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Heterotopic Gray Matter


KEY FACTS

Terminology

  • Heterotopia (HTP)

  • Arrested/disrupted migration of groups of neurons from periventricular germinal zone (GZ) to cortex

Imaging

  • Ectopic nodule or ribbon, isointense with gray matter on every MR sequence

  • Periventricular, subcortical/transcerebral, molecular layer

  • Periventricular HTP located next to periventricular white matter (GZ of cerebral mantle) but not in corpus callosum (fiber tract) or next to basal ganglia (GZ of ganglionic eminence)

  • Variable: From tiny to huge, isolated to diffuse

  • Thin-slice, high-definition 3D acquisition, heavily weighted T1 provides optimal contrast and definition

  • Large nodular HTP: Often thinned, polymicrogyric-looking overlying cortex

Top Differential Diagnoses

  • Tuberous sclerosis

  • “Closed-lip” schizencephaly

  • Tumors

Pathology

  • Periventricular nodular HTP often genetic when diffuse

    • FLNA gene commonly involved (required for cell migration to cortex) on Xq28

  • Band HTP: Mild form of type 1 (classic) lissencephaly (agyria/pachygyria/double cortex)

    • Predominantly posterior lissencephaly/band HTP: Deletion LIS1 located on 17p13.3

    • Predominantly anterior lissencephaly/band HTP: Deletion DCX = double cortin on Xq22.3-q23

Axial T2WI MR in a 6-year-old girl with refractory epilepsy shows massive right posterior subcortical heterotopia (HTP) containing cortex-like GM, WM, cerebrospinal fluid spaces
, and blood vessels
. The mass may suggest a tumor, but the hemisphere is small. Note the thin overlying cortex.

Axial DTI color FA map (same patient) depicts the complete disorganization of WM in and around nodular subcortical HTP
. Red, R-L; green, A-P; blue, S-I fiber orientation. Other hues imply intermediate directions.

Axial 3D T2WI MR shows subcortical band heterotopia in a 12-year-old boy. The symmetric HTP
lies below an intermediate layer of WM
. It is thicker posteriorly
, in keeping with LIS1 mutation (autosomal recessive). The overlying cortex looks essentially normal.

Axial T1WI MR in the same patient coregistered with magnetoencephalography (MEG) shows that the MEG spikes (triangles) originate from HTP, yet both the HTP and overlying cortex participate in the epileptogenic loop.

TERMINOLOGY

Abbreviations

  • Heterotopia (HTP); malformations of cortical development (MCD)

Synonyms

  • Gray matter heterotopia; double cortex = band HTP

Definitions

  • Heterotopias: Group of MCD characterized by presence of excessive neurons (either isolated or clustered) in subcortical white matter (WM)

    • Reflects arrested/disrupted migration of groups of neurons anywhere from periventricular germinal zone (GZ) to cortex

IMAGING

General Features

  • Best diagnostic clue

    • Nodule, clump of nodules or ribbon of tissue isointense with gray matter (GM) on every MR sequence but in wrong location

  • Location

    • 3 groups according to location/distribution of heterotopic neurons

      • –

        Neuronal heterotopia (characterized by individual misplaced neurons in WM)

      • –

        Nodular heterotopia (nodules of GM within WM)

      • –

        Band (laminar) heterotopia (double cortex)

    • GM heterotopia can be diffuse or localized; occur anywhere from ependyma to pia

      • –

        Diffuse: Subcortical laminar (band) heterotopia, extensive bilateral periventricular nodular heterotopia

      • –

        Localized: Subependymal (uni- or bilateral), subcortical (nodular, laminar), or unilateral transmantle extension from subependymal region to cortex

  • Size

    • Variable: From tiny to huge, isolated to diffuse

  • Morphology

    • Periventricular nodular heterotopia (most common)

      • –

        Focal/multifocal asymmetric GM indentation of ventricle

      • –

        Diffuse vs. anterior vs. posterior

    • Band heterotopia = laminar HTP, double cortex

      • –

        Thick symmetric subcortical GM band with thin cortex

    • Subcortical nodular heterotopia

      • –

        Focal HTP nodules, often single

        • □

          Large nodular HTP: Often thinned, polymicrogyria-like overlying cortex

      • –

        Multinodular, swirling GM mass continuous both with cortex and ventricular surface; contain GM, WM, sometimes pia, vessels, and CSF

    • Associated subcortical and periventricular HTP

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