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Hemimegalencephaly is a rare neurological condition in which one half of the brain, or one side of the brain, is abnormally larger than the other. The unusual and enlarged brain tissue causes frequent seizures, usually followed by mental retardation.

Malformations of cortical development (known sometimes as cortical dysplasias) occur during the development of the brain when neurons do not migrate and organize themselves normally. This can lead to intractable epilepsy.

Because seizures may be difficult to control with anticonvulsant medicine or other therapies, surgery is often the most successful treatment to control the seizures. There are several different surgical treatment options considered, including focal resections and hemispherectomies.

If the affected side of the brain is surgically removed, the remaining healthy side of the brain may gradually take over the functions whose control has been lost. The ability for one area of the brain to take over the function of others is known as "plasticity".

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