Sometimes referred to as the Arnold-Chiara malformation, it is a complex malformation of the brain and spine characterized often by myelomeningocoele nd hydrocephalus, asub-type of spina bifida (in which the embryonic caudal neuropore fails to close and spinalcord protrudes from back), reduced posterior fossa and descent of the brain stemand cerebellum. Frequentlyassociated with agenesis of the corpus callosum. It takes on a number of different forms and arises from a variety of determinants. Type II is most common form and is congenital in origin and arises from structural defects during fetal development as a consequence of genetic mutation or lack of vitamins and other nutrients in the maternal diet. Type I occurs during the postnatal development of the brain and skull, with the result that it is not manifest until late childhood or adulthood. This type can arise, for example, if an excessive amount of spinal fluid is drained from the lumbar or thoracic due to injury or exposure to toxins. Functional consequences vary, for example, from balance and coordination problems to muscle weakness to difficulties with swallowing. The overall incidence is 1 in 1000 live births. Is names is associated with the pathologist Hans Chiari (1851-1916) who reported a case in 1891. However, there were relevant descriptions prior to Chiari.
See Brain stem, Cerebellum(anatomy), Cerebellum (disorders), Congenital abnormalities, Corpus callosum, Dandy-Walker syndrome, Hydrocephalus (or hydrocephaly)