Unlike lissencephaly, which is defined by a reduction in brain folding, polymicrogyria is characterised by excessive small convolutions in the cortex. Sometimes, however, if the convolutions are very small it can be difficult to distinguish between these two diseases on an MRI scan. There are numerous types of polymicrogyria which are classified according to the anatomical location of the folding. For instance, bilateral frontal polymicrogria affects both sides of the cortex at the front.
Patients with polymicrogyria present with a very wide range of traits that include: developmental delay, motor dysfunction, language problems, learning difficulties and epilepsy. Like other brain abnormalities, polymicrogyria can be caused by problems during pregnancy (e.g. lack of oxygen or interruption of the blood supply to the brain), and genetic causes. Mutations in three genes are known to cause polymicrogyria; TUBB2B, GPR56 and SRPX2