The affected hemisphere may have focal or diffuse neuronal migration defects, with areas of polymicrogyria, pachygyria, and heterotopia.

Enlargement of all or parts of one cerebral hemisphere.

human_phenotype

HP:0007206

http://purl.obolibrary.org/obo/hp.owl

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.