Chiari malformation is a congenital (present at birth) defect occurring in the back of the head where the brain and spinal cord connect.
There are four types of Chiari malformations.
Type 1 occurs when the base of the skull and upper spinal area do not form properly, a type 1 Chiari malformation commonly goes unnoticed until problems arise in the adolescent or adult years of life. The headaches most typical of Chiari I malformations are usually located at the back of the head, and are often made worse by exertion.
Type 2 is the most common and is caused by part of the back of the brain shifting downward through the bottom of the skull. Type 2 Chiari malformations are typically seen in infants who are born with spina bifida, a neurological condition that causes a portion of the spinal cord and the surrounding structures to develop outside, instead of inside, the body. Type 2 Chiari malformations can also be associated with hydrocephalus, a condition in which there is an overproduction or lack of absorption of the cerebral spinal fluid (CSF) that is found inside of the ventricles (fluid-filled areas) inside of the brain. The increased fluid causes the pressure inside of the head to increase and the skull bones to expand to a larger-than-normal appearance.
Type 3 Chiari malformations occur when the back of the brain protrudes out of an opening in the back of the skull area.
Type 4 Chiari malformations occur when the back of the brain fails to develop normally.