Chiari malformations are congenital structural defects in the base of the skull and back portion of the brain called the cerebellum. In a normally formed skull and brain, the cerebellum and brain stem are located directly above an opening in the skull, called the foramen magnum, which allows the spinal cord to pass through and connect to the brain stem. In Chiari malformations, the cerebellum and/or brain stem extend through the foramen magnum and into the cavity that holds the spinal cord. As a result, the function of the cerebellum and brainstem and the flow of cerebrospinal fluid are impeded. Chiari malformations are typically the result of structural defects that occur during fetal development, but the malformations can be caused by gene mutations.
Chiari malformations can be broken down into subtypes, based on severity. Type I is the least severe Chiari malformation, and while it occurs during fetal development, it is typically not diagnosed until adolescence or adulthood. Type II, also called the Arnold-Chiari malformation or classic Chiari malformation, is characterized by both the cerebellum and brainstem extending into the spinal canal through the foramen magnum, as well as the underdevelopment or absence of the nerve tissue that connects both sides of the cerebellum. Type II is typically associated with an open neural tube defect called a myelomeningocele, which occurs when the spinal canal and spine do not close during fetal development. Type II is the leading cause of death for infants with a myelomeningocele.
Type III, the most severe and rare Chiari malformation, occurs when the brain stem and cerebellum extend through a defect, or abnormal opening, in the back of the skull, rather than through the foramen magnum. The membranes that surround the cerebellum and brain stem may also extend through the defect in the skull. Type IV is characterized by the incomplete development of the cerebellum in addition to defects in the skull. Chiari malformations are not fatal conditions, but they can cause severe, life-limiting complications. Chiari malformations are associated with additional fetal anomalies such as agenesis of the corpus callosum, hydrocephalus, and neural tube defects.
Chiari malformations are common, and an exact incidence rate is difficult to determine due to the increased use of diagnostic tools such as MRI, CT Scan, and x-ray. It has been estimated that Chiari malformations occur in 1 in 1,000 live births, but Chiari malformations are believed to be more common than that incidence rate suggests. There is almost no recurrence risk for Chiari malformations in subsequent pregnancies.