Specific Imaging Findings. In bilateral perisylvian polymicrogyria (BPP), MRI demonstrates an abnormal configuration of the cortical ribbon and irregular gray-white matter junction suggestive of multiple small gyri bilaterally around the sylvian fissures. The mildest forms of perisylvian polymicrogyria involve part of the perisylvian cortex, usually the posterior region, while the most severe forms extend beyond the perisylvian area to the frontal, occipital, and temporal lobes with a perisylvian gradient (i.e. maximal severity in the perisylvian cortex). The spectrum of cortical morphology is wide: the cortex may be normal to thick with a delicate or coarse appearance, and the cortical surface may range from bumpy and irregular to smooth. Moreover, the appearance of BPP depends on the stage of maturity/myelination of the brain: in unmyelinated regions, the inner surface of the polymicrogyric cortex looks thin and finely undulated, while in myelinated areas it looks thicker and relatively smooth. The underlying white matter usually appears decreased in volume. The sylvian fissures have an abnormally verticalized orientation extending far more posteriorly than normal into the parietal regions and may be abnormally forked along their course. The fronto-temporo-parietal opercula present an abnormally open appearance, which is frequently associated with an overlying large anomalous venous structure. CT may show abnormal sylvian fissures and thickened adjacent cortex.
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