= normal variant of focal enlargement of subarachnoid space in posteroinferior aspect of posterior fossa

Cause: ? delayed fenestration of Blake pouch

• incidental finding
• cisterna magna ≥10 mm (on midsagittal image)
  • free communication with 4^th ventricle + cervical subarachnoid space (confirmed with CSF flow study)

Pertinent negatives:

• intact vermis + normal 4^th ventricle
• absence of hydrocephalus
• occasionally enlarged posterior fossa

DDx:

1. Blake pouch cyst (hydrocephalus)
2. Isolated inferior vermian hypoplasia