Core Terms & Classification

Chiari malformation type I (CM-I)

Chiari malformation type I is the most common form, in which only the cerebellar tonsils descend below the skull's base opening, often without symptoms.

In short: Chiari malformation type I (CM-I)

It involves descent of the cerebellar tonsils alone.
It is the most common type and often found incidentally.
Symptoms, if any, may appear in older childhood or adulthood.
It can be associated with syringomyelia.

What it is

Chiari malformation type I (CM-I) is the most common form of Chiari malformation, defined by downward descent of the cerebellar tonsils (but not the brainstem) below the foramen magnum. It is frequently discovered incidentally and may cause no symptoms.

Understanding Chiari malformation type I (CM-I)

In CM-I, the cerebellar tonsils extend below the skull's base opening, typically by a threshold amount on MRI (often cited as 5 mm in adults, 3 mm in children, though debated). Unlike the more severe types, CM-I does not involve the brainstem and is often not present with symptoms at birth; when symptoms occur, they may emerge in later childhood or adulthood, classically as Valsalva-induced occipital headaches. CM-I is commonly found incidentally on imaging done for other reasons. It can be associated with syringomyelia (a fluid-filled cavity in the spinal cord), which is an important consideration. The natural history of asymptomatic CM-I is generally benign.

Why it matters

Understanding CM-I helps families see that the most common form of Chiari is often mild or asymptomatic, that symptoms (when present) typically appear later in life, and that its association with syringomyelia is an important thing to evaluate.

Common questions

What defines Chiari malformation type I?

Descent of the cerebellar tonsils alone below the skull's base opening.

Is it usually symptomatic?

Often not — it is frequently found incidentally.

Can it cause syringomyelia?

Yes — it can be associated with a spinal cord syrinx.

Bottom line

Chiari malformation type I is the most common form, involving tonsillar descent alone, often incidental, and sometimes associated with syringomyelia.

More terms in Core Terms & Classification

Chiari malformation type II (CM-II)Chiari malformation type II is a more severe form, present at birth, in which the cerebellum and brainstem descend, almost always associated with spina bifida.
Chiari malformation type III (CM-III)Chiari malformation type III is a rare, severe form in which hindbrain structures extend into a sac at the back of the skull or upper neck.
Chiari malformation type IV (CM-IV)Chiari malformation type IV is a rare, severe condition involving an underdeveloped cerebellum, now considered by many an outdated or separate term.
Chiari 0 malformationChiari 0 malformation is a variant in which a person has syringomyelia and CSF flow problems at the skull-spine junction despite little or no tonsillar descent.
Chiari 1.5 malformationChiari 1.5 malformation is a variant between types I and II, with tonsillar descent plus downward displacement of the brainstem.

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