Arachnoid cyst (fetal presentation)

A fetal arachnoid cyst is term given when an arachnoid cyst is diagnosed in utero. For a general discussion of arachnoid cysts refer to the parent article.


They can be classified as being primary or secondary .

  • primary (congenital) arachnoid cysts: result from a benign accumulation of clear fluid between the dura and the brain substance throughout the cerebrospinal axis in relation to the arachnoid membrane and do not communicate with the subarachnoid space.
  • secondary (acquired) arachnoid cysts: result from hemorrhage, trauma, and infection and usually communicate with the subarachnoid space

As in the adult population, in utero arachnoid cysts tend to be more common supratentorially.


Recognized associations include:

Radiographic features

Antenatal ultrasound

They are typically seen as an anechoic simple cystic lesion within the fetal brain. A definitive diagnosis can sometimes be difficult on ultrasound. They usually should not communicate with the ventricles.


  • when large, they may cause symptoms by compression of the adjacent brain
  • however, they do not cause brain maldevelopment, and, if treated before irreversible brain damage occurs, a good outcome may be anticipated
  • hemorrhage can occasionally complicate them, either within or adjacent to the cyst 
  • arachnoid cysts in sites such as the quadrigeminal cistern or suprasellar cistern may cause hydrocephalus, owing to compression of the adjacent structures and impairment of drainage of cerebrospinal fluid

Differential diagnosis 

Arachnoid cysts must be distinguished from other supratentorial cysts that include (fetal intracranial cystic lesions):

  • porencephalic cysts: communicate with the lateral ventricle
  • interhemispheric cysts: e.g., those associated with agenesis of the corpus callosum
  • dorsal cysts of holoprosencephaly
  • arteriovenous malformations (vein of Galen aneurysms): demonstrates an arterial color flow