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Chiari malformations are classified into four types based on the formation and severity of the malformation:
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| Type I: The cerebellar tonsils and lower vermis herniate into the spinal canal, sometimes reaching the third cervical vertebra. The medulla and fourth ventricle are in normal or slightly lower positions. About 50% have syringomyelia, usually without myelomeningocele. |
| Type II: The most common type, where the cerebellum, medulla, and fourth ventricle herniate into the spinal canal. The medulla overlaps with the upper cervical spinal cord, the pons is elongated and thinned, and the median aperture of the fourth ventricle and the aqueduct are narrowed due to adhesions, leading to obstructive hydrocephalus. Often associated with myelomeningocele. |
| Type III: The most severe type, with characteristics of Type II, often combined with high cervical or occipital encephalocele. |
| Type IV: Characterized by cerebellar hypoplasia without downward displacement. |
Explanation
The pathological changes of Chiari malformations include the following:
① The cerebellar tonsils herniate through the foramen magnum into the spinal canal, sometimes filling the entire cisterna magna;
② The medulla is elongated and herniates into the spinal canal along with the lower part of the fourth ventricle, compressing and flattening the medulla and upper cervical spinal cord;
③ The median aperture of the fourth ventricle and the aqueduct are narrowed due to adhesions, leading to obstructive hydrocephalus;
④ The downward displacement of the brainstem and cerebellum causes elongation of cranial nerves V-VII;
⑤ Intracranial structures around the foramen magnum adhere, obstructing the subarachnoid space and forming cysts;
⑥ If the midbrain is displaced downward, it may be associated with obstruction of the pontine and lateral cisterns.
This malformation is often associated with various other malformations, including cranial base and foramen magnum malformations, occipital encephalocele, spina bifida, myelomeningocele, and syringomyelia.
