Now a myelomeningocele is typically associated with a condition called an Arnold Chiari II malformation, which is where cerebellar and brainstem tissue slip down into the foramen magnum, the opening at the base of the skull.
In upward herniation, there’s a displacement of the cerebellum upward, through a notch in the tentorium cerebelli, while in tonsillar herniation, parts of the cerebellum slip down through an opening in the skull called the foramen magnum.