Chiari malformations (CMs) are congenital structural defects of the skull that force part of the brain (the cerebellum and brainstem) through the hole in the bottom of the skull where the spine exits. This causes pressure that affects both the cerebellar function of the cerebellum, which controls balance and coordination, and the brainstem, which regulates vital neurologic functions such as breathing and heart rate. The condition also blocks the flow of cerebrospinal fluid to and from the brain. CM affects around 1 of 1000 live births in the United States and long-term mortality is as high as 50%. The most common types of CM are type I (limited protrusion of the cerebellum), which is often asymptomatic in childhood although problems may develop later in life. Type II CM, usually diagnosed in children, involves a more extensive protrusion and worse symptoms; it may be associated with another type of congenital defect called myelomeningocele, where a portion of the spinal cord protrudes through an opening in the back. CMs are treated by surgical removal of bone from the skull and/or spine to create more space and relieve pressure on the brainstem.