The commonest causes of noncommunicating hydrocephalus in infancy are developmental obstructions at the aqueduct of Sylvius or in the region of the foramina of Luschka and Magendie. Malformation of the aqueduct is most frequently observed, but atresia of the foramina of Luschka and Magendie or functional blockage secondary to an Arnold-Chiari malformation are not uncommon.
Embryologically, the foramen of Magendie is usually formed by the fourth month of fetal life. At this stage of development the cerebellum is dumbbell-shaped with an underdeveloped vermis in the midline connecting two primitive cerebellar hemispheres. Since fluid has already started to form within the cerebral vesicles, the failure of normal formation of the posterior foramina leads to fluid accumulation in the fourth ventricle, inhibiting further development of the cerebellum. As the posterior fossa is thus enlarged because of the cystic dilatation of the fourth ventricle, the developing occipital lobes of the cerebrum cannot push
FOWLER FD, ALEXANDER E. Atresia of the Foramina of Luschka and Magendie: A Cause of Obstructive Internal Hydrocephalus. AMA Am J Dis Child. 1956;92(2):131–137. doi:10.1001/archpedi.1956.02060030125004
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