Inverse cerebellum and occipital encephalocele. A dorsal fusion defect uniting the Arnold-Chiari and Dandy-Walker spectrum.

The malformation of inverse cerebellum and occipital encephalocele is situated morphologically between the Arnold-Chiari and Dandy-Walker malformations. In the three reported cases, hydrocephalus was not present, concomitant malformations of the lamina terminalis were present in two, and polymicrogyria was found in all three. The primary defect in the malformation is a complex occipital encephalocele composed of miniature hemispheres connected to the brain stem by an extension of the midbrain tectum. The cerebellar folia extend ventrally and cover the basilar artery. We propose that the encephalocele arises through the processes of overgrowth and dysraphism and thus falls into the organogenetic malformations of Yakovlev. Supporting this theory is the consistent observation of the duplicate hemispheres in the encephalocele and the hydromyelia in the examined spinal cords.