Chiari II malformation

Chiari II malformation, also known as Arnold-Chiari malformation, is a relatively common congenital malformation of the spine and posterior fossa characterised by myelomeningocoele (lumbosacral spina bifida aperta) and a small posterior fossa with a descent of the brainstem and cerebellar tonsils. Numerous associated abnormalities are also frequently encountered.  

The Chiari II malformation is often thought of a more severe form of the commoner Chairi I malformation. However, it is now understood that these entities are the end points of distinct disease processes with some overlapping imaging findings. Chiari III and IV are discussed in their respective articles.

Chiari II malformations are encountered relatively commonly with an incidence of ~1:1000 live births 7. When a child is born with a myelomeningocoele the vast majority (~95%) have an associated Chiari II malformation. 

Given the wide range of anatomical severity as well as a large number of associated abnormalities which are sometimes encountered, it should be no surprise that the clinical presentation of patients with Chiari II malformations is also varied both in character and severity. The presentation can be divided according to the age of the individual (although most will have life long sequelae) as follows 7:

While Chiari I malformation is thought to result from a small posterior fossa, Chiari II occurs due to in utero malformation of the spine and cranial structures resulting in characteristic displacement of the medullafourth ventricle, and cerebellum through the foramen magnum

As almost all neonatal patients with Chari II have a myelomeningocoele it has been suggested that the underlying aetiology is that of in utero CSF leak due to open spinal dysraphism. Older patients with Chiari II without a myelomeningocoele are thought to have had either a smaller neural tube defect or subsequent closure of the defect in utero.

Classical signs described on ultrasound include

There may also be evidence of fetal ventriculomegaly due to obstructive effects as a result of downward cerebellar herniation. Additionally, many of the associated malformations (e.g. corpus callosal dysgenesis) may be identified. 

MRI is the modality of choice for detecting and characterising the full constellation of findings associated with Chiari II malformations. The key features are discussed below, whereas the wide range of associated abnormalities (see above) are discussed separately. 

  • small posterior fossa with a low attachment of the tentorium and low torcula
  • the brainstem appears 'pulled' down with an elongated and low lying fourth ventricle
  • the tectal plate appears beaked: inferior colliculus is elongated and points posteriorly, with resulting angulation of the aqueduct which results in aqueductal stenosis and hydrocephalus
  • cerebellar tonsils and vermis are displaced inferiorly through foramen magnum which appears crowded

Treatment of patients with Chiari II malformation is complex due to the variable form and severity of malformations:

  • myelomeningocoele repair and management of neurogenic bladder
    • is being performed on the in utero fetus at some centres in select cases to improve outcomes 9
  • ventricular shunting (usually ventriculoperitoneal) 
    • hydrocephalus usually requires shunting and can help ameliorate cranial nerve and brainstem dysfunction
  • craniovertebral decompression
    • may also be required in neonates which brainstem dysfunction if hydrocephalus is not present or symptoms and signs do not improve with shunting
    • older patients with hind brain herniation or syringohydromyelia may also benefit

The Chiari malformations were first described in 1891 by Hans Chiari, Austrian pathologist (1851-1916), see the article on Chiari malformations for further details. 

The differential is predominantly one of definition, and the term Chiari type II is often inappropriately used to designate a variety of malformations. Provided both a myelomeningocoele and brainstem descent are present the diagnosis is usually straight forward 7:

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Article information

rID: 5786
Synonyms or Alternate Spellings:
  • Arnold-Chiari malformation
  • Chiari type II malformation
  • Chiari 2 malformation
  • Chiari type II malformations
  • Arnold-Chiari malformations
  • Chiari 2 malformations

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