The grey matter heterotopias are a relatively common group of conditions characterised by interruption of normal neuronal migration from near the ventricle to the cortex, thus resulting in "normal neurons in abnormal locations" 2. They are a subset of disorders of cortical formation 3-4.
Grey matter heterotopias can be divided macroscopically into:
- nodular heterotopias
- diffuse heterotopias
These are discussed in detail separately. The rest of this article concerns itself with a general overview.
Although heterogeneous, some forms are X linked as such there is an overall predilection.
Patients most commonly present with partial seizures in the second decade of life. Additionally, and depending on extent, children may demonstrate developmental delay or mental retardation 3. Grey matter heterotopias are seen with greater frequency in patient with other congenital central nervous system anomalies, and these associated anomalies will also affect symptomatology. Associated anomalies include 3,10:
- agenesis of the corpus callosum
- Chiari II malformation
- basilar cephalocoeles
Grey matter heterotopias are believed to be due interruption of the normal migration of neurons from the periventricular telencephalic germinal matrix to the cortex and may be due to either genetic abnormalities or infection / trauma.
Neuroblasts proliferate in the germinal matrix between 7 and 8 weeks of gestation. Migration take place from 8 to 26 weeks gestation, and is maximal between 8 and 16 weeks 10.
As is the case with most imaging of the central nervous system, MRI is the modality of choice in assessing heterotopic grey matter due to its as yet unsurpassed contrast resolution.
Antenatal and neonatal ultrasound has difficulty identifying heterotopic grey matter as the echogenicity of such grey matter is not sufficiently different from the surrounding white matter 6.
Grey matter heterotopia has slightly higher density than the surrounding white matter and can thus be seen if sufficiently large. Thin or small regions may not be apparent.
On MRI the heterotopic tissue follows grey matter on all sequences. Their margins are often indistinct. Careful examination of the remainder of the brain is necessary to identify associated anomalies.
In general MR spectroscopy demonstrates a decrease in NAA/Cr ratio in the heterotopic grey matter compared to normal control subjects. It is important to note that comparing the spectrographic trace to the 'normal appearing contralateral side' is not necessarily valid as a control as metabolic abnormalities in patients with malformations of cortical development may be widespread7.
fMRI (BOLD imaging) can demonstrate activation in heterotopic nodules and these can match epileptogenic EEG discharges 8.
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