Chiari malformation is considered a congenital condition,
although acquired forms of the condition have been diagnosed. A German
pathologist, Professor Hans Chiari, first described abnormalities of the
brain at the junction of the skull with the spine in the 1890s. He
categorized these in order of severity; types I, II, III, and IV. The
term "Arnold-Chiari" was latter applied to the Chiari type II
malformation. These malformations, along with syringomyelia and
hydromyelia, two closely associated conditions, are described below.
The cerebellum controls the coordination of motion, and is
normally located inside 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 cerebrospinal fluid (CSF) filled space located in
front of the cerebellum (and behind the brainstem). All of these
structures are located just above the foramen magnum, the largest
opening at the base of the skull through which the spinal cord enters
and connects to the brainstem.
The prevalence in the general population has been estimated at
slightly less than one in 1000. The majority of these cases are
asymptomatic. Chiari malformations are often detected coincidently among
patients who have undergone diagnostic imaging for unrelated reasons.
Types of Malformations
Type I Chiari malformation
This malformation occurs during fetal development and is
characterized by downward displacement by more than four millimeters, of
the cerebellar tonsils beneath the foramen magnum into the cervical
spinal canal. This displacement may block the normal pulsations of CSF
between the spinal canal and the intracranial space. This form of Chiari
malformation may be associated with syringomyelia/hydromyelia. It is
diagnosed more commonly in adolescents or adults.
Anomalies of the base of the skull and spine are seen in 30-50
percent of patients with Chiari I malformation. These include:
of the upper part of the spine into the base of the skull with
resulting compression of the brainstem.
- Bony union of the first
level of the spine (C1) to the base of the skull.
- Partial bony
union of the first and second levels (C1 fusion to C2) of the spine.
deformity (congenital union or fusion of levels of the spine within the
neck with possible associated maldevelopment of levels of the cervical
- Cervical spina bifida occulta (bony defect in the
posterior part of the spine).
- Scoliosis – present in 16-80
percent of hydromyelia patients; especially in children with immature
Many people with Chiari I malformation have no symptoms. However,
any of the following symptoms may occur, alone or in combination. Some
of the symptoms are related to the development of a syrinx (a fluid
filled cavity in the spinal cord).
- Severe head and neck pain
- An occipital headache felt at the base of the skull that is
made worse by coughing, sneezing, or straining
- Loss of pain and temperature sensation of the upper torso and
arms (as a result of a syrinx)
- Loss of muscle strength in the hands and arms (as a result of a
- Drop attacks – collapsing to the ground due to muscle weakness
- Balance problems
- Double or blurred vision
- Hypersensitivity to bright lights
Type II Chiari malformation
This malformation is characterized by downward displacement of
the medulla, fourth ventricle, and cerebellum into the cervical spinal
canal, as well as elongation of the pons and fourth ventricle. This type
occurs almost exclusively in patients with myelomeningocele.
Myelomeningocele is a congenital condition in which the spinal cord and
column do not close properly during fetal development, resulting in an
open spinal cord defect at birth. Other abnormalities associated with
myelomeningocele include hydrocephalus, cardiovascular abnormalities,
imperforate anus as well as other gastrointestinal abnormalities, and
The symptoms associated with a Chiari II malformation can also be
caused by problems related to myelomeningocele and hydrocephalus. These
- Alteration in the pattern of breathing, including periods of
apnea (brief periods of cessation of breathing)
- Depressed gag reflex
- Involuntary, rapid, downward eye movements
- Loss of arm strength
Type III Chiari malformation
This malformation includes a form of dysraphism with a portion of
the cerebellum and/or brainstem pushing out through a defect in the
back of the head or neck. These malformations are very rare and are
associated with a high early mortality rate, or severe neurological
deficits in patients that survive. If treatment is undertaken, then
early operative closure of the defect is necessitated. Hydrocephalus,
which is commonly present, must also be treated through shunting.
Additional severe birth defects are often present, which may
require extensive treatment. Infants with Chiari III malformation may
have life-threatening complications.
Type IV Chiari malformation
This malformation is the most severe form and the rarest. The
cerebellum fails to develop normally. There may be other associated
malformations of the brain and brainstem. Most babies born with this
malformation do not survive infancy.
When CSF forms a cavity or cyst within the spinal cord, it is
known as syringomyelia or hydromyelia. These are chronic disorders
involving the spinal cord developing, expanding or extending over time.
As the fluid cavity expands, it can displace or injure the nerve fibers
inside the spinal cord. A wide variety of symptoms can occur, depending
upon the size and location of the syrinx. Loss of sensation in an area
served by several nerve roots is one typical symptom, as is the
development of scoliosis.
Syringomyelia can arise from several causes. Chiari malformation
is the leading cause of syringomyelia, although the direct link is not
well understood. It is thought to be related to the interference of
normal CSF pulsations caused by the cerebellar tissue obstructing flow
at the foramen magnum.
This condition can also occur as a complication of trauma,
meningitis, tumor, arachnoiditis, or a tethered spinal cord. In these
cases, the syrinx forms in the section of the spinal cord damaged by
these conditions. As more people are surviving spinal cord injuries,
increased cases of post-traumatic syringomyelia are being diagnosed.
Hydromyelia is usually defined as an abnormal widening of the
central canal of the spinal cord. The central canal, a very thin cavity
in the middle of the spinal cord, is a remnant of normal development.
- Loss of sensitivity, especially to hot and cold
- Muscle weakness and spasticity
- Loss of bowel and bladder control
- Motor impairment
- Chronic pain
- Headaches (often concurrent with the Chiari malformation)
There are several tests that can help diagnose and determine the
extent of Chiari malformation and syringomyelia.
