Arachnoid cysts: entrapped collections of cerebrospinal fluid variably communicating with the subarachnoid space.

Despite decades of investigation and discussion of the mechanisms involved in the pathophysiology of arachnoid cysts, fundamental issues concerning these entities remain poorly defined and controversial. Cine-mode magnetic resonance imaging (MRI) has shown two patterns of cerebrospinal fluid (CSF) flow within the cavity in patients harbouring arachnoid cysts. Some cysts present a harmonic flow with a patent flow entry zone. All these patients had intermittent, non-progressive and non-localizing symptoms requiring no surgery according to our criteria. The second pattern of CSF flow is more chaotic and is characterized by the presence of swirls throughout the entire cardiac cycle. This pattern is associated with a more disabling clinical picture. Some of these patients required surgical treatment. During surgery, an endoscope was used for inspection purposes revealing, above all, that arachnoid cysts always and variably communicate with the subarachnoid space. The CSF enters the cyst either through a patent flow entry zone or through minute perforations in areas more loosely packed of the arachnoid network that behave as a flexible mesh able to modify the area of flowing CSF. The slipstreams of CSF within arachnoid cysts may not be channelled properly leading to possible damage of the surrounding brain parenchyma.