Lissencephaly
Neonates with lissencephaly may present with seizures. The brain of a patient with lissencephaly shows a smooth cortical surface. Pathological examination of the brain surface in patients with lissencephaly may show agyria (absence of gyri) or pachygyria (few broad and flat gyri) or polymicroglia. Lissencephaly are classified into types I, II, III, and IV, based on the morphology of the brain and associated brain anomalies. The only common finding among the different types of lissencephaly is that the brain surface looks smooth. Lissencephaly is diagnosed by CT or MRI of the brain. Magnetic resonance imaging is the study of choice.

Lissencephaly type I

Lissencephaly type I results from a complete arrest of cortical neuronal migration between 12- and 16-weeks gestation (Figure 47.1 [A]). The MRI of the brain has a figure-8 appearance on axial images (Figure 47.1 [A]). This appearance results from the smooth brain surface, large and vertically placed Sylvian fissure, hypoplastic operculum, and enlarged ventricles. The MRI of the brain also shows that the cerebral cortex has 2 bands of cortical gray matter (Figure 47.1 [A]), an outer layer that is thin and an inner layer that is thick. The outer and inner layers are separated by a zone of white matter. Cerebral anomalies often associated with lyssencephaly type I are hypoplasia of the corpus callosum, colpocephaly (enlargement of the occipital horns of the lateral ventricles), and brainstem hypoplasia. The cerebellum and third and fourth ventricles are normal. Neonates with lissencephaly type I do not have ocular or muscle abnormalities.
Lyssencephaly type I may occur as: (1) isolated lissencephaly syndrome (no specific dysmorphysm); (2) Miller-Dieker syndrome (specific dysmorphysm and deletion of the distal part of the short arm of chromosome 17); and (3) Norman-Robert syndrome (dysmorphic features but no chromosome 17 abnormality). Miller-Dieker syndrome is probably the most common.

Miller-Dieker syndrome
Neonates with Miller-Dieker syndrome have characteristic facial features: microcephaly with bitemporal narrowing, vertical ridging and furrowing in the central forehead (especially when crying), small nose with antiverted nostrils, upslanting palpebral fissures, protuberant upper lip, thin vermilion border of upper lip, and micrognathia (Figure 47.1 [B]).
A deletion in the p13 region of chromosome 17 is present in patients with Miller-Dieker syndrome. Parents of patients with Miller-Dieker syndrome should undergo genetic evaluation to determine whether they are carriers of a balance translocation of the terminal fragment of chromosome 17 onto a chromosome in the 13-15 group.

Figure 47.1.— Miller-Dieker syndrome. [A] T1-weighted MRI axial image demonstrates the typical figure-8 appearance. The cortical surface is smooth with no secondary sulci. Thin outer cortical band separated from a thick inner cortical band by a zone of white matter (best appreciated in the regions of the sylvian groove). [B] Facial characteristics of Miller-Dieker syndrome.