NeuroradiologyLissencephaly
means "smooth brain" and describes a condition of complete absence of gyri or poor gyral formation. Complete lissencephaly is synonymous with agyria; the incomplete form is synonymous with agyria/pachygyria. The term complete lissencephaly or agyria should be used to refer only to type I lissencephaly. This condition is frequently due to a defect on chromosome 17. Lissencephaly represents a neuronal migration anomaly in which the cortex is ultimately composed of a thin outer layer of neurons, a "cell-sparse zone" and an inner thick layer of arrested neurons.
Affected children are severely retarded, microencephalic and hypotonic and develop refractory seizures within the first year of life.
MR (Fig.1) reveals a thickened smooth cortex with a straight greywhite matter interface, and open and vertically oriented sylvian fissures giving a typical figure-of-eight appearance to the brain. The thin outer layer of cortex is separated from a thick deeper layer of arrested neurons by a "cell-sparse zone" that resembles normal white matter. The posterior segments of the ventricular system are enlarged. The brain stem is often hypoplastic with a fetal appearance. A second form is type II lissencephaly frequently found in WalkerWarburg syndrome. It has a different MR picture, consisting of thickened cortex with shallow sulci, a characteristic irregular greywhite matter junction ("cobblestone" appearance), hydrocephalus, hypomyelination of the white matter, and callosal agenesis associated with microphthalmia. Also, see agyriapachygyria complex.
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a, b. MR images T1-weighted (a) and T2-weighted (b). Complete absence of gyri is seen (agyria), a thick smooth cortex, and a "truncated" aspect of the white matter that lacks the usual digitated appearance.
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Lissencephaly, Fig.1 (a) | | Lissencephaly, Fig.1 (b) | |