One of the most common central nervous system
defects is a neural tube defect. The neural tube is a portion
of the developing embryo that becomes the brain and the spinal
cord. If the neural tube fails to close properly, the result may
be spina bifida. Prenatal screening for neural tube defects can
be based on maternal serum screening or on routine ultrasound.
Approximately 80% of babies with open spina bifida and 90% of
babies with anencephaly have elevated maternal serum AFP (alpha
fetoprotein). Sonography may identify up to 90% of babies with
a spina defect and nearly 100% of babies with anencephaly. It
is important to keep in mind that this detection rate is largely
due to the expertise of the sonographer performing the procedure.
Extensive counseling and amniocentesis are recommended when a
NTD is identified.
Approximately 1/800-1/1,000 individuals in the US are born
with neural tube defects. Neural tube defects are multifactorial
conditions, which means that both a genetic and environmental
components are involved in the etiology of the condition. In
90% of cases, there is no family history of neural tube defects,
so most parents are unaware that they may have a predisposition
for having a child with a neural tube defect. It is also estimated
that approximately 10% of babies with spina bifida have an underlying
chromosome problem. If the baby were to have an underlying chromosome
abnormality as a cause of the spina bifida, and we would expect
the baby to have more significant health concerns than babies
with an isolated spina bifida.
Neural tube defects are described using often very complicated
terms. These terms help healthcare professionals describe the
specific nature of each defect. For example, the meninges is
the membranes that surround the brain and spinal cord while
the encephalon refers to the brain itself, and the prefix myelo-
refers to the spinal cord itself. Therefore a meningocele is
the protrusion of the meninges (membranes surround the brain
and/or spinal cord) while a meningoencephalocele involves the
protrusion of the membranes as well as the brain itself.
Anencephaly
and Exencephaly 
Anencephaly and exencephaly are very severe forms
of a neural tube defect and can often be diagnosed in the first
trimester. These neural tube defects are identified when the
cranial vault (portion of the skull which holds the brain) cannot
be seen. Often times the face itself, including the orbits (eyes),
can be seen. When brain appear flat, the term anencephaly is
used. When brain appear as an irregular bulging structure, the
term exencephaly is usually preferred. Unfortunately the prognosis
for these babies is extremely poor. Polyhydramnios is frequently
seen with this condition, as well as spina bifida and cleft
lip and palate.
This is a neural tube defect where the meninges (membranes that
surround the brain and spinal cord) as well as the brain protrude
through a hole in the skull. This is sometimes called an encephalomeningocele.
In Caucasians, meningoencephalocele is usually located at the
back portion of the skull. Large encephaloceles can be diagnosed
rather easily in the second or occasionally even in the first
trimester. Smaller lesions may remain undiagnosed. Establishing
their prognosis prenatally consists in evaluating the impact of
the lesion on brain development. In cases of an isolated meningocele
or small and isolated encephalocele the prognosis may be virtually
normal. Ruling out associated abnormalities is essential, since
encephalocele may be part of a multiple malformation such as Meckel-Gruber
syndrome.
Myelomeningocele is often identified on a second
trimester ultrasound with a collection ultrasound findings including
a “lemon sign”, Arnold-Chiari malformation, and open
spinal defect. A “lemon sign” is when the head shape
is slightly pinched in at the sides of the forehead of the baby
and an Arnold-Chiari malformation is the displacement of the hindbrain
into the spinal column. The problems associated with a myelomeningocele
are variable, and it is known that they are associated with the
size and position of the opening. In general, the higher the defect
on the spine the more severe the handicap. Most myelomeningoceles
are located in the lumbarsacral region of the spine. Problems
associated with myelomeningoceles usually include paralysis of
the lower limbs and incontinence of the bowel and bladder. If
the spina bifida is isolated, the majority of individuals have
normal intelligence.
One should keep in mind that spina bifida occulta, which has
a better neurological outcome than myelomeningoceles can also
be detected by ultrasound, and should be identified as such. Spina
bifida occulta involves the meninges without involvement of the
spinal cord itself.
Hydrocephalus
Aqueductal
stenosis is the most common form of obstructive hydrocephaly
and results from narrowing of the aqueduct of Sylvius, which
connects the third and fourth ventricles.
Aqueductal stenosis may be inherited as an X-linked
recessive condition, a component of an autosomal
recessive syndrome, or a multifactorial disorder. Also,
intraventricular hemorrhage, intrauterine infection, and compression
by adjacent masses may be possible etiologies of this condition.
