Core Terms & Classification

Tonsillar herniation (tonsillar ectopia)

Tonsillar herniation is the downward extension of the cerebellar tonsils below the skull's base opening, the hallmark of Chiari malformation type I.

In short: Tonsillar herniation (tonsillar ectopia)

It is the downward displacement of the cerebellar tonsils.
It is the defining feature of Chiari malformation type I.
It is measured on MRI relative to the foramen magnum.
The amount of herniation does not reliably predict symptoms.

What it is

Tonsillar herniation (also called tonsillar ectopia or tonsillar descent) is the downward extension of the cerebellar tonsils — the lower, rounded parts of the cerebellum — below the level of the foramen magnum. It is the hallmark of Chiari malformation type I.

Understanding Tonsillar herniation (tonsillar ectopia)

The cerebellar tonsils normally sit above the foramen magnum; in Chiari I, they extend below it into the upper spinal canal. On MRI, the degree of descent is measured relative to a line across the foramen magnum (the basion-opisthion, or McRae's, line), with descent beyond a threshold (commonly 5 mm in adults, 3 mm in children) typically used to define Chiari I — though this is debated. Crucially, the amount of herniation often does not correspond to symptoms: some people with marked descent have none, while others with minor descent have significant symptoms. The tonsils' shape (pointed or peg-like) may also be described. Tonsillar herniation is the central radiologic feature evaluated in Chiari I.

Why it matters

Understanding tonsillar herniation helps families interpret imaging reports and see why the measurement is only part of the picture — symptoms, CSF flow, and the overall situation matter more than the millimeters of descent alone.

Common questions

What is tonsillar herniation?

The downward extension of the cerebellar tonsils below the skull's base opening.

How is it measured?

On MRI, relative to a line across the foramen magnum.

Does more herniation mean worse symptoms?

No — the amount does not reliably predict symptoms.

Bottom line

Tonsillar herniation is the downward descent of the cerebellar tonsils below the skull's base opening, the hallmark of Chiari I, but it does not alone predict symptoms.

More terms in Core Terms & Classification

Cerebellar tonsilsThe cerebellar tonsils are the lower, rounded portions of the cerebellum that descend below the skull's base opening in Chiari malformation type I.
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.
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.

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