Chiari malformation

noun

Chi·​ari malformation kē-ˈär-ē- 

: a congenital abnormality of the hindbrain in which the cerebellum and often parts of the brain stem protrude into the spinal canal through the foramen magnum

Blocked spinal fluid can cause hydrocephalus or, as is more common in Chiari malformation, a fluid-filled cyst known as a syrinx.

—Rebecca Voelker, The Journal of the American Medical Association

Note: Chiari malformation is usually classified into four types. Type 1 involves protrusion only of the lower part of the cerebellum into the spinal canal and is often asymptomatic. Type 2 is the Arnold-Chiari malformation. In type 3, the cerebellum is completely displaced into the spinal canal. In type 4, the cerebellum fails to develop completely.