Cerebrospinal fluid collection enclosed within layers of arachnoid membrane (i.e., extraaxial location inside skull but outside brain)

Usually single, extraaxial, thin walled, smoothly marginated, avascular, anechoic

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  2/3 supratentorial (i.e., adjacent to cerebral hemispheres)

  
  
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  1/3 in posterior fossa (i.e., adjacent to cerebellum)

Exerts mass effect displacing adjacent brain parenchyma or major cerebral vessels

Underlying brain sonographically normal in majority of cases, but adjacent calvarium may be thinned, scalloped

Porencephalic cyst: Replaces destroyed brain

Schizencephaly

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  Cleft in brain substance, wedge-shaped rather than round, absent cavum

Interhemispheric cyst/AVID

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  A symmetric v entriculomegaly with i nterhemispheric cyst and d ysgenesis of corpus callosum

Dandy-Walker malformation

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  Posterior fossa cyst in continuity with 4th ventricle

Always check Doppler in apparent cyst to rule out vascular malformation

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  Vascular lesions have worse prognosis

Monitor for growth of cyst (occurs in ~ 20% of cases)

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  Rarely, may exhibit rapid growth and cause obstructive hydrocephalus

Look for other abnormalities

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  Prognosis good if isolated abnormality

> 90% discovered after 20-weeks gestation