Arachnoid cyst in the region of the fourth ventricle mimicking clinical picture of normal pressure hydrocephalus.

Arachnoid cysts in the region of the fourth ventricle are rarely reported. Two cases with intraventricular arachnoid cysts (one in the fourth and one in the right lateral) manifesting normal pressure hydrocephalus have been described in the previous literature. Here we report a clinically similar case in which the cyst was wedged in cerebellar vermis and compressed the fourth ventricle. Slow enlargement of the cyst could explain the clinical picture. The removal of the cyst resulted in a marked improvement of the symptoms despite the lack of significant changes in ventricular size.