Lissencephaly type II probably results from a defect that involves the external basal lamina and the external layer of the cortex. It is believed that the external basal lamina and the external layer of the cortex normally function as a boundary that prevents the fusion of adjacent gyri and keeps the cortical layer from invading the meninges. A defect in these structures leads to penetration of the cortical tissue into the meninges and fusion of the adjacent gyri.
The MRI of the brain shows smooth brain surfaces but the typical figure-8 appearance and the 2-band appearance of the cortex that are characteristic of type I lissencephaly are less prominent or not present. The cerebral cortex of a neonate with lissencephaly type II shows a very irregular gray-white matter junction. Lissencephaly type II is often associated with cerebellar abnormalities, hydrocephalus, hypoplasia of the corpus callosum, and hypomyelination. Neonates with lissencephaly type II have ocular anomalies and congenital muscular dystrophy.
Lyssencephaly type II may occur with: (1) Walker-Warburg syndrome (Figure 48.1), and (2) Fukuyama congenital muscular dystrophy. Fukuyama congenital muscular dystrophy has more hypomyelination and more muscular involvement than Walker-Warburg syndrome.

A	B	C

Figure 48.1.— Walker-Warburg syndrome. [A] Microphthalmia, [B] lissencephaly, and [C] agenesis of the cerebellar vermis.