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210
Chapter 5 Anomalies of Dorsal Induction
Table 5–7. PROGNOSIS FOR FETUSES WITH
CEPHALOCELE
Prognosis Better Prognosis Worse
Cranial meningocele Cranial meningoencephalocele
Small nubbing of
dysplastic glial or
neuronal tissue in hernia
Cephalocele diameter
<5 cm
No associated anomalies Concurrent microcephaly or
Normal ventricles In utero ventricular enlargement
is a result of a series of interrelated time-dependent defects
in the development of the ventricular system;
Larger portions of clearly
recognizable brain in hernia
Cephalocele diameter >5 cm
holoprosencephaly
131
essentially,
the inability to maintain distention of the ventricular
system because of CSF leaks results in the lack of development of the posterior fossa, as well as abnormalities of
neural and calvarial development.
Etiology
The etiology for the Chiari II malformation is the open
NTD itself. The etiology for the NTD has been described
above.
Associated Anomalies
Associated brain abnormalities seen with Chiari II malformation can be divided into those of the skull, which
includes a small posterior fossa, low-lying tentorium
cerebelli, and enlarged foramen magnum; those involving
the cerebral hemispheres, such as polymicrogyria, cortical
heterotopias, and dysgenesis of the corpus callosum;
and those involving the posterior fossa, such as descent
of the cerebellar vermis through the foramen magnum,
displacement of the superior cerebellum through the
tentorium, and aqueductal stenosis. Table 5–8 has a
complete list of associated anomalies.
80% to 90% of children with meningomyelocele and the
Chiari II anomaly will develop hydrocephaly.
123
Approximately
128
There are
multiple reasons for the hydrocephaly in Chiari II, such as
obstruction of the outlet of the fourth ventricle, blockage
of the cerebellar aqueduct, and obstruction at the level
of the abnormal tentorium.
are congenital scoliosis or kyphosis, hip deformities, and
clubfoot.
122 , 132
131
Other non-CNS anomalies
Sonographic Diagnosis
TVS can image the rudimentary neural tube by the sev-
enth postmenstrual week of gestation (see Chapter 2 ). The
vertebral column can be imaged by the 9th to 10th postmenstrual week; using a median plane, the posterior fetal
contour, including the covering skin, can be imaged well
enough to detect major neural tube abnormalities.
From the late first to the early second trimester, the fetal
spine can be scanned in the three planes (sagittal, coronal,
and transverse) using conventional 2D imaging; however,
due to fetal position and/or maternal body habitus, not
all planes may be accessible in all cases. However, 3D US
presents obvious advantages ( Figure 5–34 ). In the sagittal plane, the vertebral column appears as two echogenic,
parallel lines, flaring toward the upper cervical spine and
converging toward the sacrum ( Figure 5–35 ).
coronal plane, it appears as two or three (depending on the
depth of the scan) parallel bands of echoes corresponding
to the body, one on each side of the posterior vertebrae
arch ( Figure 5–36 ). In the transverse plane, the intact
vertebral arch is represented by a triangular configuration
of three echoes, forming a closed circle around the neural
canal ( Figure 5–37 ).
115
45 , 133 , 134
135
In the
AB C
Figure 5–33. A patient with a normal maternal serum alpha-fetoprotein. The US revealed a closed neural tube defect. These pictures were taken at
31 weeks, 2 days. (A) The fetal head shape and the cerebellum and posterior fossa appear normal. (B) A sagittal view of the lumbosacral region of the
fetal spine shows a large bulge ( arrow ). The fetal skin is contiguous, covering the myelomeningocele. (C) Coronal section shows the splayed vertebrae
as the result of the spinal defect.
