Classic Dandy-Walker malformation

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  Cerebellar vermis is completely or partially absent

  
  
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  Cystic dilatation of 4th ventricle (4V)

  
  
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  Enlarged posterior fossa with elevated tentorium

  
  
  
  
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    Tentorium is roof of posterior fossa and separates cerebellum from occipital lobe

  
  

4V dilated and communicates directly with cisterna magna

Posterior fossa cyst (often quite large)

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  Elevates tentorium (seen best on sagittal view)

If partial vermis present, it is rotated and displaced superiorly

Ventriculomegaly is common feature

~ 70-90% have additional anomalies

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  Other brain anomalies common (above tentorium)

Fetal MR is additive (better sagittal view, other brain anomalies)

Persistent Blake pouch cyst

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  Vermis is elevated but otherwise normal

Mega cisterna magna

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  Cerebellum otherwise normal

Arachnoid cyst

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  Mass effect on otherwise normal cerebellum

Aneuploidy in ~ 50%

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  Trisomy 18,13,9; monosomy X (Turner syndrome)

Associated with syndromes

Try 3D to obtain multiplanar sagittal view if not able to see this plane by 2D ultrasound

Use transvaginal ultrasound if fetus is cephalic

Measure vermis length on sagittal view

Measure cerebellar diameter for any anomaly in posterior fossa