Epilepsy: Functional Hemispherectomy

What Is a Functional Hemispherectomy?

The largest part of the brain, the cerebrum, can be divided down the middle lengthwise into two halves, called hemispheres. A deep groove splits the left and right hemispheres, which communicate through a thick band of nerve fibers called the corpus callosum. (Each hemisphere is further divided into four paired sections, called lobes - the frontal, parietal, occipital and temporal lobes.)

The two different sides or hemispheres are responsible for different types of thinking. Most individuals have a distinct preference for one of these styles of thinking and tend to have one side of the brain function much more than others. For example, left hemisphere thinkers are logical, analytical, objective, while right hemisphere thinkers are intuitive, creative, subjective, holistic thinkers.

A functional hemispherectomy is a procedure in which portions of one hemisphere - which is used the least - are removed, and the corpus callosum is cut. This disconnects communication between the two hemispheres, preventing the spread of seizures to the functional side of the brain.

Who Is a Candidate for a Functional Hemispherectomy?

This procedure generally is used only for people who do not experience improvement in their condition after taking medication and who have severe, uncontrollable seizures beginning in a non-functioning hemisphere. This type of epilepsy often occurs in young children who have an underlying disease, such as Rasmussen's encephalitis or Sturge-Weber syndrome, which has damaged the hemisphere.

What Happens Before Surgery?

Candidates for functional hemispherectomy undergo an extensive pre-surgery evaluation-including seizure monitoring, electroencephalography (EEG) and magnetic resonance imaging (MRI). These tests help the doctor identify the damaged hemisphere and confirm it as the source of the seizures.  An intracarotid amobarbital test, also called a WADA test, is done to determine which hemisphere is dominant for critical functions such as speech and memory. During this test, each hemisphere is alternately injected with a medication to put it to sleep. While one side is asleep, the awake side is tested for memory, speech and ability to understanding speech.

What Happens During Surgery?

A functional hemispherectomy requires exposing the brain using a procedure called a craniotomy. ("Crani" refers to the skull and "otomy" means "to cut into.") After the patient is put to sleep (general anesthesia), the surgeon makes an incision (cut) in the scalp, removes a piece of bone and pulls back a section of the dura, the tough membrane that covers the brain. This creates a "window" in which the surgeon inserts special instruments for removing brain tissue. Surgical microscopes are utilized to give the surgeon a magnified view of the brain structures. During the procedure, the surgeon removes portions of the affected hemisphere, often taking all of the temporal lobe but leaving the frontal and parietal lobes. The surgeon also gently separates the hemispheres to access and cut the corpus callosum. After the tissue is removed, the dura and bone are fixed back into place, and the scalp is closed using stitches or staples.