Types of Chiari malformation

The types of Chiari malformation are separate malformations, numbered in the order Hans Chiari described them in the 1890s. They are not stages of one disease, and a person with Chiari I does not progress to Chiari II1.

Chiari I

Chiari I is a downward displacement of the cerebellar tonsils alone through the foramen magnum, with no other brain malformation. It is by far the most common type. It is usually found in older children and adults, often on a scan done for another reason, and many people with it never have symptoms2.

On MRI, Chiari I is defined as tonsillar descent of at least 5 mm below the foramen magnum, or 3 mm with signs of crowding at the base of the skull3. US guidelines describe the threshold as 3 to 5 mm or more4. The cause is a posterior fossa, the compartment at the back of the skull, that is too small for a normally sized cerebellum1.

Chiari 0 and Chiari 1.5

Chiari 0 describes a syrinx in the cervical spinal cord with less than 5 mm of tonsillar descent. The proposed explanation is crowding at the foramen magnum that blocks CSF flow even though the tonsils are not very low1.

Chiari 1.5 describes descent of the tonsils together with the lower part of the brainstem, marked by the position of the obex, a landmark on the back of the brainstem. In a 2026 study of 147 patients across the range from Chiari 0 to 1.5, the higher types had more obstruction of CSF flow and more often had a syrinx5.

Both terms are used less consistently than the numbered types, and they mainly matter because they appear on reports.

Chiari II

Chiari II involves the cerebellum and the lower brainstem, which are displaced further than in Chiari I. It occurs together with myelomeningocele, an open form of spina bifida in which the spinal cord is exposed at birth21. It is diagnosed on prenatal ultrasound or soon after birth and is managed as part of the care of spina bifida. The name Arnold-Chiari malformation properly belongs to this type, although it is often applied to all of them.

Chiari III and IV

Chiari III is a rare and severe malformation in which part of the cerebellum and brainstem protrudes into a sac at the back of the skull or upper neck, an encephalocele1. Chiari IV, added by Chiari in 1896, describes underdevelopment of the cerebellum (cerebellar hypoplasia) rather than displacement, and many authors no longer group it with the other types. Both are diagnosed in infancy.

Acquired tonsillar descent

Tonsils can also be pushed or pulled down by a pressure imbalance between the skull and the spine, rather than by a small posterior fossa1:

• Raised pressure inside the skull, from idiopathic intracranial hypertension, hydrocephalus or a mass, pushes the cerebellum down.
• Low pressure in the spine, from a spinal fluid leak or a lumbar shunt that drains too much fluid, pulls it down.

Both can produce an MRI appearance identical to Chiari I and a similar headache brought on by coughing3. The treatment is directed at the pressure problem, and once it is corrected the descent can resolve. Decompression surgery at the foramen magnum doesn't treat the cause and can make things worse, which is why abnormal CSF pressure must be excluded in everyone with Chiari I and headache3.

Tonsils that have moved lower between two scans are a reason to look for an acquired cause.

How common each type is

Chiari I is found on MRI in 0.24% to 2.6% of people4, and symptomatic Chiari I is much less common than the incidental form6. Chiari II affects people born with myelomeningocele. Types III and IV are rare.

References

1. Rosenblum JS, Pomeraniec IJ, Heiss JD. Chiari malformation (update on diagnosis and treatment). Neurol Clin. 2022;40(2):297-307. doi:10.1016/j.ncl.2021.11.007 Source ↩
2. National Institute of Neurological Disorders and Stroke. Chiari malformation. BrainFacts.org. Accessed October 3, 2026. Source ↩
3. Headache Classification Committee of the International Headache Society. The International Classification of Headache Disorders, 3rd edition: 7.7 Headache attributed to Chiari malformation type I (CM1). Cephalalgia. 2018;38(1):1-211. Accessed October 3, 2026. Source ↩
4. Bauer DF, Niazi T, Qaiser R, et al. Congress of Neurological Surgeons systematic review and evidence-based guidelines for patients with Chiari malformation: diagnosis. Neurosurgery. 2023;93(4):723-726. doi:10.1227/neu.0000000000002633 Source ↩
5. Mirzayeva LS, Uçar M, Cindil E, Budak SN, Kuzucu P. Integrated neuroimaging approach to the Chiari malformation spectrum from type 0 to 1.5: correlations between CSF flow impedance, tonsillar blackout sign, obex position, and clinico-anatomical features. J Neuroradiol. 2026;53(2):101527. doi:10.1016/j.neurad.2026.101527 PubMed ↩
6. Holste KG, Muraszko KM, Maher CO. Epidemiology of Chiari I malformation and syringomyelia. Neurosurg Clin N Am. 2023;34(1):9-15. doi:10.1016/j.nec.2022.08.001 PubMed ↩

Related pages

• Chiari malformationChiari malformation: the cerebellar tonsils extend below the skull into the spinal canal. Types, how common it is, symptoms, MRI diagnosis, surgery and outlook.
• MRI and diagnosis of Chiari malformationHow Chiari malformation is diagnosed: the MRI criteria, what the millimeters mean, whole-spine imaging, CSF flow studies and conditions that look the same.
• Syringomyelia in Chiari malformationSyringomyelia with Chiari malformation: what a syrinx is, why it forms, its symptoms, scoliosis in children, and what decompression surgery achieves.
• Symptoms of Chiari malformationChiari malformation symptoms: the cough headache, brainstem and cerebellar symptoms, syrinx symptoms, signs in children and when to get urgent help.