• In types I, II and III of Arnold-Chiari malformation, herniation of cerebellar tonsils with a displaced, deformed brain stem (MRI, below left) is observed; primary cause is thought to be congenital hypoplasia of occipital bone
• Always associated with hydrocephalus; neurologic symptoms may not develop until adult life or adolescence and may include headaches due to increased intracranial pressure, progressive cerebellar ataxia, progressive spastic quadriparesis
• 50% of Arnold-Chiari malformation patients develop syringomyelia around third to fourth decade (below right)
• New research has suggested that the syrinx forms secondarily from the altered hydrodynamics of CSF flow due to the Arnold-Chiari malformation

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