NEURAL TUBE DEFECTS EVALUATION TREATMENT
Category: Child Health
Abstract : EVALUATION AND TREATMENT: MEDICAL AND SURGICAL Neurologic lesions The
myelomeningocele is a saccular protrusion containing a neural placode bathed in
CSF. The surface of the sac is covered by arachnoid but no dura or skin. The sac
appears velvety red or yellow with thin fragile vessels imbedded in the
arachnoid. The nerve roots pass cases, the spinal cord is attached to the
superio
EVALUATION AND TREATMENT: MEDICAL AND SURGICAL
Neurologic lesions
The
myelomeningocele is a saccular protrusion containing a neural placode bathed in
CSF. The surface of the sac is covered by arachnoid but no dura or skin. The sac
appears velvety red or yellow with thin fragile vessels imbedded in the
arachnoid. The nerve roots pass cases, the spinal cord is attached to the
superior aspect of the sac.
The myelomeningocele has many other associated CNS
anomalies that require attention.
Chiari II malformation
Symptomatic Chiari II malformation can occur
anytime after birth. The symptomatic Chiari II presentation can be as subtle as
new hoarseness and pneumonia or as obvious as a progressive quadriparesis. A
brain and cervical cord MRI in patients with myelomeningocele invariably
demonstrates a Chiari II malformation with a herniated vermis and syringomyelia.
Very few patients require decompression after their first year of life for a
symptomatic Chiari II malformation. The surgeon must first and foremost check to
see if the shunt apparatus is functioning. Most of the time, a partial or
complete obstruction of a VP shunt (based on a shunt tap or surgical
exploration) is the etiology of the new brainstem findings. A shunt malfunction
causes the hindbrain to herniate and compress the cord, thus causing many of the
new findings. Timely repair of the shunt leads to a good outcome with reversal
of most deficits.
Hindbrain anomalies
Pathophysiology of Chiari
malformations (CMs) has fascinated neurosurgeons and provided a constant stream
of literature on the presentation and presumed etiology for the past century.
Although originally thought to be a rare neuroembryological disorder associated
with NTD, CMs have been recognized with increased frequency in the past 5
decades. The number of patients seen for this disorder has increased since the
widespread application of MRI. Another increase in patient referrals has
occurred more recently as with improved understanding of the rather wide
spectrum of clinical presentation.
In 1883, John Cleland published
“Contribution to the study of spina bifida, encephalocele and anencephalus” in
the Journal of Anatomy and Physiology. Cleland made several novel observations
regarding hindbrain malformations on infant autopsy specimens. He described an
elongated brainstem and cerebellar vermis, which protruded into the cervical
canal in a full-term infant with spinal bifida and craniolacunae. Eight years
later, Hans Chiari, professor of morbid anatomy at Charles University in Prague,
published similar observations on congenital anomalies in the cerebellum and
brain stem and commented on the a priori contributions of Cleland. Chiari
further separated his patients into 3 different classifications of hindbrain
abnormality, and to ensure no confusion, the descriptions were accompanied by
beautiful and detailed illustrations first in 1891, and then later in
1896.
Many textbooks and papers still refer to these hindbrain
malformations as Arnold-Chiari malformations. However, the name Arnold-Chiari
malformation is not historically accurate. The relatively minor contribution of
Arnold to the understanding of this malformation was a report in 1894, which
consisted of a description of 1 infant with a teratoma and cerebellar
herniation. It was really students of Arnold, namely Schwalbe and Gredieg, in
1907, who erroneously suggested the term Arnold-Chiari Malformation.
Unfortunately, this 1907 article failed to correctly attribute the rather
significant contributions of Cleland. The subsequent 93 years have not corrected
this misnomer. Attempts to name this malformation, Cleland-Arnold-Chiari or
Cleland-Chiari malformation have not succeeded. Therefore, for the remainder of
this article, the author adheres to a more historically accurate term and refers
to these hindbrain anomalies simply as Chiari malformations or CMs.
The
different CMs of the hindbrain were later classified as Chiari types I-III,
terms that have been employed in a relatively consistent manner over the last
century. These lesions are at the extreme end of the spectrum, and patients with
these anomalies are difficult to treat from a surgical perspective.
Type
I is described as downward herniation of the cerebellar tonsils through the
foramen magnum.
