Heterotopia

Heterotopia are groups of neurons displaced in the cerebral hemispheric white matter (Fig. 13.19). Common sites are the periventricular zones, centum semiovale, and the subcortical white matter. Rarely, tiny groups of neurons form nodules on the cortical surface (brain warts). Neuroglial nests may also occur in the leptomeninges.

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figure 13.19

Heterotopia. A 10-year-old mentally retarded epileptic boy. A. Nodular heterotopias in the hemispheric white matter and periventricular zone are associated with agenesis of the corpus callosum. B. Laminar heterotopia in the subcortical white matter in a 40-year-old epileptic man (Weil stain). C. Neuronal and glial nodule in the leptomeninges (brain wart) (cresyl violet).

figure 13.20

Agenesis of the corpus callosum and septum pellucidum in a 19-year-old woman. She was the second twin, delivered at 7.5 months' gestation. On transverse sections, thick myelin bundles of Probst extend from the cingular gyri to the rectus gyri to form the medial walls of the lateral ventricles. The ventricles are slit-like with dorsal extensions (bat-wing shape). The septum pellucidum is absent.

figure 13.21

Cavum septum pellucidum, an incidental autopsy finding.

figure 13.20

Agenesis of the corpus callosum and septum pellucidum in a 19-year-old woman. She was the second twin, delivered at 7.5 months' gestation. On transverse sections, thick myelin bundles of Probst extend from the cingular gyri to the rectus gyri to form the medial walls of the lateral ventricles. The ventricles are slit-like with dorsal extensions (bat-wing shape). The septum pellucidum is absent.

figure 13.21

Cavum septum pellucidum, an incidental autopsy finding.

An enlarged third ventricle with a dorsal extension An occasional lipoma, hamartoma, or meningioma in the midline

Hemimegalencephaly

In hemimegalencephaly, one cerebral hemisphere is considerably larger than the other, displaying broad convolutions, a thick cortex, and heterotopia. Histologic anomalies include a disorganized cortical lamination, the presence of cytomegalic and ballooned neurons, and glial proliferation with dysplastic, swollen, and glassy astrocytes.

MALFORMATIONS OF THE MIDLINE STRUCTURES AND VENTRICLES

Agenesis of the corpus callosum (Fig. 13.20). This malformation is characterized by:

• The presence of thick myelin bundles of Probst (uncrossed callosal fibers) extending longitudinally from the lateral ventricles to the fornices

• The absence of cingulate gyri

• A radial arrangement of the convolutions in the medial aspects of the hemispheres

• Widely separated lateral ventricles displaying an angulated dorsal extension (bat-wing shape)

Agenesis of the corpus callosum occurs sporadically and may be associated with chromosomal aberrations and inherited metabolic disorders (see Table 13.2). Usually, it is clinically silent and diagnosed incidentally on MRI or at autopsy. Clinical manifestations, such as seizures and mental retardation, when present, are attributed to associated malformations.

Cavum septum pellucidum, an incidental neuroim-aging or autopsy finding, is a cystic cavity bounded by the two membranes of the septum, the crura of the for-nices, and the corpus callosum. Its posterior extension is the cavum vergae (Fig. 13.21).

Malformations of the Ventricles

Several forms are distinguished:

• Membranous occlusion of the foramen Monro

• Adhesions between the caudate nucleus and the corpus callosum or between opposite walls of the anterior, temporal, or occipital horns

• Obliteration of the third ventricle by fusion of the thalami

Neuronal Heterotopia

figure 13.22

Cerebellar Malformations. Partial cerebellar agenesis. A. The right cerebellar hemisphere is absent. The origin of cranial nerve roots is asymmetrical. The left lower nerve roots emerge through the cerebellar folia. B. The inferior vermis is absent. The gap between the hemispheres is covered with leptomeninges. On dorsal aspect, the vermis is fused with the hemispheres. Orientation of convolutions is abnormal.

figure 13.22

Cerebellar Malformations. Partial cerebellar agenesis. A. The right cerebellar hemisphere is absent. The origin of cranial nerve roots is asymmetrical. The left lower nerve roots emerge through the cerebellar folia. B. The inferior vermis is absent. The gap between the hemispheres is covered with leptomeninges. On dorsal aspect, the vermis is fused with the hemispheres. Orientation of convolutions is abnormal.

• Atresia and stenosis of the aqueduct of Sylvius, a leading causes of congenital hydrocephalus

• Membranous occlusions of the foramina Luschka and Magendie of the fourth ventricle, which are inconstant features of the Dandy-Walker malformation

MALFORMATIONS OF THE CEREBELLUM

The spectrum of cerebellar malformations ranges from gross anomalies to discrete histologic derangements. The malformations are conveniently grouped as (a) cerebellar agenesis, hypogenesis, and hypoplasia; (b) minor histologic anomalies; and (c) dysplastic gangliocytoma.

Clinically, gross malformations present with congenital cerebellar ataxia. The symptoms appear in early infancy, with delays and difficulties sitting and standing and a lack of coordination of simple movements. Gait, if it develops, is severely unsteady and broad-based. Speech is impaired.

Agenesis, Hypogenesis, and Hypoplasia

Agenesis of the cerebellum is rare. Partial agenesis may involve the hemisphere (neocerebellum) or the vermis (paleocerebellum) (Figs. 13.22 and 13.23). Partial or total absence of the vermis is a feature of the DandyWalker malformation (see the section Congenital Hydrocephalus).

Cerebellar hypoplasia refers to underdevelopment of parts or of the whole cerebellum (see Fig. 13.23). Some cases have been reported in nonketotic hyperglycinemia.

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