Chiari malformations affect females more often than males.
Scientists once believed that Chiari malformations occurred in only 1 in every 1,000 births. But the increased use of diagnostic imaging techniques such as CT scans and MRIs suggests that the condition may be much more common.
Accurate estimates are difficult to make. That's because some children who are born with the condition either never develop symptoms or don't develop symptoms until they reach adolescence or adulthood.
Causes of Chiari Malformations
Chiari malformations are usually caused by structural defects in the brain and spinal cord. These defects develop during fetal development.
Due to genetic mutations or a maternal diet that lacked certain nutrients, the indented bony space at the base of the skull is abnormally small. As a result, pressure is placed on the cerebellum. This blocks the flow of the cerebrospinal fluid. That's the fluid that surrounds and protects the brain and spinal cord.
Most Chiari malformations occur during fetal development. Much less commonly, Chiari malformations can occur later in life. This can happen when an excessive amount of cerebrospinal fluid is drained away because of:
exposure to toxic substances
Types of Chiari Malformations
There are four types of Chiari malformations:
Type I. This is by far the most commonly observed type in children. In this type, the lower part of the cerebellum -- but not the brain stem -- extends into an opening at the base of the skull. The opening is called the foramen magnum. Normally, only the spinal cord passes through this opening.
Type I is the only type of Chiari malformation that can be acquired.
Type II. This is usually only seen in children born with spina bifida. Spina bifida is the incomplete development of the spinal cord and/or its protective covering.
Type II is also known as "classic" Chiari malformation or Arnold-Chiari malformation. In type II Chiari malformation, both the cerebellum and the brain stem extend into the foramen magnum.
Type III. This is the most serious form of Chiari malformation. It involves the protrusion or herniation of the cerebellum and brain stem through the foramen magnum and into the spinal cord. This usually causes severe neurological defects. Type III is a rare type.
Type IV. This involves an incomplete or undeveloped cerebellum. It sometimes is associated with exposed parts of the skull and spinal cord. Type IV is a rare type.
In addition to spina bifida, other conditions sometimes associated with Chiari malformations include:
Hydrocephalus. An excessive buildup of cerebrospinal fluid in the brain.
Syringomyelia. A disorder in which a cyst develops in the spinal cord's central canal.