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 th ese entities remain poorly defined and controversial. Cine-mode magnetic re sonance imaging (MRI) has shown two patterns of cerebrospinal fluid (CSF) f low within the cavity in patients harbouring arachnoid cysts. Some cysts pr esent a harmonic flow with a patent flow entry zone. All these patients had intermittent, non-progressive and non-localizing symptoms requiring no sur gery according to our criteria. The second pattern of CSF flow is more chao tic and is characterized by the presence of swirls throughout the entire ca rdiac cycle. This pattern is associated with a more disabling clinical pict ure. Some of these patients required surgical treatment. During surgery, an endoscope was used for inspection purposes revealing, above all, that arac hnoid cysts always and variably communicate with the subarachnoid space. Th e CSF enters the cyst either through a patent flow entry zone or through mi nute perforations in areas more loosely packed of the arachnoid network tha t behave as a flexible mesh able to modify the area of flowing CSF. The sli pstreams of CSF within arachnoid cysts may not be channelled properly leadi ng to possible damage of the surrounding brain parenchyma.