Pathogenesis of the Arnold-Chiari malformation: the significance of hydrocephalus and aqueduct stenosis.

In 24 cases of spina bifida, correlations were made comparing the degree of hydrocephalus with the cross-sectional area of the supratentorial compartment, posterior fossa, hindbrain prolapse, size of myelocele, and degree of fibrovascular reaction at the base of the brain. The size and postion of the aqueduct and fourth ventricle were compared with the degree of hydrocephalus. The results demonstrate that the aqueduct stenosis is usually due to external compression of the mesencephalon. The results also suggest that the size of the posterior fossa and hindbrain prolapse are secondary to hydrocephalus. It is postulated that the hydrocephalic process may be initiated by fibrovascular occlusion of the basal subarachnoid space.