Chapter 5 Anomalies of Dorsal Induction
211
Table 5–8. CHIARO-ASSOCIATED CENTRAL
NERVOUS SYSTEM MALFORMATIONS
Malformation
Disorders of the skull
Lückenschädel of the skull
Small posterior fossa
Low-lying tentorium cerebelli with large incisura
Scalloping of the petrous bone
Shortening of the clivus
Enlargement of the foramen magnum
Disorders of the cerebal hemisheres
Polymicrogyria
Cortical heterotopias
Dysgnesis of the corpus callosum
Large massa intermedia
Disorders of the posterior fossa
Descent of the cerebellar vermis through the foramen
magnum
Caudal displacement of pons and medulla
Rostral displacement of superior cerebellum through the
tentorium
Kinking of the brainstem
Loss of pontine flexure
Aqueductal stenosis or forking
Beaking of the tectum
Reproduced, with permission, from McLone DG, Dias MS. The Chiari II
malformation: Cause and impact. Childs Nerv Syst. 2003;19:540–550.
Prenatal diagnosis of spina bifida is possible before
the 12th postmenstrual week by noting irregularities of
the bony spine or a bulging within the posterior contour
of the fetal back in a sagittal view ( Figure 5–38 ).
136
On
transverse sections, the open spine has a U shape, and in
the coronal section, the affected bony segment shows a
divergent configuration replacing the normal parallel lines
of the normal vertebral arches ( Figures 5–39 to 5–48 ).
The diagnostic sensitivities for the prenatal sonographic
detection of open myelomeningocele are reported to be
between 80% and 90% and even higher prior to the knowledge of the MSAFP results.
137 – 139
Determining the site
and the extent of the spinal lesion is important because
they correlate with the neurologic outcome of the fetus.
The higher and the larger the lesion, the more severe the
neurologic dysfunction the neonate will have. The vertebral level can be assessed in a sagittal view of the spine by
(1) counting up from the last ossified vertebral segment
(S4 in the second and S5 in the third trimester) (2) assuming that the last rib corresponds to T12, and the top of the
iliac wing corresponds to L5 to S1.
Kollias et al
140
reported that the pathology and the in utero
140
Using this method,
US assessment of the spinal level agreed in 64% of cases,
and in an additional 14% it correlated to within one vertebra of the lesion. Three-dimensional US can be used to
count the ribs and determine the level of the spinal defect
( Figures 5–42 and 5–43 ).
Spina bifida occulta refers to a spinal anomaly that is
covered with skin and hence with no exposed neural tissue.
If only the spinous process and neural arch of the vertebrae
are affected, this is usually an asymptomatic condition,
rarely diagnosed in utero, and not associated with elevated
MSAFP. However, commonly spina bifida occulta is used as
Vertebral body
C
Figure 5–34.
Canal
Using 3D sonography, the spine is displayed along three orthogonal scanning planes: sagittal ( A ), transverse ( B ), and coronal ( C ).
Skin
BA
Lamina
212
Chapter 5 Anomalies of Dorsal Induction
Canal
AB
Skin
C
Figure 5–35. A volume of a normal fetal spine is displayed using 3D tomography. (A) Transverse section of the fetal abdomen at the level of the stom-
ach; the parallel lines are placed 2 mm from each other. (B)–(D) Display of three sagittal sections of the fetal spine at 2 mm from each other. Note that
the posterior contour of the spine (skin) is seen in its entirety. The bony components of the vertebrae are seen and are also parallel to each other.
a general term to include conditions such as tethered spinal
cord, lipomyelomeningocele, lipomeningocele, thickened
filum terminale, fatty filum terminale, diastematomyelia
(split spinal cord), diplomyelia, and dermal sinus tract
(with involvement of the spinal cord); these conditions can
be associated with significant neurologic dysfunction.
A
D
After the 12th week, there are well-established intracranial sonographic findings that can enhance the detection of spina bifida, namely, the “lemon” sign, the “banana
141 , 142
sign,”
and hydrocephaly ( Figures 5–44 , 5–45 , 5–46 ,
5–47 , and 5–48 ). The lemon sign refers to deformity
of the frontal bone, and the banana sign refers to the
Vertebral
body
B
Ribs
C
Figure 5–36.
the same distance between the sections in (B)–(D). (B)–(D) Three coronal sections of the fetal spine at 1.5 mm from each other. (B) A coronal section
demonstrating the vertebral bodies; due to the normal curvature of the spine, only a part of the vertebral bodies of the fetus are seen. (C), (D) Two more
anterior coronal sections displaying the fetal ribs.
