Arnold-Chiari malformation

Arnold-Chiari malformation, sometimes referred to as Chiari malformation or ACM, is a congenital anomaly of the brain.Arnold-Chiari Malformation II occurs in almost all children born with both spina bifida and hydrocephalus, but ACM I is typically seen in children and adults without spina bifida. The scale of severity is rated I - IV, with IV being the most severe.
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noun
congenital condition where the base of the skull is malformed, allowing parts of the cerebellum into the spinal canal