Core Terms & Classification
Chiari malformation
Chiari malformation is a group of structural conditions in which part of the back of the brain extends down through the opening at the base of the skull toward the spinal canal.
In short: Chiari malformation
- It involves brain tissue (usually the cerebellar tonsils) sitting lower than normal, through the skull's base opening.
- There are several types, ranging from common and often mild to rare and severe.
- Many people have no symptoms and learn of it incidentally on imaging.
- Symptoms, when present, often relate to crowding and disrupted cerebrospinal fluid flow at the skull-spine junction.
What it is
Chiari malformation (CM) is a group of conditions in which structures at the back and base of the brain — most often the lower part of the cerebellum (the cerebellar tonsils), and in some types the brainstem — extend downward through the foramen magnum, the large opening at the base of the skull, toward or into the upper spinal canal. It is named after Hans Chiari, who described several types in the 1890s.
Understanding Chiari malformation
Rather than a single disease, Chiari malformation is a spectrum of related conditions classified by type (I, II, III, and IV, plus newer variants such as 0 and 1.5). The most common form, type I, involves only the cerebellar tonsils and is often discovered incidentally; more severe types are present at birth and can involve the brainstem and other structures. The crowding at the skull-spine junction can disrupt the normal flow of cerebrospinal fluid (CSF), which is thought to underlie many symptoms and the associated condition syringomyelia. Importantly, the amount of herniation often does not match the severity of symptoms.
Why it matters
Understanding that Chiari malformation is a spectrum — not one condition — helps families see why experiences vary so widely, from no symptoms at all to significant problems, and why the type and the individual situation, rather than the label alone, guide care and decisions.
Common questions
What is Chiari malformation?
A group of conditions where part of the back of the brain extends down through the skull's base opening.
Is it one disease?
No — it is a spectrum with several types of differing severity.
Does everyone with it have symptoms?
No — many people have no symptoms and find it incidentally.
Bottom line
Chiari malformation is a spectrum of conditions in which part of the back of the brain extends through the skull's base opening, ranging from often-incidental to severe.
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