Type II malformation is herniation of the cerebellar
vermis and brainstem below the foramen magnum. Type II malformation also has
kinking of the cervicomedullary junction, an upward trajectory of the cervical
nerve roots, and associated syringomyelia. The medulla often protrudes below the
foramen magnum and into the spinal canal, compressing the cervical cord. The
medulla then buckles dorsally and forms a medullary kink" Also, the fourth
ventricle often is below the foramen magnum, and the midbrain tectum forms a
sharp corner on midsagittal MRI and looks like a beak. Type II malformations are
the subject of this section.
Type III malformation is essentially a
posterior fossa encephalocele or a cranium bifidum with herniation of the
cerebellum through the posterior fossa bone and is a more severe neural tube
defect.
The only deviation from the consistent terminology is the eponym
Chiari type IV malformation. The Chiari type IV malformation consists of
cerebellar hypoplasia, not herniation, and is no longer considered a Chiari
malformation.
Description and diagnostic studies
A CM II is downward
displacement of the cerebellar vermis, fourth ventricle, and brainstem below the
foramen magnum into the cervical canal. In recent years, the terms hindbrain
herniation, displacement, descent, and ectopia have been used synonymously in a
wide range of posterior fossa conditions. From a historical point of view (prior
to MRI), the diagnosis of CM II most often was made using autopsy, air or
contrast myelogram, or CT/myelography. Thus, the diagnosis was made
infrequently, although all patients with myelomeningocele were thought to have a
CM II.
Currently, radiological diagnosis is made using MRI. The crucial
measurement in relation to descent of the hindbrain and vermis below the foramen
magnum usually is assessed on sagittal section of MRI. The hindbrain or vermis
displacement is measured from a straight line drawn between the basion to the
opisthion of the foramen magnum. A perpendicular line dropped from the
basion/opisthion line to the vermis tip is considered the extent of the
herniated brain.
Syringomyelia is a cavitation of the spinal cord whose
walls are comprised of glial tissue, whereas hydromyelia is a cavitation or
dilatation of the central canal lined by ependyma. The author uses the term
syringomyelia in this chapter instead of the more descriptive term
syringohydromyelia to avoid generating scientific and semantic confusion. The
association of CM II with syringomyelia varies from 80-90%, depending on the
patient population studied.
Syringomyelia, the common finding associated
with CM, is derived from the Greek words, syrinx (meaning tube or pipe) and
muelos (meaning marrow). Estienne, from France, first described the spinal cord
cavitation called syringomyelia in human cadavers in 1546. In 1824, Charles
Ollivier d'Angers provided the very descriptive name syringomyelia to the
cylindrical dilatation of the spinal cord, which in his illustrative case
report, communicated with the fourth ventricle. In 1892, Abbe and Coley from New
York performed a myelotomy to drain the syrinx cavity. This was the first
recorded surgical procedure to treat syringomyelia.
Hindbrain
malformations are the leading cause of syringomyelia. This cavitation of the
spinal cord usually is gradually progressive and can cause neurologic
deterioration over time. The fluid in the syrinx is identical to the CSF found
elsewhere in the subarachnoid space; therefore, theories based on aberrant CSF
physiology are invoked to explain the relationship of syringomyelia in patients
with CM II. Nevertheless, the pathophysiologic mechanisms that cause these 2
disorders are not well understood. Many excellent theories have been suggested,
however, none have been conclusively proven or universally accepted. Examination
of the spinal cord in many neonates with myelomeningocele reveals atrophic or
poorly developed anterior horn cells, incomplete posterior horns, and small
nerve roots.
Initial examination
The initial neurologic examination of
a neonate born with a neural tube defect should focus on the neurologic sequelae
of the NTD. Specifically, evaluate (1) site and level of the lesion, (2) motor
and sensory level, (3) presence of associated hydrocephalus, (4) presence of
associated symptomatic hindbrain herniation (eg, CM II), and (5) presence of
associated orthopedic deformity.
The lesion is first examined after the
birth of a neonate. Myelomeningocele is a consequence of failed closure of the
dorsal neural tube. Thus, the lesion appears as a red, raw neural plate
structure devoid of dura and skin covering. The sac comprising arachnoid laced
with thin, fragile vessels can be filled with CSF escaping from the central
canal. A meningocele, in contradistinction, does not have neural tissue in the
sac and usually has a nearly complete skin covering.
