Arachnoid cysts are collections of CSF most commonly found in the suprasellar region, sylvian fissure, over the convexity of the hemisphere, and more rarely in the cisterna magna of the posterior fossa.
Their cause is unknown. Some are associated with a previous infective meningitis and subsequent adhesions whilst those in the Sylvian fissure appear to be related to temporal lobe agenesis.
Most arachnoid cysts are asymptomatic and are often occur as incidental findings on CT. On occasion, they may present with mass effects or epilepsy.
Only symptomatic cysts are treated, either by cystoperitoneal shunt or by marsupialisation.
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