An Arnold-Chiari type II malformation:

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Arnold-Chiari malformation is a developmental deformity of the hindbrain.A type I malformation is not associated with a lower neural tube defect and occurs when the cerebellar tonsils herniate into the vertebral canal.If symptomatic,this malformation is usually characterized by headache and neck pain,hydrocephalus,and visual disturbances.Signs of type II malformation are visible beginning in infancy and result from extension of the medulla and cerebellum through the foramen magnum.This malformation often produces progressive hydrocephalus,deafness,weakness of eye movements,as well as facial and neck muscle weakness.In addition,it is nearly always accompanied by incomplete closure of the lower neural tube.

An Arnold-Chiari Type II Malformation