Subependymal grey matter heterotopia

Subependymal grey matter heterotopia, also known as periventricular heterotopia, is the most common form of grey matter heterotopia and is characterized by nodules of grey matter located immediately beneath the ependyma of the lateral ventricles. It can be divided according to morphology into ²:

• unilateral focal
• bilateral focal
• bilateral diffuse: an undulating band of grey matter surrounding the ventricles

Epidemiology

Although the majority of cases are sporadic, some are X-linked recessive (Xq28) ³. Affected females have relatively mild cognitive deficits and tend to develop epilepsy later. Many affected males are spontaneously aborted, presumably due to cardiovascular defects. Those that survive have more profound disability ³.

Clinical presentation

They most commonly are associated with epilepsy and sometimes developmental delay ².

Pathology

As with other grey matter heterotopias, subependymal heterotopia is thought to result from interruption of normal neuronal migration. For further discussion please refer to grey matter heterotopia. It is also possible that some cases result not from halted migration but from abnormal proliferation ³.

The nodules themselves consist of clusters of neurons and supporting glial cells ³. It is interesting to note that they are more commonly seen on the right, thought to be due to the later migration of neurons on the right ².

In X-linked cases the abnormal gene product is filamin-1, a protein which cross-links intracellular actin ³. In addition filamin-1 also has an important role in vascular development.

Radiographic features

Although faintly visible on CT, are rarely on ultrasound (only if very large), MRI is the modality of choice in assessing patients with subependymal heterotopia.

Ultrasound

Subependymal heterotopic nodules are slightly hyperechoic compared to normal white matter and they can be seen to bulge into the lumen of the ventricles (thus visible as undulation of the ventricular margin) ³.

CT

CT demonstrates non-enhancing, non-calcified tissue with attenuation similar to normal grey matter along the ventricular margins, bulging into the lumen.

MRI

Antenatal MRI

Late during gestation the diagnosis of subependymal heterotopia is relatively straight forward. Prior to 26 weeks gestation the presence of normal telencephalic periventricular germinal matrix does make identification difficult, as does fetal movement ³.

Postnatal MRI

Small nodules of grey matter are seen immediately deep to the ependymal layer, elevating it and distorting the outlines of the ventricles. They are most frequently seen in the region of the trigones and occipital horns ². The remainder of the brain appears normal, with normal adjacent white matter, normal deep grey matter nuclei and normal overall morphology of the brain ¹.

The nodules follow grey matter on all sequences including on post contrast sequences where they, like normal grey matter, do not enhance ².

For a discussion of fMRI and MR spectroscopy please refer to the article heterotopic grey matter.

Differential diagnosis

• normal periventricular grey matter
  • caudate nucleus
  • thalamus
• normal germinal matrix (8-26 weeks of gestation) ³
  • no irregularity of ventricular wall
  • heterotopic nodules may be lower in T2 signal
• subependymal giant cell astrocytoma
  • usually enhance brightly with contrast
  • usually located at or near foramen of Monro
  • usually in the context of tuberous sclerosis
• subependymal tuber of tuberous sclerosis
  • usually calcified (except in early childhood)
  • higher T2 signal than normal grey matter
  • usually other stigmata of TS are present
• subependymal hemorrhage (ultrasound and antenatal MRI) ³
  • although may appear very similar, follow-up imaging will demonstrate expected evolution