Functional hemispherectomy is any procedure which disables the function of one cerebral hemisphere but does not remove the hemisphere itself.
Rasmussen was the first epilepsy surgeon to develop the functional hemispherectomy technique and is the most common technique today. The temporal lobe is removed but the frontal pole and occipital pole are preserved. This provides access to connections in the front and back of the hemisphere and the midbrain which the surgeon cuts and allows the surgeon to perform a complete corpus callosotomy. The brain that is left behind is living because the veins and arteries which provide its blood supply are not cut. For this reason, the remaining part of the brain may still seize, but because the axonal connections are severed, the seizures do not spread and have no effect.
Risk of this procedure include incomplete disconnection. Incomplete disconnection rates have been reported between 7 – 52%, thus requiring reoperation. Hydrocephalus is also a post-operative risk, with 23% of children developing hydrocephalus after functional hemispherectomy.