posterior fossa, falx (interdigitation of
sulci, low-lying tentorium and transverse sinuses), large massa
intermedia, cerebral hemispheres (hydrocephalus, dysgenesis of
corpus callosum, heterotopias, malformations of cortical development,
and distortion of the limbic system). 125-127 It is believed
that leakage of CSF through the spinal defect leads to a cascade
of events resulting in a small underdeveloped posterior fossa
followed by hindbrain compression and obstruction to CSF low,
which leads to additional cerebral and calvarial abnormalities. 126
Some believe that failure of neural tube closure is not just a
single event, but rather is only part of a spectrum of developmental
abnormalities not yet fully deined but that result in multiple
additional cerebral and somatic abnormalities. 125
Of newborns with spinal NTDs, it was found that 80% had
isolated NTDs. he remaining 20% had additional anomalies
including chromosomal abnormalities or sporadic abnormalities
involving virtually all systems. 123,128 An autopsy study found that
68% of cases with spina biida had non-CNS anomalies, most
commonly skeletal (club feet), but also involving kidneys,
gastrointestinal tract, abdominal wall, and other structures. 129
A recent multicenter prenatal ultrasound study found chromosome
abnormalities in 6.2% of cases (most commonly, trisomy
18 and triploidy) and structural abnormalities in 4.6% (most
commonly heart defects). 130
he actual spinal abnormality of open NTD can be very diicult
to see at screening ultrasound, but the defect leaks CSF containing
alpha-fetoprotein (AFP), which may become detectable in
maternal serum (MS-AFP). his was the basis of earlier serum
AFP screening for open defects. Maternal serum levels greater
than 2.5 MoM at 16 to 18 weeks detected 88% of anencephalic
fetuses and 79% of fetuses with open NTDs but were also found
in 3% of normal fetuses. However, all closed, skin-covered NTDs
were missed by MS-AFP screening. 131
In the 1980s it was found that open NTDs are virtually always
associated with readily detected ultrasonographic cranial changes
of Chiari II malformation in the second trimester (Fig. 34.14,
Video 34.6). his is unlike closed spina biida, in which the brain
and posterior fossa are normal and the spinal abnormality can
be detected only with careful direct examination of the spine
(Fig. 34.15). 132 Improvements and standardization and use of
these cerebral signs now allow second-trimester detection of
open spina biida in 88% to 96% of cases, and even 100% in
some expert hands. Many believe that second-trimester ultrasound
has replaced MS-AFP screening for open lesions (anencephaly,
encephalocele, open spina biida). 130
Many second-trimester cranial signs of spina biida have been
suggested (Table 34.3). he most commonly used are obliteration