Dandy-Walker cyst upward herniation: the role of magnetic resonance imaging and double shunts.

Upward transtentorial herniation of a Dandy-Walker cyst is a potentially fatal complication, particularly after solitary shunting of the lateral ventricle for associated hydrocephalus. Two patients with upward transtentorial herniation of their Dandy-Walker cysts are presented to illustrate the diagnostic value of magnetic resonance (MR) imaging with the demonstration of unique 'keyhole' and 'snail' signs on the axial and sagittal MR images, respectively. In both patients, double and simultaneous shunting en-Y of a lateral ventricle and the posterior fossa cyst resulted in the resolution of the herniation syndrome.