This group of disorders of cortical neuronal migration may arise from primary agenesis and also from secondary destruction of the cortex at an early stage of formation (Evrard et al. 1989). The most extreme manifestation is a smooth brain (lissencephaly) but usually there is some evidence of gyri and sulci. Pachygyria implies some evidence of primary and secondary gyration (Fig. 1). Polymicrogyria may also coexist.
Type I lissencephaly is the severest end of the range, with a thick, four-layered cortex and maximal developmental arrest. Although often sporadic, a dysmorphic syndrome, the Miller- Dieker syndrome, which is associated with a deletion on chromosome 17p13 (Van Tuinen et al. 1988; Ledbetter et al. 1992), is the best described example of type I lissencephaly. The deletion may be small and the dysmorphology absent.
Type II lissencephaly has a thin, disorganized cortex and is a feature of the Walker–Warburg syndrome (Pagon et al. 1983).
There are more than 15 syndromes associated with this group of disorders, some clearly genetic and others of uncertain cause (Winter and Baraitser 1991). They are discovered by CT or MRI. The range of clinical manifestations varies widely, with focal unilateral pachygyria presenting as focal epilepsy at the milder end of the spectrum (Andermann et al. 1987). The classification of this group of disorders is developing rapidly (Dobyns and Truwit 1995).
Origin: Brain's Diseases of the Nervous System