Chiari malformation

a congenital anomaly in which the cerebellum and medulla oblongata, which is elongated and flattened, protrude into the spinal canal through the foramen magnum. It is classified into three types according to severity: type I involves prolapse of the cerebellar tonsils into the spinal canal without elongation of the brainstem; type II is the Arnold-Chiari malformation (q.v.); and type III consists of complete herniation of the cerebellum to form an occipital encephalocele. It may be accompanied by hydrocephalus, spina bifida, syringomyelia, and mental defects. Called also Chiari deformity. 2. Arnold-Chiari m.