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.
The clinical presentation of hemimegalencephaly is usually with early onset intractable focal seizures and hemiplegia. Seizure onset is typically in the first year of life, often in the neonatal period. Developmental, cognitive and motor impairments are frequent and significant in most cases.
CAUTION developmental and cognitive outcome may be worse in children with uncontrolled seizures in early life, especially if epileptic spasms or generalized seizure types appear, therefore proactive seizure control is important, this may include epilepsy surgery, if seizures are not controlled with medication.