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Paediatric Imaging

Heterotopia

(singular, heterotopion), collections of normal neurons in abnormal locations and caused by an arrest of migration of the neurons to the cerebral cortex. This results in the presence of nodular masses in the subependymal regions of the lateral ventricles, subependymal heterotopia, or in the subcortical white matter, subcortical hetertopia. Symmetrical band-like or laminar heterotopias (Fig.1) may also be seen in the white matter of the cerebral hemispheres producing a 'double-cortex', also known as X-linked lissencephaly which has been found to be caused by a mutation of chromosome Xq22.3.

Though genetic causes may occur, the cause of most heterotopias is unknown, however, the insult occurs not later than the 5th month of gestation. Maternal ingestion of mercury and alcohol have been implicated in the condition.

The more common subependymal heterotopia may be single or multiple (Fig.2). They tend to be located in the region of the ventricular trigones, temporal and occipital horns. These are rarely familial, however, if they are large in number and line along the entirety of the lateral ventricles, they may be caused by a mutation of chromosome Xq28. Subcortical heterotopias may appear as multinodular, swirling or curvilinear masses of grey matter. The overlying cortex is often abnormal with thin cortex and shallow sulci. Associated congenital malformations such as agenesis of the corpus callosum are frequent. The affected hemisphere may also be small. The clinical manifestations of grey matter heterotopia is very variable.

Patients with isolated subependymal heterotopia usually present with a seizure disorder in the second decade of life. Patients with focal subcortical heterotopia have a variable motor and intellectual disturbance depending on the size and site of the heterotopion. Almost all will develop seizures.

Patients with band heterotopia may present at any age with variable developmental delay and seizure disorder, however, normal patients with only mild seizures disorders have been described.

The hallmark of all of these abnormalities is that they follow grey matter signal on all MRI sequences and do not enhance after administration of intravenous contrast. This helps distinguish them from the hamartomata of tuberous sclerosis.

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Fig.2

Sagittal T1-weighted (a) and axial T2-weighted (b) image showing multiple irregular nodules in the subependymal region in the ventricular walls, typical of heterotopia.
Heterotopia, Fig.1
Heterotopia, Fig.2 (a)
Heterotopia, Fig.2 (b)