Volume of a normal fetal spine displayed using 3D tomography. (A) Sagittal section. The longitudinal lines are 1.5 mm from each other,
D
Chapter 5 Anomalies of Dorsal Induction
Lamina
213
A
Vertebral body
C
Figure 5–37.
(B)–(D) Three axial (transverse) sections of the fetal spine are 2 mm from each other. These transverse sections through the fetal spine demonstrate the
triangular configuration of the lamina and vertebral bodies enclosing the spinal canal. Note the skin covering the vertebrae.
A volume of a normal fetal spine is displayed using 3D tomography. (A) Sagittal section: The lines are 2 mm from each other.
B
Canal
D
A
B
C
Figure 5–38. A fetus with a sacral spinal defect at 12 postmenstrual weeks. (A) Sagittal section demonstrating a defect over the sacral area of the spine
( arrow ). (B), (C) Because of the gestational age, the typical “lemon” sign is not seen; however, the cerebellum is not clearly seen.
214
Chapter 5 Anomalies of Dorsal Induction
AB
Figure 5–39. Transabdominal sonography shows a sacral defect of a fetus at 30 postmenstrual weeks. (A) Transverse sections through the spinal
defect. Note that the defect is covered by a thin membrane; no skin is covering the defect. In addition, the vertebra has a U-shape. (B) Sagittal section
showing the extent of the spinal lesion ( arrow ). (C) The cerebellum is not seen, as it has completely herniated.
abnormal shape of the flattened cerebellum, which obliterates the cisterna magna. Blumenfeld et al
143
reported
on the diagnosis of NTDs between 12 and 17 weeks by
using the banana and lemon signs. In one case followed
serially from 10 weeks the cerebellum initially appeared
normal, by 12 weeks there was a mild convexity of the
cerebellum, and by 14 postmenstrual weeks, the typical
banana and lemon signs were present. Although only a
single report is currently available regarding the earliest appearance of these cranial findings, it seems that
the “lemon” and “banana” signs may be imaged from 14
postmenstrual weeks. Therefore, from the early second
trimester these indirect cranial findings may be used
to enhance the detection of open NTDs. Because these
findings may be subtle at 14 to 15 postmenstrual weeks,
a follow-up scan later on in the second trimester may
be indicated in cases at risk. The lemon sign is present
in virtually all cases between 16 and 24 postmenstrual
weeks, but after 24 postmenstrual weeks of gestation, the
lemon sign is a less reliable marker and is present in only
13% to 50% of fetuses with spinal defects.
132 , 144 – 146
It is
theorized that the loss of the lemon sign with advancing
gestational age in fetuses with spina bifida is the result of
maturation and “strengthening” of the fetal skull.
contrast, cerebellar abnormalities with obliteration of the
cisterna magna are present all through gestation in 95%
to 100% of cases,
weeks, cerebellar absence is more commonly seen than
the banana sign.
junction with a spinal defect are referred to as the Chiari
II malformation. This malformation is present in almost
every case of thoracolumbar, lumbar, and lumbosacral
myelomeningocele. In other words, the Chiari II malformation is exclusively found in cases of open spina bifida.
The major features include (1) inferior displacement of
the medulla and fourth ventricle into the upper cervical
canal; (2) elongation and thinning of the upper medulla
and lower pons and persistence of the embryonic flexures
of these structures, (3) inferior displacement of the lower
cerebellum through the foramen magnum (banana sign);
and (4) a variety of bony defects of the foramen magnum,
occiput, and upper cervical vertebrae.
probably results from either the hindbrain malformation
that blocks the flow of CSF through the fourth ventricle
or posterior fossa or from aqueductal stenosis that may be
present in 40% to 75% of the cases.
C
144 , 146 – 148
although after 24 postmenstrual
132
These indirect cranial findings in con-
11
The hydrocephaly
11 , 12
144 , 145
In
A
Figure 5–40. Two different cases of spina bifida covered by skin. (A), (B) Sagittal sections showing the bulging defect covered by skin .
