Definition:

Hemimegalencephaly is a rare brain malformation caused by anomalous neuronal and glial proliferation or differentiation, with an abnormally enlarged and dysplastic hemisphere. It can occur as an isolated finding, or as part of a syndrome, such as hypomelanosis of Ito, tuberous sclerosis complex, epidermal nevus syndrome or Klippel-Trenaunay syndrome. Patients have intellectual delay, hemiparesis and severe epilepsy. Drug resistant epilepsy is often treated with a hemispherectomy. If resective surgery is impossible, vagus nerve stimulation (VNS) can be used for seizure palliation. The etiology of hemimegalencephaly is not clear. Some authors consider abnormal neuroepithelial cell lineage as the primary cause.