What is Arnold–Chiari Malformation?

Chiari malformation was originally known as "Arnold–Chiari malformation", named after the two researchers who first discovered the condition. The condition is now more commonly referred to as Chiari malformation, as some clinicians reserve the term "Arnold–Chiari" to describe the paediatric form (Type II) of the disorder.

What is Chiari Malformation?

Chiari malformation is a neurological disorder that is characterised by abnormalities in the area where the brain and spinal cord meet – this is roughly the area at the base of the skull.

In Chiari, the bottom portion of the brain becomes compressed into the spinal column. There are many possible reasons this could happen. One is that the size of the growing skull does not keep up with the growing brain. Another is that the skull may have a slightly abnormal shape that has gone undetected. Because this type of Chiari malformation arises with growth and development, its symptoms are most often noticed in adulthood. This type of compression is referred to as Chiari malformation Type I. Brain tissue compressed into the spinal column will also block the regular flow of cerebral spinal fluid (CSF) to the area, and this also contributes to the symptoms experienced.

The cerebellum is located at the base of the brain and this is the region most affected by Chiari malformation. The cerebellum is responsible for balance and walking, and some Chiari symptoms have a noticeable effect on these motor skills. 

The Mayo Clinic in the USA has a website that describes in more detail Chiari malformation, symptoms and best forms of treatment. It is worth having a look.

How many types of Chiari malformation are there?

There are four types of Chiari malformation and they have varying degrees of severity. Both the brain and spinal cord can be affected in Chiari. Type 1 is the least severe, Type IV is the most severe. In brief, they are characterised as follows:

Type I: This is the most common form of Chiari malformation. A lot of people with Type I do not know they have it until they are teenagers or adults, though it can also be detected in young children. Usual signs are severe headache brought on by sudden coughing, sneezing or straining.

Type II: This form is more severe as a greater amount of tissue is compressed into the spinal column. Type II Chiari malformation usually results in a form of spina bifida called myelomeningocele, where the backbone and spinal cord do not close properly before birth. Type II Chiari malformation can be detected during pregnancy with ultrasound.

Type III & Type IV: These are the most severe forms of Chiari malformation, with a portion of the lower brain or brainstem exposed through an opening in the skull (Type III), or where the brain has not formed properly (Type IV). The prognosis for survival with Type III is severe, complicated by other life-threatening conditions; babies born with the Type IV condition often die in infancy.

See our extended list of Symptoms and Treatment page for more information.