Abnormal arachnoid mater morphology Arachnoid cyst 2011-03-31T12:46:59Z UMLS:C0078981 Arachnoid cysts Fluid-filled sac located in membrane surrounding brain or spinal cord An extra-parenchymal and intra-arachnoidal collection of fluid with a composition similar to that of cerebrospinal fluid. HP:0100702 Arachnoid cysts can be found on the brain, or on the spine. Intracranial arachnoid cysts usually occur adjacent to the arachnoidal cistern. Spinal arachnoid cysts may be extradural, intradural, or perineural. SNOMEDCT_US:33595009