Chiari malformation

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This page is first because it matters most. If any of these is happening, contact your care team, an urgent care line, or emergency services — do not wait for a scheduled appointment.

These are the guides most people read first. Each one is written to be understood without a medical background, and none of them assumes you already know the vocabulary.

Eighteen of the 265 terms in the full dictionary — the ones most likely to appear on a report or be said in a consultation.

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.

Arnold-Chiari malformation

Arnold-Chiari malformation is another name for Chiari malformation, used especially for type II and sometimes for the group as a whole.

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.

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 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.

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malformation type IV is a rare, severe condition involving an underdeveloped cerebellum, now considered by many an outdated or separate term.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 malformation

Chiari 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 malformation

Chiari 1.5 malformation is a variant between types I and II, with tonsillar descent plus downward displacement of the brainstem.

Complex Chiari malformation

Complex Chiari malformation refers to Chiari with additional craniocervical junction abnormalities, often requiring surgery beyond standard decompression.

Foramen magnum

The foramen magnum is the large opening at the base of the skull through which the brainstem and spinal cord connect, the site of herniation in Chiari malformation.

Craniocervical junction (craniovertebral junction)

The craniocervical junction is the region where the skull meets the upper spine, the area involved in Chiari malformation and related conditions.

Posterior fossa

The posterior fossa is the compartment at the back and base of the skull that holds the cerebellum and brainstem, often small or crowded in Chiari malformation.

Congenital Chiari malformation

Congenital Chiari malformation is Chiari that is present from birth, arising from how the skull and brain developed.

Acquired Chiari malformation

Acquired Chiari malformation is a less common form that develops after birth due to another condition affecting pressure or space around the brain.

Symptomatic Chiari malformation

Symptomatic Chiari malformation is Chiari that is causing symptoms, as opposed to being an incidental, symptom-free finding.

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Independent organisations that support people with this condition. AURORA is not affiliated with any of them and does not receive anything for listing them.