Hemimegalencephaly is a rare malformation involving one side of the brain. It may occur alone or in association with other syndromes such as Proteus syndrome, tuberous sclerosis, linear sebaceous nevus syndrome, neurofibromatosis, Sturge-Weber syndrome, or Klippel-Trenaunay syndrome. Children with this disorder may have a large, asymmetrical head accompanied by seizures, partial paralysis, and impaired cognitive development. Because the seizures associated with hemimegalencephaly are difficult to treat with anticonvulsant medications, a surgery called hemispherectomy is often the most successful treatment. The cause of hemimegalencephaly is not fully understood, but involves a disturbance of cells early in development and likely involves genes involved in patterning and symmetry.

Yes, hemimegalencephaly and the symptoms associated with it can range from mild to severe. Individuals with hemimegalencephaly and normal intellectual development have been described in the medical literature. Some researchers have suggested that mildly affected individuals are less likely to be identified as having this condition.

Because the seizures associated with hemimegalencephaly are difficult to control with anti-epileptic medications, most patients undergo surgery to separate one hemisphere of the brain from the other (hemispherectomy). There are several surgical options to consider. One is functional hemispherectomy which involves severing the nerves and tissue connecting one side of the brain to the other, leaving the brain within the skull. Another is a complete or anatomic hemispherectomy in which the affected side of the brain is surgically removed. These surgeries are usually performed by a neurosurgeon with experience treating epilepsysyndromes. Over time, the remaining side of the brain may take over the functions lost.