Core Terms & Classification

Hindbrain herniation

Hindbrain herniation is the downward displacement of structures at the back of the brain through the skull's base opening, the central feature of Chiari malformation.

In short: Hindbrain herniation

"Hindbrain" refers to the cerebellum and brainstem.
"Herniation" means these structures extend lower than normal.
It is the defining feature of most Chiari malformations.
The extent and structures involved vary by type.

What it is

Hindbrain herniation is the downward displacement of structures of the hindbrain — the cerebellum and brainstem — through the foramen magnum at the base of the skull. It is the central, defining feature of most Chiari malformations and is sometimes used as another name for the condition.

Understanding Hindbrain herniation

The hindbrain (rhombencephalon) includes the cerebellum and the brainstem. In Chiari malformation, part of this region extends below its normal position through the skull's base opening: in type I, the cerebellar tonsils; in type II and the 1.5 variant, the brainstem as well. The degree of herniation and which structures are involved define the type and influence the clinical picture. Describing Chiari malformation as "hindbrain herniation" emphasizes this shared underlying feature across the spectrum, even though the types differ in severity. Notably, the amount of herniation does not reliably predict symptoms.

Why it matters

Understanding hindbrain herniation helps families grasp the common thread across Chiari malformations — downward displacement of the back of the brain — while recognizing that the structures involved and the extent vary by type and do not alone determine symptoms.

Common questions

What is the hindbrain?

The cerebellum and brainstem at the back and base of the brain.

What does herniation mean here?

That these structures extend lower than normal through the skull's base opening.

Is it the defining feature of Chiari?

Yes — for most types.

Bottom line

Hindbrain herniation is the downward displacement of the cerebellum and brainstem through the skull's base opening, the defining feature of most Chiari malformations.

More terms in 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.
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.

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