Brainstem auditory evoked potential (BAER): An electrical
test to examine the function of the hearing apparatus and brainstem
connections. This is used to determine if the brainstem is working
Computed tomography scan (CT or CAT scan): A diagnostic
test that creates an image by computer reconstruction of x-rays; it is
particularly good at defining the size of the cerebral ventricles and
showing an obvious blockage. It is less effective for analysis of the
posterior fossa or the spinal cord.
Magnetic resonance imaging (MRI): A diagnostic test that
produces three-dimensional images of body structures using magnetic
fields and computer technology. It can provide an accurate view of the
brain, cerebellum and the spinal cord, is very good at defining the
extent of malformations, and distinguishing progression. The MRI
provides more information than the CT scan when analyzing the back of
the brain and spinal cord, and is usually the preferred test.
Myleogram: An x-ray of the spinal canal following injection
of a contrast material into the CSF space; can show pressure on the
spinal cord or nerves due to malformations. This test is performed less
Somatosensory evoked potentials (SSEP): An electrical test
of the nerves involved in sensation, which gives some information about
peripheral nerve, spinal cord, and brain function.
Treatment of Chiari malformations and syringomyelia is very
dependent on the exact type of malformation, as well as progression in
anatomy changes or symptoms.
Chiari I malformations that are asymptomatic should be left
alone. There is no indication for "prophylactic" surgery on these. If
the malformation is defined as symptomatic, or is causing a syrinx,
treatment is usually recommended.
Chiari II malformations are treated if the patient is
symptomatic, and physicians have determined that there are no
complications from hydrocephalus. In some patients, consideration of a
tethered cord is also explored. In many infants who become symptomatic
from a Chiari II malformation, the symptom onset and progression are
severe and rapid, and this requires an urgent or emergency approach.
Surgical treatment of these malformations depends on the type of
malformation. The goal of surgery is to relieve the symptoms, or stop
the progression of the syrinx or symptoms.
Chiari I malformations may be treated surgically with only local
decompression of the overlying bones, decompression of the bones and
release of the dura (a thick membrane covering the brain and spinal
cord), or decompression of the bone and dura and some degree of
cerebellar tissue resection.
Decompression is performed under general anesthesia. It consists
of removing the back of the foramen magnum and often the back of the
first few vertebrae to the point where the cerebellar tonsils end. This
provides more space for the brainstem, spinal cord, and descended
cerebellar components. 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
divert the fluid from inside the spinal cord to outside the cord, or be
directed to either the chest or abdominal cavity. These procedures can
be done together or separately.
Chiari II decompression is treated similarly, but is usually
restricted to decompressing the tissues in the spinal canal and leaving
the back of the skull alone.
The goal of Chiari surgery is:
- Optimal decompression of nerve tissue
- Reconstruction of normal CSF flow around and behind the
The benefits of surgery should always be weighed carefully
against its risks. Although some patients experience a reduction in
their symptoms, there is no guarantee that surgery will help every
individual. Nerve damage that has already occurred usually cannot be
reversed. Some surgical patients need repeat surgeries, while others may
not achieve symptom relief.
Glossary of Terms
Aperture: An opening or space between bones or within a
Basilar impression: When the base of the skull is pushed
“up” into the skull.
Brainstem: The portion of the brain connecting the spinal
cord with the midbrain/cortex, which houses the controls of many basic
functions such as breathing, swallowing, eye movement.
Cavity: An open area or space, such as a sinus within a
Central canal: A small tubular cavity in the center of the
spinal cord which is normally not dilated.
Cerebellum: The portion of the brain which is in the
posterior fossa. It is involved in coordination of movements.
Cranium: All of the bones of the skull except for the
Dura: The covering over the surface of the brain and spinal
Dysraphism: Developmental abnormalities in the midline
region of the back, with incomplete fusion or malformation of a seam,
such as in all forms of spina bifida.
Foramen magnum: The opening at the base of the skull
through which the spinal cord passes.
Hydrocephalus: A condition in which excess cerebrospinal
fluid (CSF) builds up within the ventricles (fluid-containing cavities)
of the brain and may increase pressure within the head.
Medulla oblongata: Lowest part of the brainstem, located
just above the spinal cord, at the exit from the skull. Provides control
for breathing and heart function.
Neurocranium: Referring to the braincase of the skull.
Pons: Band of nerve fibers linking the medulla oblongata
and the cerebellum with the midbrain.
Posterior fossa: The cavity in the back part of the skull
which contains the cerebellum, brainstem, and cranial nerves 5-12.
Shunt: A tube which drains spinal fluid from one space to
another body cavity.
Spasticity: Increased tightness or tone in the arms and/or
legs, making one less flexible and possibly causing the arms and/or legs
Splanchocranium: Referring to the facial bones of the
Suture: The saw-like edge of a cranial bone that serves as
joint between bones of the skull.
Syrinx (syringomyelia, hydromyelia): These terms all refer
to a fluid filled cavity in the spinal cord.
Tonsils: The portion of the cerebellum that protrudes into
the spinal canal, which may become elongated.
Vallecula (of cerebellum): The longitudinal hollow on the
inferior surface of the cerebellum, between the hemispheres, in which
the medulla oblongata rests.
Ventricles: Fluid filled cavities within the brain. The
spinal fluid is produced by a gland in the ventricle (about two pints
each day) and circulates through the ventricles and over the surface of
the brain into the veins. If there is a block in the system, the fluid
can build up and cause hydrocephalus.
Vermis: The narrow middle zone between the two hemispheres
of the cerebellum; the portion projecting above the level of the
hemispheres on the upper surface is called the superior vermis; the
lower portion, sunken between the two hemispheres and forming the floor
of the vallecula, is the inferior vermis.
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