B
Chapter 5 Anomalies of Dorsal Induction
215
A
C
B
D
E
F
Figure 5–41. Arnold-Chiari type II malformation at 19 postmenstrual weeks. (A) An occipital–1 section of the brain demonstrating the dilation of the
posterior horns and the extremely thin cerebellum, C. (B) An oblique–1 section showing the dilated ventricular system. Note the thin hyperechoic choroid
plexus above the thalamus, T. Also note that the most dilated horn of the lateral ventricle is the posterior horn, OH. (C) The median section of the vertebral column showing the cystic appearance of the sacral meningocele ( white arrow ). (D) The transverse section of the lesion. The (white long arrow) points
at the membranous coverage of the meningocele. Note the open vertebra ( small white arrow ). (E), (F) A view from the side of the aborted specimen.
216
Figure 5–42.
defect can be determined by counting the ribs and/or vertebral bodies. (B) The image looks at the spine from the inside, revealing the vertebrae and the
interverteal spaces.
Chapter 5 Anomalies of Dorsal Induction
T2
T4
T6
T8
T10
T12
L 1-5
S 1-5
A
By using the 3D radiograph mode or bone display, the ribs and vertebrae are displayed. (A) Using this modality, the level of the spinal
B
Recently, three other sonographically detectable
supratentorial abnormalities that can be imaged with
transabdominal sonography (TAS) when scanning in the
axial plane during 2nd/3rd trimester have been described
in fetuses with Chiari II malformation ( Figure 5–49 ).
149 – 151
The first is the ventricular point, which refers to a pointed
deformity of the posterior horn when scanning axially at
the level of the lateral ventricles; this deformity has previ-
T12
Lumbo
Sacral
ously been described in the magnetic resonance literature
( Figure 5–49A ).
152 , 153
Callen et al
149
found the ventricular
point in 70% of fetuses with myelomeningocele, although
they believe that the real prevalence may be closer to that
described by Levine et al
152
of 92%. Another important
factor about the ventricular point is that it was seen in 75%
of fetuses younger than 24 weeks of gestation. The second
is tectal beaking, or abnormal elongation of the tectum
AB
Figure 5–43.
localized by starting the count at the 12th vertebra. (B) The divergent vertebrae are seen at the level of the defect.
The 3D radiograph or bone display of a fetus at 18 weeks, 3 days, with a large lumbosacral spinal defect. (A) The level of the defect is
Chapter 5 Anomalies of Dorsal Induction
AH
d
3
217
A
Figure 5–44. Arnold-Chiari type II malformation at 24 postmenstrual weeks. Diagnosis: sacral spina bifida. (A) Median section of the lower spine
demonstrating the bulging membranes of the meningomyelocele measuring 3.9 × 3.1 × 3.7 cm ( arrow ). (B) The “lemon” sign of the deformed bone at
the temporal region is shown, as well as dilation of the anterior horns (AH) and the third ventricle.
( Figure 5–49B ); this finding has previously been described
on magnetic resonance imaging (MRI) of neonates with
Chiari II malformation.
154 , 155
The tectum is located in the
dorsal region of the mesencephalon; it consists of the
superior and inferior colliculi. The superior colliculi has
visual receptors, and the inferior has auditory receptors.
Callen et al
150
reported on tectal beaking when scanning
in the axial plane; this is the same findings reported by
others when using MRI. In their study, this finding was
present in 66% of cases with Chiari II malformation; it
was seen with equal frequency before or after 24 postmenstrual weeks, and even in fetuses with normal-size
ventricles, tectal beaking could be seen. Tectal beaking
correlates with the severity of the spinal defect as well
as being the most likely cause of the oculomotor impairment in children with Chiari II defect.
150 , 155
The third
abnormality is the interhemispheric cyst, although this
can be seen in other conditions, such as interhemispheric
arachnoid cyst, dorsal cysts associated with callosal dysgenesis, pineal cyst, and cavum vela interpositi, as well as
in normal fetuses ( Figure 5–49C ).
