Chiari II malformation, also known as Arnold-Chiari malformation, is a relatively common congenital malformation of the spine and posterior fossa characterized by myelomeningocele (lumbosacral spina bifida aperta) and a small posterior fossa with descended 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 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.
Chiari II malformations are encountered relatively commonly with an incidence of ~1:1000 live births 7. When a child is born with a myelomeningocele, 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 lifelong sequelae) as follows 7:
- young adult
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 a 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 a myelomeningocele are thought to have had either a smaller neural tube defect or subsequent closure of the defect in utero.
- cranial vault
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 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.
- 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
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 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 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.
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 myelomeningocele and brainstem descent are present, the diagnosis is usually straightforward 7:
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- 3. Naidich TP, Pudlowski RM, Naidich JB et-al. Computed tomographic signs of the Chiari II malformation. Part I: Skull and dural partitions. Radiology. 1980;134 (1): 65-71. Radiology (abstract) - Pubmed citation
- 4. Curnes JT, Oakes WJ, Boyko OB. MR imaging of hindbrain deformity in Chiari II patients with and without symptoms of brainstem compression. AJNR Am J Neuroradiol. 10 (2): 293-302. AJNR Am J Neuroradiol (abstract) - Pubmed citation
- 5. Naidich TP, Mclone DG, Fulling KH. The Chiari II malformation: Part IV. The hindbrain deformity. Neuroradiology. 1983;25 (4): 179-97. - Pubmed citation
- 6. Dipietro MA, Venes JL, Rubin JM. Arnold-Chiari II malformation: intraoperative real-time US. Radiology. 1987;164 (3): 799-804. Radiology (abstract) - Pubmed citation
- 7. Maixner WJ. Spina Bifida, Management and Outcome. Springer. (2008) ISBN:8847006511. Read it at Google Books - Find it at Amazon
- 8. Osborn A, Blaser S, Salzman K. Encyclopedia of Diagnostic Imaging. AMIRSYS. (2008) ISBN:0721629059. Read it at Google Books - Find it at Amazon
- 9. Adzick NS. Fetal surgery for myelomeningocele: trials and tribulations. Isabella Forshall Lecture. (2012) Journal of pediatric surgery. 47 (2): 273-81. doi:10.1016/j.jpedsurg.2011.11.021 - Pubmed
Related Radiopaedia articles
Congenital spinal abnormalities
- spinal dysraphism
- Chiari malformations