Hindbrain compression with cerebellar herniation through foramen magnum

Open spina bifida present in almost every case

Posterior fossa compression is key finding

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  Small or obliterated cisterna magna (CM)

  
  
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  Cerebellum loses normal bilobed morphology

  
  
  
  
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    Only finding with mild compression

  
  
  
  
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  Banana sign when severe compression

  
  
  
  
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    Cerebellum wraps around midbrain

  
  

Ventriculomegaly (often progressive)

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  55% at diagnosis and > 90% at delivery

Frontal bone concavity (lemon sign) is nonspecific finding that often resolves in 3rd trimester

Loss of intracranial translucency can be finding at time of nuchal translucency screening

Aqueductal stenosis

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  Progressive ventriculomegaly with macrocephaly

  
  
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  CM not obliterated

Small cerebellum from other causes

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  Cerebellar hypoplasia: Small bilobed cerebellum

  
  
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  Rhombencephalosynapsis: Globular cerebellum

Associated with aneuploidy

↑ maternal serum α-fetoprotein on quad screen results

Newborn needs immediate postnatal ONTD surgery

80% need ventriculoperitoneal shunt

Do not wait for banana sign to suspect Chiari 2

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  Obliteration of CM is best clue

Look meticulously for spina bifida when Chiari 2 seen

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  Might be subtle and without sac