High Quality Content by WIKIPEDIA articles! Lissencephaly, which literally means smooth brain, is a rare brain formation disorder caused by defective neuronal migration during the 12th to 24th weeks of gestation, resulting in a lack of development of brain folds (gyri) and grooves (sulci). It is a form of cephalic disorder. Terms such as `agyria` (no gyri) or `pachygyria` (broad gyri) are used to describe the appearance of the surface of the brain. Children with lissencephaly are severely neurologically impaired and often die within several months of birth.