Hemimegalencephaly

Hemimegalencephaly or unilateral megalencephaly is a congenital disorder in which there is hamartomatous overgrowth of all or part of a cerebral hemisphere. The affected hemisphere may have focal or diffuse neuronal migration defects, with areas of polymicrogyria, pachygyria, and heterotopia.

Magnetic resonance imaging (MRI) reveals abnormal gyration, ventriculomegaly, colpocephaly, an \"occipital sign\" (displacement of the occipital lobe across the midline), and increased volume and T2 signal of white matter, in addition to the overall increased size of the involved hemisphere.