Specific Imaging Findings. Lissencephaly is characterized by absent (agyria) or decreased (pachygyria) cortical convolutions, resulting in a smooth cerebral surface. The affected cortex is abnormally thick, usually measuring 10-15 mm in agyria with almost no visible sulcation, and 6-9 mm in pachygyria (also called “incomplete lissencephaly”) with the presence of only a few shallow sulci and broad gyri. Complete agyria or pachygyria are unusual, and most cases are a combination of agyria and pachygyria. The thick, smooth cortex characteristically shows a peripheral stripe of high T2 signal and low CT attenuation, corresponding to sparse cell zone. The white matter is very thin and may be reduced to periventricular areas only. Several different patterns of lissencephaly have been described depending on the severity of the gyral simplification and the gradient along the anterior to posterior axis, with good correlation between the phenotypic spectrum and the underlying genetic abnormality. The most severe pattern is characterized by complete agyria with smooth cerebral surface and absent opercularization (“figure eight” configuration of the brain). Children with LISI and TUBA1A mutations have predominant posterior lissencephaly with a posterior to anterior (P>A) gradient. TUBAIA mutations may also show perisylvian pachygyria with typical dysgenesis of the anterior limb of the internal capsules. Children with DCX mutations have predominant anterior lissencephaly, with anterior to posterior (A>P) gradient. Other lissencephaly-associated abnormalities may include microcephaly, commissural abnormalities (in particular callosal anomalies), brainstem abnormalities (such as severe hypoplasia of the pons and medulla) and abnormalities of the cerebellum (in particular hypo/dysplasia of the vermis).
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