1. There are four types.
Type 1, downward herniation of cerebellar tonsils of at least 3-5 mm below the foramen magnum. A study of posterior cranial fossa and CSF volumes showed them to be reduced compared with normal patients, and hindbrain overcrowding results in direct compression of tissue (Milhorat et al 1999). Clinical symptoms thus relate to CSF disturbances and direct compression of nervous tissue; they include headaches, pseudotumour-like episodes, a syndrome resembling Menieres disease, lower cranial nerve signs and spinal cord disturbances without syringomyelia. It is mostly seen in young adults.
Type 2 (Arnold-Chiari) is the most important and includes; thoracolumbar myelomeningocoele, hypoplasia of posterior fossa, and displacement of cerebellar vermis, brainstem and fourth ventricle. It presents in infancy or early childhood.
Type 3, dilated fourth ventricle with cervical meningomyelocoele.
Type 4, cerebellar hypoplasia.
2. May be associated with syringomyelia or syringobulbia, other skeletal abnormalities of skull base and cervical vertebrae, or myelomeningocoele. Syringomyelia is the term for an expanding, longitudinal cystic cavity within the spinal cord. In the communicating variety, there is continuity between the syrinx and the CSF in the central canal.
3. May need decompression of foramen magnum or cervical spine.
4. Progressive hydrocephalus may occur which needs shunt surgery.
5. Obstructive sleep apnoea (Doherty et al 1995).
7. Sudden unexpected death may occur.A 27 year old presented to the emergency room with headache for which he was given a variety of sedatives and analgesics; 4 h later he was drowsy, and at 5 h he developed pulmonary oedema,VF and died. Postmortem showed severe cerebral oedema and herniation of the cerebellar tonsils (Rocker et al 1995).Two fatal cases of unexpected respiratory arrest occurred in two children with previously undiagnosed Chiari type 1 malformation (Martinot et al 1995).
8. Recurrent pulmonary aspiration (Nathadwarawala et al 1992).
9. May exhibit abnormal autonomic control.
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