2 months baby with a large meningocele
•Chiari I malformation
- most common
- peg-like cerebellar tonsils displaced into the upper cervical canal through the foramen magnum
•Chiari II malformation
- displacement of the medulla, fourth ventricle and cerebellum through the foramen magnum
- usually with associated with a lumbosacral spinal myelomeningocoele
•Chiari III malformation
- features similar to Chiari II but with an occipital and / or high cervical encephalocoele
•Chiari IV malformation
- severe cerebellar hypolasia without displacement of the cerebellum through the foramen magnum
- probably a variation of cerebellar hypoplasia
Arnold chiari type II malformation:
- Usually accompanied by a lumbar myelomeningocele leading to partial or complete paralysis below the spinal defect.
- As opposed to the less pronounced tonsillar herniation seen with Chiari I, there is a larger cerebellar vermian displacement.
- Low lying torcular herophili, tectal beaking, and hydrocephalus with consequent clival hypoplasia are classic anatomic associations.
- The position of the torcular herophili is important for distinction from Dandy-Walker syndrome in which it is classically upturned.
- This is important because the hypoplastic cerebellum of Dandy-Walker may be difficult to distinguish from a Chiari malformation that has herniated or is ectopic on imaging.
- Colpocephaly may be seen due to the associated neural tube defect.
Reference : Pediatric brain and spine, an atlas of MRI and spectroscopy. Springer Verlag. (2005) ISBN:3540213406
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