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IV. Posterior Fossa Anomalies

1. Arnold-Chiari malformation - small posterior fossa

The term "Chiari malformation" encompasses a complex of abnormalities of the brainstem and cerebellum. Four types have been described. Type II, called the Arnold-Chiari malformation, is the most common and is almost always associated with polymicrogyria, meningo­myelocele, and hydrocephalus. There is a probable pattern of multifactorial inheritance. The Arnold-Chiari malformation is characterized by:

a) misshapen midline cerebellum with downward extension of cerebellar vermis through foramen magnum

b) elongation of medulla, pons and 4th ventricle, with kinking of medulla.

c) "beak"-shaped malformation of colliculi

d) aqueductal stenosis, leading to hydrocephalus


Radiology showing Arnold-Chiari malformation - the arrow indicates the cerebellar vermis extending downward
Click here to enlarge