Fetal arachnoid cyst

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.

Pathology

They can be classified as being primary or secondary ².

• primary (congenital) arachnoid cysts : result from 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 haemorrhage, trauma, and infection and usually communicate with the subarachnoid space

Location

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

Associations

Recognised associations include

• dysgenesis of the corpus callosum
• fetal ventriculomegaly
• neurofibromatosis type I

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.

Complications

• 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.
• they can be occasionally complicated by haemorrhage, 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 

• porencephalic cysts : comminucate 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 arterial colour flow

See also

• fetal intracranial cystic lesions