Sonographic findings of aqueductal stenosis include third ventricular
and lateral ventricular dilatation. This dilation is usually
severe and progressive, and it usually can be identified during
the second trimester. However, if the etiology is intraventricular
hemorrhage or infection, the abnormality may not be present
until the third trimester (or even evolve after birth). Fetuses
with aqueductal stenosis have a good survival rate, because
they usually do not have associated abnormalities. However,
neurological outcome varies. Males with X-linked hydrocephalus
generally have a less favorable prognosis, and flexion abnormalities
of the thumb.
Communicating hydrocephalus, also known as external
hydrocephalus, is the result of obstruction of the flow of Cerebral
Spinal Fluid (CSF) outside the ventricles or an impaired reabsorption
of CSF. Findings of communicating hydrocephalus are dilatation
of the subarachnoid
space and the lateral, third, and fourth ventricles. The
etiology of this condition is unknown, and familial transmission
is rare. If the communicating hydrocephalus is isolated, the
prognosis is better than with other forms of hydrocephalus.
If other anomalies are present, the prognosis may depend on
the type of anomaly.
Ventriculomegaly
Ventriculomegaly is among the most common brain abnormalities
that can be picked up by routine ultrasound. Fetal ventriculomegaly
is the result of the accumulation of cerebrospinal fluid in
the ventricles, or chambers, in the brain. Within the brain,
there are four ventricles that allow the fluid to circulate
from one chamber to another through several channels. If a blockage
occurs that prevents this normal flow of fluid, the ventricles
or channel above the blockage will become dilated. The result
is enlarged ventricles (ventriculomegaly). This blockage can
lead to hydrocephaly when the head size becomes increased to
accommodate the excess fluid. There are many causes of ventriculomegaly
such as an underlying chromosome abnormality, genetic syndrome,
developmental defect of the fetal brain, or a fetal infection
or hemorrhage. Often it is difficult to determine the cause.
Ventriculomegaly is associated with a wide spectrum of abnormalities
ranging from isolated "mild" ventriculomegaly with
a normal neurological outcome to extremely severe, complex cerebral
malformations.
Moderate ventriculomegaly is a nonspecific prenatal sonographic
feature that may be "physiological" or may be the
hallmark of a severe abnormality. The size of the lateral ventricles
is remarkably stable throughout gestation after 22 weeks, the
95th centile corresponding to 10 mm and the 98th to 12 mm. We
tend to consider that ventriculomegaly is moderate when it does
not exceed 15 mm. However, this definition is somewhat arbitrary,
since postnatal outcome is probably related to the presence
or absence of structural brain abnormalities rather than to
the size of the ventriculomegaly.
Prenatal diagnosis of moderate ventriculomegaly should prompt
a chromosomal abnormality or a severe fetal infection such as
toxoplasmosis or cytomegalovirus to be ruled out by performing
amniocentesis and/or by maternal serology. However, ventriculomegaly
due to cytomegalovirus is usually associated with extracerebral
sonographic abnormalities. Associated abnormalities and spina
bifida should also be ruled out at initial evaluation. A careful
follow-up based on sonography and third trimester MRI is required
to ascertain the morphological normality of the central nervous
system.
Infants with moderate nonprogressive ventriculomegaly, without
associated structural abnormalities, and with normal growth
of the head and brain ascertained by US and MRI, are expected
to do well postnatally. However, one should be aware that occasionally,
our diagnostic tools may fail to spot subtle brain abnormalities
that have yet to make a significant clinical impact.
Dandy-Walker malformation
Dandy-Walker malformation (DWM) is a constellation
of abnormalities consisting of complete or partial absence of
the cerebellar
vermis, cyst in the posterior
fossa, and hydrocephalus. However, hydrocephalus is not
usually present at birth but does develop within the first months
of infancy. DWM accounts for approximately 5 to 12 percent of
all cases of hydrocephalus. The etiology of DWM may be a inherited
disorder, infection, teratogen
exposure, maternal condition, chromosome abnormality, or isolated
sporadic event. Both CNS anomalies and extra-CNS anomalies are
frequently associated with DWM. Prognosis for fetuses with a
DWM is worse than with other types of hydrocephaly.
Holoprosencephaly
Holoprosencephaly results from failure of the
hemispheres of the brain to cleave into the cerebral and lateral
hemispheres. This defect occurs at approximately 6 to 10 weeks
gestation, so it is conceivable to identify this defect before
the fetus is viable. Holoprosencephaly can be divided into three
categories, alobar, semilobar, and lobar depending on the degree
of cleavage. Alobar holoprosencephaly is the most severe form.
