Holoprosencephaly with retroprosencephalic extracerebral cyst.

The authors report holoprosencephaly combined with an enormous extracerebral glio-ependymal cyst in the superior cistern of a 10-week-old boy. The cyst had compressed the holosphere at the floor of the anterior fossa and had caused herniation of the cerebellar vermis and tonsils at the foramen magnum, along with ascending cervical roots. This uncommon combination of lesions clinically may mimic hydranencephaly or hydrocephalus.