Malformations of cortical development and epilepsy, part 1: diagnosis and classification scheme.

Malformations of cortical development (MCDs) are a common cause of epilepsy, although seizures are not always the most prominent neurologic manifestation of these disorders. In localization-related epilepsy, certain features should create a strong suspicion that an MCD is the underlying cause; these include developmental delay and static focal neurologic deficits, a family history of developmental delay or epilepsy, frequent seizures from onset, and episodes of focal status epilepticus. MCDs can be classified according to a number of different criteria emphasizing clinical phenotype, imaging findings, pathology, or genetic defects. The overall classification of MCDs is based on the 3 fundamental events of cortical formation: 1) proliferation of neurons and glia in the ventricular and subventricular zones; 2) multidirectional migration of immature but postmitotic neurons to the developing cerebral cortex; and 3) cortical organization. Among the most common and distinct syndromes and entities affecting patients with MCDs and epilepsy are focal cortical dysplasia, hemimegalencephaly, tuberous sclerosis, classical lissencephaly, periventricular nodular heterotopia, focal subcortical heterotopia, polymicrogyria, and schizencephaly, all of which are discussed herein.