What is Hemimegalencephaly?

Hemimegalencephaly (HME), also know as unilateral megalencephaly, is a rare malformation characterized by the enlargement of one side of the brain. The affected hemisphere may have focal or diffuse neuronal migration defects, with areas of polymicrogyria, pachygyria, and heterotopia.


Children with this disorder may have a large, asymmetrical head accompanied by seizures, partial paralysis, often associated with cognitive or behavioral disabilities. Seizures in association with HME often begin in early infant life including an association with infantile spasms.


Anti-seizure medications usually are not effective and surgery is often recommended to control the seizures. If the affected side is surgically removed (anatomic hemispherectomy) or disconnected from the other brain structures (functional hemispherectomy), the remaining side of the brain may gradually take over the functions normally performed by the affected side.


The prognosis for children with hemimegalencephaly is poor.

1. Satoa N, Yagishitac A, Obad H, et al. Hemimegalencephaly: A Study of Abnormalities Occurring Outside the Involved Hemisphere. American Journal of Neuroradiology 2007; 28: 678-682