III. Forebrain Anomalies
Forebrain anomalies (Many of these are associated with mental retardation)
1. Polymicrogyria (SEE IMAGE) - increased number of small irregular gyri; not necessarily accompanied by mental retardation; commonly seen with the Arnold-Chiari malformation. The cortical architecture is abnormal, typically with 4 layers (instead of 6 layers normally found.)
2. Changes in the volume of brain tissue: megalencephaly (abnormally large volume) or microcephaly (brain smaller than normal without degenerative lesions). Microcephaly is more common and is associated with many inherited syndromes. Defects in five genes have been associated with familial forms of microcephaly. One of these genes, ASPM, is a major determinant of cerebral cortical size in several species.
3. Heterotopias - clusters of neurons in abnormal locations, usually in white matter; generally result from a failure of migration of nerve cells from the subependymal region to the cortex. These abnormal foci of neurons may cause seizures.