Chiari Malformation

Pearls in the Diagnosis of Chiari Malformation [Reference 3]

  • Chiari I: Ectopia with or without cyst formation
  • Chiari II [Arnold-Chiari]: Caudal displacement of the fourth ventricle, peg-shaped and fused tonsils, cranio-cervical junction anomalies such as Klippel-Feil, +/- meningocele (almost all)
  • Chiari III-IV: Cerebellar hypoplasia or occipital encephalocele

 

Chiari I [Reference 2]

  • Peglike, pointed tonsils displaced into upper cervical canal
  • At least 6 mm displacement in the first decade of life
  • 10-30 years: 5 mm displacement
  • 30-80 years: 4 mm displacement

Associated anomalies:

Brain

  • Usually none

Ventricles

  • Mild-moderate hydrocephalus [20-25%]

Spinal cord

  • Syringomyelia in 30-60% of all patients, 60-90% in symptomatic patients

Skeletal anomalies in 25%

  • Basilar invagination [25-50%]
  • Klippel-Feil [5-10%]
  • Atlanto-occipital assimilation [1-5%]
  • Retroflexed dens

Malformations:

  • Cerebellar tonsillar ectopia [>5 mm]

Associated Findings:

  • Hydrocephalus
  • Syringohydromyelia
  • Craniocervical dysgenesis

 

Chiari II [Reference 3]

Skull & Dura

  • Calvarial defects [lacunar skull or luckenschadel]
  • Small posterior fossa with low-lying transverse sinuses
  • Falx hypoplasia
  • Heart-shaped incisura with hypoplastic tentorium
  • Enlarged foramen magnum
  • Concave clivus, petrous ridges

Brain

  • Inferiorly displaced vermis
  • Medullary spur and kink
  • Beaked tectum
  • Interdigitated gyri
  • Cerebellum “creeps” around brainstem and herniates superiorly through the widened incisura
  • Other associated anomalies: callosal dysgenesis, heterotopias, polymicrogyria, stenogyria

Ventricles

  • Whole system: hydrocephalus in 90%
  • Fourth ventricle compressed, elongated and low
  • Third: Enlarged massa intermedia, may be high-riding in corpus callosum absent
  • Lateral: colpocephaly, scalloped, pointed walls

Spine & cord

  • Myelomeningocele in nearly 100%
  • Syringohydromyelia 50-90%
  • Diastematomyelia
  • Sementation anomalies in <10%, incomplete C1 arch
  • Spinal dysraphism

Clinical

  • Open spina bifida in nearly all cases
  • Raised blood alphafetoprotein

Malformations:

  • Lumbosacral myelomeningocele
  • Small posterior fossa and large foramen magnum
  • Downward displacement of cerebellum, pons, medulla
  • Cervicomedullary “kink”
  • Anterolateral displacement of cerebellum around brainstem
  • Tectal “beaking”

Common associated findings:

  • Agenesis of the corpus callosum
  • Hydrocephalus

Modified from Reference 1

 

Chiari III [Reference 3]

  • Same findings as in Chiari II, but with associated occipital / high cervical encephalocele

 

References:

  • 1. Greenberg MS. (1991) Handbook of Neurosurgery, Chiari Type I-II Malformations on MR, p 134.
  • 2. Osborne AG. Diagnostic Neuroradiology, Mosby-Yearbook, St. Louis, p 18.
  • 3. Pomeranz SJ and Smith PJ. (1998) Gamuts & Pearls in Neurologic MRI, Chiari I Malformations, pp 32-33.