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Atresia of the aqueduct of Sylvius is a malformation comparable to the myelodysplastic states observed in instances of spinal dysraphism. It may vary in degree from an absolute state incompatible with survival of the newborn to such minor degrees as are asymptomatic. Absolute atresia of the aqueduct of Sylvius produces internal hydrocephalus in utero with pressure disintegration of the convexity of the cerebral hemispheres. Atresia of the aqueduct of Sylvius may occur as an isolated anomalous state without the Arnold-Chiari complex.In 17 instances of meningomyelocele complicated by progressively enlarging internal hydrocephalus, the Arnold-Chiari complex was present and the aqueduct of Sylvius was abnormal.In 2 cases, the aqueduct was dilated, the obstruction to the intracerebral Mow of cerebrospinal fluid being the fourth ventricle which was filled with cerebellar folia and choroid plexus packed into the foramen magnum.In 9 cases, the aqueduct of Sylvius was elongated and reduced to a narrow vertical slit. No evidence of associated atresia or dysplasia was present from a study of serial sections. The focal point of obstruction to the intracerebral How of cerebrospinal fluid in these cases was the stenosis of the iter.In 6 cases, the aqueduct of Sylvius showed evidence of atresia and the mid-brain, pons and medulla oblongata showed evidence of elongation. The focal point of obstruction to the intracerebral flow of cerebrospinal fluid was the atretie aqueduct.The Arnold-Chiari complex observed in association with spina bifida and hydrocephalus in infants and children is characterized by elongation of the pons and medulla oblongata, occlusion of the fourth ventricle by cerebellar folia and choroid plexus, and by displacement of the medulla oblongata and inferior portions of the cerebellum into the cervical spinal canal. A significant associated finding in the majority of the cases is the vertical narrowing and elongation of the non-atretic aqueduct of Sylvius which is the focal point of obstruction in the intracerebral flow of cerebrospinal fluid. This type of Arnold-Chiari complex is, I believe, the result of traction at an early formative period resulting in the displacement of the anlage of the caudal portions of the hindbrain into the cervical spinal canal. The invariable association of this variety of Arnold-Chiari complex with myelomeningocele speaks for such a dynamic mechanism. This type of Arnold-Chiari complex has not been observed without a coexistent myelomeningocele.The Arnold-Chiari complex observed with platybasia and/or basilar impression or without obvious cause does not present all of the features of the Arnold-Chiari complex seen in association with myelomeningocele. Tumors of the brain stem and long-standing increased intracranial pressure may also so alter the relationship of the neuraxis to the foramen magnum that the resultant deformity may simulate an Arnold-Chiari deformity.