Arnold-Chiari-Malformation Typ 2

Infant status
post myelomeningocele repair. Sagittal T1 MRI without contrast of the brain shows a small posterior fossa with downward cerebellar tonsil herniation and a small fourth ventricle. There is kinking of the spinal cord at the cervico-medullary junction. There is also agenesis of the body and rostrum of the corpus callosum.The diagnosis was Chiari II malformation and agenesis of the corpus callosum.
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Chiari II
malformation • Chiari II malformation with obstructive hydrocephalus and spinal dysraphism - Ganzer Fall bei Radiopaedia
Khedr, D. Chiari II malformation with obstructive hydrocephalus and spinal dysraphism. Case study, Radiopaedia.org. (accessed on 21 Oct 2022) https://doi.org/10.53347/rID-99561 • CC by-nc-sa 3.0 (modified)
Newborn with
a lumbar myelomeningocele. Lateral radiograph of the skull shows a disordered arrangement of the inner table of the skull.The diagnosis was lacunar skull in a patient with a Chiari II malformation.
pediatricimaging.org • CC-by-nc-sa 4.0
Newborn with
a myelomeningocele. Sagittal T1 MRI without contrast of the brain (left) shows a small posterior fossa with downward cerebellar tonsil herniation and a small fourth ventricle. There is kinking of the spinal cord at the cervico-medullary junction. Sagittal (above right) and axial (below right) T2 MRI without contrast of the spine shows a low-lying conus medullaris with the spinal cord nerve roots terminating in a posteriorly located cerebrospinal fluid filled sac which is not covered by skin at the level of the L5-S1 vertebral bodies.The diagnosis was Chiari II malformation with a myelomeningocele.
pediatricimaging.org • CC-by-nc-sa 4.0
Radiopaedia • CC-by-nc-sa 3.0 • de

Chiari II malformations are relatively common congenital malformation of the spine and posterior fossa characterized by myelomeningocele (lumbosacral spina bifida aperta) and a small posterior fossa with descent of the brainstem and cerebellar tonsils and vermis. Numerous associated abnormalities are also frequently encountered.

Terminology

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

It should be noted that the term Arnold-Chiari malformation should no longer be used; see Chiari malformations history and etymology section for more information.

Epidemiology

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

Clinical presentation

Given the wide range of anatomical severity, as well as a large number of associated abnormalities that 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 lifelong sequelae) as follows :

Pathology

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 a characteristic displacement of the medulla, fourth ventricle, and cerebellum through the foramen magnum.

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

Associations

Radiographic features

Antenatal ultrasound

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

MRI is the modality of choice for detecting and characterizing 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.

Posterior fossa
  • 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: the inferior colliculus is elongated and points posteriorly, with resulting angulation of the aqueduct which results in aqueductal stenosis and hydrocephalus
  • the cerebellar tonsils and vermis are displaced inferiorly through the foramen magnum, which appears crowded
Spine

Treatment and prognosis

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

  • myelomeningocele repair and management of neurogenic bladder
    • is performed on the in utero fetus at some centers in select cases to improve outcomes
  • 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 with brainstem dysfunction if hydrocephalus is not present or symptoms and signs do not improve with shunting
    • older patients with hindbrain herniation or syringohydromyelia may also benefit

History and etymology

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

Differential diagnosis

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 myelomeningocele and brainstem descent are present, the diagnosis is usually straightforward :

Siehe auch:
und weiter:
Assoziationen und Differentialdiagnosen zu Arnold-Chiari-Malformation Typ 2:
Chiari-Malformation
Typ 1
Wikipedia • CC BY-SA 3.0
Myelomeningozele
 
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pädiatrische
Erkrankungen des Liquorsystems
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