Hemimegalencephaly

Hemimegalencephaly, also called unilateral megalencephaly, is a rare congenital disorder in which all or part of one cerebral hemisphere is abnormally enlarged. Its main feature is severe, frequent seizures that are hard to control, sometimes numbering in the hundreds a day and usually beginning soon after birth, although onset can come later in infancy or, rarely, early childhood. Other signs include an enlarged or asymmetrical head, developmental delay, and progressive weakness and loss of vision on one half of the body. The condition is linked to somatic activation of the AKT3 gene, and is thought to arise from excessive early proliferation of neurons and overgrowth of the developing cortex. It is suspected in infants with intractable seizures, shown on CT or ultrasound as an enlarged, distorted hemisphere, and sometimes found before birth. Medicines occasionally help, but the main treatment is surgery to remove or disconnect the affected hemisphere, ideally early in life while brain plasticity is greatest.