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(CC) Image: Lefèvre and Mangin, 2010
Gyrification from a clinical perspective: Normal adult human cortical surface (left), polymicrogyria (center) and lissencephaly (right).

Polymicrogyria (abbrev. PMG, also known as cobblestone lissencephaly or microgyria) is a neurodevelopmental disorder with abnormal gyrification of the cerebral cortex. Instead of the few and large convolutions (gyri) typical for normal brain development, multiple small convolutions occur, as a result of which the brain surface appears pebbled. Histologically, polymicrogyria is characterized by a strong reduction (or even the complete lack) of layer V of the cerebral cortex. Further diagnostic criteria include white matter reduction, ventricular enlargement, and hypoplasia of the brainstem and cerebellum (Pabuşçu et al., 2002; Jansen and Andermann, 2005). At least two genes have been found to be associated with polymicrogyria — GPR56 (Piao et al., 2004) and AHI1 gene, though in the latter case, polymicrogyria is only one aspect of a wider disorder, Joubert syndrome (Dixon-Salazar et al., 2004). Polymicrogyria is also frequently associated with deletion 22q11.2 syndrome (Robin et al., 2006), possibly due to perinatal cardiovascular effects (Barkovich et al., 1995).