Online Module: Chiari Malformations

Online Module:
Chiari Malformations
About the term
To say “Chiari malformations” is slightly
misleading. The Chiari malformations actually
consist of four defined types of hindbrain
abnormalities, each distinct from the others.
The first two types, especially type 1, will be
briefly reviewed here. The 3rd and 4th types of
Chiari malformations are exceedingly rare and
will only briefly be mentioned.
The Chiari Malformation
The term usually refers to “Type 1 Chiari
Malformation,” which is classically described as
“adult-onset Chiari” (avg. age of presentation is
~40 yrs) with downward displacement of the
cerebellar tonsils through the foramen magnum.
Although this is a common radiographic feature of
the condition, it is not a prerequisite for diagnosis.
Type 1 Chiari
Most common presenting complaint is
suboccipital headache
Neck pain, subjective weakness, numbness, loss of
temperature sensation also ~40-60% incidence
Most common presenting sign is hyperactive
lower extremity reflexes
“Cape”-like sensory loss, nystagmus (downbeat), gait
disturbance, upper extremity weakness, etc., are also
all very common (~30-50% incidence of each).
Type 1 Chiari – why should you care?
Patients who are identified early and receive
early treatment have the best response to
surgical intervention. Because of the variable
constellation of signs and symptoms associated
with type 1 Chiari, it is regularly missed or
Type 1 Chiari - imaging
MRI is diagnostic test of choice
Can show compression of brain stem at foramen
magnum (common, and significant finding)
 Hydrocephalus can be present
 Syringomyelia
 Descent of cerebellar tonsils through foramen
Importance probably related to brainstem compression at
foramen magnum; nevertheless, this is classic finding
associated with type 1 Chiari.
Type 1 Chiari malformation
T1 weighted MRI w/o
contrast, sagittal view; in
this outstanding picture
of a patient with type 1
Chiari, you see:
(1) cerebellar tonsils well
below the foramen
(2) syringomyelia
(3) compression of
Type 1 Chiari – cerebellar tonsils
The cerebellar tonsils normally ascend as we age;
in normal adults, the tonsils usually do not
descend through the foramen magnum (or
descend a very small amount), but in Chiari 1
patients descent is the norm. Lack of tonsillar
descent is an extremely sensitive marker;
therefore, in a patient with presentation that can
be consistent with type 1 Chiari and cerebellar
tonsil protrusion through the foramen magnum
> 3mm, the pt. needs neurosurgery referral.
Operative Results
The most commonly-performed surgery is
suboccipital craniectomy (essentially opens up
the foramen magnum), with or without C1
laminectomy and dural graft patch.
Patients with pain as primary complaint respond best
to surgery; weakness less responsive, but overall
~80% of patients report favorable results.
Presence of muscle atrophy, ataxia, and duration of
symptoms >2 yrs all associated with poorer outcome.
Type 2 Chiari
Type 2 Chiari malformation is also referred to as
“Arnold-Chiari malformation.”
Presents in childhood
Usually the younger it presents, the more severe the
Usually associated with myelomeningocele!!!
(The USMLE loves this)
Arnold-Chiari malformation
Signs/symptoms secondary to brainstem and
lower cranial nerve dysfunction.
Findings (best seen on MRI):
Caudal displacement of posterior fossa structures,
including cervicomedullary junction, pons, medulla,
4th ventricle, and cerebellar tonsils. Classically, the
cervicomedullary junction is described as having a
“kink-like deformity.”
Arnold-Chiari malformation
Associated findings
Hydrocephalus (VERY common – requires shunt)
 Syringomyelia
 Agenesis/dysgenesis of corpus callosum
Operative goals similar to type 1, but these
patients do not do as well (and the younger their
presentation, the worse the general outcome).
Type 3 Chiari - Rare and severe (usually not
compatible with life); basically, posterior fossa
structures end up everywhere except where they
should be.
Type 4 Chiari – Cerebellar hypoplasia without
For USMLE purposes, it’s good to understand
the major characteristics of and differences
between type 1 Chiari and type 2 Chiari, as it
seems to show up a lot (probably because
people mix them up a lot).
Understand that type 1 Chiari malformation has
an extremely variable presentation. If you keep
it on your radar in patients who present with
these symptoms, you can be a patient’s hero!

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