Walsh & Hoyt: Ectopias of Neural Tissue

In addition to hypothalamic hamartomas, well-defined masses of ectopic gray matter are occasionally encountered in the centrum semiovale during routine neuropathologic examinations. This heterotopic gray matter the presence of cortical neurons in an abnormal location is being increasingly appreciated since the advent of MR imaging. Gray matter heterotopia is thought to result from abnormal neuronal migration and is considered part of a broad group of anomalies including agyria, pachygyria, polymicrogyria, schizencephaly, and unilateral megalencephaly. Three patterns of heterotopia are recognized: (a) a nodular form composed of subependymal masses of gray matter often localized at the corners of the lateral ventricles, (b) a laminar form separated from both the cortex and ventricle walls by thick layers of white matter, and (c) a band form (""double cortex"") where there is a continuous band between the lateral ventricles and cortex.