Although the names of two authors (Arnold and Chiari) were originally linked to the description of malformations at the medullary-spinal junction, Chiari must take most credit for providing a detailed description of this condition.
The cerebellar tonsils lie below the level of the foramen magnum (cerebellar ectopia). This may not produce symptoms.
(in symptomatic patients):
Syringomyelia 1 Hydromyelia J
Hydrocephalus (10%) (occurs less often than Chiari originally described)
Part of the cerebellar vermis, medulla and 4th ventricle extend through the foramen magnum, often to the midcervical region. The lower cranial nerves are stretched and the cervical nerve roots run horizontally or in an upward direction.
Part of the cerebellum and medulla lie within a cervico-occipital meningomyelocele.
Cerebellar hypoplasia - best considered as a separate entity.]
Syringomyelia Hydromyelia Spina bifida - meningomyelocele, diastomatomyelia
Cervical fusion (Klippel-Feil) Cranial Hydrocephalus (85%) Aqueduct stenosis and forking Small posterior fossa Basilar impression Fusion of both thalami Fusion of the superior and inferior colliculi Microgyria
Hypoplastic tentorium cerebelli and falx Skull lacuniae - vault thinned or defective Others
Developmental anomalies of the cardiovascular, gastrointestinal and genitourinary systems in 10%
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