Definition:

Chiari malformations, also known as Arnold-Chiari syndrome, is a group of syndromes consisting of different kinds of pathologic conditions of the posterior fossa development. They are congenital in most cases, caused by structural defects in the brain spinal cord which may involve genetic mutations or lack of proper vitamins or nutrients in the maternal diet. Less frequently, Chiari malformations are acquired after birth. Causes of acquired Chiari malformations involve injuries, exposure to harmful substances, infections. More rarely, chronic subdural hematoma can be the cause of progressive caudal descent of the cerebellar tonsils. Chiari malformations were classified by Hans Chiari in 1891, into four groups. Chiari malformation type I (CM I), the most frequent of Chiari malformations, is characterized by the inferior displacement of cerebellar tonsillas through the foramen magnum. This leads to different symptoms and clinical features, such as headaches, syringomyelia, and hydrocephalus. CM II is characterized by displacement of the parts of the inferior vermis, pons, and medulla oblongata together with elongation of the fourth ventricle. Most cases are associated with myelomeningocele. CM III is characterized by an occipital or cervical encephalocele along with the intracranial abnormalities seen with CM II malformation and a wide foramen magnum. It causes severe neurological defects. CM IV is characterized by marked cerebellar hypoplasia or aplasia.