Classifications
The degree of cerebral cortex malformation caused by genetic mutations is classified by the degree of malposition and the extent of faulty grey matter differentiation.
Neuronal migration disorders are generally classified into three groups:
- lissencephaly/subcortical band heterotopia
- cobblestone
- ‘other’ heterotopias
The ‘other’ types are associated with corpus callosum agenesis or cerebellar hypoplasia while the cobblestone lissencephalies are associated with eye and muscle disorders.
Classical lissencephaly, also known as type I or generalized agyria-pachygyria, is a severe brain malformation of a smooth cerebral surface, abnormally thick (10-20mm) cortex with four layers, widespread neuronal heterotopia, enlarged ventricles, and agenesis or malformation of the corpus callosum. Classical lissencephaly can range from agyria to regional pachygyria and is usually present along with subcortical band heterotopia (known as ‘double cortex’ to describe the circumferential bands of heterotopic neurons located beneath the cortex). Subcortical band heterotopia is a malformation slightly different from lissencephaly that is now classified under the agyria-pachygyria-band spectrum because it consists of a gyral pattern consistent with broad convolutions and an increased cortical thickness. The established classification scheme for lissencephaly is based on the severity (grades 1-6) and the gradient.
- Grade 1: generalized agyria
- Grade 2: variable degree of agyria
- Grade 3: variable degree of pachygyria
- Grade 4: generalized pachygyria
- Grade 5: mixed pachygyria and subcortical band heterotopia
- Grade 6: subcortical band heterotopia alone
- Gradient ‘a’: from posterior to anterior gradient
- Gradient ‘b’: from anterior to posterior gradient
Grade 1 and Grade 4 are very rare. Grade 2 is observed in children with Miller-Dieker syndrome (a combination of lissencephaly with dysmorphic facial features, visceral abnormalities, and polydactyly). The most common lissencephaly observed, consisting of frontotemporal pachygyria and posterior agyria, is Grade 3. Another malformation worth mentioning because of its connections to pachygyria is polymicrogyria. Polymicrogyria is characterized by many small gyri separated by shallow sulci, slightly thin cortex, neuronal heterotopia and enlarged ventricle and is often superimposed on pachygyria.
Read more about this topic: Pachygyria