In cases of the Arnold-Chiari malformation, the foramen of Magendie has been variously reported as being present or absent. All authors agree, however, that the caudal extremity of the cerebellar component of the malformation is bound down by dense fibrous adhesions in which the choroid plexus of the fourth ventricle is included.1 Since the choroid plexus normally passes through the foramen of Magendie, it is apparent that in the presence of such adhesions considerable difficulty in identification of the foramen may be encountered. Alvord2 summarizes the position in the following words: "The foramen of Magendie is elongated, and frequently extends well below the level of the foramen magnum; it may be patent, or it may be occluded by fibrous adhesions between the cerebellum and medulla at or below the foramen magnum. Within these fibrous adhesions choroid plexus is found."
Some authors have described other conditions of the foramen
PEACH B. Cystic Prolongation Of Fourth VentricleAn Anomaly Associated With the Arnold-Chiari Malformation. Arch Neurol. 1964;11(6):609–612. doi:10.1001/archneur.1964.00460240041005