Overview
Chiari malformation (CM) describes the downwards displacement of one or both cerebellar tonsils through the foramen magnum with or without associated intracranial or extracranial defects such as hydrocephalus and encephalocele.
The presentation of CM can vary from palsies of the lower cranial nerves (cranial nerves IX, X, XI, or XII), to cerebellar signs, upper motor neurone signs, and lower motor neurone signs.
The only type of Chiari malformation known as an ‘Arnold-Chiari’ malformation is Chiari malformation type II.
Causes
Chiari malformations are thought to be due to reduced or absent development of the posterior fossa due to congenital or acquired disorders.
Types
Chiari malformation type I
- Cerebellar tonsils are displaced into the upper spinal cord
- Least severe clinically and often found incidentally, patients may have headaches and/or neck pain
- Associated with syringomyelia
Chiari malformation type II (Arnold-Chiari malformation)
- The only type of Chiari malformation known as an ‘Arnold-Chiari’ malformation
- The medulla, 4th ventricle, and cerebellum are displaced into the upper spinal cord
- Associated with hydrocephalus (due to obstructed cerebrospinal fluid flow) and myelomeningocele
- Characterised by paralysis below the level of the myelomeningocele and features of hydrocephalus
Chiari malformation type III
- The cerebellum with or without the brainstem are herniated into an occipital sac known as an encephalocele
- Severe neurological deficits
Chiari malformation type IV
- Now considered obsolete as it described severe cerebellar hypoplasia and was similar to cerebellar agenesis.
- Previously considered to be associated with myelomeningocele, but many argued that this would be classed as a Chiari II malformation with an absent cerebellum
- Incompatible with life
