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The term polymicrogyria defines an excessive number of small and prominent convolutions spaced out by shallow and enlarged sulci, giving the cortical surface a lumpy aspect. Cortical infolding and secondary, irregular, thickening due to packing of microgyri are visible on MRI although mild forms are difficult to recognize on neuroimaging. Two histological types are recognized. In unlayered polymicrogyria, the molecular layer is continuous and does not follow the profile of the convolutions, and the underlying neurons have radial distribution but no laminar organization. In four-layered polymicrogyria, there is an intermediate layer of intracortical laminar necrosis with consequent impairment of late migration and post migratory disruption of cortical organization. The two subtypes do not necessarily have a distinct origin as both may coexist in contiguous cortical areas.
Polymicrogyria can be focal or diffuse, unilateral or bilateral. It can occur as an isolated lesion, in association with other brain malformations such as heterotopia or white matter lesions, or as part of several multiple congenital anomaly/ mental retardation syndromes. The extent of polymicrogyria varies from focal polymicrogyria in otherwise normal brain to diffuse polymicrogyria with multiple other brain abnormalities. Similarly, the spectrum of clinical manifestations ranges from normal individuals with only selective impairment of cognitive function and no or easily controlled epilepsy to patients with severe encephalopathies and intractable epilepsy.
Several syndromes featuring bilateral polymicrogyria have been described, including bilateral perisylvian polymicrogyria, bilateral parasagittal parietooccipital polymicrogyria, bilateral frontal and frontoparietal polymicrogyria and unilateral perisylvian or multilobar polymicrogyria. These different forms might represent distinct entities that reflect the influence of regionally expressed developmental genes.