Chiari 1

KEY FACTS

TERMINOLOGY

• Chiari 1 malformation (CM1): Constellation of findings (not

disease, not simply measurement)

• No clear consensus definition of what constitutes CM1

○ Traditional: Elongated, peg-shaped cerebellar tonsils

extend below foramen magnum (FM) into upper cervical

spinal canal

○ 5-mm criterion for tonsillar position (TP) below FM is

flawed criterion

– TP is morphometric distribution, also changes with

time

– TP plus shape/configuration (elongated, pointed)

– TP also risk factor for syrinx (lower the tonsils, higher

the risk)

○ "Crowded" posterior fossa + compressed CSF spaces

– +/- obex displaced below FM

○ Evaluate skull base, upper cervical spine

– Posterior fossa may be small, shallow (especially

children)

– Short clivus, craniovertebral junction assimilation

anomalies common

TOP DIFFERENTIAL DIAGNOSES

• Normal variant (normal-shaped tonsils below FM)

• Intracranial hypotension

○ Critical not to mistake this for CM1

• Acquired tonsillar herniation (don't call "acquired Chiari 1")

• "Complex Chiari malformation" (neurosurgery: "Chiari 1.5")

○ Tonsillar herniation complicated by other abnormalities

(caudal descent of brainstem with low-lying obex, bony

anomalies, such as "retroflexed" odontoid)

CLINICAL ISSUES

• Up to 50% of CM1 is asymptomatic

○ Headache, ataxia, downbeat nystagmus, progressive

spastic quadriparesis

• Caution: Look for findings of intracranial hypotension

before making diagnosis of CM1

(Left) Sagittal graphic

demonstrates pointed peg-like

tonsils ﬈ extending below

foramen magnum, elongating

the normally positioned 4th

ventricle. Note low position of

obex ﬊. (Right) Sagittal T2WI

in a 23-year-old man with

classic Chiari 1 malformation

shows low-lying pointed

tonsils ﬇, obex below the

foramen magnum st, and

hyperintensity in the upper

cervical cord ſt that may

represent "presyrinx" state,

nucleus gracilis.

(Left) Sagittal T1WI MR shows

normal 4th ventricle position

and appearance. The fastigium

st is in normal position,

helping to distinguish from

Chiari 2 malformation. There

is inferior displacement of the

ectopic cerebellar tonsils ſt

through the foramen magnum

with odontoid process

retroflexion and clivus

foreshortening. (Right) Axial

T2WI MR confirms inferior

displacement of ectopic

cerebellar tonsils ſt through

the foramen magnum,

producing foramen magnum

crowding.

Trauma, and Stroke

Brain: Pathology-Based Diagnoses: Malformations,

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