Anatomy - Chiari Malformation
What is Chiari Malformation?
The Chiari (kee-ar-ee) malformations
are congenital abnormalities of the posterior fossa
(base of brain where the spinal column joins the skull).
This usually causes a protrusion of the cerebellum through
the bottom of the skull (foramen magnum) into the spinal
canal. This results in a poor circulation of cerebrospinal
fluid from the brain to the spinal cord.
Professor Hans Chiari was a German pathologist who
first described these abnormalities of the brain at
the junction of the skull with the spine in 1890. Another
doctor, Arnold, later added to the type II description,
hence the name “Arnold-Chiari Malformation.” Of the
four classifications of Chiari, only two types of the
malformation have practical importance, commonly referred
to as Chiari type I and Chiari type II.
Type I Chiari malformation
The Chiari I malformations are a result of the smallest
herniation of an otherwise normal hindbrain. The tonsils
would normally be round but often become elongated as
they protrude down the spinal canal. Diagnosis can be
difficult because not all patients will have the classical
sign of deeply herniated tonsils. The Chiari II is usually
found in children with spina bifida or myelomeningocele.
Not only is part of the cerebellum unusually low and
lying below the bottom of the skull, but the brain stem
can be malformed in several ways. This malformation
is part of a much greater malformation of the nervous
system. This type of Chiari malformation is correctly
referred to as “Arnold-Chiari” malformation.
What symptoms can it cause?
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 syrinx)
- 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 characterised 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 genitourinary abnormalities.
The symptoms associated with a Chiari II malformation
can also be caused by problems related to myelomeningocele
and hydrocephalus. These symptoms include:
- 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
How is it evaluated?
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 properly.
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 frequently now.
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
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