Pathology of an arachnoid cyst. Case report.

Accidental death brought to autopsy a 19-year-old girl with asymptomatic arachnoid cyst which had expanded from the region of the interpeduncular cistern so as to replace the third ventricle and extend into both alteral ventricles. Associated with this expansion was a significant degree of hydrocephalus, compression of the walls of the lateral ventricle, and obliteration of the normal structures of the floor of the third ventricle. The histological structure of the cyst wall and its relationship to the normal arachnoid are defined and found to consist of a reduplication of the normal arachnoid membrane resulting in a space within the arachnoid tissue. It is suspected but not proven that the cyst was congenital, resulting from an abnormality in development. Some suggestions are offered regarding the mechanism for its enlargement.