This case demonstrates transtentorial posterior fossa pseudocyst communicating with the dilated right lateral ventricle complicated by supratentorial hydrocephalus and compression of the cerebellum resulting in its downward descent and tonsillar herniation in keeping with acquired Chiari I malformation. 

The mass effect and combined impedance of CSF flow at the foramen magnum and distal aqueductal obstruction results in marked supratentorial hydrocephalus.

Tonsillar herniation is assessed measuring its descent from the line that connects the inner margins of the foramen magnum (basion to opisthion). It measures 18 mm in this case (above 5 or 6 mm is considered a Chiari I malformation).