Findings of this type include absence of the interhemispheric
fissure, a single primitive ventricle, fused thalami,
and absent third ventricle, olfactory
bulbs, and optic tracts. Semilobar holoprosencephaly is
characterized by partially separated cerebral hemispheres and
a single ventricular cavity. The interhemispheric fissure is
well-developed in lobar holoprosencephaly; however, there is
some fusion of structures. Facial anomalies, consisting of aplasia
or hypoplasia
of midline structures such as the nose are characteristic of
holoprosencephaly. Prognosis depends on the type of holoprosencephaly
and the presence of associated anomalies. Chromosome abnormalities,
especially trisomy 13 and 18, may be associated with holoprosencephaly.
For this reason, if holoprosencephaly is identified, amniocentesis
is recommended.
Choroid plexus cysts
Choroid plexus cysts are believed to result
from the accumulation of CSF and cellular debris within the
neuroepithelial
folds. These cysts are found in approximately 50% of individuals
of all ages in serial autopsy. These cysts can generally be
visualized in the second trimester and usually resolve by 24
weeks gestation. Isolated choroid plexus cysts are usually benign.
However, serial scans should be used to monitor the size of
the cyst and exclude the development of hydrocephalus. If the
cyst does not resolve, is unusually large, or other abnormalities
are detected, the risk of the fetus having a chromosome abnormality,
particularly trisomy 18, increases.
Cephaloceles are defects in the cranium through
which intracranial contents can protrude. When meninges are
protruding, the term cranial meningocele is often used, and
when a portion of the brain is protruding it is called an encephalocele.
These defects are subdivided into frontal, parietal,
and occipital,
depending on the location of the defect, with occipital cephaloceles
being the most common. Hydrocephalus is frequently associated
with the presence of a cephalocele, and spina bifida and microcephaly
are also associated findings. A paracranial
mass on ultrasound may be suggestive of a cephalocele, but one
should try to identify the actual cranial defect. This may not
be possible, but the defect should be differentiated from a
cystic
hygroma or scalp edema.
The prognosis of fetuses with a cephalocele is variable depending
on the presence of brain in the sac, hydrocephalus, and microcephaly.
If the brain is involved the prognosis may be quite poor. However,
approximately 60% of individuals with a cranial meningocele
are of normal intelligence.
Craniostenosis
Craniostenoses are a group of disorders in which the plates
of the cranium (skull) in children fuse prematurely, resulting
in deformities of the skull of present at (or even before) birth.
There are many different types of craniostenosis determined
by which plates are fused. Scaphocephaly (also known as dolichocephaly)
is a malformation of the skull which is long and narrow. Plagiocephaly
is a malformation of the skull most commonly caused by the plates
of one side of the forehead (coronal sutures) closing prematurely
resulting in a flattening on one side of the skull and a slight
bulging on the other. This is similar to positional deformation
of the skull sometimes due to the child sleeping on only one
side. Brachycephaly may be caused by the premature closure of
the coronal sutures on both sides of the forehead resulting
in a head which is disproportionately wide. Brachycephaly is
a feature of several genetic syndromes such as Apert, Crouzon,
Pfeiffer, and Carpenters syndromes. Trigonocephaly is the result
of premature closure the plates at the center of the forehead
(metopic suture), resulting in a longer head shape from front
to back and possible ridging of the forehead.
Craniostenoses occur in approximately 1/2,000-1/2,500 live
births. The prognosis is usually good for these children providing
other abnormalities or syndromes are ruled out and surgical
treatment is provided when necessary. It is important to determine
whether other abnormalities are present and whether or no the
craniostenoses is related to a genetic condition. This is crucial
to accurately determine the prognosis.
REFERENCES
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Nyberg, D.A., Mahony, B.S., and Pretorius, D.H.
The Spine and Neural Tube Defects in Diagnostic Ultrasound of
Fetal Anomalies. Mosby-Year Book, Inc: St. Louis, 1990. Pp 147-202.
Prenatal Diagnosis of Central Nervous System
Abnormalities. Curr Probl Obstet Gynecol Fertil March/April,
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Romero, R., Pilu, G., Jeanty, P., Ghidini, A.,
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M. C. Aubry, J. P. Aubry and M. Dommergues. Sonographic prenatal
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