Chiari II Malformation

Findings:
The axial MR images show colpocephaly and a towering cerebellum extending through a widened tentorial hiatus. The sagittal images show a small posterior fossa, associated with flattening and elongation of the fourth ventricle. The tectum is pointed.

Discussion:
This constellation of findings is characteristic of Chiari II malformation. Chiari II malformation is a relatively common congenital CNS anomaly which represents an abnormality of neural tube closure at aproximately 4 weeks of gestation. The most common anomaly due to defective neural tube closure is anencephaly. The basic abnormality in Chiari II malformation is inadequate developmant of the fourth ventricle with a resultant small posterior fossa. The associated findings are largely due to the small posterior fossa. Myelomeningocele is almost universally present with this anomaly. The condition can be hereditary and familial. Chiari II is in no way related to Chiari I.
Associated intracranial findings (variable and rarely all present):
    -hydrocephalus, narrow aqueduct, large foramen magnum
    -lacunar skull- inner table scalloping- resolves
    -scalloped posterior petrous/clivus
    -low tent, wide hiatus
    -hypoplastic falx with gyral interdigitation
    -vertical straight sinus
    -lateral vents parallel with colpocephaly and squaring of frontal horns
    -80% absent septum pellucidum
    -hourglass 3rd vent with large massa intermedia
    -long, low and small 4th vent
    -beaked tectum with large QP cistern
    -large caudate heads
    -towering cerebellum
Extracranial findings:
   ->99% with myelomeningocele
   -70% with deficient C1 posterior arch
   -restrictive dural band at craniocervical junction
   -20% with diastematomyelia (20%)
   -50% hydromyelia 
   -thoracolumbar kyphosis

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