Chiari type 1 deformity classification

Chiari type 1 deformity classification

Nishikawa et al. classified Chiari malformation type I (CM-I) according to the mechanism of ptosis of the brain stem and cerebellum, based on a morphometric study of the posterior cranial fossa (PCF) and craniovertebral junction (CVJ). Surgery was performed to manage the mechanism of the hindbrain ptosis. They calculated the volume of the PCF (VPCF) and the area surrounding the foramen magnum (VSFM) and measured the axial length of the enchondral parts of the occipital bone (occipital bone size) and the hindbrain. According to these measures, they classified CM-I into type A (normal VPCF, normal VSFM, and normal occipital bone size), type B (normal VPCF, small VSFM, and small occipital bone size), and type C (small VPCF, small VSFM, and small occipital bone size). Foramen magnum decompression (FMD) (280 cases) was performed on CM-I types A and B. Expansive suboccipital cranioplasty (ESCP) was performed on CM-I type C. Posterior craniocervical fixation (CCF) was performed in cases with CVJ instability. Lysis of the adhesion and/or sectioning of the filum terminale was performed on cases with tethered cord syndrome. Both ESCP and FMD had a high rate of improvement of neurological symptoms (87%) and recovery rate. There was only a small number of complications. CCF had a high rate of improvement of neurological symptoms (88%) and joint stabilization. In the management of Chiari malformation, appropriate surgical methods that address ptosis of the hindbrain should be chosen. Each surgical approach resulted in a good improvement of neurological symptoms 1).

Valentini et al. suggested defining an association of Chiari type 1 deformity plus untreated sagittal synostosis, a new subtype of complex CM1. For the high percentage of complications and multiple procedures needed to solve the CM1, they advise identifying by 3D-CT scan these children before performing craniovertebral decompression (CVD). They suggest also that if left untreated, sagittal synostosis may lead to the delayed occurrence of a challenging subset of CM1 2).

Chiari malformation Type 1.5 (CM 1.5) was defined as the association of Chiari malformation Type I (CM I) and brainstem herniation.

Although CM 1.5 patients presented with brainstem herniation and more severe tonsillar herniation, other clinical and imaging features and surgical outcomes were similar to CM I patients. Liu et al. think CM 1.5 is just a subtype of CM I, rather than a unique type of Chiari malformations 3).

Taylor et al. identify two subtypes, crowded and spacious, that can be distinguished by MRI appearance without volumetric analysis. Earlier age at surgery and the presence of syringomyelia are more common in the crowded subtype. The presence of the spacious subtype suggests that crowdedness alone cannot explain the pathogenesis of Chiari I malformation in many patients, supporting the need for further investigation 4).

see Pediatric Chiari type 1 deformity.

see Chiari type 1 deformity and syringomyelia.


Nishikawa M, Bolognese PA, Kula RW, Ikuno H, Takami T, Ohata K. Surgical Management of Chiari Malformations: Preliminary Results of Surgery According to the Mechanisms of Ptosis of the Brain Stem and Cerebellum. J Neurol Surg B Skull Base. 2021 Apr;82(2):264-272. doi: 10.1055/s-0039-1697977. Epub 2019 Sep 30. PMID: 33816049; PMCID: PMC8009696.

Valentini LG, Saletti V, Erbetta A, Chiapparini L, Furlanetto M. Chiari 1 malformation and untreated sagittal synostosis: a new subset of complex Chiari? Childs Nerv Syst. 2019 Jul 21. doi: 10.1007/s00381-019-04283-0. [Epub ahead of print] PubMed PMID: 31327038.

Liu W, Wu H, Aikebaier Y, Wulabieke M, Paerhati R, Yang X. No significant difference between Chiari malformation type 1.5 and type I. Clin Neurol Neurosurg. 2017 Mar 30;157:34-39. doi: 10.1016/j.clineuro.2017.03.024. [Epub ahead of print] PubMed PMID: 28384597.

Taylor DG, Mastorakos P, Jane JA Jr, Oldfield EH. Two distinct populations of Chiari I malformation based on presence or absence of posterior fossa crowdedness on magnetic resonance imaging. J Neurosurg. 2017 Jun;126(6):1934-1940. doi: 10.3171/2016.6.JNS152998. Epub 2016 Sep 2. PubMed PMID: 27588590.

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