151 , 156
Wong et al
151
found
that 43% of fetuses with an open spinal defect had the
B
interhemispheric cyst seen; they speculate in their article
that this cyst is part of a dilated third ventricle that is unable
to expand anteriorly due to the massa intermedia, so it
expands posteriorly. Lack of visualization of the intracranial
translucency between 11 and 13 postmenstrual weeks has
recently been described by Chaoui
157 , 158
et al as a new sign
of open spinal defect ( Figure 5–50 ). The fourth ventricle
is identifiable between 11 and 13 weeks as an intracranial
translucency in the customary sagittal section used to
measure the nuchal translucency. In their study, intracranial translucency could be seen in all normal fetuses;
however, in the four cases in which a spinal defect was
diagnosed during the second trimester, the intracranial
translucency could not be seen.
Three-dimensional US has an important role in the
assessment of fetuses with Arnold-Chiari II malformation;
the volume that is obtained can be displayed in multiples
types of display, such as multiplanar or orthogonal planes
( Figure 5–34 ), tomographic US imaging ( Figures 5–35 , 5–36 ,
5–42 , 5–43 , 5–51 , 5–52 , and 5–53 ), radiograph mode (bone),
and surface rendering ( Figures 5–42 and 5–43 ), which may
facilitate the diagnosis of Arnold-Chiari II malformation.
218
Chapter 5 Anomalies of Dorsal Induction
A
B
C
Figure 5–45. A transabdominal scan in a patient at 18 weeks, 3 days with elevated maternal serum alpha-fetoprotein showing the most common
sonographic findings in cases of Chiari II malformation. (A) The fetal head clearly shows the “lemon” sign, which is a deformity of the frontal bones.
(B) In the posterior fossa, the cerebellum ( arrow) shows the “banana” sign, which is the result of an inferior displacement of the cerebellum and vermis
obliterating the cisterna magna. (C) Sagittal view of the spine showing the large spinal defect ( arrow ).
Risk of Recurrence
The risk of spina bifida in the general population is
159
∼0.3%.
increases. If one of the parents has spina bifida, the risk to
the offspring is as high as 4.5%.
However, once there is a family history, this risk
159
If a previous full sibling
is affected, the recurrence risk is ∼2% to 5%; if two previous
siblings are affected, this risk increases to as high as 11%.
The recurrence risk among half-siblings is ∼0.5% to 0.8%.
The risk for second-degree relatives (eg, grandparents and
grandchildren, uncles and aunts, nephews and nieces) is
0.5%, and for third degree (eg, first cousins), it is similar to
that of the general population.
15
Differential Diagnosis
The differential diagnosis of meningomyelocele includes
sacrococcygeal teratoma or a mass affecting the spine
that for Chiari II malformation includes aqueductal
stenosis.
Prognosis
Spinal dysraphism and Chiari II malformation are not
lethal anomalies, although they are associated with
a significant amount of morbidity and mortality.
Approximately 14% of all children will die within the
first 5 years of life in spite of aggressive treatment.
15
Many of the deaths are related to shunt complications
15
(malfunction and infection); among those with brainstem dysfunction secondary to the Chiari II malformation leading to respiratory and swallowing problems, the
mortality rate rises to 35%.
as the result of recurrent urinary tract infection has been
the most common cause of death among individuals who
survive to adulthood.
the hydrocephaly and shunt complications. About 70%
of patients will have IQ >80; however, only about half of
patients will be able to live independently as adults.
Independent mobility is related to the level of the defect.
For those individuals with lesions above L2, there is
160 – 162
In the past, renal failure
162
Cognitive outcome is related to
160 , 161
Chapter 5 Anomalies of Dorsal Induction
219
A
C
B
D
E
Figure 5–46.
thoracic spina bifida. (A) Horizontal transabdominal section of brain demonstrating the dilated lateral ventricles and the dangling choroid plexus
( arrow ). (B) Transvaginal scan using the mcoronal–1 section showing the dangling choroid plexus within the dilated ventricles ( arrow ). (C) The trans-
abdominal scan with a 3.5 MHz probe demonstrates the faint outlines of the meningomyelocele ( arrow ) . ( D) The transvaginal picture was created by a
5MHz probe, which easily images the lesion at the 11th thoracic vertebra ( arrow ). (E) The aborted fetus with the lesion of the thoracic meningomyelocele
and spina bifida.
Arnold-Chiari type II malformation diagnosed at 16 postmenstrual weeks by transabdominal and transvaginal sonography. Diagnosis: