Gray matter heterotopias are neurological disorders caused by clumps of gray matter (nodules of neurons) located in the wrong part of the brain. A grey matter heterotopia is characterized as a type of focal cortical dysplasia. The neurons in heterotopia appear to be normal, except for their mislocation; nuclear studies have shown glucose metabolism equal to that of normally positioned gray matter. The condition causes a variety of symptoms, but usually includes some degree of epilepsy or recurring seizures, and often affects the brains ability to function on higher levels. Symptoms range from nonexistent to profound; the condition is occasionally discovered as an incidentaloma when 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. Fatalities are practically unknown, other than the death of unborn male fetuses with a specific genetic defect. Preliminary Material: Neurological Development of the Human Fetus
The development of the brain in the human fetus is extraordinarily complex and is still not fully understood. Neural matter originates in the outer, ectodermic layer of the gastrula;  thus, it originates from the cell layer primarily responsible for skin, hair, nails, etc., rather than from the layers that develop into other internal organs.
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 seem to develop more severe symptoms than women with similar formations. In general, band heterotopia, also known as double cortex syndrome, 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 intellectual disability; 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. Periventricular or subependymal
Periventricular means beside the ventricle, while subependymal (also spelled subepydymal) means beneath the ependyma; because the ependyma is the thin epithelial sheet lining the ventricles of the brain, these two terms are used to define heterotopia occurring directly next to a ventricle. This is by far the most common location for heterotopia. Patients with isolated subependymal heterotopia usually present with a seizure disorder in the second decade of life. Subependymal heterotopia present in a wide array of variations.