Schizencephaly, or "split brain," is a neurological disease caused by abnormal development of the brain, leading to the characteristic appearance of abnormal clefts in either one or both cerebral hemispheres. The exact etiology is unknown, although it is classified as a type of neuronal migration disorder and thought to be due to a defect in development that occurs during the period of one to seven months of fetal gestation.
Schizencephaly may have different forms. The appearance of the abnormal schizencephalic brain varies depending on the size and extent of the clefts. Clefts may be unilateral or bilateral and usually extend from the surface of the brain to the fluid-filled ventricles. Clefts are usually located next to the Sylvian fissure, but may be located in any part of the hemispheres. Separation of the walls of the cleft is referred to as open-lip schizencephaly, whereas apposed walls are referred to as closed-lip schizencephaly.
Schizencephaly differs from porencephaly, another developmental disorder that is due to early injuries to the developing fetal brain. Porencephaly results from injured brain tissue that subsequently dissolves and leaves a fluid-filled area known as a porencephalic cyst. This cyst can resemble the cleft seen in schizencephaly. Whereas schizencephaly is thought to be a primary disorder of development or neuronal migration, porencephaly is thought to be due to secondary brain damage, although the distinction is not entirely clear. Some theories of schizencephaly also propose early brain injury as contributory, but at an earlier stage of development than in porencephaly. Differentiation between the two often requires brain imaging such as magnetic resonance imaging (MRI) to identify the nature of the brain tissue lining the cleft. In porencephaly, scar tissue and white matter is often present, whereas in schizencephaly, gray matter lines the cleft.
Peter T. Lin MD, The Gale Group Inc., Gale, Detroit,