Arnold‐Chiari Malformation with Associated Analogous Malformation of the Midbrain

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Abstract
In 3 infants dying within 9 weeks of birth, a malformation of the midbrain was seen, in which the caudal extremity of the tectum formed a pyramidal caudal projection of tissue overlying the anterior medullary velum, and resembling the caudal projection of cerebellar and medullary tissue of the Arnold-Chiari malformation. A meningomyelocele was present in all. Two showed a severe hydrocephalus, this being slight in the third. Microgyria was present in 1 case, and 2 showed the classical Arnold-Chiari malformation. One case revealed a malformation of the choroid plexus of the 4th ventricle, while another showed a complex of malformations. The occurrence of this midbrain deformity in one case with only slight hydrocephalus and no Arnold-Chiari malformation and the existence of a septum of glial tissue between it and the anterior medullary velum suggests that mechanical pressure against the crura of the tentorium was not a factor. In 2 cases, the malformed tissues contained irregular ependymal lined channels, and in all 3, encompassed a projection of subarachnoid space. These observations and the presence of malformations elsewhere in the brain, suggest a developmental origin for the midbrain lesion, and supports the evidence indicating a similar origin for the Arnold-Chiari malformation.

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