Pathology

Grossly, the hydrocephalic skull is thin, and the inner table is uneven, resembling digital marking. The cerebral hemispheres are markedly enlarged and soft. The sulci are shallow, and the convolutions appear small. The floor of the third ventricle is thin and distended. On sections, the ventricles are enormously enlarged, and the cerebral wall may be reduced to 1 cm or less. This reduction involves chiefly the white matter, which is unmy-elinated and distensible; the width of the cortex is relatively preserved (see Fig. 13.33). Histologically, the table 13.5.

Cerebral Malformations That Cause Hydrocephalus

Atresia/Stenosis of aqueduct of Sylvius Dandy-Walker malformation

Partial/total absence of vermis Cystic transformation of 4th ventricle Inconstant atresia of foramina Magendie and Luschka Large posterior fossa Arnold-Chiari Type 2 malformation Displacement of cerebellar tonsils, inferior vermis and elongaged medulla into the spinal canal Small posterior fossa Meningomyelocele Chiari Type 1 malformation

Cerebellar tonsils located below the foramen magnum ependymal lining is disrupted, and the subependymal astrocytes show focal fibrillary proliferation.

Obstructive congenital hydrocephalus is most often due to cerebral malformations, seldom to cranial anomalies or a congenital neoplasm (Table 13.5).

figure 13.34

Dandy-Walker malformation in a 4-year-old boy. At about 2 to 4 weeks of age, it was noticed first that his head was larger than normal. At 2 months of age, the head circumference was 48 cm (normal 40 cm). The sutures were widely separated and the fontanels tense. A ventriculoperitoneal shunt was placed, which 2 months later was replaced with a ventriculocaval shunt. Despite surgical intervention, the infant progressively deteriorated. His head rapidly enlarged. He became blind and paralyzed. A. Base of the skull shows a markedly enlarged posterior fossa. B. The inferior vermis is absent and a thin membrane roofs the ventricle. C. At pontine level, the large cystic fourth ventricle is bounded by severely atrophic cere-bellar folia and a thin layer of white matter (myelin stain). D. The foramin Luschka is occluded by a fibrous membrane lined with ependymal cells.

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