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To say “Chiari malformations” is slightly misleading. The Chiari malformations actually consist of four defined types of hindbrain abnormalities, each distinct from the others.
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The first two types, especially type 1, will be briefly reviewed here. The 3 rd and 4 th types of
Chiari malformations are exceedingly rare and will only briefly be mentioned.

The
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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.
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Although this is a common radiographic feature of the condition, it is not a prerequisite for diagnosis.

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Most common presenting complaint is suboccipital headache
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Neck pain, subjective weakness, numbness, loss of temperature sensation also ~40-60% incidence
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Most common presenting sign is hyperactive lower extremity reflexes
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“Cape”-like sensory loss, nystagmus (downbeat), gait disturbance, upper extremity weakness, etc., are also all very common (~30-50% incidence of each).

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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 misdiagnosed.

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MRI is diagnostic test of choice
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Can show compression of brain stem at foramen magnum (common, and significant finding)
Hydrocephalus can be present
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Syringomyelia
Descent of cerebellar tonsils through foramen magnum
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Importance probably related to brainstem compression at foramen magnum; nevertheless, this is classic finding associated with type 1 Chiari.

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T1 weighted MRI w/o contrast, sagittal view; in this outstanding picture of a patient with type 1
Chiari, you see:
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(1) cerebellar tonsils well below the foramen magnum
(2) syringomyelia
(3) compression of brainstem
(3)
(2)
(1)

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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.

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The most commonly-performed surgery is suboccipital craniectomy (essentially opens up the foramen magnum), with or without C1 laminectomy and dural graft patch.
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Patients with pain as primary complaint respond best to surgery; weakness less responsive, but overall
~80% of patients report favorable results.
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Presence of muscle atrophy, ataxia, and duration of symptoms >2 yrs all associated with poorer outcome.

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Type 2 Chiari malformation is also referred to as
“Arnold-Chiari malformation.”
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Presents in childhood
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Usually the younger it presents, the more severe the condition.
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Usually associated with myelomeningocele!!!
(The USMLE loves this)

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Signs/symptoms secondary to brainstem and lower cranial nerve dysfunction.
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Findings (best seen on MRI):
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Caudal displacement of posterior fossa structures, including cervicomedullary junction, pons, medulla,
4 th ventricle, and cerebellar tonsils. Classically, the cervicomedullary junction is described as having a
“kink-like deformity.”

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Associated findings
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Hydrocephalus (VERY common – requires shunt)
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Syringomyelia
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Agenesis/dysgenesis of corpus callosum
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Operative goals similar to type 1, but these patients do not do as well (and the younger their presentation, the worse the general outcome).

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Type 3 Chiari - Rare and severe (usually not compatible with life); basically, posterior fossa structures end up everywhere except where they should be.
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Type 4 Chiari – Cerebellar hypoplasia without herniation.

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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!