Gray matter heterotopia is a neurological condition which is characterized by abnormal locations of nerve cell bodies in the brain. It is a type of cortical dysplasia which results in the presence of nodular masses of grey matter in the subependymal regions of the lateral ventricles, or in the subcortical white matter. Grey matter heteropia is caused by arrested migration of neurons to the cerebral cortex during embryological development. When neurons that are supposed to form part of the cerebral cortex fail to climb to the end of their ladder correctly, they permanently wind up in the wrong location. Although neurons in heterotopia appear to be normal, they are wrongly situated.
The disease causes a variety of symptoms, but usually includes some degree of epilepsy or recurring seizures, and often affects the brain's ability to function on higher levels. The condition is occasionally discovered by brain imaging performed for an unrelated problem and has no apparent ill effect on the patient. At the other extreme, heterotopia can result in severe seizure disorder, loss of motor skills, and mental retardation. Symmetrical band-like or laminar heterotopias may also be seen in the white matter of the cerebral hemispheres producing a "double-cortex", also known as X-linked lissencephaly which has been found to be caused by a mutation of chromosome Xq22.3. Although genetic causes may occur, the cause of most heterotopias is unknown; nevertheless, the insult occurs not later than the 5th month of gestation. Maternal ingestion of mercury and alcohol have also been implicated in the condition.
Gray matter heterotopia are common malformations of cortical development. Affected patients are generally divided into three groups, depending on the location of the formation: subependymal, subcortical, and band heterotopia. In addition, especially with heterotopia that are genetically linked, there are gender differences, men suffering more severe symptoms than women with similar formations. In general, band heterotopia are seen exclusively in women; men with a mutation of the related gene (called XLIS or DCX) usually die in utero or have a much more severe brain anomaly. Symptoms in affected women vary from normal to severe developmental delay or mental retardation; the severity of the syndrome is related to the thickness of the band of arrested neurons. Nearly all affected patients that come to medical attention have epilepsy, with partial complex and atypical absence epilepsy being the most common syndromes. Some of the more severely affected patients develop drop attacks.