Computed tomographic signs of Chiari II malformation. II: Midbrain and cerebellum.

In the Chiari II deformity, the following defects can be visualized by computed tomography: partial or complete fusion of the corpora quadrigemina into a tectal beak or spur and invagination of the midline cerebellum to receive this spur; marked upward, transincisural growth of the cerebellum to produce a supratentorial, "extra-axial" mass effect (especially after shunting); enlargement of the pericerebellar cistern about the "towering" cerebellum, and infolding of the anterior border of the towering cerebellum to overlap the vermis and the more caudal portions of the cerebellum. Growth of the cerebellar hemispheres around the brain stem results in (a) overlapping of more of the cerebral peduncles than usual, which sometimes separates the midbrain from the hippocampus; (b) overlapping of more of the lateral aspect of the pons than usual, causing partial filling in of the cerebellopontine angle (CPA) cisterns, presenting as bilateral CPA masses, and (c) wedging of the cerebellar margin between the lateral aspect of the brain stem and the adjacent free tentorial margin, so that the cerebellar hemispheres acquire an anteriorly pointed configuration.