Core Terms & Classification

Cerebellar tonsils

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

In short: Cerebellar tonsils

They are the lowest, rounded parts of the cerebellum.
Their downward descent defines Chiari malformation type I.
They sit near the brainstem and upper spinal cord.
Their position is the key measurement in Chiari I.

What it is

The cerebellar tonsils are the lower, rounded lobes on the underside of the cerebellum, located near the midline at the back of the brain. Their downward displacement below the foramen magnum is what defines Chiari malformation type I.

Understanding Cerebellar tonsils

Normally positioned within the skull above the foramen magnum, the cerebellar tonsils sit close to the brainstem and the top of the spinal cord. In Chiari I, they extend downward through the foramen magnum into the upper spinal canal, where they can crowd these structures and impede the flow of cerebrospinal fluid. Their position — how far they descend below the foramen magnum — is the central measurement used in diagnosing and describing Chiari I, and their shape (rounded versus pointed/peg-like) may also be noted. Understanding the tonsils helps families see exactly which part of the brain is involved in Chiari I.

Why it matters

Understanding the cerebellar tonsils helps families see precisely which structure descends in Chiari I, why its position is measured on imaging, and how its crowding of nearby structures can relate to symptoms and CSF flow.

Common questions

What are the cerebellar tonsils?

The lower, rounded parts of the cerebellum.

What is their role in Chiari?

Their downward descent defines Chiari malformation type I.

Why is their position measured?

It is the key measurement in diagnosing Chiari I.

Bottom line

The cerebellar tonsils are the lower, rounded parts of the cerebellum whose downward descent below the skull's base opening defines Chiari malformation type I.

More terms in 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.
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.

Browse all 265 Chiari malformation definitions

Read the Chiari malformation patient guides

Not sure which page you need? Put words to it with the symptom navigator

What AURORA is building for Chiari malformation