These surgeries involve cutting and dividing fiber bundles that connect portions of the brain. The rationale is to separate the area of the brain generating the seizures from the normal brain.
The corpus callosum is one of the main fiber bundles that connect the two hemispheres. When debilitating, generalized seizures or falling-type seizures start on one side of the brain and quickly spread to the other, patients may be candidates for this palliative procedure, meaning that although seizures may improve, they usually do not disappear. A large part of this fiber bundle may be cut.
Functional hemispherectomy involves separating the abnormal hemisphere from the normal one by disconnecting fibers that communicate between the two. Often, some portions of the abnormal brain are surgically removed in order to perform this disconnection.