Paediatric Imaging

Lissencephaly

smooth brain surface, caused by a paucity of gyri on the surface of the brain. Complete agyria (Fig.1) is synonymous with complete lissencephaly. Agyria-pachygyria refers to the presence of a combination of smooth areas and broad flat gyri, also known as incomplete lissencephaly. Both of these entities are now known as classical lissencephaly which is caused by an arrest of the migration of neurons from the germinal matrix zone to the surface of the brain. The brain is smooth with a markedly thickened cortex with vertically oriented sylvian fissures giving the brain the figure of eight appearance, similar to that of a normal foetus prior to 24 weeks gestation before the cortical gyri begin to form. There is a thin outer layer of cortex with a cell-sparse zone lying immediately deep to this, followed by a thick inner layer of neurons.

Some patients with classical lissencephaly have mutations of either 17p13.3 (Miller-Dieker syndrome), or chromosome Xq22.3-23 (X-linked lissencephaly). Many of these patients have incomplete lissencephaly with areas of pachygyria and agyria. The agyria may be most severe in the frontal lobes in those with X-linked lissencephaly, whereas in the Miller-Dieker syndrome the agyric areas are seen in the parieto-occipital regions.This group of patients usually present early in life with hypotonia and seizures, with later development of limb and oropharyngeal spasticity.

A second group of lissencephalies is known as cobblestone lissencephaly which is caused by overmigration of neurons. This latter group has a high incidence of associated muscular dystrophy and myelination delay. Ocular abnormalities and hydrocephalus may also be present.The cerebral cortex is thickened with a cobblestone appearance of its inner margin usually in the parieto-occipital regions. This may be associated with areas of polymicrogyria in the frontal lobes. A number of defined congenital muscular dystrophies fall into this group including Fukuyama congenital muscular dystrophy, Walker Warburg syndrome and muscle-eye-brain disease. Specific features associated with Walker Warburg syndrome include posterior encephalocoele, congenital hydrocephalus, agenesis of the corpus callosum and eye abnormalities. Vermian hypogenesis and cerebellar polymicrogyria may also be seen. Eye abnormalities are less common in patients with Fukuyama type congenital muscular dystrophy.

A third group of lissencephalies is caused by reduced proliferation of neurons and glia in the germinal zones resulting in microlissencephaly in which the brain is small and there are too few gyri and abnormally small sulci. These patients have microcephaly. Radial microbrain and microcephalia vera (true micrencephlay) have now been reclassified into this group.

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Fig.1

T2-weighted MR image of a child with complete absence of cerebral gyri, the typical appearance of a "pancake" brain of lissencephaly.
Lissencephaly, Fig.1