Malformed Cerebral Hemispheres
Cerebral hemispheres may be large, small, or asymmetric; the gyri may be absent, unusually large, or multiple and small.
In addition to the grossly visible malformations, microscopic sections of normal-appearing brain may show disorganization of the normal laminar neuronal arrangement. Localized deposits of gray matter may be present in regions normally occupied only by white matter (heterotopic gray matter).
Malformations of the cerebral hemispheres may be due to genetic or acquired causes. Acquired causes include infections (eg, cytomegalovirus), and vascular events that interrupt the blood supply to the developing brain.
Microcephaly or macrocephaly, moderate to severe motor and intellectual disability, and epilepsy often occur with these defects.
Treatment is supportive, including anticonvulsants, if needed.
Holoprosencephaly occurs when the embryonic prosencephalon does not undergo segmentation and cleavage. The anterior midline brain, cranium, and face are abnormal. This malformation may be caused by defects of the protein produced by the sonic hedgehog gene. Severely affected fetuses may die before birth.
Treatment is supportive.
Lissencephaly consists of an abnormally thick cortex, diminished or absent gyral pattern on the surface of the brain, reduced or abnormal lamination of the cerebral cortex, and often diffuse neuronal heterotopias. This malformation is caused by abnormal neuronal migration, the process by which immature neurons attach to radial glia and move from their points of origin near the ventricle to the cerebral surface. Several single-gene defects may cause this anomaly (eg, LIS1).
Affected infants may have intellectual disability and seizures (often infantile spasms—see Infantile Spasms).
Treatment is supportive; survival depends on seizure severity and the presence of other complications including swallowing dysfunction, apnea, and difficulty clearing oropharyngeal secretions.
Polymicrogyria, in which the gyri are small and overabundant, also involves abnormal neuronal migration. Other common findings include simplified or absent cortical lamination in affected regions, heterotopic gray matter, a hypoplastic or absent corpus callosum and septum pellucidum, and malformations of the brain stem and/or cerebellum. The structural abnormalities may be diffuse or focal. The most common area of focal involvement is the perisylvian fissure (bilaterally or unilaterally).
Polymicrogyria is highly associated with schizencephaly (see Porencephaly : Schizencephaly), in which there are abnormal slits, or clefts, in the cerebral hemispheres. Numerous causes of polymicrogyria have been identified, including a number of single-gene mutations (eg, of SRPX2), and primary maternal infection with cytomegalovirus (ie, in which the mother has no prior immunity—see Congenital and Perinatal Cytomegalovirus Infection (CMV)). The most common clinical manifestations are seizures, intellectual disability, and spastic hemiplegia or diplegia.
Treatment is supportive.