[0] Arachnoid cyst [1] Common [2] Multiple possible mechanisms including leptomeningitis, trauma, and malformation. [3] About half of the cases occurs along the Sylvian fissure, other common locations include the cerebellar pontine angle and quadrigerminal area, vermian area, sellar and suprasellar area. [4] May be clinically silent. Symptomatic cases have manifestations of a gradually expansile mass leading to increased intracranial pressure. May cause hydrocephalus, particular the cerebellar cysts. [6] Most of them have a thin, collgeneous cyst wall lined by stratified and compressed meningothelial (arachinoid) cell. [5] 
The size is variable but some can attain a very large size. They are fluid filled cysts with thin, transparent wall. The cyst can be well distincted from the leptomeninges and dura. The fluid is usually clear and colorless.