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Hemimegalencephaly (or malformations of cortical development) is a rare neurological condition in which one-half of the brain, or one side of the brain, is abnormally larger than the other. The unusual and enlarged brain tissue causes frequent seizures, usually followed by mental retardation.
Malformations of cortical development (known sometimes as cortical dysplasias) occur during the development of the brain when neurons do not migrate and organize themselves in a normal fashion. This can lead to intractable epilepsy.