Cortical heterotopias: animal models and human disease.

Cortical heterotopia is defined as the misplacement of a group of neurons d isplaced to a precise localization in the neocortex and results from pertur bed migration along the glial guide, either because of glial destruction or molecular anomalies. Heterotopic neurons are rarely dispersed but are rath er grouped in nodules or bands. Heterotopic masses may lie in an ependymal or subcortical localization depending on whether they result from lack of m igration or an arrested migration. Heterotopias can also occur in intra-cor tical or extra-cortical localizations. The cause of heterotopia remains to be elucidated Two genes situated on chromosome X have been implicated but n on-genetic forms attributable to antenatal ischemia or toxic aggression dur ing fetal development have also been observed. The presence of heterotopia is usually associated with epilepsy and sometimes with mental retardation. Seizures may be initiated within the heterotopic region then propagate via long projections to the neocortex which may also be malformed.