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 types of seizures associated with lissencephaly include:

persisting spasms
focal seizures
tonic seizures
atypical seizures
atonic seizures Other possible symptoms of lissencephaly include telecanthus, estropia, hypertelorism, varying levels of mental retardation, cerebellar hypoplasia, corpus callosum aplasia, and decreased muscle tone and tendon reflexes. Over 90% of children affected with lissencephaly have seizures.Patients with subcortical band heterotopia (another disorder associated with pachygyria) typically have milder symptoms and their cognitive function is closely linked to the thickness of the subcortical band and the degree of pachygyria present. Causes
Pachygyria is caused by a breakdown in the fetal neuronal migration process due to genetic or possibly environmental influences. The affected cerebral cortex will typically have only four developed layers instead of the normal six. One of the best known and most common types of neuronal migration disorders is lissencephaly, a diffuse cortical malformation relating directly to agyria and pachygyria. Incomplete neuronal migration during the early fetal brain development is the precursor to lissencephaly. Should neurons follow an abnormal migration during development possible cortical malformations include classical lissencephaly (as stated above) and subcortical band heterotopia with an agyria-pachygyria band spectrum.