An Unusual EEG Pattern Associated with a Complex Malformation of Cortical Development.

A 5-month-old infant with macrocephaly and dysmorphism presented with seizures. Magnetic resonance imaging of the brain showed fused lateral ventricles and the absence of mid-line structures (inter-hemispheric fissure, septum pellucidum, and corpus callosum). A large dorsal cyst was seen, pushing the thin strip of the remaining fused cerebral cortex anteriorly. Imaging was suggestive of alobar holoprosencephaly [Figure 1a]. The electroencephalogram (EEG) showed a decreasing gradient of electrical potentials from the frontal electrode to the occipital electrode, with an almost isoelectric line over the most posterior leads [Figure 1b]. The electrical isolation of the posterior electrodes from the thin strip of the cerebral cortex by the huge dorsal cyst might be considered as the most plausible hypothesis for this unusual EEG pattern.[12]

Figure 1

(a): (Left) Sagittal T2 – A strip of cerebral parenchymal tissue over the frontal region (small arrow). A dorsal cyst occupies the rest of the supratentorial compartment (big arrow). (a): (Right) Axial T2 – Single ventricle (small arrow) and the absence of mid-line structures (inter-hemispheric fissure, septum pellucidum, and corpus callosum). The cerebral cortex is fused and pushed anteriorly by the large dorsal cyst (big arrow). (b): EEG in the bipolar longitudinal double banana (left) and referential common average (right) montages. Note the decreasing gradient of potentials from the frontal to occipital electrode chains, with an almost isoelectric recording in the most posterior leads, especially on the bipolar montage (arrows)

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