There are 4 types of Arnold-Chiari malformations, each with a different prognostic significance.
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Findings
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Type
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cerebellar tonsils below the level of the foramen magnum
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1
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cerebellar vermis, medulla and IVth ventricle below the level of the foramen magnum
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2
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cervico-occipital meningomyelocoele, with parts of the cerebellum and medulla in the cavity
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3
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cerebellar hypoplasia AND malformation type 1, 2 or 3
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4
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Associations with the Different Malformation Types
Type 1:
(1) 50% with syringomyelia and/or hydromyelia
(2) 10% have hydrocephalus
Type 2:
(1) 90% with syringomyelia, dysraphism and Klippel-Feil syndrome
(2) 80% with hydrocephalus, aqueduct stenosis and cerebellar malformations
(3) 10% with other visceral malformations
