Grey matter heterotopia is a malformation of cortical development, where cortical cells (grey matter) are present in inappropriate locations in the brain, due to interruption in their migration to their correct location in the cerebral cortex. Grey matter heterotopia may be unilateral or bilateral, singular or multiple, separate or contiguous. The most common form is bilateral periventricular nodular heterotopia (grey matter heterotopia lining the lateral ventricles). Grey matter heterotopia can also occur in subcortical white matter (subcortical nodular heterotopia). Grey matter heterotopia can co-occur with other structural abnormalities. Unilateral periventricular nodular heterotopia can co-occur with subcortical nodular heterotopia and with polymicrogyria. Bilateral periventricular nodular heterotopia can co-occur with cerebellar vermis hypoplasia and hypoplasia of the corpus callosum. Hippocampal sclerosis can co-occur with heterotopia.
The clinical presentation of grey matter heterotopia depends on the extent of the malformation, and whether the malformation is unilateral or bilateral, or associated with other brain malformations or congenital malformations elsewhere in the body. Developmental, cognitive and motor impairments can occur, and are more severe in those with bilateral or extensive heterotopia or other brain abnormalities. Seizures are common, seen in 80-90% of cases. There is a female predominance in cases of bilateral periventricular nodular heterotopia.