Heterotopia

greek for "ectopic grey matter", collections of normal neurones in an abnormal location, outside the cortex. Heterotopia is classified as a neuronal migration anomaly and is due to an arrest or damage of migration of the primitive neuroblasts from the germinal matrix to the forming cortex, along the radial glioneuronal fibres. Heterotopia almost always produces seizure disorders, with some variability in clinical presentation.

The anomalies may be focal or diffuse, and subependymal, cortical or mixed in location. For clinical and prognostic purposes they are divided into:

subependymal heterotopia;

subcortical heterotopia;

diffuse grey matter heterotopia, also called band heterotopia or "double cortex" heterotopia.

MR is superior to CT scan in the detection and definition of this kind of anomaly (Fig.1). On MR heterotopia remains isointense with grey matter in all imaging sequences and does not show contrast enhancement.