Arnold-Chiari malformation

noun

Ar·​nold-Chi·​ari malformation ˈär-nᵊld-kē-ˈär-ē- 

: a congenital abnormality in which the lower surface of the cerebellum and the lower brain stem protrude into the spinal canal through the foramen magnum

She was born with Arnold-Chiari malformation, a rare and random birth defect that occurs, according to one study, in about 8 of 100,000 people. A deformity of the base of the skull and spine, it can cause chronic pain and loss of balance.

—Arthur Allen, The Washington Post

Note: Arnold-Chiari malformation often occurs with spina bifida and hydrocephalus. It is classified as a type of Chiari malformation.