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.

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

It involves descent of the cerebellum and brainstem.
It is almost always associated with myelomeningocele (spina bifida).
It is present at birth and often accompanied by hydrocephalus.
It is also called Arnold-Chiari malformation.

What it is

Chiari malformation type II (CM-II), also called Arnold-Chiari malformation, is a more severe form present at birth, in which both the cerebellar tonsils and parts of the brainstem (and fourth ventricle) are displaced downward through the foramen magnum. It is almost always associated with myelomeningocele, a form of spina bifida.

Understanding Chiari malformation type II (CM-II)

CM-II is more complex than type I, involving downward displacement of the brainstem (medulla, pons, fourth ventricle) as well as the cerebellum, and it is intrinsically linked to spina bifida — nearly all children with myelomeningocele have CM-II. It is frequently accompanied by hydrocephalus and other brain findings. Symptoms can arise from brainstem and lower cranial nerve dysfunction, including breathing and swallowing problems, and in some infants the onset can be rapid and severe, requiring urgent care. Because CM-II is part of the spectrum of spina bifida, its management is integrated into the comprehensive care of children with that condition. CM-II is identified in the context of spina bifida, often before or at birth.

Why it matters

Understanding CM-II helps families of children with spina bifida see why this more severe form is part of that condition, why it is monitored from birth, and why breathing or swallowing changes in affected infants warrant urgent attention.

Common questions

What defines Chiari malformation type II?

Descent of the cerebellum and brainstem, almost always with spina bifida.

Is it present at birth?

Yes — it is a congenital condition.

What is it also called?

Arnold-Chiari malformation.

Bottom line

Chiari malformation type II is a more severe, congenital form involving the cerebellum and brainstem, almost always associated with spina bifida.

More terms in Core Terms & Classification

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

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