Hemimegalencephaly (HME), or unilateral megalencephaly, is a rare congenital disorder affecting all or a part of a cerebral hemisphere.

[6] It is a disorder related to excessive neuronal proliferation and hamartomatous overgrowth affecting the cortical formation.

[7] The excessive proliferation is postulated to occur early and to possibly continue beyond the normal proliferative period.

Epidermal growth factor is thought to play an important role in the excessive proliferation and the pathogenesis of HME.

However, it has a high mortality [recent citations needed], and there have been reports of a vegetative state and seizures resuming, this time in the healthy hemisphere.