Chiari Malformation
Medical Disclaimer
What is it?
Chiari malformation is an abnormality that encompasses four separate, distinct
development malformations. While these malformations can 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.
Chiari Malformation Anatomy
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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.
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Types of Malformations
Syringomyelia
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.
More about Syringomyelia
Type I Chiari Malformation
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 Malformation
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 Malformation
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 Malformation
Type IV Chiari malformations are associated with an underdeveloped cerebellum.
Both Type III and IV Chiari malformations are rare.
What Causes a Chiari Malformation?
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.
Clinical Presentation - Symptoms
Patients with the Chiari Malformation present with one or more signs
and symptoms of brainstem, cranial nerve, cerebellar or spinal cord dysfunction
including:
- Headache (especially cough headache)
- Neck pain
- Disordered eye movements
- Trigeminal dysesthesia
- Sensorineural hearing loss
- Tinnitus
- Dizziness
- Dysphagia
- Sleep apnea
- Nystagmus
- Dysarthria
- Ataxia
- Dysethesias/numbness
- Sensory loss
- Weakness, greater in the upper than lower extremities
Diagnosis
Magnetic resonance imaging (MRI)
is the test of choice in diagnosing the Chiari
Type 1 Malformation. The Chiari Malformation is present when the tonsils
are herniated 5 mm or more below the foramen magnum. If the cerebellar tonsils
are herniated from 3-5 mm, the Chiari malformation is diagnosed if there are
associated changes such as cervicomedullary kinking, elongation of the 4th
ventricle, or syringomyelia.
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 be done
together or at separate sittings.
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