Lissencephaly is the arrest of neuronal migration causing either developmental failure of sulci and gyri (agyria) or development of abnormal broad and flat gyri with abnormal shallow sulci (pachygyria). Lesions typically do not appear in isolation. Polymicrogyria is a similar disorder that results in small, disorganized gyri. It is unknown whether this is due to abnormal neuronal migration or cortical dysplasia.