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