Clinical History: 29 year old with possible atypical absence seizures.
Diagnosis: Gray Matter Heterotopia
Discussion: Gray matter heterotopia is defined as abnormally positioned but otherwise normal neurons. These cells are found anywhere along their normal migration path along radial glial fibers from ependyma to cortex. Multiple variations are described including focal or diffuse, and nodular or band. The lesions always appear isodense and isointense to normal gray matter. No enhancement is demonstrable. Clinically patients present with seizures, in some cases these are intractable. Isolated familial occurrences are reported though the majority are sporadic. Nuclear studies have shown glucose metabolism equal to that of normally positioned gray matter.
In the case presented focal nodular gray matter heterotopia is shown on proton, T2, T! and T1 contrasted images. The abnormal neuronal collection is seen in a subependymal position along the superolateral border of the mid left lateral ventricle. Differential consideration might include a subependymal nodule of tuberous sclerosis. However no other associated abnormalities and no abnormal calcifications are present. Subependymal nodules in Tuberous Sclerosis do not follow gray matter signal.
References: Osborne,AG, Diagnostic Neuroradiology, Mosby, pp.44-52,1994.
Kamurok,K & Tenokuchi,Y, Familial and PeriventricularNodular Heterotopia, Brain & Development 15:237-41, 1993.
Huttenlocher,PR, Taravath,S & Mojtahedi,S,Periventricular Heterotopia and Epilepsy, Neurology 44:51-55,1994.
Miura, K, et al., Magnetic Resonance Imaging and PositronEmission Tomography of Band Heterotopia, Brain & Development15:288-90, 1993.
Submitted by:Jeffrey L. Sunshine, M.D.Jonathan S. Lewin, M.D.Neuroradiology

Heterotopic Grey Matter

Heterotopic Gray Matter
Heterotopic gray matter is caused by arrested migration of neurons from periventricular germinal zone to cortex. It can be inherited or acquired (maternal trauma, infection or toxin). It can be band-like (laminar) (Figure A) or nodular (Figure B). Nodular heterotopias can be focal or diffuse. Subependymal heterotopias are the most common, seen as nodules indenting the ventricles. Heterotopic gray matter is isodense with gray matter on CT and isointense with gray matter on MRI. Band heterotopias resemble double cortex, the inner heterotopic band may be thick or thin and continuous or discontinuous. Overlying cortex may be dysplastic or it may be normal, depending on the amount of heterotopic gray matter. Calcification is very rarely seen in the heterotopic gray matter.
Figure : Heterotopic Gray Matter. A, Axial MR image showing a band of heterotopic gray matter bilaterally (Arrows) with thin overlying cortex, giving an appearance of double cortex. B, Axial MR image showing subependymal nodular heterotopic gray matter (Arrows) lining both lateral ventricles.

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