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| Patient Education -- Chiari Malformation |
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Chiari Malformation
Chiari malformation is an anomaly that encompasses four
separate development malformations. While these malformations
may present at any point in life, they have a very particular
set of associated symptoms. These malformations along
with syringomyelia, a distinct yet closely associated
condition, are described below.
Anatomy
The cerebellum is normally located at the base of the
skull, in what is referred to as the posterior fossa.
Usually, the cerebellum is composed of two lateral halves,
or hemispheres, and a narrow central portion between these
hemispheres, known as the vermis. Along the under surface
of the hemispheres, there are two small protrusions called
the tonsils. The fourth ventricle is a space filled with
cerebrospinal fluid (CSF) located in front of the cerebellum
that communicates with the central canal of the spinal
cord. All of these structures are located inside the skull
above the foramen magnum, which is the largest opening
at the base of the skull through which the spinal connects
to the brain. The cerebellum normally controls coordination
of movement.
Types of Malformations
Syringomyelia: refers to a cavitation, or hollowing out,
of the center of the spinal cord. While "syringomyelia"
is commonly associated with Chiari malformations, the
use of this word is not entirely appropriate. More often
the spinal cord cavitation is actually a hydromyelia.
Hydromyelia, in contrast to syringomyelia, is an expansion
of the spinal canal secondary to increased spinal fluid
pressure within the canal. With hydromyelia, there is
no loss of normal spinal cord tissue but rather compression
from the inside of the cord. In either case, spinal cord
function can be significantly affected.
Type I Chiari malformations: are characterized
by downward displacement of the tonsils of the cerebellum
through the foramen magnum into the upper portions of
the spinal canal. This form of Chiari malformation is
associated with hydromyelia and usually presents in adolescents
or adults. In adolescents, the main symptoms are due to
this hydromyelia - a flexed paralysis of the arms, loss
of pain and temperature sensation of the upper torso and
arms, as well as loss of muscle strength in the hands
and arms. Adults typically complain of an occipital headache
felt at the base of the skull that is made worse by coughing.
They also experience neck and arm pain, sometimes in a
cape-like distribution.
Type II Chiari malformations: are characterized
by displacement of the cerebellar vermis, tonsils, the
fourth ventricle, and the medulla, which is a section
of the brainstem, through the foramen magnum. Type II,
also known as Arnold-Chiari malformation, is more highly
associated with hydromyelia than Type I Chiari malformations
and is almost always associated with myelomeningocele.
A meningomyelocele is a congenital failure of the closure
of the spinal cord and column in the lumbar region during
fetal development. Other associated abnormalities include
hydrocephalus (increased fluid pressure within the skull),
cardiovascular abnormalities, imperforate anus as well
as other gastrointestinal abnormalities, and genitourinary
abnormalities. Symptoms of this malformation are usually
obvious and present shortly after birth and may include
brief periods of cessation of breathing; depressed gag
reflex; involuntary, rapid, downward eye movements; and
loss of arm strength.
Type III Chiari malformations: consist of displacement
of the cerebellum and portions of the brainstem into a
meningocele. In this case, the meningomyelocele involves
the occiput or cervical spine. The tissue surrounding
the brain and spinal cord protrude into the meningocele.
Type IV Chiari malformations: are associated with
an underdeveloped cerebellum. Both Type III and IV Chiari
malformations are rare.
Cause
While the cause of Chiari malformation is not exactly
known, several potential explanations exist. Most theories
indicate that an increase in pressure above the cerebellum
will result in its displacement. The cerebellum is forced
downward into the foramen magnum not unlike a cork into
the neck of a bottle. Magnetic resonance imaging (MRI)
allows physicians to accurately view the cerebellum and
the associated structures and determine what form and
to what extent the malformation has progressed.
Surgical Treatments
There are many ways to treat Chiari malformations, but
all require surgery. Surgical therapy for Chiari malformation
consists of either local decompression or diversion of
CSF pressure. Decompression is performed under general
anesthesia and consists of removing the very base of the
back of the skull as well as the back of the first few
vertebrae to the point where the cerebellum descends.
This effectively widens the opening of the foramen magnum
to provide more space for the brainstem, spinal cord,
and descended cerebellar components. The dura mater, a
thick membrane surrounding the brain and spinal cord,
is also opened. A tissue graft is often spliced into this
opening to provide even more room for the unimpeded passage
of CSF. Occasionally, the cavity within the spinal cord
resulting from hydromyelia can be drained with a diverting
shunt tube. This tube can be directed from the spinal
cord cavity to either the chest wall or the abdominal
wall. These procedures can be done together or at separate
sittings.
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