Open neural tube
defects should be immediately covered with a saline-moistened sponge to avoid
rupture of the sac and drying of the exposed neural placode. The neonate is
maintained and examined in the prone or lateral recumbent position. An IV is
placed, and feedings are held until a full assessment can be completed. The
neonate is treated with systemic antibiotics consisting of ampicillin at
meningitic doses and gentamicin. Common neonatal organisms, such as group B
streptococci, and nosocomial organisms must be prevented from entering the CSF,
especially through a leaking myelomeningocele.
The neonatologist,
pediatric geneticist, pediatric neurosurgeon, and pediatric orthopedist should
immediately evaluate the child. Possible cardiac abnormalities are evaluated
with ultrasound. An initial ultrasound of the head to evaluate for hydrocephalus
also may be performed. Urologic examination by ultrasound followed by a complete
pediatric urologic evaluation may be performed initially or at a later date.
Orthopedic evaluation is performed shortly before discharge, as up to 10% of
neonates with an NTD may have hip dislocations. In addition, presence of a varus
or valgus extremity disorder is documented. A higher motor level lesion, such as
L3-L4, can predispose some children to hip dislocations due to the unopposed hip
flexors.
The pediatric neurosurgeon carefully evaluates the patient to
assess the site and type of lesion, including assessment of lower extremity
function. Evaluate the symmetry of the motor and sensory levels affected by the
NTD. Flaccid paralysis below the L4 level may reveal a strong psoas, but not hip
adduction, knee hyperextension, or foot inversion deformities. Flaccid paralysis
of the foot with a weak gastrocnemius-soleus complex may result in foot
dorsiflexion deformities.
Attention to the anus helps to assess sacral
nerve root function. Flaccid musculature in the S2-4 region often presents with
a flat buttocks, absence of a well-developed gluteal cleft, and a patulous anus
with no anal wink. The thoracic or lumbar region may have a large hump due to
kyphosis or scoliosis of the spine; this can be so severe that it impedes the
ability to place skin flaps over the NTD
Head ultrasound can be performed
during the neonatal period to evaluate the extent of ventricular enlargement.
Initially, the ventricles may be normal or only slightly enlarged. However,
after the NTD is closed surgically, the ventricles often enlarge. Incidence of
hydrocephalus associated with myelomeningocele ranges from 80-95%. In 2 studies
performed in the 1980s and 1990s, approximately 85-90% of all patients with NTD
required a VP shunt for progressive hydrocephalus. The highest incidence in
shunt dependence occurs in thoracic lesions; the lowest incidence occurs in
sacral lesions. The risk of shunt revision in this population may be no
different from that of other children with shunts. Approximately 40-50% of all
children with NTDs require shunt revision in the first year and approximately
10% every year after that.
An MRI may reveal defects in cellular
migration in the cerebral cortices. These include gray matter heterotopia,
schizencephaly, gyral abnormalities, agenesis and thinning of the corpus
callosum, abnormal thalami, and abnormal white matter
findings.
Meaningful surgical treatment of myelomeningocele was not
undertaken until the invention of the shunt valve by Holter in the 1950s. Prior
to that, closure of a myelomeningocele was possible, but the ensuing
uncontrolled hydrocephalus decreased the chance of survival. In the 1980s, the
US Department of Health and Human Services issued the Baby Doe directive,
stating that medical and surgical treatment could not be withheld simply because
a neonate is handicapped. Although the directive was struck down, the decision
to operate on NTD in neonates was already an accepted practice in the United
States. Furthermore, outcome studies by McClone, Shurtleff, and others presented
a more positive outcome than had previously been thought for these
children.
Timing of myelomeningocele repair
In the 1960s, the birth of
a patient with myelomeningocele was a neurosurgical emergency, and immediate
closure of the defect was required. Studies have subsequently shown that closure
within 48 hours was both safe and effective. A study by Charney et al comparing
delayed closure (3-7 d) to immediate closure (<48 h) showed little difference
in survival, ventriculitis, or worsening paralysis. The implications of this
study were immense: surgeons could plan a deliberate but thorough workup on a
neonate with an NTD. Parents would have time to ask questions and be acclimated
to the intensive surgical therapy that was about to commence. In the author's
Children's Hospital setting, a great deal of time is spent performing a detailed
workup and counseling parents. Closure is performed on the next available
elective operative time, usually within 72 hours after birth.
