| Chiari Type | Anatomical Anomaly | Frequency of Hydrocephalus | Associated CNS Anoimalies |
| I | Tonsillar herniation ≥5 mm below plane of foramen magnum with moderate frequency of syringomyelia | Low | Multiple |
| II | Caudal herniation of vermis, brainstem, and fourth ventricle with high frequency of syringomyelia | High | Associated with myelomeningocele and multiple brain anomalies |
| III | Occipital encephalocele containing dysmorphic cerebellum and brainstem | N/A | N/A |
| IV | Hypoplasia or aplasia of cerebellum | N/A | N/A |
Anomalies associated with Chiari I malformation
- Syndromic craniosynostosis: These disorders include Apert’s and Crouzon’s syndromes
- Endocrinopathies: These include achondroplasia, acromegaly growth hormone deficiency, and osteopetrosis or Paget’s disease.
- Neurocutaneous disorders: These disorders include NF1, acanthosis nigricans, blue rubber bleb nevus syndrome, giant congenital melanocytic nevi, LEOPARD syndrome, and Waardenburg syndrome.
- Beckwith-Wiedemann syndrome
- Ehlers-Danlos syndrome
- Craniocervical anomalies: These include basilar invagination, atlantoaxial assimilation and Klippel-Feil syndrome.
- Acquired tonsillar descent: Elevation in ICP due to conditions such as brain tumors and hydrocephalus may cause acquired tonsillar descent (56).
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