Hemimegalencephaly true SNOMEDCT_US:253170008 Enlargement of all or parts of one cerebral hemisphere. The affected hemisphere may have focal or diffuse neuronal migration defects, with areas of polymicrogyria, pachygyria, and heterotopia. human_phenotype Hemimegalencephaly is a malformation of cortical development, where there is an abnormally large and malformed cerebral hemisphere, with abnormal cortical formation, white matter volume and ventricular morphology. The other hemisphere is usually normal, but may be smaller in size. Hemimegalencephaly can co-occur with other structural abnormalities including enlargement of the ipsilateral cerebellar hemisphere and brainstem, polymicrogyria and grey matter heterotopia. It can occur in tuberous sclerosis, linear nevus sebaceous syndrome (50% have hemimegalencephaly), hypomelanosis of Ito, neurofibromatosis, Proteus syndrome and in Klippel-Trenaunay-Weber syndrome. HP:0007206 MSH:D065705 UMLS:C0431391 Malformations of Cortical Development