Calcified congenital arachnoid cyst with heterotopic neuroglia in wall.

Two unique findings, advanced calcification and ectopic neuroglia, were encountered in the wall of a giant congenital arachnoid cyst occurring in a 40 year old woman. The cyst almost totally filled the supratentorial subdural space of the left hemicranial cavity, was not connected with the subarachnoid space, and thus developed intra-arachnoidally. Its congenitally derived nature was supported by the unique finding of heterotopic neuroglia in the wall. Congenital arachnoid cyst must be distinguished from various cystic lesions within the central nervous system, including the neuroepithelial cyst which can arise throughout the neuraxis. It is suggested that pathogenesis of the congenital arachnoid cyst is related to aberrant flow of the ventricular cerebrospinal fluid into the developing leptomeninx in the process of differentiation of the subarachnoid space. The tract or pouch may occur within the arachnoid mater, and the cyst is formed intra-arachnoidally when the former filled with fluid is closed off from the subarachnoid space.