Operative
approach Any major procedure on a neonate with myelomeningocele must be
performed in such a fashion as to avoid hypovolemia, hypothermia, and airway
compromise. Operative techniques vary by institution but, in general, the goal
is similar: to circumnavigate the neural placode without injuring any of the
neural elements. Once that is completed, the neural placode is placed into the
spinal canal.
The next step entails the identification and dissection of
the dura. The neural placode is covered by the dura by a watertight closure. If
the dura is absent, as sometimes occurs, the muscle fascia is reflected off the
muscle and used to create a watertight tube to enclose the neural placode. Skin
closure is achieved by mobilizing the skin from the underlying paraspinal fascia
in an avascular plane. The skin is then closed in layers, and an attempt is made
to ensure little tension is placed on the wound. The skin may look somewhat pale
immediately after closure, especially if the slightest bit of tension is present
on the wound. Care is taken to avoid necrosis or ischemia of the skin flap. The
skin closure is protected with a sterile dressing.
Shunt placement during
myelomeningocele closure Approximately 20% of all patients with
myelomeningoceles have significant hydrocephalus at birth; placement of a shunt
during the same operation for closure of a myelomeningocele is entirely
reasonable. At the author's institution, patients who manifest ventriculomegaly
after birth undergo shunt placement after myelomeningocele closure but while
under the same anesthetic. Shunt placement not only decreases future anesthetic
risk, but also it decreases the chance of CSF leaking through the
myelomeningocele closure.
Treatment of Chiari II malformations In CM II,
decompression of the posterior fossa and/or cervical cord, with its variable
anatomy, is surgically challenging and requires an experienced surgeon. The
torcular can come in low near the foramen magnum, the cerebellum often is
adherent to the medulla, and there are many venous sinuses. Catastrophic blood
loss is the major risk when a sinus is inadvertently opened. Prior to
decompressing a CM II, ensure the shunt is functioning. CT scan findings can be
misleading, as ventricles can remain small despite an obstruction in the shunt.
Shunt tap or exploration is the most reliable test prior to embarking on a
Chiari decompression.
The main signs and symptoms of a CM II that
requires decompression are those of brainstem compression. For example, neonates
can have stridor, central apnea, dysphagia, quadriparesis, or failure to thrive.
Patients may have subtle signs, such as worsening strabismus, nystagmus,
myelopathy, or aspiration of unclear etiology. Symptomatic CM II is the leading
cause of death in our patients with myelomeningocele. (Approximately 30% of
children die that develop brainstem symptoms when <5 y.) Symptomatic
deterioration from a Chiari II can constitute a neurosurgical emergency and,
despite urgent decompression, children can die from hindbrain compression.
Patients who fare the worst are those who have ventilatory difficulties shortly
after birth. Autopsies on these clinically challenging patients often show
brainstem anomalies, such as disorganized brainstem nuclei, as well as cortical
and subcortical abnormalities.
Signs and symptoms of problematic CM II in
neonates include the following:
• Stridor with vocal cord paralysis
•
Central apnea
• Aspiration
• Dysphagia
• Hypotonia
• Progressive
brainstem function
• Myelopathy
• Hypotonia, quadriparesis
• Nystagmus,
strabismus, progressive
• Swallowing difficulties, poor
suck
Lipomyelomeningocele
Although this skin-covered NTD deserves an
entire article of its own, a few salient points should be included here. The
neonate often presents with a skin-covered mass above the buttocks. The natural
history of these lesions consists of eventual neurologic deterioration.
Appropriate prophylactic surgical treatment of these lesions can halt the
progression of the neurologic deficits and improve neurologic function, and the
risk of surgery in skilled hands is quite low.
The surgical goal in
treating these lesions is to detach the lipoma of the buttocks from the lipoma
that emerges through the dura, fascia, and bony defect. The technique requires
the surgeon to identify normal anatomy and travel down to the location where the
lipoma pierces the dura and enters the spinal cord. Often with use of
microsurgical technique and/or a carbon dioxide laser, the lipoma is
disconnected from the spinal cord. All of the lipoma need not be removed. Take
care to leave some lipoma on the cord in order to avoid injuring the underlying
neural substrate. The filum terminale also is divided to further untether the
cord. A patulous graft is then placed over the dural opening to establish a pool
of CSF around the cord to help prevent retethering.
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