Giant Posterior Fossa Arachnoid Cyst

Posterior fossa arachnoid cysts comprise 20 to 30% of intracranial arachnoid cysts. The characteristic appearance with magnetic resonance imaging (MRI) is a nonenhancing cystic lesion situated in the cerebellopontine angle or region of the foramen magnum that is hypointense on T1-weighted images and hyperintense on T2-weighted images. The fourth ventricle and cisterna magna are formed normally but may be displaced by the mass effect of the cyst. Less commonly, the cyst may be located in the quadrigeminal cistern or dorsal to the cerebellum. Rarely, these cysts may attain considerable size before producing symptoms. The differential diagnosis of a posterior fossa arachnoid cyst includes the Dandy-Walker malformation, mega cisterna magna, pilocytic astrocyto-Fig. 1. Sagittal T2-weighted MRI showing a hyperintense cyst filling the posterior fossa and extending into the rostral spinal canal. [PUBLICATION ABSTRACT]